Page 1

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY

CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024

Number 3657, 51 pp., 21 figures, 1 table

August 28, 2009

The Perinate Skull of Byronosaurus (Troodontidae)

with Observations on the Cranial Ontogeny of

Paravian Theropods

GABE S. BEVER1 AND MARK A. NORELL2

ABSTRACT

The skulls of two perinate paravians from Ukhaa Tolgod, Djadoktha Formation, Mongolia, are

described here. The skulls are nearly unique in their combination of ontogenetic age and

preservational quality and provide us with the first look at the morphology of such important

anatomical regions as the rostrum, palate, and braincase at or near the onset of postnatal

development in a nonavian paravian coelurosaur. Based on a number of derived characters, the

skulls are allocated to a derived position within Troodontidae that is outside the clade consisting of

Saurornithoides mongoliensis, Saurornithoides junior, Troodon formosus, and probably Sinornithoides

youngi. A single synapomorphy, presence of a lateral maxillary groove, supports the Ukhaa perinates

as Byronosaurus. The comparative morphology of the Ukhaa perinates with adult troodontids

indicates a number of significant postnatal transformations (e.g., elongation and flattening of the

rostrum, increase in the number of maxillary and dentary teeth, restructuring of the occipital plate

and paroccipital process). These comparisons demonstrate that many characters historically

considered important for phylogenetic and taxonomic assessments of adult maniraptorans are

present at a relatively early stage of ontogeny. Differences in the developmental timing of various

cranial characters have important implications for interpreting the fossil record as well as for

understanding the role heterochrony has played in the evolution of derived coleurosaurs, including

birds. The ontogenetic information provided by the Ukhaa perinates also allow us to comment on

the enigmatic paravian Archaeornithoides deinosauriscus, which has been considered both the sister

taxon to Avialae and a juvenile specimen of the troodontids Saurornithoides mongoliensis and

Byronosaurus jaffei. We found no unique characters that support a priviledged relationship of

Archaeornithoides deinosauriscus with avialans and only weak character support for this taxon as a

basal troodontid—there is no known character evidence supporting it as a juvenile of either

Saurornithoides or Byronosaurus.

Copyright E American Museum of Natural History 2009

ISSN 0003-0082

1 Division of Paleontology, American Museum of Natural History (gbever@amnh.org).

2 Division of Paleontology, American Museum of Natural History (norell@amnh.org).

INTRODUCTION

The remains of embryonic, neonate, and

juvenile dinosaurs are emerging from the fossil

record with increasing frequency (e.g., Norell

et al., 1994; Mateus et al., 1997; Xu et al.,

2001; Reisz et al., 2005; Goodwin et al., 2006;

Schwarz et al., 2007; Balanoff and Rowe,

2007; Balanoff et al., 2008; Kundrát et al.,

2008). This generalized trend is less obvious

within Theropoda, where the number of

known specimens representing these relatively

early stages of skeletal ontogeny—especially

those with well-preserved cranial material—

continues to be extremely limited (Rauhut and

Fechner, 2005). Our understanding of skeletal

development and its phylogenetic patterns in

this important clade, which includes the origin

of birds, is equally limited. Establishing the

nature and pattern of skeletal development,

and the phylogenetic distribution of the

features defining that development, is neces-

sary to fully understand the characters

through which we interpret the evolutionary

history of this, or any, group. The implications

of this understanding—or lack thereof—range

from the accuracy with which specimens of

different ontogenetic ages are identified taxo-

nomically, to the precision with which the

characters we use to build phylogenetic

hypotheses are defined, to our ability to

recognize transformations in developmental

patterns. In other words, our understanding of

skeletal development and its phylogenetic

patterns has implications for our ability to

effectively carry out many of the core objec-

tives of morphological research. The only way

to establish ourselves on the right side of these

implications is by finding new ontogenetically

and phylogenetically informative specimens

and describe them in detail.

During the 1993 field season of the

American Museum–Mongolian Academy of

Sciences joint expeditions (Novacek et al.,

1994), a weathered theropod nest was encoun-

tered at the rich Late Cretaceous fossil locality

of Ukhaa Tolgod. This nest contained an

embryo of a near hatchling oviraptorid

(Norell et al., 1994, 2001a). Associated with

the clutch of oviraptorid eggs were two skulls

of a nonoviraptorid theropod, the extremely

small size and morphology of which suggests

they are either late-stage embryos or hatch-

lings (i.e., perinates). These specimens were

noted in a short paper (Norell et al., 1994)

where they were referred to as dromaeosaur-

ids.

The purpose of this paper is to provide a

detailed description of these two skulls, and in

the process, critically assess both their taxo-

nomic status and importance for our under-

standing of ontogenetic and phylogenetic

transformations in theropod cranial morphol-

ogy. The relatively high preservational quality

of these skulls provides us with the first

opportunity to study several aspects of cranial

anatomy in a highly derived, nonavian thero-

pod. For example, the presence of a well-

preserved braincase in one of these specimens

gives us our first look at many features of the

otic capsule not previously described in non-

avialan Maniraptora. The Ukhaa perinate

skulls also allow us to comment on the status

of other small, but more fragmentary,

Mongolian theropods. Chief among these is

Archaeornithoides deinosauriscus, which is

based on a juvenile nonavian theropod that

originally was given a privileged relationship

with birds (Elzanowski and Wellnhoffer, 1992,

1993).

OCCURRENCE

The perinate skulls (IGM 100/972 and IGM

100/974; figs. 1–4) were found in a weathered

nest of at least six eggs in the Late Cretaceous

Xanadu sublocality of Ukhaa Tolgod,

Djadoktha Formation, Mongolia (Loope

et al., 1998; Norell et al., 1994; Dingus

et al., 2008; fig. 5). This number probably

represents only part of the total number of

eggs originally deposited in the nest. Our

experience at Mongolian Djadoktha and

Djadoktha-like localities (Norell et al., 1995)

and the observations of others (Sabath, 1991;

Mikhailov et al., 1994) show that nests of this

type usually contain 10 or more eggs. Because

embryonic remains of an oviraptorid theropod

were found inside one of the eggs (Norell et

al., 1994) and other occurrences of adult

oviraptorids associated with eggs of this

eggshell type are documented (Osborn, 1924;

Currie et al., 1993; Norell et al., 1995; Dong

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AMERICAN MUSEUM NOVITATES

NO. 3657

and Currie, 1996), the nest is inferred to be

that of an oviraptorid dinosaur.

TAXONOMIC IDENTIFICATION

The presence of numerous teeth, a close

packing of the dentition within the dentary

near the rostral tip of the lower jaw, a distinct

groove for the neurovascular foramina on the

buccal surface of the dentary, a dorsoventrally

flattened internarial bar, absence of a basi-

sphenoid recess, and a short, largely apneu-

matic paroccipital process are synapomor-

phies present in the perinate skulls that

diagnose them as Troodontidae (Xu et al.,

2002; Makovicky et al., 2003; fig. 6). Within

Troodontidae, these specimens are allocated

to the clade comprised of all known troodon-

tids besides Sinovenator changii, Mei long, and

probably Jinfengopteryx elegans based on the

derived presence of an extensive supraantor-

bital process of the lacrimal, a supraorbital

shelf of the lacrimal, a wedge-shaped nasal-

frontal suture, a highly pneumatized base of

the cultriform process, pneumatized basipter-

ygoid processes, and a constricted neck of the

occipital condyle. Within this derived clade,

the perinates lack a series of synapomorphies

that would support their inclusion in the clade

comprised of Troodon formosus, Saurornitho-

ides mongoliensis, and Saurornithoides junior,

and probably Sinornithoides youngi. The syna-

pomorphies supporting the monophyly of

Saurornithoides, Troodon, and Sinornithoides

Fig. 1. Right (A, A!) and left lateral (B, B!) views of IGM 100/972, a Late Cretaceous perinate

troodontid from Ukhaa Tolgod, Mongolia.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

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to the exclusion of the Ukhaa perinates

include a medially recurved dentary symphy-

sis and serrated teeth. Absence of these

synapomorphies does not support the alloca-

tion of the Ukhaa perinates to Byronosau-

rus jaffei—the possible sister taxon to the

Saurornithoides-Troodon-Sinornithoides clade

and the only other named taxon in this area

of the tree—but it does support the Ukhaa

perinates as either Byronosaurus jaffei, the

sister taxon to Byronosaurus jaffei, or the sister

taxon to the Saurornithoides et al. clade.

Byronosaurus jaffei was diagnosed as a new

troodontid based on the presence of unser-

rated teeth, an interfenestral bar that is not

recessed from the plane of the maxilla, and a

shallow groove along the buccal margin of the

maxilla (Norell et al., 2000; Makovicky et al.,

2003). The Ukhaa perinates possess unser-

rated teeth and a buccal maxillary groove but

Fig. 2. Dorsal (A) and ventral (B) views of IGM 100/972.

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AMERICAN MUSEUM NOVITATES

NO. 3657

clearly have a recessed interfenestral bar.

Unserrated teeth are no longer restricted to

Byronosaurus within Troodontinae—present

in Mei long (Xu and Norell, 2004), EK

troodontid IGM 100/44, and Urbacodon item-

irensis (Averianov and Sues, 2007). Based on

this distribution and the lack of serrations in

other paravians, such as Buitreraptor gonzale-

zorum (Makovicky et al., 2005), Rahonavis

ostromi (Forster et al., 1998), and Archaeop-

teryx lithographica, unserrated teeth may be

plesiomorphic for Troodontidae (with the

serrated teeth of Sinovenator changii, Sinorni-

thoides youngi, Troodon formosus, and both

species of Saurornithoides representing a con-

vergently derived condition within troodon-

tids). As noted above, the lack of serrated

teeth restricts the Ukhaa perinates from the

Saurornithoides-Troodon-Sinornithoides clade.

The presence of a buccal maxillary groove

and recessed interfenestral bar can be inter-

preted a number of ways. A distinct buccal

groove is a derived character within troodon-

tids found only in Byronosaurus jaffei and the

perinate skulls and thus supports a close

phylogenetic relationship between these speci-

mens. The plesiomorphic retention of a re-

cessed interfenestral bar may represent intra-

Fig. 3. Right (A, A!) and left lateral (B, B!) views of IGM 100/974, a Late Cretaceous perinate

troodontid from Ukhaa Tolgod, Mongolia. The partial braincase preserved with this specimen was removed

and prepared separately.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

5

specific variation within Byronosaurus jaffei

(possibly a postnatal ontogenetic transforma-

tion). It also is possible that a buccal maxillary

groove is diagnostic of a more inclusive clade

that includes Byronosaurus jaffei and the

Ukhaa perinates, with an unrecessed interfe-

nestral bar optimized as an autapomorphy of

Byronosaurus jaffei. In the latter interpretation,

the perinate skulls could be referred to a new

species of Byronosaurus diagnosed by the

presence of the Byronosaurus synapomorphy

(i.e., buccal maxillary groove) and the plesio-

morphic lack of the Byronosaurus jaffei auta-

pomorphy (i.e., recessed interfenestral bar).

Considering the early ontogenetic age of the

perinate skulls and the lack of a clear autapo-

morphy, we are conservative and refrain from

naming a new species based on these specimens.

Ontogenetic transformations derived from

comparisons between the Ukhaa perinates

and the holotype of Byronosaurus jaffei are

optimized

at the taxonomic level of

Byronosaurus rather than Byronosaurus jaffei.

This does not necessarily affect the implications

Fig. 4. Dorsal (A, A!) and ventral (B, B!) views of IGM 100/974.

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AMERICAN MUSEUM NOVITATES

NO. 3657

of these interpretations but does convey our

recognition that these transformations are

likely to be informative at a deeper position

on the troodontid tree than Byronosaurus jaffei.

DESCRIPTION

The preservational nature of the Ukhaa

perinates restricts comparison of most stan-

dard cranial measurements. The two skulls,

however, are nearly identical in size based on

qualitative comparison and the few compara-

ble measurements available (table1). For

example, greatest length of the maxilla in

IGM 100/972 and IGM 100/974 (measured

buccally) is 18.9 and 19.2 mm respectively,

which is approximately the same as Troodon

formosus (18 mm; Varricchio et al., 2002) and

slightly smaller than the perinate holotype of

Archaeornithoides deinosauriscus (24.5 mm;

Elzanowski and Wellnhofer, 1992, 1993).

Greatest length of the right maxilla in the

adult holotype of Byronosaurus jaffei is

approximately 97 mm, which is approximately

82% (5.53) greater than the perinate maxillae.

The length of the entire upper jaw (tip of

rostrum to caudal margin of maxilla) in IGM

100/974 is approximately 25 mm. The ratio

between this length and greatest skull length is

0.5–0.6 in Archaeopteryx lithographica, Saur-

ornithoides mongoliensis, and Velociraptor

mongoliensis (Elzanowski and Wellnhofer,

1993). Based on this index, greatest skull

length in IGM 100/974 was approximately

5 cm (the same as estimated for Archaeor-

Fig. 5. Map of Mongolia showing the geogra-

phic position of Ukhaa Tolgod (Late Cretaceous,

Djadoktha Formation).

Fig. 6. Phylogenetic relationships of selected paravian taxa (after Makovicky et al., 2003). The inferred

position of the Ukhaa Tolgod perinates as Byronosaurus is based on a series of derived characters (see text).

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

7

nithoides deinosauriscus; Elzanowski and

Wellnhofer, 1993).

IGM 100/972 (figs. 1, 2) is composed of a

preorbital rostral region that includes at least

portions of the midline vomer, right ectopter-

ygoid, and right and left premaxillae, maxillae,

nasals, frontals, lacrimals, jugals, palatines,

and pterygoids. No elements caudal to the

orbit are present. The preserved elements lay

in loose articulation and most are slightly

displaced. Two small pieces of eggshell adhere

to the right rostral region. The mandibular

rami are preserved back to and including the

mandibular fenestrae. The preserved mandib-

ular elements are preserved in articulation and

include at least portions of the right and left

dentaries, splenials, prearticulars, angulars,

and surangulars.

IGM 100/974 (figs. 3, 4) is more complete

than IGM 100/972 in that an unbroken right

frontal is preserved, as are details of the

secondary palate and braincase. The articu-

lated shape of the rostrum exhibits little

distortion. The bones of IGM 100/974, how-

ever, are less well preserved than the corre-

sponding elements of IGM 100/972. The

individual braincase elements, which include

the right exoccipital/opisthotic, left prootic,

and midline parabasisphenoid were removed

and prepared individually. Postdentary bones

of both lower jaws are present but poorly

preserved in IGM 100/974.

Our description is based on a composite

of IGM 100/972 and IGM 100/974. Cases

of ambiguity are indicated. Comparisons

with the type and referred specimen of

Byronosaurus jaffei are based on Norell et al.

(2000), Makovicky et al. (2003), and new

observations. Comparisons with Sinovenator

changii, Mei long, Sinornithoides youngi,

Saurornithoides junior, and Troodon formosus

are based on Xu et al. (2002), Xu and Norell

(2004), Russell and Dong (1993), Barsbold

(1974), Norell et al. (2009), and Currie (1985),

Currie and Zhao (1993), respectively. Com-

parisons with Saurornithoides mongoliensis are

based on Barsbold et al. (1987), Makovicky et

al. (2003), Norell et al. (2009), and personal

observations. Additional comparisons are

cited independently.

ROSTRUM AND PALATE

PREMAXILLA: The paired premaxilla is at-

tenuate rostrally (figs. 7, 8; more so than the

referred specimen of Byronosaurus jaffei, IGM

100/984). As in other troodontids, the pre-

maxilla is significantly smaller than the

external nares (Xu et al., 2002). The premax-

illa contacts the opposing premaxilla medially,

maxilla caudolaterally, and nasals caudome-

dially. The lateral surfaces are heavily degrad-

ed; however, an identical sutural contact with

the maxilla positioned rostral to the longitu-

dinal midpoint of the narial opening is visible

on both specimens. The degraded nature

probably reflects fragility of perinate bone,

as there is little chance the bone was subjected

to surficial erosion. Both specimens were

collected inside concretions and the tapho-

nomic processes at the site are thought to be

burial alive (Dingus et al., 2008).

The nasal process is thin and defines the

rostrodorsal margin of the external naris. It

cannot be determined definitively whether

this process separates the nasals rostrally as

TABLE 1

Measurements (mm) of the perinate skulls of Byronosaurus from Ukhaa Tolgod

* estimated. The two measurements for length of the maxillary fenestra in IGM 100/974

refer to the right and left sides respectively.

IGM 100/972

IGM 100/974

Length from preorbital bar to tip of rostrum

22.6

25.4

Length of maxilla

18.9

19.2

Length of upper tooth row*

20.8

21.5

Length of dentary tooth row*

?

21.1

Length of antorbital fenestra

8.7

8.3

Length of antorbital fenestra (com)

16.0*

16.9

Length of external nares

7.6

7.9

Length of maxillary fenestra

5.0

7.3, 5.0

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AMERICAN MUSEUM NOVITATES

NO. 3657

in Byronosaurus jaffei, Saurornithoides junior,

and Velociraptor mongoliensis (AMNH 6515).

It is apparent that the nasal process meets the

premaxillary process of the nasal rostral to the

caudal boundary of the external naris and

therefore contributes to the dorsoventrally

flattened internarial bar. A flattened internar-

ial bar is derived for Troodontidae but also is

found in ornithomimids and the alvarezsaur

Shuuvia deserti.

The maxillary process is short and pitted as

in the adult. Also in agreement with

Byronosaurus jaffei and other troodontids

besides Sinovenator changii, the maxillary

process does not exclude a nasal-maxillary

contact as it does in adult dromaeosaurs and

ornithomimids. The floor of the narial cham-

ber is formed largely by the narial ramus of

the premaxilla, with only a short, caudal,

maxillary contribution. This morphology is

similar to Troodon formosus but contrasts

somewhat to Byronosaurus jaffei, in which

the maxilla contributes more significantly to

the narial floor. A short but distinct sagittal

ridge runs along the floor of the nasal

chamber. The ridge terminates caudally above

a notch that is formed between the opposing

premaxillae and likely marks the contact with

the vomer. A longitudinal trough excavates

the floor of the narial chamber on either side

of the sagittal crest. It is unclear whether this

excavation represents a simple concavity or a

penetration of the narial ramus.

EXTERNAL

NARIS: The paired external

naris is approximately 8.0 mm long and 3.5

mm tall giving it a long and elliptical appear-

ance (fig. 7). Its relative length is abbreviated

compared to that of Byronosaurus jaffei,

which extends for more than half the maxil-

lary tooth row (this expanded adult condition

previously was described as an autapomorphy

of Mei long [Xu et al., 2002] but may be basal

for Troodontidae). As in other troodontids,

with the exception of Sinovenator changii and

Mei long, the caudal margin of the external

naris fails to overlap with the antorbital

fenestra, although it does overlap with the

maxillary fenestra. The nasal and maxillary

processes of the premaxilla delineate the

rostral margin of the naris (dorsally and

ventrally, respectively), while the nasal forms

the posteriorly convex caudal boundary. The

ascending process of the maxilla contributes to

the caudoventral narial margin—an apparent-

ly derived condition present in all troodontids

besides Sinovenator changii, where the plesio-

morphic exclusion of a maxillary participation

Fig. 7. Left lateral views of the rostrum in IGM

100/972 (A) and IGM 100/974 (B) and a dorsolat-

eral view of the right side of the rostrum in IGM

100/974 (C).

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

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in the narial border by the premaxilla is

retained.

MAXILLA: The triangular maxilla (lateral

view; fig. 7) includes a prominent ascending

process, alveolar process, and palatal shelf.

The maxilla contacts the premaxilla rostrome-

dially, vomer medially, nasal and lacrimal

dorsally, and the jugal, ectopterygoid, and

palatine caudally. The ascending and alveolar

processes are thin compared to those of

Byronosaurus jaffei and dromaeosaurids—

more comparable in relative size to those of

Troodon formosus (Russell, 1969; Currie,

1985). The ascending process in both the

perinate and adult of Byronosaurus narrows

distally to a fingerlike terminus that fails to

contact the rostral ramus of the lacrimal (a

narrow contact occurs in Troodon formosus;

Currie, 1985). The perinate ascending process

differs from that of the adult by projecting

caudodorsally along a curved rather than

linear trajectory (i.e., the ascending process is

vertical as in other embryonic or neonate

theropods [Norell et al., 1994; Dal Sasso and

Signore, 1998; Rauhut and Fechner, 2005]).

The alveolar ramus is laterally concave and

narrows caudally into a longitudinal facet that

lies medial to the jugal. The lateral concavity

houses a series of foramina that correspond

with maxillary tooth positions and likely

transmitted branches of the maxillary nerve

(CN V) and associated vasculature from the

supraalveolar canal as described for Troodon

formosus (Currie, 1985).

The interfenestral bar (pila interfenestralis)

is distinctly recessed from the lateral maxil-

lary surface (fig.7). As noted above, the

recessed condition is plesiomorphic within

Fig. 8. Dorsal (A) and ventral (B) views of the rostrum in IGM 100/974.

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AMERICAN MUSEUM NOVITATES

NO. 3657

Troodontidae and is present in all troodontids

except the holotype of Byronosaurus jaffei.

The overall shape of the perinate interfenestral

bar differs distinctly from that of the adult,

which is relatively shorter and wider. The

interfenestral bar in the perinate and adult

is slightly concave rostrally and oriented

along a rostrodorsal-caudoventral angle. This

morphology likely is plesiomorphic for

Byronosaurus as it is present in Troodon

formosus, Saurornithoides mongoliensis, and

dromaeosaurids. It differs from the autapo-

morphically straight and vertical interfenestral

bar of Sinovenator changii (Xu et al., 2002).

The caudal margin of the perinate interfenes-

tral bar lacks the distinct emarginations

marking the caudal openings of the narial

passage (dorsally) and interfenestral canal

(ventrally) in the adult. The openings them-

selves, however, are present and described

with the antorbital and maxillary fenestrae

below. The interfenestral bar lies dorsal to the

9th or 10th maxillary tooth—well in front of

the same landmark in Byronosaurus jaffei and

other troodontids (e.g., above the 20th max-

illary tooth [approximately] in Sinovenator

changii). The position is similar to the

dromaeosaur condition (e.g., above maxillary

tooth 4 or 5 in Velociraptor mongoliensis

[Barsbold and Osmólska, 1999], 5 or 6

in Deinonychus antirrhopus [Ostrom, 1969a,

1969b], and probably 3 in Dromaeosaurus

albertensis). These comparisons are compli-

cated by the larger, less closely packed teeth of

dromaeosaurs.

The perinate maxillae have extensive, but

thin, palatal shelves that contribute signifi-

cantly to the formation of a complete second-

ary palate (fig. 8). The shelves form the rostral

margin of the choanae caudally (as in

Byronosaurus jaffei, Velociraptor mongoliensis,

and Tsaagan mangas; the palate is largely

unknown in other troodontid taxa, but from

CT data it is apparent this architecture also is

present in both species of Saurornithoides

[Norell et al., 2009]). The shelves are individ-

ually concave and appear to join along the

sagittal midline to form (in part) a conspicu-

ous vomerine ridge that terminates rostrally

just behind the tip of the snout. This

termination probably occurs on the palatal

surface of the premaxillae (the premaxilla-

maxilla suture is indistinct). The height of the

vomerine ridge may be exaggerated due to

mediolateral compression—the right maxilla is

pushed slightly under the left—but the ridge

itself appears to be real. The dorsal surface of

the palatal shelves helps floor the antorbital

cavity and maxillary antrum and is described

with those structures below.

NASAL: The paired nasals exhibit a mid-

length constriction and thus a slight hourglass

shape in dorsal view (fig. 8A). The nasals are

wider rostral to this constriction where they

contact the ascending processes of the maxilla

ventrally (fig. 7). Like the other rostral bones,

the nasal is proportionally shorter than in

adult paravians (including Byronosaurus jaf-

fei). A distinct midline ridge achieves its

greatest development directly above the nares.

In lateral view, the dorsal margin of the nasal

is arched rostrally but grades into a nearly flat

surface caudally. This rostral arch forms a

distinct nasal boss (fig. 7B, C) that may reflect

an expansion of the underlying nasal cavity

(visible only in IGM 100/974). This boss is not

present in Byronosaurus jaffei but is to some

degree in Sinovenator changii and Mei long.

The nasal narrows caudally to a fingerlike

process that cups the triangular rostral margin

of the frontal (fig. 8A). The shape of this

suture compares closely with Byronosaurus

jaffei and Saurornithoides junior (Barsbold,

1974), whereas a transverse suture is present in

Mei long, dromaeosaurs, and basal avialans.

The rostral margin of the nasals is deeply

concave where it forms the caudal margin of

the external nares. Despite a dramatic post-

natal elongation of the snout, the perinate

nasals agree with those of the adult in

terminating just behind the preorbital bar.

A sharp demarcation between the dorsal

and lateral nasal surface is not apparent

except where the nasal overlies the ascending

process of the maxilla. In this region, the

lateral margin is deflected, so it faces dorsally

rather than laterally and forms a small shelf

(fig. 8A). This deflected condition is similar to

that described for Sinovenator changii (Xu et

al., 2002) and more dramatic than the slightly

deflected condition of most other theropods,

including Byronosaurus jaffei. A row of small

foramina lies within the nasal on the dorsal

surface of the rostrum. These foramina paral-

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

11

lel the nasal-maxillary suture as in Byrono-

saurus jaffei, Troodon formosus, Velociraptor

mongoliensis (Sues, 1977), Tsaagan mangaas

(Norell et al., 2006), and Deinonychus antir-

rhopus (Ostrom, 1969b). The foramina likely

are not pneumatic as a pneumatic recess of the

nasal is not visible (unknown in troodontids

even with CT data; Witmer, 1997a; Norell et

al., 2009). The nasal overlies the rostral

process of the lacrimal caudally. The lacrimal

contact extends for nearly half the preserved

length of the nasal.

LACRIMAL: The paired lacrimal is a slen-

der, T-shaped element (fig. 9). The lacrimal

contacts the nasal rostromedially, frontal

medially and caudomedially, and the jugal

(and possibly palatine) ventrally. The ventral

ramus (preorbital bar) separates the orbit

from the antorbital fossa and is compressed

rostrocaudally with a transverse breadth that

is expanded relative to the slender dromaeo-

saur condition (Ostrom, 1969b; Sues, 1977).

The shaft of the preorbital bar twists slightly

and widens ventrally before contacting the

jugal. A ridge, comparable to that described

for Troodon formosus (Currie, 1985), extends

down the rostrolateral margin. The vertical

angle of the preorbital bar in both perinate

skulls supports the same morphology in the

adult, whose vertical orientation was consid-

ered to possibly be the result of postmortem

distortion (Makovicky et al., 2003). A vertical

preorbital bar is a potential synapomorphy of

Byronosaurus as it is angled in other troodon-

tids (e.g., Saurornithoides junior and Mei long).

The ventral articulation with the jugal occurs

along a horizontal surface that is expanded

relative to the shaft. The vertically oriented,

lateral facet described for Troodon formosus

(Currie, 1985) is not evident in Byronosaurus,

although, a small, medial articulation with the

palatine may have been present in the peri-

nates. The foramen described as opening

caudolaterally near the lacrimal-jugal contact

in Bryonosaurus jaffei (Makovicky et al.,

2003), and likely housing a diverticulum of

the antorbital sinus (Witmer, 1997a), is not

apparent in either perinate skull.

The lacrimal exhibits extensive dorsal expo-

sure that is exaggerated caudally where a

shelflike process overhangs the rostrodorsal

margin of the orbit (derived condition known

in Byronosaurus jaffei, Troodon formosus, and

Saurornithoides junior; fig. 9A). The caudal

margin of the supraorbital process (caudal

ramus) lies in a transverse plane that is well

posterior to the caudal margin of the nasal (as

in the adult; Makovicky et al., 2003). The

supraantorbital process (rostral ramus) also is

extensively developed but, in contrast to the

dorsoventrally compressed caudal ramus, is

cylindrical in cross-sectional shape. The ros-

tral ramus forms the entire dorsal margin of

the antorbital fenestra in IGM 100/972,

whereas in IGM 100/974 the same process

forms only the caudal half of this margin (with

the rostral half formed by the nasals). This

may reflect postmortem damage or variation

in the extent of ossification. The rostral ramus

is longer than the caudal ramus in agreement

with Byronosaurus jaffei, Troodon formosus,

Saurornithoides junior and mongoliensis, and

Sinornithoides youngi and in contrast to Mei

long, dromaeosaurs, and avialans. The lateral

margin of the rostral ramus is visible dorsally,

whereas its medial margin is overlain by the

nasal.

A lacrimal boss (fig. 9A, B) overhangs the

preorbital bar and although more distinct in

IGM 100/974 (where it results in a triangle-

shaped dorsal surface), it is less prominent in

the perinates than in Byronosaurus jaffei. A

recess lies below this boss in the junction

between the rostral ramus and preorbital bar.

The recess delineates the caudodorsal corner

of the antorbital fossa and likely housed a

diverticulum of the antorbital sinus (recessus

pneumaticus lacrimalis; Witmer, 1997a). A

foramen lies in the deepest part of the recess in

the left lacrimal of IGM 100/972 and the only

visible lacrimal of IGM 100/974 (right), and

just outside the recess in the right lacrimal of

IGM 100/972. This position corresponds to

the nasal aperture of the nasolacrimal canal as

described for Byronosaurus jaffei (Makovicky

et al., 2003). The typical course in theropods,

including Troodon formosus (Currie, 1985), is

for the nasolacrimal canal to penetrate the

lacrimal on the medial surface of the rostral

ramus and pass posterolaterally (Sampson and

Witmer, 2007). The perinate foramen is thus

interpreted to be the caudal opening of the

nasolacrimal canal. It should be noted that the

lacrimal of Troodon formosus lacks any lateral

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AMERICAN MUSEUM NOVITATES

NO. 3657

Fig. 9. Lacrimal and surrounding region of IGM 100/972 (A, A! right; B, B! left) and IGM 100/974 (C,

C!, right).

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

13

apertures; Currie, 1985; Witmer, 1990). As

noted by Makovicky et al. (2003), a nasolac-

rimal duct (or at least its osteological corre-

late) previously was considered absent in

troodontids (Currie and Dong, 2001), suggest-

ing its presence in Byronosaurus may be

derived within troodontids. The condition in

Mei long and Sinovenator changii is difficult

to interpret, but a nasolacrimal canal also

appears to be present in Troodon formosus and

both species of Saurornithoides. There is no

evidence of a groove on the medial surface of

the lacrimal (or the nasal) in either of the

Ukhaa perinates indicative of a suborbital

nasal gland (Witmer, 1997a), however, these

surfaces are largely obscured by matrix

making their absence far from definitive.

FRONTAL: The right and left frontal in

IGM 100/972 are represented by poorly

preserved fragments that lie above the orbit

and meet each other along a clearly visible

midline suture. These fragments extend ros-

trally to a position adjacent to the anterior

orbital margin. Their articulation with the

nasals is not retained. A nearly complete,

although displaced, right frontal is present in

IGM 100/974 (figs. 3, 10). This element is

more complete than in Byronosaurus jaffei and

thus provides novel information for this part

of the troodontid tree. The frontal is elongate

with a strongly curved lateral margin that

contributes significantly to the dorsal orbital

boundary. The medial margin, which formed

the apparently unfused suture with the oppos-

ing frontal, is relatively straight but with a

slight medial bow. The frontal is narrow

rostrally but with an overall triangular shape

comparable to the frontal of Troodon formosus

(Currie, 1985). The caudal margin, which in

Troodon formosus articulates with the parietal

medially and contributes to the rostral margin

of the supratemporal fenestra laterally, is

rounded in outline and lacks a clear distinc-

tion between the parietal suture and supra-

temporal boundary. This is in contrast to the

heavily angled caudal margin of Troodon

formosus and likely is due, at least in part, to

the ontogenetic age of IGM 100/974. For

example, the frontoparietal fontanelle, which

closes postnatally in at least some ratite birds

(Balanoff and Rowe, 2007), may not be

completely closed in the Ukhaa perinates.

The caudodorsal surface of the frontal lacks

the distinct sculpturing that marks the rostral

origins of the temporal musculature in

Troodon formosus. The dorsal surface, in

general, is more convex and bulbous than

that of Troodon formosus—frontal shape

apparently reflects the shape of the forebrain

early in ontogeny before thickening and

flattening during postnatal growth. The su-

praciliary rim is preserved as a slightly everted

crest bounded laterally by a shallow but

distinct trough. This margin reveals no distinct

foramina or fine grooves that may reflect the

presence of a birdlike supraorbital nasal gland

(Gauthier, 1986; Witmer, 1997a) as described

for Troodon formosus (Currie, 1985). The

osteological correlates of a supraorbital nasal

gland also are lacking in Sinornithoides youngi

(Russell and Dong, 1993). Also in apparent

agreement with Sinornithoides youngi and the

observations of Makovicky and Norell (2004),

the lateral frontal margin in IGM 100/974

shows no evidence of a separate prefrontal

ossification as described for Troodon formosus

(Currie, 1985). The vertical lamina of the

Fig. 10. Dorsal view of right frontal of

IGM 100/974.

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AMERICAN MUSEUM NOVITATES

NO. 3657

frontal that borders the lacrimal in

Sinovenator changii is absent in IGM 100/

974, Byronosaurus jaffei, Mei long, and more

derived troodontids.

JUGAL: The jugal is best preserved on the

right side of IGM 100/972 (figs. 1A, 11). As in

the adult, it is thin, elongate, horizontally

oriented and forms visible contacts with the

maxilla and palatine rostromedially, ectopter-

ygoid and pterygoid caudomedially, and

lacrimal rostrodorsally. The jugal forms the

caudoventral corner of the external antorbital

fenestra and the ventral orbital margin. The

jugal’s contribution to the antorbital fossa is

dorsoventrally flattened and contains a dis-

tinct fossa. This fossa rotates slightly as it

extends caudally so that it opens dorsolater-

ally as a troughlike groove below the orbit

(fig. 11). The fossa gradually shallows as the

jugal assumes a mediolaterally compressed

shape near its ectopterygoid contact. The fossa

likely housed a portion of the antorbital sinus

(at least rostrally). There is no visible pene-

tration that would suggest internal pneumati-

zation (as in Deinonychus antirrhopus [Witmer,

1997a], Tsaagan mangaas [Norell et al., 2006],

and a large number of other theropod taxa—

including Saurornithoides junior [Norell et al.,

in 2009]).

ECTOPTERYGOID: The troodontid ectopter-

ygoid is not well known and is unknown for

Byronosaurus jaffei, so its presence in IGM

100/972 provides important new information.

The perinate ectopterygoid (fig. 11) is triradi-

ate with prominent jugal, pterygoid, and

maxillary processes as in dromaeosaurs and

other closely related maniraptorans (Ostrom,

1969b; Sues, 1978; Currie, 1995). The lateral

jugal process has a strong caudal hook. The

medial pterygoid flange is large and inflated

relative to that of Saurornithoides junior and

adult dromaeosaurs (Currie and Zhao, 1993).

A ventrally positioned recess marks the

position through which a diverticulum (likely

from the suborbital air sac) pneumatized an

internal cavity within this process (Currie and

Zhao, 1993; Witmer, 1997a). A similar ventral

recess is present in Saurornithoides junior;

however, it is blind with no associated internal

pneumatic cavity. A dorsal trough lies medial

and rostral to the inflated part of the

pterygoid flange. This trough deepens caudal-

ly and in this sense is similar to the conspic-

uous ‘‘pit’’ on the dorsal surface of the

ectopterygoid in Saurornitholestes langstoni

(Sues, 1978) and Deinonychus antirrhopus

(Ostrom, 1969b). This dorsal recess also may

be associated with the suborbital sinus (more

likely for the dorsal than ventral recess; with

the latter possibly formed from a middle ear

sac or novel diverticulum; Witmer, 1997a).

The dorsal and ventral recesses do not

communicate in Byronosaurus (in agreement

with Deinonychus antirrhopus). The dorsal

recess could not previously be scored in any

troodontid, but its presence appears to be

derived for Deionychosauria with an apo-

morphic loss in Dromaeosaurus albertensis.

VOMER: The vomer is a midline element

that contacts the maxillae rostrolaterally and

palatines laterally (fig. 9B, C). The vomer is

mediolaterally compressed where it forms the

medial margin of the choanae. The vomer is

preserved behind the secondary palate, but its

caudal terminus is not preserved in either of

the perinates. The relationship of the vomer to

the vomerine ridge (fig. 8B) is unclear. Also

unclear is whether the vomer extended the

entire length of the palate to contact the

premaxillae rostrally as in many nonavian

theropods (e.g., Allosaurus fragilis [Madsen,

1976], Velociraptor mongoliensis [Barsbold and

Osmólska, 1999], Tsaagan mangaas [Norell et

al., 2006]); however, as noted above, the

vomerine ridge terminates just behind the tip

of the snout on the premaxilla. The vomer in

Byronosaurus jaffei was interpreted to termi-

nate at a position caudal to the external nares

(Makovicky et al., 2003).

PALATINE: The palatines are poorly pre-

served in Byronosaurus jaffei making their

presence in the perinates especially important.

The paired palatine (fig. 12) is a relatively flat

element lying medial and just rostral to the

preorbital bar of the lacrimal. It contacts the

alveolar process of the maxilla laterally, jugal

caudolaterally, vomer medially, and probably

the pterygoid caudomedially—the latter artic-

ulation is not preserved in either perinate

specimen, probably due to displacement of the

pterygoids. The most distinctive feature is a

large, rostrally positioned U-shaped notch

that forms the caudal choanal margin and is

defined by a vomeropterygoid process medi-

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

15

Fig. 11. Right dorsolateral view of the caudal half of IGM 100/972 showing the right ectopterygoid and

jugal, the caudal ends of both pterygoids, and the rostral end of the parasphenoid process of

the parabasisphenoid.

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AMERICAN MUSEUM NOVITATES

NO. 3657

Fig. 12. Left dorsolateral view of IGM 100/972. Note the broad lacrimal shelf and well-preserved

left palatine.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

17

ally and maxillary process (facies articularis

maxillaris) laterally. Both processes are signif-

icantly broader mediolaterally than the fin-

gerlike processes of Deinonychus antirrhopus

(Ostrom, 1969b; Witmer, 1997a). The rostral

margins of both processes slant caudome-

dially. A dorsal ridge along the caudal margin

of the choana slopes medially and laterally to

the base of the vomeropterygoid and maxillary

processes, respectively.

The maxillary process contains a short,

dorsal ridge that parallels the lateral margin

of the choana and delimits a shallow trough.

A foramen lies at the caudal end of the trough

directly adjacent to a prominent, caudome-

dially directed cavity. This cavity (recessus

pneumaticus palatinus) agrees in general

position with the same structure in Deinony-

chus antirrhopus (Ostrom, 1969b), Veloci-

raptor mongoliensis (Norell et al., 2004),

Archaeopteryx lithographica (Elzanowski and

Wellnhofer, 1996; Witmer, 1997a), and

Saurornithoides mongoliensis (Norell et al.,

2009), and is interpreted as having housed a

diverticulum of the antorbital sinus. The single

foramen adjacent to the recess also is inter-

preted to be pneumatic in origin and likely

opens into an internal pneumatic cavity. A

single pneumatic foramen on the dorsal

surface of the palatine is known in a small

number of coelurosaur taxa (e.g., Tyran-

nosaurus rex; AMNH 5027) but was previous-

ly undescribed in a troodontid (Witmer,

1997a).

The caudal margin of the palatine has a

transverse orientation medially but curves

posteriorly to form a lateral jugal process.

The long medial surface and distinct pterygoid

process of other theropods (e.g., Deinonychus

antirrhopus; Ostrom, 1969b) is absent. The

jugal process is shorter than in Deinonychus

antirrhopus. The medial surface of this process

is deeply excavated—probably by the antorbi-

tal sinus. The resultant fossa is not visible in a

strict dorsal view but opens ventrolaterally.

The fossa is positioned caudoventral and

lateral to the aforementioned recessus pneu-

maticus palatinus. The lengths of these two

pneumatic fossae overlap but are not wholly

confluent—being separated by a small longi-

tudinal ridge. A much larger dorsal ridge

separates these cavities from a small but

distinct fossa that excavates the caudomedial

surface of the jugal process. This fossa is an

osseous signature of the m. pterygoideus pars

dorsalis and is considerably smaller than the

same fossa in Deinonychus antirrhopus (possi-

bly reflecting the small size and early ontoge-

netic age of the Ukhaa perinates). The lack of

a distinct pterygoid process eliminates an

osseous demarcation of the space housed by

the dorsal pterygoideus laterally and subsidi-

ary palatal fenestra medially (compare to

Archaeornithoides deinosauriscus [Elzanowski

and Wellnhofer, 1993; fig. 4c]).

PTERYGOID: The pterygoids are not visible

in IGM 100/974 and are not preserved in

Byronosaurus jaffei. The right and left ptery-

goid are present in IGM 100/972 and are

preserved (at least approximately) in their

expected life positions at the caudal end of the

palate (figs. 11, 12). Some displacement, how-

ever, certainly has occurred making the exact

positional relationships and bony contacts

difficult to interpret. The dorsal surface of

the left pterygoid is visible for most its length,

although its medial surface is embedded in

matrix. The rostral extent of the right ptery-

goid and the entire ventral surface of both

pterygoids are obscured by matrix. A short

length of a dorsoventrally compressed bone

that tapers to a point is visible through the left

choana. This element appears to be displaced

and may represent the broken rostral tip of the

right pterygoid.

The pterygoids do not meet along the

midline caudally and thus a long interpter-

ygoid vacuity is retained. It is unclear whether

a midline contact occurred rostrally; however,

based on the shape of the left pterygoid, such a

contact is unlikely. If a rostral contact did

occur, it probably did not form at the extreme

tip (as in Allosaurus fragilis), but rather in

the anterior half of the pterygoid with

subsequent rostral divergence. The pterygoids

of Saurornithoides mongoliensis lack a medial

contact (Norell et al., 2009). The tapered

rostral tip of the pterygoid is cylindrical in

cross-sectional shape. This shape transforms

caudally into a dorsoventrally compressed

plate. This platelike morphology is most

obvious where the pterygoid contacts the

ectopterygoid along a distinct caudolateral

flange (the rostral margin of this ‘‘pterygoid

18

AMERICAN MUSEUM NOVITATES

NO. 3657

flange’’ may also contact the palatine), and is

similar to that of Deinonychus antirrhopus

(Ostrom, 1969b) and Dromaeosaurus alberten-

sis (Currie, 1995). The caudal end of the flange

curves dorsolaterally to form a flat, poster-

omedially facing facet that articulates with the

basipterygoid process of the basisphenoid. A

well-developed pterygoid flange previously

was scored only for Saurornithoides mongo-

liensis within Troodontidae (Turner et al.,

2007a, 2007b). A thin, ventrolaterally project-

ing process defines a lateral notch that accepts

the pterygoid ramus of the quadrate rostral to

the basipterygoid articulation.

ANTORBITAL

AND

MAXILLARY

FENESTRA:

The lateral rostral surface is characterized by

a large, polygonal external antorbital fenestra

and a smaller, triangular maxillary fenestra

(subsidiary antorbital fenestrae; see Witmer,

1997a). These fenestrae (figs. 1, 3, 7, 9, 13) are

less elliptical and elongate than those of

Byronosaurus jaffei and other troodontids

whose long and low maxillary fenestra was

considered a derived shape shared with or-

nithomimosaurs (Witmer, 1997b).

The external antorbital fenestra is delineat-

ed by the lacrimal caudally (preorbital bar)

and caudodorsally (supraantorbital process),

and the maxilla rostrally (interfenestral bar)

and ventrally (alveolar process). The nasal

contributes to the dorsal margin—between the

supraantorbital process of the lacrimal and

ascending process of the maxilla. The rostral

tip of the maxillary process of the jugal

emerges from beneath the distal end of the

preorbital bar to form the caudodorsal corner

of the fenestra. Because the interfenestral bar

is recessed, the external antorbital fenestra

technically extends to the ascending ramus of

the maxilla, with the maxillary fenestra lying

medial to it. To ease comparisons with the

adult (where the bar is not recessed), we

describe the rostral and caudal limits of the

perinate antorbital and rostral fenestrae re-

spectively based on the position of the

interfenestral bar. The internal antorbital

fenestra is not well ossified leaving a broad

communication between the antorbital and

nasal cavities. The antorbital fossa (as defined

by Witmer, 1997b) is relatively narrow me-

diolaterally. The lacrimal antorbital fossa is

characterized by a dorsal and ventral widening

Fig. 13. Left caudolateral view of the rostrum

of IGM 100/972 showing the rostral walls of the

antorbital fossa and maxillary antrum.

2009

BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

19

of the preorbital bar (fig. 9A). The dorsal

widening forms a distinct pocket (recessus

pneumaticus lacrimalis) that housed a diver-

ticulum of the antorbital sinus as well as the

caudal aperture of the nasolacrimal canal (see

above). No pneumatic foramen or internal

pneumatic cavities are associated with this

recess (also lacking in Troodon formosus). The

lacrimal pneumatic recess is delineated medi-

ally by a short, oblique crest (forming the

caudodorsal corner of the internal antorbital

fenestra) and laterally by the vertical ridge

that extends along the rostrolateral margin of

the preorbital bar (fig.9). The maxillary

antorbital fossa is characterized by a ventral

widening of the interfenestral bar. As noted

above, this surface is penetrated by a foramen

and canal (fig.13; interfenestral canal of

Makovicky et al., 2003). The foramen actually

is a slitlike opening that begins in the dorsal

surface of the alveolar ramus of the maxilla

and widens rostrodorsally where it extends

into the base of the interfenestral bar. A series

of small openings pierce the maxilla along this

slitlike structure culminating with the large

interfenestral canal between the antorbital

fossa and maxillary antrum. The smaller

foramina, which penetrate the alveolar ramus

of the maxilla, are interpreted as having

communicated branches of the maxillary

nerve (CN V) and its associated vasculature

into the supraalveolar canal. The interfenes-

tral canal likely communicated a diverticulum

of the antorbital sinus, although portions of

the maxillary vasculature also may have

traversed this opening. The fenestra’s ventral

position indicates the m. pterygoideus pars

dorsalis extended well into the antorbital fossa

but did not fill it—the neurovascular bundle

consistently lies dorsal to this muscle in sau-

ropsids (Witmer, 1997a). The interfenestral

canal is positioned more ventrally in the peri-

nates than in the holotype of Byronosaurus

jaffei.

The floor of the antorbital cavity is largely

unossified. The choana lies directly adjacent to

the antorbital fossa (as in Archaeopteryx

lithographica), and the length of this opening

corresponds almost exactly to the length of the

antorbital cavity (fig. 12). The palatal ramus

of the maxilla partially floors this space

rostrally and laterally. This floor is slightly

more extensive in IGM 100/972 than IGM

100/974, but this may reflect the region’s poor

preservation in the latter specimen. The dorsal

surface of the palatine contributes to the floor

of the antorbital cavity (contribution restrict-

ed to the caudal margin). The palatine

contribution to this floor is characterized by

three distinct fossae described above—two of

which are excavated by pneumatic diverticula

of the antorbital sinus and the third is an

osseous signature of the dorsal pterygoideus

(fig. 12). The palatal ramus of the maxilla

extends medially from the interfenestral canal

to define the rostral choanal margin and meet

the vomer near the cranial midline. Just lateral

to the vomer contact, a maxillary flange

curves dorsally to form the medial wall of a

large rostrocaudal communication between

the antorbital cavity and maxillary antrum

(fig. 13). The opening is interpreted as the

caudal fenestra of the maxillary antrum with

its medial wall formed by the postantral strut

(Witmer, 1997b). The dorsal margin of the

strut curves laterally to meet the interfenestral

bar above the caudal antral fenestra. The

diameter of the fenestra narrows rostrally.

Fenestration of the postantral strut was not

reported previously in troodontids (known

largely in tyrannosaurids; absent in Ornitho-

lestes hermanni and Deinonychus antirrhopus

Witmer, 1997a). The postnasal fenestra is

preserved in IGM 100/974 medial to the

caudal ramus of the lacrimal and dorsal to

the palatine. The medial margin of this broad

communication between the antorbital fossa

and the orbit, however, is not visible in either

specimen and probably was delimited by the

caudal extent of the cartilaginous nasal

capsule (Witmer, 1995, 1997a).

The external maxillary fenestra is delineated

solely by the maxilla and lies at, rather than

behind, the rostral border of the antorbital

fossa, in contrast to Saurornithoides mongo-

liensis and a variety of dromaeosaurs and

other theropods. The rostral margin of the

maxillary fenestra, which is formed by the

lateral ramus of the ascending process of the

maxilla, is less rounded in both the perinates

and adults of Byronosaurus than in Troo-

don formosus—in this respect, Byronosaurus

more closely resembles Saurornithoides junior

(Barsbold, 1974; Currie, 1985). The interfenes-

20

AMERICAN MUSEUM NOVITATES

NO. 3657

tral bar forms the caudal margin of the

maxillary fossa, but as noted above, the

antorbital and maxillary fossae communicate

lateral to the bar due to its recessed position.

The interfenestral canal is confluent rostrally

with a distinct groove in the floor of the

mediolaterally deep maxillary antrum. A simi-

lar groove is located in the medial wall of the

maxillary antrum rostral to the caudal antral

aperture. These excavations may penetrate the

floor and medial wall of the maxillary antrum.

A series of foramina are present on the roof of

the palate; however, these are largely directed

laterally toward the alveolar ramus. The ceiling

of the maxillary antrum extends medial to the

ascending maxillary ramus, but a distinct

epiantral recess near the junction between the

interfenestral bar and postantral strut as found

in tyrannosaurids does not appear to be present.

The medial wall of the maxillary antrum is

complete (as in Ornitholestes hermanni and

Deinonychus antirrhopus; Witmer, 1997a).

The front wall of the maxillary antrum (pila

promaxillaris) is pierced by a relatively large

opening that communicates rostrally with a

deep fossa lying beneath the rostral margin of

the nasal passage. This fossa is interpreted to

be the promaxillary recess, with its communi-

cation with the maxillary antrum being the

fenestra communicans. The fenestra commu-

nicans was not reported previously in troo-

dontids, but its presence is known in such taxa

as Allosaurus fragilis and Deinonychus antir-

rhopus (Witmer, 1997a). The right and left

promaxillary recesses are widely confluent

with each other medially and with the external

nares rostrodorsally. The maxillary shelf

separating the promaxillary recess (ventrally)

from the nasal passage (dorsally) is penetrated

by a large opening on the left side of IGM

100/972 that may be a preservational artifact

(the shelf is extremely thin).

The fenestra promaxillaris, which is present

as a slitlike opening in the caudal margin of

the ascending ramus of the maxilla in Troodon

formosus (Currie, 1985; Witmer, 1997a),

Sinornithoides youngi (Russell and Dong, 1993),

Saurornithoides junior (Barsbold, 1974), and

Sinovenator changii as well as in Archae-

opteryx lithographica, Deinonychus antirrho-

pus, and Velociraptor mongoliensis (Witmer,

1997a), is not immediately visible in IGM

100/972. The fenestra was scored as absent in

Saurornithoides mongoliensis and unknown in

Mei long (Turner et al., 2007a, 2007b). The

internal surface of the ascending ramus

adjacent to the promaxillary recess is inflated

and appears to house a cavity (or potentially

multiple cavities). There is no visible com-

munication between this cavity and the

maxillary antrum (i.e., no visible fenestra

promaxillaris), but a medial communication

with the promaxillary recess is present. At

least the rostral portion of this cavity is

interpreted as the vestibular bulla, which is

present in Troodon formosus (Currie, 1985;

Witmer, 1997b). The vestibular bulla and its

associated cavities in the ascending ramus of

the maxilla may be pneumatized by a

diverticulum entering through the medial

communication with the promaxillary recess

or they may be pneumatized by a diverticu-

lum from the maxillary antrum (with the

requisite fenestra promaxillaris concealed by

matrix). If a promaxillary fenestra is present

then it is recessed behind the lateral ramus of

the ascending maxillary process, as in Troodon

formosus and in contrast to Sinornithoides

youngi and Saurornithoides junior (Witmer,

1997a). The rostral terminus of the supraalveo-

lar canal is present along the maxillary margin

near or within the premaxilla-maxilla suture.

The foramen is visible rostrally but not

laterally. The same opening was described in

Byronosaurus jaffei as the subnarial foramen

(Makovicky et al., 2003), which previously had

been considered absent in troodontids (Sereno,

2001). A subnarial foramen also is present in

the purported troodontid Archaeornithoides

deinosauriscus (Elzanowski and Wellnhofer,

1993) and in the alvarezsaur Shuuvia deserti.

BRAINCASE

Elements of the otic capsule and neurocra-

nium preserved in IGM 100/974 include the

right exoccipital/opisthotic and prootic, and

the midline parabasisphenoid. No other brain-

case elements, including ossifications of the

interorbital septum were recovered.

EXOCCIPITAL/OPISTHOTIC: The exoccipital/

opisthotic is a thin, irregularly shaped com-

pound element composed of the caudal

exoccipital and rostral opisthotic (fig. 14). A

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

21

small length of the suture between the two

elements is retained dorsomedially (above the

foramen magnum; fig. 14A, B).

The caudal surface is marked by a broadly

concave medial margin that represents a

relatively large foramen magnum. Based on

this surface only, the foramen magnum is less

elliptical than that of the adult—although it

does appear to be taller than wide in

agreement with the adult condition and that

of most troodontids (e.g., Sinovenator changii)

but differing from the subcircular foramen

magnum of Troodon formosus (Currie and

Zhao, 1993). An exoccipital/opisthotic contri-

bution to the occipital condyle is present as a

small knob. It is unclear whether this contri-

bution met that of the opposing exoccipital/

opisthotic to prevent basioccipital participa-

tion in the foramen magnum. The occipital

condyle is not preserved in the adult holotype

of Byronosaurus jaffei; however, a midline

contact of the exoccipital/opisthotics at the

occipital condyle is absent in Troodon for-

mosus and uncommon in coelurosaurs

(Osmólska et al., 1972; Currie, 1985). The

perinate occipital condyle lacks a distinct

neck (fig. 14C)—in agreement with Troodon

formosus, Saurornithoides mongoliensis, and

Saurornithoides junior.

Lateral and slightly ventral to the occipital

condyle is a broad surface penetrated by three

foramina. The two medial-most foramina

communicate directly with the endocranial

floor and likely transmitted branches of the

hypoglossal nerve (CN XII). There is no

evidence of the third hypoglossal foramen

present in Itemirus medularis, Troodon for-

mosus, and Hesperornis regalis (Kurzanov,

1976; Elzanowski, 1991; Currie and Zhao,

1993). The first (medialmost) hypoglossal

foramen is positioned slightly dorsal to, and

is distinctly larger than, the second (in

agreement with Byronosaurus jaffei, Troodon

formosus, and birds). Lateral and slightly

dorsal to the hypoglossal foramina is a

foramen comparable in size to the largest of

the hypoglossal foramina. This opening is

confluent with the postnatal remnant of the

metotic fissure (cavum metoticum) and thus is

the jugular (vagus) foramen. The relative

position of this opening differs from that of

adult Bryonosaurus jaffei and juvenile Struthio

camelus where the jugular foramen lies dis-

tinctly below the dorsomedial hypoglossal

foramen and roughly in line with the smaller,

posterolateral foramen of CN XII. The

external surface through which the hypoglos-

sal and vagus nerves pass is slightly concave,

but is not the bowl-like depression of more

basal tetanurans, some dromaeosaurs, and

Oviraptor philoceratops (Turner et al., 2007a).

The absence of additional foramina in the

caudal surface of the occipital plate suggests

the jugular foramen transmitted both the

vagus (CN X) and spinal accessory (CN XI)

nerves. The path of the glossopharyngeal

nerve (CN IX) is unclear, but it either exited

through the jugular foramen or traversed the

more rostral fenestra pseudorotunda (fenestra

cochleae; see below). The latter condition

generally is assumed in nonavian theropods

(Currie, 1997), whereas CN IX generally exits

with CN X in Rhea americana, Struthio

camelus, and crocodilians (Müller, 1961;

Iordansky, 1973; Bellairs and Kamal, 1981;

Walker, 1985). A groove or foramen in the

prevagal strut (crista tuberalis of many au-

thors; metotic buttress of Walker, 1985;

metotic strut of Witmer, 1990; Baumel and

Witmer, 1993; see discussions in Gower and

Weber [1998] and Sampson and Witmer

[2007]) rostrolateral to the jugular foramen

marks an independent path of the glossopha-

ryngeal nerve in many neognath birds and at

least some nonavian coelurosaurs, including

Troodon formosus (Witmer, 1990; Currie and

Zhao, 1993; Sampson and Witmer, 2007). As

noted by Mackovicky et al. (2003), a distinct,

but unidentified, foramen is present in the left

prevagal strut of the holotype of Byronosaurus

jaffei but is absent from the right side of the

same skull. A small groove, however, is

present on the rostral face of the right strut

of this specimen. Considering the comparative

framework, it appears likely this foramen and

groove in Byronosaurus jaffei mark the pas-

sage of the glossopharyngeal nerve out of the

cavum metoticum. The same path of CN IX,

therefore, is inferred for the perinates—with

the associated bony signature developing as

the ossified prevagal strut thickens during

postnatal ontogeny.

The ventral margin of the exoccipital/

opisthotic extends away from the occipital

22

AMERICAN MUSEUM NOVITATES

NO. 3657

Fig. 14. Right exoccipital/opisthotic of IGM 100/974 in caudal (A, A!), rostral (B, B!) and lateral (C,

C!) views.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

23

condyle past the position of the hypoglossal

and jugular foramina to form a distinct lateral

surface. This surface contributes to a promi-

nent notch that is confluent rostrally with the

fenestra pseudorotunda and whose dorsal

margin is formed by the paroccipital process

(fig. 14). The notch is less prominent in the

adult where the paroccipital process is dorso-

ventrally deeper (see below). The ventrolateral

process of the exoccipital/opisthotic, which

flanks the basal tubera of the basioccipital in

many theropods (Rauhut, 2004), is absent in

IGM 100/974. A modestly expanded surface

that borders the reduced and medially posi-

tioned basal tubera exists in Byronosaurus

jaffei, but unlike in most other theropods (e.g.,

Gallimimus bullatus; Osmólska et al., 1972),

this surface fails to narrow to a discrete

ventrolateral process. The absence of discern-

ible sutures in the holotype of Byronosaurus

jaffei further complicates the development of

this region because this surface may be an

extension of the basioccipital rather than

exoccipital/opisthotic.

The paroccipital process in both IGM 100/

974 and Byronosaurus jaffei exhibit the rela-

tively short, rostrocaudally compressed, and

dorsoventrally deep morphology typical of

troodontids. This shape is less exaggerated in

the perinate than in the holotype. A broad yet

shallow depression separates the base of the

paroccipital process from the foramen mag-

num (also present in Mei long; Xu and Norell,

2004). A low, horizontal ridge excludes this

depression from the more ventral concavity

housing the hypoglossal and jugular foramina,

whereas in the adult these depressions are fully

confluent. The ventral margin of the parocci-

pital process in the perinate and adult extends

laterally at a subtle downward angle (in

agreement with Troodon formosus; Currie

and Zhao, 1993) and lies along a transverse

plane that approximates the dorsoventral

midline of the foramen magnum (in contrast

to Troodon formosus whose ventral margin lies

below the foramen magnum). The distal

extremity of the paroccipital process is twist-

ed, so that the dorsal border is rostral to the

ventral border giving the perinate and adult

process surfaces that are oriented caudodor-

sally and rostroventrally (fig. 14C; this torsion

is more extreme in Byronosaurus jaffei).

The paroccipital process lacks any hint of a

foramen on either its rostral or caudal surface

and exhibits no degree of inflation. This is in

contrast to the slightly inflated process of the

adult whose base is pierced by a pair of rostral

foramina (Makovicky et al., 2003). This

external morphology suggests the paroccipital

process of Byronosaurus jaffei was pneuma-

tized, at least to some degree, by a diverticu-

lum associated with a posterior (caudal)

tympanic recess—a feature common in de-

rived theropods (Witmer, 1997b) but one

considered secondarily lost in troodontids

(Currie and Zhao, 1993; Turner et al., 2007a,

2007b). The presence of a cavity within the

paroccipital process of Byronosaurus jaffei

associated with the rostral foramina and

therefore inferred to be pneumatic is con-

firmed by CT data (fig. 15). The paroccipital

process of Saurornithoides junior contains a

similar pneumatic signature (Norell et al.,

2009). The distal end of the perinate process is

not distinctly expanded as in Byronosaurus

jaffei, Troodon formosus, and Mei long, and

lacks the diagonal ridge that divides the

process into medial and lateral components

in the adults. The degree to which these

features are dependent on pneumatization of

the paroccipital is not clear; however, Mei long

was described as having an apneumatic

process that exhibits distal expansion.

The rostral surface of the paroccipital

process is smooth and gently concave

(fig. 14C), but lacks the well-defined columel-

lar sulcus of Byronosaurus jaffei, adult speci-

mens of Velociraptor mongoliensis (Norell et

al., 2004), and Dromaeosaurus albertensis

(Currie, 1995). The rostral concavity continues

medially as a broad and relatively deep

depression in the lateral surface of the

exoccipital/opisthotic that widens to include

the area directly above the fenestra pseudo-

rotunda (fig. 14C). The rostral concavity and

its associated medial fossa are interpreted as

having housed the pneumatic sac associated

with the posterior tympanic recess (Walker,

1985). If correct, this sac, whose origins are

unclear (Witmer, 1997b; Rauhut, 2004), was

present in the caudal margin of the perinate

tympanic cavity despite its failure to invade

and inflate the paroccipital process (a condi-

tion similar to that described for the basal

24

AMERICAN MUSEUM NOVITATES

NO. 3657

theropod Syntarsus rhodesiensis; Raath, 1985).

This medial fossa is partially divided into

dorsal and ventral compartments resulting in

an hourglass shape when viewed laterally and

indicating at least some morphological com-

plexity at this early stage of postnatal ontog-

eny. The rostral margin of this fossa is sharply

delimited and, in contrast to the adult, does

not extend onto the surface above the crista

interfenestralis (accessory tympanic recess;

Norell et al., 2001b). Broad surfaces posi-

tioned caudodorsal and rostrodorsal to the

medial concavity of the posterior pneumatic

cavity probably represent articular facets for

the squamosal and quadrate, respectively (as

in Troodon formosus; Currie and Zhao, 1993).

A relatively diminutive posterior tympanic

recess therefore is present in Byronosaurus

(following Makovicky et al., 2003; contra the

matrix of Turner et al., 2007a, 2007b). The

recess has an osseous signature early in

postnatal ontogeny as it excavates a promi-

nent cavity in the perinate opisthotic above

the fenestra pseudorotunda. The perinate

paroccipital process, however, does not con-

tain a pneumatic cavity nor is the area above

the crista interfenestralis pneumatized (both of

which are in contrast to the adult condition of

Byronosaurus jaffei).

The caudal surface of the exoccipital/

opisthotic dorsal to the foramen magnum is

broadly convex but with a distinct concavity

directly above the paroccipital process

(fig. 14A). This concavity may be related to

the insertion of transversospinalis muscles

(Tsuihiji, 2005), whereas the broad convexity

reflects the general expansion of the underly-

ing inner ear. The inner ear contains a deeply

excavated cavum vestibulare that reaches (but

does not penetrate) the caudodorsal margin of

the exoccipital/opisthotic and continues into

the prootic through a large, ovoid, rostral

opening. The medial wall of the vestibule

contains a small horizontal ridge that partially

divides the chamber into dorsal and ventral

portions—this partial division may reflect the

relative positions of the utriculus and sacculus

within the vestibular cavity.

Dorsal to the rostral opening of the

vestibule, the roof of the inner ear is excavated

by a deep and well-defined depression that is

partially divided into caudomedial and ros-

Fig. 15. Horizontal slice (A) through the

braincase of the holotype of Byronosaurus jaffei

(IGM 100/983) (B) showing the presence of

pneumatic cavity in the base of the paroccipital

process. The cavity is associated with a pair of

rostral foramina and is inferred to result from

invasion of the paroccipital process by diverticula

of a posterior tympanic sinus residing in the

posterior tympanic recess. The pneumatic cavity

in the paroccipital process is largely infilled with a

mineral inclusion that renders as a bright white in

the figure. The white, horizontal line through the

braincase (B) shows the position of the slice (A).

Rostral is to the left, dorsal to the top.

2009

BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

25

trolateral halves by an oblique ridge in its

floor. This recess probably represents, at least

in part, a deep auriculae cerebelli that would

have housed an expanded floccular lobe (as

described in Troodon formosus; Currie and

Zhao, 1993). The caudal wall of this concavity

is marked by two circular openings—one

positioned at its lateral margin, one at its

medial margin—that mark the posterior semi-

circular canal. The vestibular cavity remains

partially open dorsally, which may correspond

to the notch along the opisthotic-prootic

suture described in juvenile birds and the

London specimen of Archaeopteryx lithogra-

phica (Walker, 1985).

The medial, lateral, and caudal walls of the

otic capsule are relatively thick. The caudal

wall separates the cavum vestibulare from the

postnatal remnant of the metotic fissure. The

small foramen inferred as transmitting the

endolymphatic duct posteromedially through

the vestibular wall into the endocranial space

in Byronosaurus jaffei (Makovicky et al., 2003)

and Troodon formosus (Currie and Zhao,

1993) is not preserved in IGM 100/974 and

likely passed through the unossified space

between the prootic, opisthotic, and epiotic

(vestibular pyramid; see below). The cavum

metoticum opens into the endocranial space

through a dorsoventrally elongate medial

aperture that narrows laterally and widens

ventrally (fig. 14C). There is no indication

this space is divided medially—in agreement

with Byronosaurus jaffei and crocodilians

(Iordansky, 1973; Rieppel, 1985) but differing

from hatchling Struthio camelus, in which the

internal opening of the jugular foramen is

positioned more medially and communicates

directly with the endocranial space (fig. 16).

Cranial nerves IX, X, XI, as well as the jugular

(posterior cerebral) vein, all are interpreted as

having passed out of the endocranial space

through this medial aperture. The medial

aperture of the cavum metoticum is recessed

laterally from the medial margin of the

vestibular eminence and foramen magnum.

This recessed position results in a depression

that leads into the medial aperture laterally

and likely housed a ganglion for CN IX, X,

and XI (fovea ganglii vagoglossopharyngealis

of Currie and Zhao, 1993). The jugular vein

presumably passed through the constricted

dorsal portion of the fovea before traveling

ventrally to enter the medial aperture through

which it was transmitted with the aforemen-

tioned cranial nerves below the otic capsule to

the lateral surface of the braincase. The slitlike

medial aperture of the cavum metoticum (in

medial view) is similar in shape to that of

Troodon formosus (Currie and Zhao, 1993)

and differs from the more rounded medial

aperture of Itemirus medularis, Dromaeosaurus

albertensis, Velociraptor mongoliensis, Tsaagan

mangas, and Bambiraptor feinbergi. There

is a slight medial constriction resulting in

an hourglass-shaped medial aperture in

Byronosaurus jaffei and Troodon formosus that

is absent in the perinate. The vestibule

communicates with the endocranial space

rostral to the medial aperture of the cavum

metoticum through what appears to be an

ovoid window that is completed rostrally by

the prootic and dorsally by the unpreserved,

probably unossified, epiotic. This window

differs from that of most known coelurosaurs,

including Byronosaurus jaffei, where this

medial opening is more triangular in shape

(vestibular pyramid)—this difference likely

reflects the reduced ossification of the otic

capsule in IGM 100/974.

The caudolateral margin of the cavum

vestibulare is delimited by an expanded lateral

surface that marks the insertion of the

footplate of the columella into the fenestra

ovalis (fig. 14B, C). This surface (crista inter-

fenestralis) is confluent with the lateral surface

of the opisthotic and prootic rather than

distinctly depressed within the middle ear.

This plesiomorphic lateral position also is

present in Byronosaurus jaffei, Sinovenator

changii, Mei long, and Saurornithoides mon-

goliensis (Barsbold et al., 1987), whereas the

derived recessed condition is expressed in

Troodon formosus, Citipati osmolskae, and

dromaeosaurids (Turner et al., 2007a,

2007b). The crista interfenestralis is not well

preserved in the perinate perhaps due to

delayed ossification of this portion of the

opisthotic. The crista interfenestralis separates

the fenestra ovalis from the cavum metoticum.

The crista interfenestralis also forms the

dorsolateral margin of a large, well-defined

notch (see rostral view). In adult Byronosaurus

jaffei and juvenile Struthio camelus, this

26

AMERICAN MUSEUM NOVITATES

NO. 3657

‘‘notch’’ is present as a large communication

between the middle ear and metotic fissure. It

is unclear what structure, if any, passed

through this space. The perilymphatic duct

of Byronosaurus jaffei was interpreted to enter

the cavum metoticum through a small, medial

foramen (Makovicky et al., 2003). A foramen

similar in size and position to the inferred

foramen perilymphaticum of the adult is

present in the IGM 100/974 and juvenile

Struthio camelus. If the perilymphatic duct

traversed the large opening, a structure whose

identity is unclear, formed the small, medial

foramen.

The prevagal strut of the perinate separates

the jugular foramen from the fenestra pseu-

dorotunda thereby dividing the lateral portion

of the cavum metoticum (fig. 14). This lateral

division of the embryonic metotic fissure is

likely plesiomorphic in Byronosaurus and

paravians in general, as it is present in more

basal tetanurans (e.g., Majungasaurus; Samp-

son and Witmer, 2007). The prevagal strut,

which may represent the ossified metotic

cartilage of birds (de Beer, 1937) and perhaps

the subcapsular process of crocodiles (Baird,

1970; see Rieppel, 1985), is relatively small in

the perinate. The ventral margin of the

fenestra pseudorotunda is open in IGM 100/

974 but may have been closed by the

caudodorsal margin of the basisphenoid as in

the adult. This is in contrast to the condition

in Archaeopteryx lithographica and many

extant birds in which the crista interfenestralis

either curves caudally to contact the rostral

face of the prevagal strut or the strut exhibits a

distinct rostroventral expansion that contacts

a vertical crista interfenestralis (the former

being present in Archaeopteryx lithographica;

Walker, 1985). Perinate Struthio camelus

exhibit a prevagal strut with a strong rostro-

ventral expansion that approximates but fails

to contact the crista interfenestralis (fig. 16). A

shallow groove extends from a position below

the otic capsule ventrolaterally along the

caudal wall of the cavum metoticum to the

internal opening of the jugular foramen. This

groove is inferred to mark the path of CN X

and CN XI through the lateral portion of the

cavum metoticum to the caudolateral surface

of the braincase.

The ventral margin of the prevagal strut

lacks any expansion that would indicate the

presence of a subotic recess (although this

recess would have resided mainly in the

unpreserved basioccipital). The subotic recess

is present in the adult Byronosaurus jaffei and

therefore may develop relatively late in post-

natal ontogeny (or at least its progression onto

the exoccipital/opisthotic occurs relatively

late). The presence of a subotic recess was

considered a synapomorphy of Troodontidae

(present in Byronosaurus jaffei, Troodon for-

mosus, Saurornithoides junior, Saurornithoides

Fig. 16. Lateral (A) and medial (B) views of an

articulated prootic and exoccipital/opisthotic in a

perinate Struthio camelus. Note the medial opening

of the metotic fissure is divided—in contrast to the

undivided metotic foramen of IGM 100/974.

2009

BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

27

mongoliensis), but is shared with some derived

ornithomimids, Velociraptor mongoliensis, and

Allosaurus fragilis. A subotic recess is absent

in basal avialans, alvarezsaurids, oviraptoro-

saurs, and most dromaeosaurids (Norell et al.,

2001b; Hwang et al., 2004; Turner et al.,

2007b).

PROOTIC: The prootic in medial and lateral

views is a triangle-shaped element whose

corners are formed by three strong processes

(fig. 17). The paroccipital ramus is positioned

caudodorsally and forms a dorsal articulation

with the exoccipital/opisthotic. The ramus is

curiously short compared to that of basal

avialians (e.g., Hesperornis regalis; Witmer,

1990; Elzanowski, 1991) and nonavialian

theropods. A short paroccipital ramus also is

present in Byronosaurus jaffei suggesting the

restricted length is a derived feature within

Coelurosauria rather than an ontogenetic

variation. Internally, the paroccipital ramus

houses (in part) the prootic contribution to the

vestibular cavity.

The articular surface with the laterosphe-

noid lies at the rostrodorsal margin of the

prootic. This surface is separated from the

paroccipital ramus by a saddle-shaped depres-

sion (fig. 17C) that lies in a position homol-

ogous to the dorsal tympanic recess of

Archaeopteryx lithographica, modern birds

(Walker, 1985; Witmer, 1990), and perhaps

the alvarezsaurid Shuuvia deserti. Considering

its presence in these taxa, it is not surprising

this area has received considerable attention in

nonavian theropods. Witmer (1997b) consid-

ered this recess present in ornithomimids, ve-

lociraptorine dromaeosaurids, and all known

avialians, whereas a much wider taxonomic

distribution was argued by Rauhut (2004),

who interpreted the dorsal tympanic recess to

be derived at a relatively deep node in

theropod evolution (Neotheropoda). A small

depression referred to as a nascent dorsal

tympanic process (Walker, 1985) characterizes

this region in Dromaeosaurus albertensis,

Archaeopteryx lithographica, and other known

troodontids. This is in contrast to the same

area in adult Velociraptor mongoliensis that

houses a large excavation bounded com-

pletely by the prootic (Sues, 1977; Norell et

al., 2004). The extent of this depression in

Byronosaurus—both the perinate and adult—

is more comparable to that of adult

Velociraptor mongoliensis and Tsaagan man-

gaas and therefore larger than the recess of

most non-metornithine theropods. No fora-

men is associated with this concavity in either

IGM 100/974 or Byronosaurus jaffei, which

agrees with Archaeopteryx lithographica

(Walker, 1985) and Chilantaisaurus ashui-

kouensis (see Rauhut, 2004). The presence of

a pneumatopore on this surface is intraspecif-

ically variable in Troodon formosus (Currie

and Zhao, 1993). A mediolaterally elongate

groove lies within this recess along the medial

margin of the perinate laterosphenoid articu-

lar surface. This groove conforms in both size

and shape to a pneumatic depression found in

perinate Struthio camelus.

The rostral margin of the prootic below the

laterosphenoid contact is marked by a deeply

excavated depression forming the caudal

margin of the trigeminal fenestra (fig. 17A).

The laterosphenoid was not recovered in

either the perinate or adult but almost

assuredly formed the rostral margin of the

trigeminal fenestra as in Troodon formosus

(Currie and Zhao, 1993; contra Currie, 1985)

and most archosaurs (Clark et al., 1993). The

depth of the lateral concavity associated with

the trigeminal fenestra in both the perinate

and adult suggest the gasserian ganglion (CN

V) was positioned extracranially in Byrono-

saurus (in agreement with Saurornithoides

junior, Saurornithoides mongoliensis, Dromaeo-

saurus albertensis, and most nonavian thero-

pods). In contrast, the gasserian ganglion of

Troodon formosus, allosaurids, tyrannosaur-

ids, and the majority of avialians (including

modern birds) lies within the endocranial

space (Madsen, 1976; Currie and Zhao,

1993; Brochu, 2003) and exits the braincase

through a pair of openings—the ophthalmic

branch passing through the laterosphenoid.

The trigeminal fenestrae of the perinate and

Byronosaurus jaffei are relatively large com-

pared to the same structure in adult specimens

of Troodon formosus and Saurornithoides

junior. A distinct rostral process forms the

ventral margin of the perinate trigeminal

fossa. The rounded lateral margin probably

represents a portion of the otosphenoidal

crest. The crest is short, but this appears to

be a reflection of ontogenetic scaling in which

28

AMERICAN MUSEUM NOVITATES

NO. 3657

Fig. 17. Left prootic of IGM 100/974 in caudal (A, A!), rostral (B, B!) and lateral (C, C!) views.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

29

the entire lateral surface of the prootic is short

relative to that of the adult, rather than

allometric development of the anterior tym-

panic recess (which delineates this recess

rostrodorsally). The rostral surface of the

prootic below the trigeminal fenestra houses

a well-defined, blind cavity in the perinate. In

an articulated skull, this surface lies adjacent

to a deeply excavated pneumatic fossa of the

parabasiphenoid that is considered part of the

anterior tympanic recess (see below). A

pneumatic diverticulum from within the ante-

rior tympanic recess likely excavated this

rostroventral cavity. The timing and progres-

sion of this structure’s development is of

phylogenetic interest as the presence of a large

otosphenoidal crest defining a lateral depres-

sion on the side of the braincase may

be a synapomorphy of a monophyletic

Troodontidae (Makovicky et al., 2003).

A small but distinct canal that transmitted

the facial nerve (CN VII) from the endocranial

space penetrates the lateral surface of the

prootic behind the trigeminal fenestra

(fig. 17C). The facial foramen lies completely

within the prootic as in Byronosaurus jaffei

and Troodon formosus and presumably trans-

mitted both the palatine and hyomandibular

branches of CN VII. The foramen opens

caudolaterally and is confluent with a shallow

groove that turns caudally and presumably

marks the path of the hyomandibular ramus

above the acoustic recess, as is typical of

maniraptorans. A subtle vertical groove ex-

tends ventrally from the area behind the facial

foramen. This structure may reflect the path

of the palatine branch (Currie, 1985). If so,

this nerve passed rostrally below the trigem-

inal fenestra rather than traversing the fenes-

tra as was speculated for Troodon formosus

(Currie and Zhao, 1993). The medial opening

of the facial canal is significantly smaller than

its lateral counterpart—perhaps reflecting the

position of the geniculate ganglion on the

lateral surface of the prootic. The facial

foramen lies at a level just above the ventral

border of the trigeminal fossa as in the

oviraptorosaur Conchoraptor gracilis (Bala-

noff and Norell, in prep.) and Struthio

camelus. This position is dorsal to that of

most adult maniraptorans (e.g., Velociraptor

mongoliensis and Dromaeosaurus albertensis)

including other troodontids (e.g., Troodon

formosus) where the facial foramen lies fully

below the trigeminal opening. The condition

in Byronosaurus jaffei is intermediate with the

facial foramen positioned in line with the

ventral margin of the trigeminal fenestra. This

difference with other maniraptorans may

reflect the relatively large trigeminal opening

in Byronosaurus. Below the facial foramen is a

small but highly rugose surface. This rugosity

lies in the same position as the terminus of the

otosphenoidal crest in Byronosaurus jaffei and

therefore may represent the early development

of this structure. The adult trigeminal and

facial openings lie in a shallow but wide

trough delineated by a strongly developed

otosphenoidal crest ventrally and a lateral

expansion of the braincase dorsally (Mako-

vicky et al., 2003). This trough is absent in the

perinate due to the lack of a well-defined

otosphenoidal crest. The facial foramen in

Byronosaurus lies dorsal to the anterior

tympanic recess rather than within it as in

both species of Saurornithoides and Troodon

formosus (Barsbold, 1974; Currie and Zhao,

1993; see Norell et al., 2009).

The caudal edge of the perinate prootic

forms the rostral border of the fenestra ovalis.

This margin is sharply delimited but is not

markedly concave at the point of insertion for

the columella auris (unlike the adult and

perinate Struthio camelus; fig. 16). This mar-

gin continues ventrally as the caudal edge of

the long, medially curved cochlear recess

(fig. 17B).

A saddle-shaped area in the caudodorsal

region of the medial surface (fig. 17A) is the

prootic contribution to the floccular recess.

Rostral to this recess, the base of the latero-

sphenoid contact is markedly convex reflect-

ing the rostrodorsal extension of the underly-

ing vestibular cavity (medial vestibular emi-

nence). There is no visible signature of the

middle cerebral vein. Caudoventral to the

floccular recess is a second inflated area

representing an emargination of the prootic

contribution to the vestibular pyramid. The

dominant feature on the medial surface is a

broad acoustic recess that houses the lateral

openings of both the facial and vestibuloco-

chlear (CN VIII) nerves. The facial foramen

lies near the rostral margin of this recess

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AMERICAN MUSEUM NOVITATES

NO. 3657

within a deep secondary fossa (facial fossa).

This fossa is slightly larger than a similar

secondary fossa (vestibulocochlear fossa) po-

sitioned caudally and housing the ganglion

and associated foramina of CN VIII. The

facial fossa is confluent with the surface lying

ventral to it and is separated from the

vestibulocochlear fossa by a vertical ridge that

becomes less distinct ventrally.

The vestibulocochlear fossa is penetrated by

two foramina that transmitted branches of CN

VIII into the inner ear. The larger of these

openings lies at the ventral margin of the

acoustic recess (at approximately the same

dorsoventral position as the facial foramen—

as noted for the Byronosaurus jaffei;

Makovicky et al, 2003). This foramen conveyed

the cochlear branch of CN VIII into the

caudodorsal margin of the cochlear recess at a

slightly downward angle. The foramen is

bordered internally by a ventral surface that

lies behind the primary chamber housing the

cochlear duct (fig. 17B). The cochlear recess,

which ossifies around the cochlear duct and is

open caudally, is bulbous, medially curved, and

long. The oblique, medial ridge partly dividing

the cochlear recess into proximal and distal

parts in the adult is not present in the perinate.

The second acoustic foramen, which trans-

mitted the vestibular branch of CN VIII, is

similar in size to the facial foramen and thus

much smaller than the cochlear foramen. The

vestibular foramen is positioned above the

cochlear foramen along the rostral margin of

the vestibulocochlear fossa. Internally, this

foramen opens onto the floor of the vestibule,

which extends into the paroccipital ramus as a

largely horizontal, cylindrical cavity (cavum

vestibulare). A relatively small secondary

chamber, which is widely confluent with the

greater vestibular cavity, is present in the

dorsolateral region of the inner ear. This

secondary chamber probably housed the

ampulla of the anterior and horizontal semi-

circular ducts. The dorsal foramen for the

anterior semicircular canal resides in the

caudal margin of the paroccipital ramus. The

canal is closed, but a groove marks its vertical

length. A strong horizontal ridge separates the

vestibular cavity from the more vertical

cochlear recess. This ridge also separates the

vestibular and cochlear foramina internally.

The vestibulocochlear fossa continues as a

deep cavity caudal and dorsal to the foramina

of CN VIII. This extension also is present in

perinate Struthio camelus, but unlike in the

ostrich, in which this cavity is pierced by a

foramen that opens caudally into the medial

margin of the vestibule, the extension in IGM

100/974 ends as a blind pocket. Two addition-

al foramina, also not present in IGM 100/974,

penetrate the floor of the vestibular cavity in

Struthio camelus. These foramina enter the

vestibule through the dorsal margin of the

facial fossa. The expanded fossae surrounding

CN VII and VIII are shared features of the

perinate Byronosaurus and Struthio camelus,

although the additional pneumatic foramina

that penetrate these fossae in the latter are

absent in the former. The anatomical origin

of these similarities and differences, as well

as their phylogenetic

polarity

within

Coelurosauria, is currently unclear.

PARABASISPHENOID: The parabasisphenoid

is an elongate element whose left side is badly

damaged exposing the internal surfaces of the

right side (fig. 18). The parabasisphenoid is

inferred as divided indistinguishably into an

anterior parasphenoid, which forms a prom-

inent rostrum, and the more compact basi-

sphenoid. The fusion of this compound

element likely occurred prenatally as it does

in the modern reptiles whose skeletal develop-

ment has been surveyed (Bellairs and Kamal,

1981). The basioccipial is not preserved, but a

small triangular surface lying at the caudo-

ventral margin of the dorsum sellae represents

the basioccipital-parabasisphenoid contact.

The triangular shape of this concave surface

suggests that at least the lateral margins of the

basicranial fenestra (de Beer, 1937) remained

unossified (as in specimens of similarly aged

Struthio camelus, but in contrast to Aepyornis

where this fontanelle closes prenatally;

Balanoff and Rowe, 2007).

The cultriform process (parasphenoid ros-

trum) is prominently elongate and tapers

rostrally to a thin apex. This structure is not

preserved in the holotype of Byronosaurus

jaffei. A small but distinct secondary process

(parasphenoid process of Colbert and Russell,

1969) lies rostral to the hypophyseal fossa and

roughly approximates the caudal margin of the

cultriform process and probably the caudoven-

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

31

Fig. 18. Parabasisphenoid of IGM 100/974 in left lateral (A, A!), right lateral (B, B!), and rostrodorsal

(C, C!) views.

32

AMERICAN MUSEUM NOVITATES

NO. 3657

tral extent of the interorbital septum (Currie,

1985). The parasphenoid process is bilaterally

symmetrical with a distinct sagittal keel. There

is no indication of a midline suture, in

agreement with Saurornithoides mongoliensis,

although the same process in a specimen of

Troodon formosus was described as paired

(Currie, 1985). The parasphenoid process also

differs from that of Troodon formosus in being

broadly rounded rather than forming a nearly

vertical, fingerlike structure (Currie, 1985;

Currie and Zhao, 1993). There is no evidence

on the dorsal surface of the cultriform process

of the narrow, longitudinal trough that houses

the trabecular cartilages of the interorbital

septum in other troodontids, dromaeosaurs,

and ornithomimids (Currie, 1985).

The base of the cultriform process is

pneumatized forming a bulbous parasphenoid

capsule. The capsule is mediolaterally con-

stricted directly rostral to the basipterygoid

process; however, there is no evidence for

a discrete basipterygoid recess (present in

Sinovenator changii but absent in Sinorni-

thoides youngi, Saurornithoides mongoliensis

and junior, and Troodon formosus). The capsule

likely is pneumatized by a diverticulum of the

anterior tympanic air sac lying within the

anterior tympanic recess (especially considering

the absence of a basisphenoid recess, which also

was implicated as pneumatizing the parasphe-

noid; see Currie and Zhao, 1993; Witmer,

1997a). Rauhut (2004) drew support for

pneumatization of the capsule through the

anterior tympanic recess based on the conflu-

ence of these cavities in Sinovenator changii.

This confluence also was described in Troodon

formosus (Currie, 1985; Currie and Zhao,

1993), and is partially present along the sagittal

midline in the Ukhaa perinate. An irregularly

shaped wall of bone positioned rostral and

ventral to the hypophyseal fossa and medial to

the basipterygoid process, however, at least

partially separates the parasphenoid capsule

and a rostral expansion of the anterior

tympanic recess. Details of this bony partition

are difficult to discern because of the presence

of a small amount of matrix inside the cultri-

form process and the probable existence of

postmortem damage to this area. Pneumatized

parasphenoid capsules are known in several

theropod

lineages including troodontids

(Barsbold, 1974; Osmólska and Barsbold 1990;

Currie, 1985), ornithomimids (Osmólska et al.,

1972; Barsbold, 1983), and therizinosaurids

(Currie, 1997; see also Kurzanov, 1976; Clark

et al., 1994; Holtz, 1994).

The right basipterygoid process is preserved

(neither are preserved in Byronosaurus jaffei).

The process has a strongly developed base and

an ovoid, rostroventrally projecting distal end

(a derived character shared with other troo-

dontids, some ornithomimids, and Apsaravis

ukhaana; Norell et al., 2001b; Hwang et al.,

2004; Turner et al., 2007a, 2007b). Its length

does not surpass the ventral margin of the

cultriform process, which makes it consider-

ably shorter than that of other troodontids.

This may be due in part to a distal break or

reduced ossification along its contact with the

pterygoid. Relative to the hypophyseal fossa,

the process is positioned further rostral than in

Troodon formosus (Currie, 1985; Currie and

Zhao, 1993). The perinate process is bulbous

suggesting pneumatization. However, as noted

above, there is no visible evidence of a

basipterygoid recess or foramina that might

have communicated pneumatic diverticula to

its interior. The distal and medial surfaces of

this structure are covered in a thin layer of

matrix that might be obscuring such an

opening(s) if one exists. Pneumatized process-

es are considered derived within Coeluro-

sauria and are present in Troodon formosus,

Sinornithoides youngi, Saurornithoides mongo-

liensis, Saurornithoides junior, oviraptoro-

saurs, and derived ornithomimids (Turner et

al., 2007a, 2007b).

The ventral surface of the parabasisphenoid

between the basipterygoid processes is only

partially preserved but exhibits no evidence of

the basisphenoid recess or more rostral sub-

sellar recess that excavate this surface in other

tetanurans (Witmer, 1997). The basisphenoid

recess is absent in other known troodontids,

the therizinosaurid Erlikosaurus andrewsi

(Clark et al., 1994) and advanced avialians.

Its loss was considered a synapomorphy of

Troodontidae (Makovicky et al., 2003).

Caudally, the ventral margin of the parabasi-

sphenoid contacts the basioccipital and agrees

with other troodontids in providing no bony

struts to brace the presumably reduced basi-

tubera (basitubera formed by both the basi-

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

33

occipital and basisphenoid is plesiomorphic

within Coelurosauria; Currie and Zhao, 1993).

The hypophyseal fossa lies in the floor of a

triangular sella turcica (differs from the more

circular structure of dromaeosaurs; e.g.,

Bambiraptor feinbergii). The fossa is deep but

did not communicate with the ventral surface

of the parabasisphenoid via a craniopharyn-

geal canal (an embryonic communication that

can be retained late into postnatal ontogeny in

a wide variety of vertebrates; Edinger, 1942;

Hauser and De Stefano, 1989). Two short

ridges flank the hypophyseal fossa laterally

and likely contacted the absent laterosphe-

noids and orbitosphenoids (as in Troodon

formosus [Currie and Zhao, 1993] and Sauror-

nithoides mongoliensis [Barsbold, 1974]), as

well as providing a surface of origin for the

retractor muscles of the eye. These ridges

culminate in the clinoid processes, which are

asymmetrically developed (left larger than

right). There is no evidence of a retractor pit

between the hypophyseal fossa and parasphe-

noid process. A small, bilaterally symmetrical

tubercle projects laterally from the surface of

the parabasisphenoid directly below these

ridges and likely functioned to increase the

surface area available to the external eye

musculature.

This lateral tubercle lies at the dorsal

margin of the anterior tympanic recess, which

excavates the lateral surface of the parabasi-

sphenoid caudal to the basipterygoid process.

The resultant cavity continues medial to the

basipterygoid process and possibly forms a

midline communication with the parasphenoid

capsule (described above). The right and left

anterior tympanic cavities form a midline

communication beneath the hypophyseal fos-

sa rostral to the cerebral carotid canals. A

second communication also may be present

behind the hypophyseal fossa and cerebral

carotid canals and in front of the dorsum

sellae as in Troodon formosus (Currie and

Zhao, 1993). This caudal confluence appears

to exist in Byronosaurus jaffei, although poor

preservation precludes confidently assessing

the extent to which a bony lamina partitions

these cavities (Makovicky et al., 2003). There

does not appear to be an osseous subdivision

of the anterior tympanic recess into a dorsal

prootic and ventral subotic region as clearly is

present in Byronosaurus jaffei. Disparity in the

degree to which the rostrodorsal (see prootic

above) and rostroventral regions are excavat-

ed in the perinate indicates at least some

vertical partitioning of the anterior tympanic

recess. Also difficult to assess is the presence/

absence of a thin wall of bone contributed by

the parasphenoid that encloses the anterior

tympanic recess and part of the middle ear sac

laterally in other troodontids, ornithomimids,

and birds (Witmer, 1997b). Such a wall is not

present in the perinate either because it had yet

to ossify, was ossified but not preserved, or

never developed.

The cranial carotid arteries were enclosed in

osseous tubes passing through the anterior

tympanic recess (the left tube is broken).

Similar tubes were described in therizinosaur

embryos and are known in some birds

(Kundrát et al., 2008). These tubes meet ventral

to the hypophyseal fossa (intrahypophyseal

recess) and probably entered the fossa through

a common canal—as in Byronosaurus jaffei,

other troodontids, and Itemirus medularis

(Kurzanov, 1976), but in contrast to the con-

dition in Dromaeosaurus albertensis (Currie

and Zhao, 1993) and Gallimimus bullatus

(Osmólska et al., 1972). The common carotid

canal of IGM 100/974 enters the hypophyseal

fossa in a more ventral position than in Struthio

camelus. There is no evidence of a separate

foramen vidiani indicating that, as in most

archosaurs including Byronosaurus jaffei, the

palatine artery split from the internal carotid

artery external to the parabasisphenoid and ran

rostrally ventromedial to the basipterygoid

processes (Currie, 1985; Walker, 1990;

Rauhut, 2004). A shaft of bone in both the

perinate and the adult angles from the rostral

surface of the dorsum sellae to buttress the

carotid canals at their apparent confluence

below the hypophyseal fossa (fig. 18A). There

is no indication of a lateral groove transmitting

the oculomotor nerve (CN III).

Broad and relatively tall dorsum sellae

delineates the caudal margin of the hypophy-

seal fossa and contribute to the floor of the

endocranial space. The dorsum sellae is heart

shaped in caudodorsal view with a ventral apex

truncated by the basioccipital contact. A faint

sagittal line divides the dorsum sellae caudally.

The line suggests fusion of right and left

34

AMERICAN MUSEUM NOVITATES

NO. 3657

ossification centers was incomplete at this stage

of ontogeny (fig. 18C). This suture also is

visible in the caudal wall of the sella turcica.

A paired abducens canal (CN VI) penetrates

the dorsum sellae near its dorsolateral margin.

The rostral abducens foramina lie ventral to the

clinoid processes, and thus open below the

origins of the external eye musculature, which

the abducens nerve innervates. There is no

broad opening of the caudal wall of the

hypophyseal fossa as described in some dino-

saurs (e.g., Piatnitzkysaurus floresi, Rauhut,

2004; Apatosaurus, Balanoff et al., in press).

The oblique orientation of the dorsum sellae

together with the hypophyseal surface gives the

basisphenoid a pyramidal shape when viewed

laterally. This orientation also indicates the

caudal floor of the perinate endocranial cavity

was deeply concave as in the adult, which

Makovicky et al. (2003) interpreted (in combi-

nation with a large foramen magnum) as

reflecting an enlarged pons and medulla

oblongata. If the hindbrain of Byronosaurus

was large relative to other derived coelurosaurs

than this expansion occurred relatively early in

ontogeny. A remarkably deep braincase floor

characterizes all known troodontids (Currie

and Zhao, 1993). The lateral margins of the

dorsum sellae contain an elongate articular

surface for contact with the prootic.

The distal end of the cultriform process is

preserved in IGM 100/972 as the only known

braincase element of this specimen (fig. 11). The

fragment includes a short length of the para-

sphenoid capsule, which compares closely with

that of IGM 100/974 (the wall of the capsule

may be slightly thicker in IGM 100/972).

MANDIBLE

DENTARY: The dentary has a subtriangular

shape (in lateral view; fig. 19) that is shared

with other troodontids (Currie, 1987), includ-

ing Byronosaurus jaffei, and differs from the

apomorphic subparallel dorsal and ventral

margins in dromaeosaurs and Archaeopteryx

lithographica. In ventral view (fig. 2A), the

dentary is straight (up to and including the

symphysis)—a condition present in Byro-

nosaurus jaffei, Sinornithoides youngi, Mei

long, Sinovenator changii, and adult dromaeo-

saurs (Ostrom, 1990). This contrasts with the

derived condition in Saurornithoides mongo-

liensis, Saurornithoides junior, and Troodon

formosus in which the dentary curves medially

at the symphysis (Currie, 1987). The perinate

dentary is expanded dorsoventrally and trans-

versely at the symphysis. The lateral surface is

marked by a shallow groove housing a row of

nutritive foramina that communicate with the

inferior alveolar canal (fig. 19). The groove

delineates a small dorsal ridge that overhangs

the remaining lateral surface of the dentary.

The groove is not as deep as in other

troodontids, including Byronosaurus jaffei,

but this is likely an ontogenetic variation.

Meckel’s groove on the medial surface of the

dentary also is relatively shallow. A groove

housing nutritive foramina was considered a

synapomorphy of Troodontidae, although

it is present in a small number of other para-

vian taxa (e.g., Buitreraptor gonzalezorum

[Makovicky et al., 2005]; Archaeornithoides

deinosauriscus; Elzanowski and Wellnhofer,

1993). The lower dentition is housed in a thin,

but deep, groove (see below). The dentary

lacks a process that extends above the

mandibular fenestra, in agreement with dro-

maeosaurs and Archaeopteryx lithographica

but differing from Mei long and Confuciu-

sornis sanctus (Chiappe et al., 1999).

SPLENIAL: As in dromaeosaurs and other

troodontids (Osmólska and Barsbold, 1990),

the perinate splenial is exposed laterally as an

extensive triangular wedge. The rostral extent

of this exposure is difficult to determine

precisely but appears to have extended for

more than half the dentary length, as in

Byronosaurus jaffei and similar to Velo-

ciraptor mongoliensis and Dromaeosaurus al-

bertensis. The splenial-angular articulation

occurs along a concave margin. The splenial

wraps around the ventral mandibular margin

and is exposed as a large plate medially. The

rostral extent of this medial exposure is

concealed by matrix, however, the visible

surface compares closely with the same

triangle-shaped exposure in Saurornithoides

junior, Saurornithoides mongoliensis, and dro-

maeosaurids (Currie, 1995) (cannot be deter-

mined in Byronosaurus jaffei; Makovicky et

al., 2003). The splenial delineates the rostro-

medial margin of the mandibular fenestra. The

caudal margin of the splenial’s medial expo-

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

35

sure inclines caudodorsally and does not fork,

which differs from the forked condition of

adult dromaeosaurs (e.g., Dromaeosaurus al-

bertensis and Deinonychus antirrhopus).

PREARTICULAR, ANGULAR, AND SURANGU-

LAR: Poor preservation obscures the intra-

mandibular joint. The prearticular is present

on the medial surface of the right mandible of

IGM 100/972 as a broad, slightly convex bone

that covers the dorsal half of the mandibular

fenestra (fig.19A). The slender, elongate

angular has a straight ventral margin that

helps form the mandibular fenestra. The

rostral one-third of the angular curves dorsal-

ly following the fenestra and forms a broad

contact with the caudoventral corner of the

dentary and the concave caudal margin of the

splenial. The angular-surangular contact lies

slightly above the ventral margin of the

mandibular fenestra. The angular meets the

prearticular medially. The straight, dorsally

convex surangular forms the roof of the

external mandibular fenestra. Anteriorly, it

meets the dentary at the rostral border of the

mandibular fenestra. Posteriorly, it forms the

caudal border of the same fenestra. The

surangular lacks the T-shaped cross-sectional

morphology exhibited by Sinovenator changii.

A foramen pierces the lateral surangular

surface directly above the midline of the

mandibular fenestra. A laterally exposed

surangular foramen is common in coeluro-

saurs although absent in Shuuvia deserti,

oviraptorosaurs, and Archaeopteryx lithogra-

phica (Turner et al., 2007a, 2007b). The

surangular foramen of Byronosaurus jaffei is

much larger but similar in position to that of

the perinates. Neither the coronoid nor the

supradentary of Dromaeosaurus albertensis

(Currie, 1995) can be observed.

MANDIBULAR FENESTRA: As in other troo-

dontids, the external mandibular fenestra

retains a basically oval shape, which is

plesiomorphic for paravians. The dorsal mar-

gin is straight, the ventral margin concave.

Laterally, the angular forms the ventral

margin, the surangular the dorsal border (with

no contribution from a caudal process of the

dentary as in Mei long). The mandibular

fenestra is proportionally larger than in adult

deinonychosaurs.

DENTITION

The premaxillae bear four teeth—as in

Byronosaurus jaffei, Velociraptor mongoliensis

Fig. 19. Lateral views of the left (A) and right (B) mandibles of IGM 100/972.

36

AMERICAN MUSEUM NOVITATES

NO. 3657

(contra Sues, 1977), and other dromaeosaurids

(Currie, 1995; Ostrom, 1990; this likely is the

plesiomorphic condition for Theropoda,

Makovicky et al., 2003).

The dental border of the maxilla contains at

least 13 tooth positions; if more are present,

there is room only for an additional one or

two teeth. This compares with 10 maxillary

teeth in adult Velociraptor mongoliensis, 15 in

Deinonychus antirrhopus (Ostrom, 1969b), nine

in Dromaeosaurus albertensis (Currie, 1985)

and more than 10 in Saurornitholestes langstoni

(Currie et al., 1990). Within Troodontidae, Mei

long contains approximately 24 maxillary teeth

(Xu and Norell, 2004), with a similar number

present in Sinovenator changii (Xu et al., 2002)

and Saurornithoides junior (Barsbold, 1974).

Sinornithoides youngi contains approximately

18 maxillary teeth (Russell and Dong, 1993)

and whereas only nine tooth positions are

preserved in Troodon formosus, much of the

maxillary tooth row of that specimen is missing

(Currie, 1985). At least eight and at most

11 maxillary teeth were described for

Archaeornithoides deinosauriscus; although,

like the Ukhaa perinates, the only known

specimen of this taxon is ontogenetically young

(Elzanowski and Wellnhofer, 1992, 1993).

Makovicky et al. (2003) noted difficulty in

determining the number of maxillary teeth in

Byronosaurus jaffei but predicted the number

may reach 30, based on spacing of the

observable teeth. These comparisons support

a relatively large number of maxillary teeth

as a derived feature of adult troodontids.

Approximately 18 or 19 tooth positions are

located on the perinate dentary, which is

significantly fewer than the conservative esti-

mate of 30 for the adult (Makovicky et al.,

2003).

The upper and lower teeth (fig.20) in

general exhibit straight, conical cusps that

are mediolaterally compressed. Some teeth are

slightly recurved and there is considerable

variation with regard to the rostrocaudal

length of the crowns (there is no obvious

trend with regards to tooth-row position for

either of these variables). Length of the tooth

affects the crown shape, with the wider teeth

having a vertical caudal margin and a poster-

oventrally sloping rostral margin. The teeth

are constricted between the root and crown

and lack any evidence of serrations—condi-

tions that differ from the unconstricted and

serrated teeth of dromaeosaurs. Absence

of serrations also characterizes the teeth

of Mei long, EK troodontid IGM 1000/44,

Archaeornithoides deinosauriscus (Elzanowski

and Wellnhofer, 1993), and Urbacodon item-

irensis (Averianov and Sues, 2007), as well as a

Fig. 20. Lateral views of the left maxillary (A),

right maxillary (B), and right dentary (C) teeth of

IGM 100/972. A fracture in the right dentary

reveals the closely appressed rostral dentition of

the lower jaw characteristic of troodontids and the

restriction of the interdental septae to the base of

the tooth roots.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

37

number of non-troodontid paravians (e.g.,

Buitreraptor gonzalezorum, Rahonavis ostromi,

Archaeopteryx lithographica, and other toothed

avialans: Turner et al., 2007b).

The dentary teeth become smaller, more

numerous, and more appressed rostrally,

which

is characteristic of troodontids

(Currie, 1987). The teeth are set in a deep

groove and separated by bony septae that

appear to extend lingually as outgrowths of

the dental groove’s labial margin (fig. 21). The

separation between the highly vascularized

interdental bone and the labial surface of

the dentary described in a specimen of

Dromaeosaurus and figured in a specimen of

Troodon formosus by Currie (1987; fig. 3b) is

not apparent in IGM 100/972 (fig. 21). There

is no sutural connection between these inter-

dental septae and the lingual margin of the

dental groove, except perhaps at the base of

the groove—as described for Troodon formo-

sus (Currie, 1987). This basal separation of the

tooth roots is maintained even at the rostral

end of the jaw where appression of the

numerous dentary teeth results in the reduc-

tion of most of the length of the interdental

septae (fig. 21C).

The phylogenetic identity of the interdental

septae in paravians is one that is not

immediately clear. Currie (1987) concluded

that the interdental plates, so obvious in

more basal coelurosaurs (e.g., tyrannosaurids;

fig. 21E), are lost in troodontids and present,

but fully fused, in dromaeosaurs (fig. 21D).

This scenario is in contrast to Varricchio

(1997b), who described troodontids as having

interdental plates. Evidence for interdental

plates in dromaeosaurs is drawn largely from

the absence of a distinct disparity between the

height of the dorsolabial and dorsolingual

margins of the dental groove. The dorsolabial

margin is significantly higher than the dorso-

lingual margin in troodontids and in taxa

where interdental plates are present unambig-

uously. The relatively tall dorsolingual margin

of dromaeosaurs, therefore, either is the result

of fusion of the interdental plates into the

labial margin of the dental groove (as argued

by Currie, 1987) or the acquisition of a derived

growth trajectory for these margins. Currie

(1987) drew additional evidence for the

retention of interdental plates in dromaeo-

saurs from a histological disparity between the

highly vascularized interdental plates and the

laminar bone of the dentary in a specimen of

Dromaeosaurus (TMP 82.19.185). This dispar-

ity, however, is not obvious in Dromaeosaurus

albertensis (AMNH FR 5356), Velociraptor

mongoliensis (IGM 100/976), Bambiraptor

feinbergi (AMNH FR 30556), or the perinate

Byronosaurus (IGM 100/972). Complicating

the issue, the interdental morphology of IGM

100/972 compares closely with that of the

Munich specimen of Archaeopteryx lithogra-

phica, which is identified as retaining true

interdental plates (Wellnhofer, 1993). Both

specimens exhibit interdental septae set

low in the dental groove and that extend from

the relatively tall labial margin of the dental

groove before expanding lingually. Depending

on the number of characters that ultimately

are shared with basal avialans, the phyloge-

netic implications of this problem may lie

more in the accuracy of character definitions

than in the phylogenetic position of dromaeo-

saurs and/or troodontids. For example, the

dromaeosaur condition is autapomorphic

among paravians regardless of whether the

apomorphy lies in the fusion of retained

interdental plates or the loss of these plates

combined with an autapomorphic growth

trajectory for the dorsolingual and dorsolabial

margins of the dentary. The same is true for

the troodontid condition, which either reflects

a derived state in which the interdental plates

are lost or simply reduced as the teeth become

autapomorphically numerous and closely ap-

pressed within the jaw. The identity and

evolutionary history of tooth implantation in

derived coelurosaurs needs to be reviewed

comprehensively—perhaps beginning with the

nature of inter- and intraspecific variation in

the histological signature of ‘‘true’’ interdental

plates.

DISCUSSION

The perinate skulls collected in association

with a nest of oviraptorid eggs and discussed

previously as dromeosaurids (Norell et al.,

1994) are herein allocated to the derived

troodontid, Byronosaurus. The only other

specimens currently allocated to Byrono-

saurus are the holotype skull of Byronosaurus

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AMERICAN MUSEUM NOVITATES

NO. 3657

jaffei and a fragmentary rostrum (IGM 100/

984), both of which are relatively large,

skeletally mature, and considered to reflect

the adult condition of this taxon (Makovicky

et al., 2003). The Ukhaa perinates preserve

cranial elements and structures that are not

preserved

in

the adult specimens of

Byronosaurus and are poorly understood in

troodontids and derived maniraptorans in

general. The ectopterygoid, pterygoid, and

cultriform process of the parabasisphenoid

were not preserved in the adult and therefore

are described for the first time here. The

frontal, lacrimal, and palatine are better

preserved in the perinate than the adult, as

are the palate and the internal morphologies

of the rostrum, braincase, and otic capsule.

ONTOGENY

The ontogenetic age of the Ukhaa perinates

provides a nearly unique opportunity to infer

postnatal changes in the cranial morphology

of a highly exclusive clade of nonavian

Fig. 21. Scanning electron microscopy images (A–C) of a portion of dentary and associated dentition

removed from the left side of IGM 100/972. The teeth are set relatively low in a dental groove but are divided

by interdental septae that appear to extend from the groove’s labial margin. Medial view of the left mandible

of Dromaeosaurus albertensis (AMNH FR 5356) (D). Note the similar height of the labial and lingal margins

of the dental groove that characterizes dromaeosaurs (Currie, 1987) among coelurosaurs and the apparent

lack of a distinct textural difference between the proposed fused interdental plates and laminar bone of the

dentary. Medial view of a partial right mandible of the tyrannosaurid Tarbosaurus baatar (IGM 100/1841)

(E) showing the distinct difference between the labial and lingual margins of the dental groove and a distinct

textural difference between the interdental plates and dentary bone.

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

39

coelurosaurs. This ontogenetic variation

serves as baseline data for future comparisons

of postnatal transformations in maniraptoran

cranial morphology and therefore is an

important step in establishing the role heter-

ochrony has played in the morphological

diversification of derived coelurosaurs (includ-

ing birds).

That stated, establishing the biological

source of observed morphological variation

requires replication (Jones and German,

2005)—variation has to be studied at two

hierarchical levels (at least) to say anything

about one. For example, an observed differ-

ence between a male and female specimen may

not represent a true sexual dimorphism but

rather reflect an individual variation that

occurs in both sexes but cannot be recognized

without adequate samples of both males and

females. The samples required for replication

often are difficult to procure for extant

vertebrate taxa much less extremely rare

extinct lineages (Bever, 2009). Observed dif-

ferences between the perinates and adults of

Byronosaurus therefore may represent phylo-

genetic variation (the specimens may represent

different species of Byronosaurus), sexual

dimorphism, polymorphism, ontogenetic trans-

formations, or some combination thereof.

Considering the disparity in size and stage

of development between the Ukhaa perina-

tes and adults of Byronosaurus, however, we

conclude that the vast majority of the

observed differences between these specimens

are likely to be highly correlated with

ontogeny (meant here in the classical

sense—all anatomical features and thus all

variation have an ontogenetic history). This

conclusion allows these comparative observa-

tions to serve as a series of hypotheses

regarding what features of the Byronosaurus

skull transform during postnatal ontogeny.

The polarity of these transformations, the

accuracy with which they predict ontogenetic

transformations in other coelurosaur taxa,

and the status of these observed variations as

ontogenetic in origin will be established/

rejected only with future study and new

specimens.

GENERAL SIZE AND SCALING: Based on

maxillary length, the Ukhaa perinates are

approximately 20% the size of the adult

holotype of Byronosaurus jaffei. The size

disparity in Byronosaurus supports the con-

clusions of Rauhut and Fechner (2005) that

perinates of derived coelurosaurs were preco-

cially large relative to more basal tetanurans

(e.g., allosaurids)—even when the adult body

size of the former is considerably smaller than

the latter. One of the most striking features of

the cranial morphology of the perinate

Byronosaurus (most obvious in IGM 100/

974) compared to that of the adult is the

disparity in the relative size of the circular

orbit. A relatively large orbit in the perinate

skull is expected considering that vertebrate

species in general retain an ontogenetic

trajectory (apparently derived at a deep node

in chordate if not deuterostome phylogeny) in

which the central nervous system develops at a

faster rate than the surrounding skeleton (see

Emerson and Bramble, 1993). This trend

predicts that skeletal structures housing com-

ponents of the central nervous system (e.g.,

orbit, braincase, trigeminal fenestra) appear

larger in juveniles and then become relatively

small during postnatal growth. In fact, both

the braincase and trigeminal fenestra of the

Ukhaa perinate IGM 100/974 is large relative

to the same structures in the adult of most

troodontids and other maniraptorans. For

example, the convex and bulbous shape of

the frontal in the perinate as compared to that

of the adult Troodon formosus (the frontal of

the adult Byronosaurus is not well preserved)

likely is a reflection of the different postnatal

growth trajectories of the frontal (positive

allometry) and the underlying telencephalon

(negative allometry). In contrast to these

expected trajectories, the trigeminal fenestra

of adult Byronosaurus retains its relatively

large size. Makovicky et al. (2003) attributed

this large size to allometry, which undoubtedly

is correct. Based on comparisons with other

troodontids and the expected relative size of

this structure if plesiomorphic growth trajec-

tories were fully retained, a large trigeminal

fenestra in adult Byronosaurus reflects a

derived transformation in the allometry of

this region. The pattern of this transformation

most closely resembles paedomorphosis

(Alberch et al., 1979; Fink, 1982). Intra-

versus extracranial positions of the gasserian

ganglion seemingly would affect the allometry

40

AMERICAN MUSEUM NOVITATES

NO. 3657

of this fenestra (an extracranial position

is interpreted for this ganglion in Byronosaurus

—in contrast to Troodon formosus; see above);

however, we know of no studies that have

examined these effects explicitly. The apo-

morphic retention of a large trigeminal fenestra

combined with a plesiomorphic decrease in the

relative size of the orbit and endocranial space

in adult Byronosaurus suggests the ontogeny

and phylogeny of these bony structures and the

components of the central nervous system with

which they are directly associated are decou-

pled at least to some degree.

ROSTRUM: Another component of the peri-

nate skull of Byronosaurus that differs mark-

edly from that of the adult is rostral shape.

The perinate rostrum is short (rostrocaudally)

and deep (dorsoventrally) with a markedly

convex dorsal margin (when viewed laterally),

whereas that of the adult is relatively elongate

and flat. This indicates that significant shape

change due to allometric growth within

the rostrum (flattening) and between the

rostrum and the rest of the skull (lengthening)

is a morphological transformation that char-

acterizes the postnatal development of Byro-

nosaurus. Postnatal elongation of the rostrum

is common among dinosaurs (Coombs, 1982;

Horner and Currie, 1994; Varricchio, 1997b;

Carr, 1999; Rauhut and Fechner, 2005), and

archosaurs in general (e.g., Hall and Portier,

1994; Monteiro et al., 1997), and therefore its

presence is plesiomorphic in Byronosaurus.

The perinate rostrum of Byronosaurus, how-

ever, is more elongate than the perinate

rostrum of more basal tetanurans (Rauhut

and Fechner, 2005), which suggests that the

growth trajectory of the snout of Byronosaurus

is derived at some position within coeluro-

saurian phylogeny. Likewise, the rostrum of

adult Byronosaurus jaffei is relatively long and

flat (i.e., more shallow dorsoventrally) com-

pared to other adult troodontids (e.g., Mei

long, Sinovenator changii, and Troodon for-

mosus; Makovicky et al., 2003). The appar-

ently apomorphic shape of the rostrum in

adult Byronosaurus therefore is the result of a

growth trajectory that begins early in ontog-

eny (probably prenatally). Whether this adult

shape ultimately is due to a derived elongation

of the plesiomorphic allometric growth trajec-

tory, or whether a change in the slope of

allometric change is driving this apomorphic

transformation in Byronosaurus cannot cur-

rently be determined.

A number of more specific characters that

exhibit variation between the perinate and

adult skull of Byronosaurus likely are correlated

with the general transformation in rostral

shape. These characters include rounding of

the premaxillary symphysis, lengthening of the

external naris, maxillary fenestra, and antorbi-

tal fenestra, a caudal shift in the position of the

rostral margin of the antorbital cavity relative

to the maxillary tooth row, a flattening of the

caudodorsal trajectory of the ascending process

of the maxilla, a shift in the primary contribu-

tion to the narial floor from the premaxilla to

the maxilla, loss of a rounded boss on the

dorsal surface of the nasals, deflection of the

lateral margin of nasals so that they face

dorsally, lengthening and flattening of the

interfenestral bar, and scooping of the caudal

margin of the interfenestral bar, so that the

caudal openings of the nasal passage and

interfenestral canal are visible laterally. The

interfenestral canal, which is interpreted as

having communicated branches of the maxil-

lary nerve (CN V) and associated vasculature

as well as a diverticulum of the antorbital sinus

from the antorbital cavity to the supraalveolar

canal and maxillary antrum respectively, has a

more ventral position in the interfenestral bar

of the perinates than the adult. This variation

may be a correlate of the elongation and

flattening of the rostrum during postnatal

growth or perhaps the relative size of the dorsal

pterygoideus muscle within the antorbital

cavity increases during postnatal ontogeny

causing a slight dorsal shift in the position of

the neurovascular bundle and its associated

interfenestral canal. These two possibilities are

not mutally exclusive as the flattening of the

rostrum in general and antorbital cavity in

particular could result in a rostral expansion of

the dorsal pterygoideus within the antorbital

cavity. This expansion could, in turn, then

result in a dorsal shift of the interfenestral

canal.

DENTITION: The small number of maxil-

lary teeth (13–15) in the Ukhaa perinates

represents either the retention of the plesio-

morphic paravian condition (based on compar-

ison with dromaeosaurs) or a secondarily

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

41

derived condition within troodontids. Based on

the hypothesized position of the Ukhaa peri-

nates within Byronosaurus, the latter interpre-

tation likely is the correct one. A third

possibility is that Byronosaurus exhibits a

significant postnatal increase in the number of

maxillary teeth. In agreement with adult

Byronosaurus jaffei, the maxillary tooth row in

the Ukhaa perinates extends approximately to

the rostral margin of the antorbital fenestra and

fails to reach the level of the preorbital bar (as in

Mei long; Xu and Norell, 2004). Therefore, if

the relatively small number of maxillary teeth in

the Ukhaa perinates reflects the presence of a

postnatal transformation in which the number

of maxillary teeth increases significantly, then it

appears to do so without a significant relative

increase in the length of the maxillary tooth row

(despite the aforementioned overall size in-

crease in the length of the maxilla). Rauhut and

Fechner (2005) noted an increase in the number

of maxillary teeth in Allosaurus fragilis during

postnatal ontogeny from 13 to 15 or 16. They

considered this increase to be the plesiomorphic

postnatal trajectory within Tetanurae, with

coelurosaurs exhibiting a derived trajectory in

which the maxillary dentition either was stable

during postnatal growth or the number of teeth

decreased (Varricchio, 1997b; Carr, 1999). A

corollary of their hypothesis is that hetero-

chrony in the form of paedomorphosis may

have served to decrease the number of teeth in

adult coelurosaurs. The number of maxillary

teeth exhibited by the Ukhaa perinates compli-

cates hypothesized trends in dental ontogeny

and phylogeny in an area of the tree important

for understanding the origin of birds—even if

the morphology of these specimens reflects

the apomorphic morphology of a new species

of Byronosaurus within troodontids rather

than evidence of a dramatic postnatal transfor-

mation.

BRAINCASE AND PNEUMATICITY: The pleth-

ora of anatomical features raises the possibil-

ity that the braincase will become increasingly

important for understanding phylogenetic

relationships among the derived coelurosarian

taxa, including birds (Currie and Zhao, 1993).

This phylogenetic potential makes under-

standing the developmental history of the

braincase and the characters it contains all

the more important.

The postnatal maturation of the vertebrate

neurocranium and otic capsule in general is

delayed relative to other cranial partitions.

This plesiomorphic ontogenetic trajectory

appears to be retained in Byronosaurus as the

braincase exhibits a number of variations

between the perinate and adult condition that

likely represent postnatal transformations.

Fusion of the exoccipital and opisthotic occurs

relatively early in ontogeny in most if not all

diapsid reptiles (de Beer, 1937; Bellairs and

Kamal, 1981) and in birds generally is

completed during prenatal development (al-

though with probable exceptions, e.g.,

Aepyornis [Balanoff and Rowe, 2007]). The

retention of a small length of suture in the

dorsomedial surface of this compound ele-

ment in IGM 100/974 suggests fusion of these

elements occurred largely, but not wholly,

during prenatal ontogeny in Byronosaurus.

The dorsal position of the sutural remnant

indicates fusion of these elements does not

occur uniformly along their entire margin. The

phylogenetic and/or functional information

correlated with the timing and topological

progression of this sutural obliteration is

unclear. The midline suture between the

paired ossifications constituting the adult

dorsum sellae also is retained in IGM 100/974.

The occipital plate undergoes considerable

postnatal restructuring. One of the primary

transformations involves the development of

an osseous flange, probably an extension of

the exoccipital, below the hypoglossal foram-

ina (this flange is absent in the perinate).

Expansion of this region correlates with a shift

in the position of the jugular foramen relative

to the hypoglossal foramina. The absence of a

ridge that in the adult partially obscures the

jugular foramen in caudal view and is formed

by the ventral margin of the paroccipital

process is one of several transformations

associated with the paroccipital process. The

paroccipital process in general changes from

being gracile in the perinate to relatively short

and robust in the adult. The postnatal

dorsoventral expansion of the paroccipital

process is most obvious distally but also

results in the concavities, which lie at the base

of the process and are partitioned by a ridge in

the perinate, becoming fully confluent in the

adult. In contrast, a ridge develops during

42

AMERICAN MUSEUM NOVITATES

NO. 3657

postnatal growth that partitions the caudal

surface of the paroccipital process into medial

and lateral components. The groove in the

rostral surface of the paroccipital process that

houses (in part) the stapedial shaft in the adult

is absent in the perinate and therefore is

inferred to develop postnatally.

One of the most notable differences between

the perinate and adult braincase is the absence

of a rostral foramen in the perinate parocci-

pital process and the inflation associated with

pneumatization by the posterior tympanic

sinus. A depression in the caudal margin of

the tympanic cavity above the fenestra pseu-

dorotunda and near the base of the para-

occipital process is interpreted as having

housed the posterior tympanic sinus. The

presence of this portion of the posterior

tympanic recess combined with the absence

of both an associated pneumatic foramen and

inflation of the paroccipital process indicates

that while the posterior tympanic sinus was

present at this stage of ontogeny it had yet to

invade the paroccipital process. The delayed

invasion of the process by a diverticulum of

the posterior tympanic sinus likely accounts

for many of the described variations of the

paroccipital process and occipital plate, and

therefore has a number of morphological

consequences.

The matrix of Turner et al. (2007a, 2007b)

reflects the presence of a posterior tympanic

recess in the basal troodontids Sinovenator

changii (Xu et al., 2002) and Mei long (Xu and

Norell, 2004), whereas the more derived

troodontids Sinornithoides youngi, Sauror-

nithoides mongoliensis, Saurornithoides junior,

Troodon formosus, and Byronosaurus jaffei

were scored as lacking this recess. The absence

of a posterior tympanic recess in these latter

taxa may be interpreted as a derived, second-

ary loss that diagnoses (in part) these five

troodontids as a clade. However, based on the

observations of Norell et al. (2009), Mako-

vicky et al. (2003), and those presented here,

Saurornithoides mongoliensis and Byrono-

saurus contain a posterior tympanic recess—

albeit one that, in agreement with Sinovenator

changii and Mei long, is reduced relative

to that of most coelurosaurs (a posterior

tympanic recess is known for ornithomi-

mids, therizinosaurs, oviraptorids, avialians,

and all dromaeosaurids; Witmer, 1997a).

Phylogenetic analyses (e.g., Makovicky et al.,

2003; Xu and Norell, 2004; Turner et al.,

2007a, 2007b) determined Byronosaurus jaffei

resides at a more derived position within

Troodontidae than Sinovenator changii and

Mei long but outside the Saurornithoides-

Troodon clade. The presence of a posterior

tympanic recess in dromaeosaurids and avia-

lans, indicates the presence of this structure in

Sinovenator changii, Mei long, Byronosaurus,

and Saurornithoides mongoliensis is plesio-

morphic, with its loss apomorphic in

Troodon formosus, Sinornithoides youngi, and

Saurornithoides junior. Based on these same

comparisons, however, the diminutive nature

of the posterior tympanic recess in Sinovenator

changii, Mei long, and Byronosaurus would be

derived, placing these taxa as morphological

intermediates between dromaeosaurs and the

more derived troodontids. The perinate pos-

terior tympanic recess of Byronosaurus is even

simpler than that of the adult in that its

paroccipital process lacks any degree of

pneumatization. This feature may be shared

with the holotype of Mei long, which also is

skeletally immature (Xu and Norell, 2004).

The delayed onset of paroccipital pneumati-

zation well into postnatal ontogeny as evi-

denced by IGM 100/974 may provide some

insight into the nature of the transformation

that results in the loss of the posterior

tympanic recess in troodontids and the role

that developmental timing might have played.

In contrast to the posterior tympanic recess,

the anterior tympanic recess is present in

the early stages of postnatal ontogeny in

Byronosaurus. Excavation of the parabasi-

sphenoid by the anterior tympanic recess in

this specimen is significant and closely resem-

bles the extent of pneumaticity present in adult

Byronosaurus jaffei and adult specimens of

other derived coelurosaurs. The dorsal extent

of the anterior tympanic recess, however,

appears to exhibit considerable ontogenetic

variation in that the lateral surface of the

prootic is not deeply excavated in the perinate

(in contrast to the adult). A correlative of this

lack of pneumaticity is the underdevelopment

of the otosphenoidal crest on the lateral

surface of the prootic. A deeply excavated

rostroventral margin in the perinate represents

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

43

the initiation of prootic excavation by the

anterior tympanic recess. Development of the

deep lateral recess, which characterizes the

lateral wall of the braincase in most derived

coelurosaurs, procedes ventral to dorsal.

Osteological correlates of the pneumatiza-

tion of the perinate rostrum by diverticula of

the paranasal sinus in general appear to

correspond more closely with the adult con-

dition than those associated with pneumatiza-

tion of the braincase (either the anterior

tympanic or posterior tympanic recesses).

For example, the presence of an expanded

cavity within the ectopterygoid of IGM 100/

972 likely is due to pneumatization of this

element by a diverticulum of the antorbital

sinus relatively early in postnatal ontogeny

followed by positive allometric growth of the

surrounding bone. The well-defined osteolog-

ical correlates of rostral pneumatization in the

Ukhaa perinates support the conclusion of

Rauhut and Fechner (2005) that this pneu-

matic system was established relatively early in

the skeletal ontogeny of nonavian theropod

dinosaurs (in agreement with extant crocodiles

and birds; Witmer, 1995).

The disparity in development timing be-

tween the paranasal and the tympanic pneu-

matic systems in perinate Byronosaurus skull

likely is due, at least in part, to differences in

the relative maturity of the bones themselves

rather than the timing of development of the

actual sinuses (as noted above, maturation of

the otic capsule and neurocranium generally

lags behind that of the dermatocranium). That

stated, it is apparent that rostral and braincase

pneumatization in Byronosaurus was not

uniform at the early posthatching stage of

development, which is somewhat in contrast

to the observations of Chure and Madsen

(1996) and speculations of Rauhut and

Fechner (2005) on the postnatal development

of allosaurids. This lack of uniformity suggests

that heterochrony may be an important vector

for the phylogenetic transformation of these

pneumatic systems and the skeletal characters

they influence.

ARCHAEORNITHOIDES

Archaeornithoides deinosauriscus Elzano-

wski and Wellnhofer, 1992, was named based

on a partial rostrum and mandible from

the Djadokhta Formation at Bayn Dzak,

Mongolia (Elzanowski and

Wellnhofer,

1993). As recently reviewed by Averianov

and Sues (2007), Archaeornithoides deinosaur-

iscus originally was hypothesized as having a

privileged phylogenetic relationship with birds

among nonavian theropods. Support for this

relationship was drawn from the absence of

interdental plates and lack of serrations on the

teeth, and the presence of a paradental groove

on the dentary and wide palatal shelves on the

maxilla. The presence of each of these ‘‘avian’’

characteristics is now known to occur within

troodontids, with Byronosaurus (both the

holotype of Byronosaurus jaffei and the

Ukhaa perinates) exhibiting all four charac-

ters. The avialan status of Archaeornithoides

deinosauriscus was challenged by Clark et al.

(2002) who indicated the holotype probably

had passed through the digestive tract of a

larger animal. These authors considered the

holotype to be a poorly preserved juvenile

specimen of a nonavialan coelurosaurian

taxon (Clark et al., 2002: 39). Currie (2000)

suggested that the holotype of Archaeo-

rnithoides deinosauriscus might be a juvenile

Saurornithoides mongoliensis, which also is

known from Bayn Dzak. This possibility was

considered by Elzanowski and Wellnhofer

(1993) but rejected based on the presence of

expanded palatal shelves of the maxillae in

Archaeornithoides deinosauriscus—shelves that

are present in basal avialans but also present

in Byronosaurus (and probably both species of

Saurornithoides). Averianov and Sues (2007)

put forth the idea that Archaeornithoides

deinosauriscus could be a juvenile Byrono-

saurus jaffei; however, they did not synony-

mize Byronosaurus under Archaeornithoides

stating that it was preferable to retain both

as distinct taxa until more information on

troodontid ontogeny was available. The dis-

covery and description of the Ukhaa peri-

nates, which are highly comparable in size to

the holotype of Archaeornithoides deinosaur-

iscus, provide us with enough new information

on troodontid skeletal ontogeny to reconsider

this enigmatic taxon.

The presence of a distinct groove on the

buccal surface of the dentary housing neuro-

vascular foramina, a relatively large number

44

AMERICAN MUSEUM NOVITATES

NO. 3657

of small teeth that are packed closely together

(most notably at the rostral end of the lower

jaw), dentary teeth that lie within a medially

open groove, and a flat internarial bar are

synapomorphies that are present in all known

troodontids and that at least could be pre-

served in the holotype of Archaeornithoides

deinosauriscus. The presence of each of these

characters in the Ukhaa perinates establishes

that in at least one troodontid lineage, these

characters are present early in postnatal

ontogeny and therefore likely present in

Archaeornithoides deinosauriscus if that taxon

is indeed a troodontid.

The morphology of the internarial bar of

Archaeornithoides deinosauriscus cannot be

discerned. The dentary teeth are larger than

those of the Ukhaa perinates and are evenly

spaced, with no obvious concentration at the

rostral end of the lower jaw. The dentary teeth

are described as sitting within distinct alveoli;

however, the lingual margin of the tooth row

is lower than the labial margin and is

separated from the tooth row by a distinct

paradental groove. The presence of an open

groove for the dentary teeth as described by

Currie (1987), therefore, may be present. The

buccal surface of the lower jaw contains

nutrient foramina lying within a distinct

groove that is delimited dorsally by a ridge,

as in the Ukhaa perinates. The groove is not as

deep as that of adult troodontids but is similar

in depth to the same structure in the Ukhaa

perinates. The size and distribution of the

dentary teeth suggests that Archaeornithoides

deinosauriscus falls outside the troodontid

clade defined by the most recent common

ancestor of Sinovenator changii and Troodon

formosus. The presence of a distinct groove on

the buccal surface of the dentary is present in

all troodontids but also is known in a small

number of non-troodontid paravians (e.g.,

Buitreraptor gonzalezorum; Makovicky et al.,

2005). If the dentary teeth actually do lie

within an open groove, this may represent

an unambiguous troodontid synapomorphy

(see above). These two characters tenta-

tively support the conclusions of Currie

(2000) and Averianov and Sues (2007) that

Archaeornithoides deinosauriscus is a troodon-

tid—but one that is outside the clade com-

prised of the other currently recognized

troodontid taxa. Further complicating the

issue of Archaeornithoides deinosauriscus is its

possession of unconstricted teeth. A continu-

ous transition between root and crown is a

derived feature within Paraves that is shared

by dromaeosaurs. If Archaeornithoides deino-

sauriscus is a basal troodontid then uncon-

stricted teeth may be plesiomorphic for

Paraves with the apomorphically reversed

condition of constricted teeth occurring in

troodontids, avialians, and

Microraptor

zhaoianus. The

perinate

holotype

of

Archaeornithoides deinosauriscus indicates that

the transformation from constricted to unre-

stricted teeth occurs relatively early in ontog-

eny, and if the presence of constricted teeth in

adult troodontids and avialans does reflect a

paedomorphic phylogenetic transformation

then the transformation likely affected the

embryological rather than postnatal develop-

ment.

Archaeornithoides deinosauriscus, despite

being based on a highly fragmentary single

specimen, provides further complexity to the

issue of ontogeny and phylogeny of paranasal

pneumaticity in nonavian theropods. The

antorbital fossa, maxillary antrum, and pre-

maxillary recess all are present and well

defined in the holotype. The palatine of

Archaeornithoides deinosauriscus, however, ex-

hibits no evidence of pneumatization—in

stark contrast to the Ukhaa perinates. The

palatine also differs from that of the Ukhaa

perinates in its possession of a more plesio-

morphic tetraradiate shape that includes a

well-defined pterygoid process. The establish-

ment of pneumatic-related rostral morpholo-

gies early in the postnatal ontogeny of

crocodilians, birds, allosaurs, and the Ukhaa

perinates, indicates that the apparent absence

of pneumatic fossae in the palatine of

Archaeornithoides deinosauriscus is a derived

condition (either the palatine in this taxon

lacks pneumatization or pneumatization is

apomorphically delayed into latter stages of

postnatal ontogeny).

ACKNOWLEDGMENTS

We gratefully acknowledge Amy Davidson

for her careful preparation of the small and

delicate specimens described in this paper and

2009

BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

45

Emily Griffiths for her help with the SEM.

Mick Ellison provided invaluable help and

advice with figures. Magdalena Borsuk-Bialy-

nicka, Institute of Paleobiology, Warsaw,

kindly provided access to Archaeornithoides.

We thank Larry Witmer and the OUmCT

facility, Ohio University, for scanning the

holotype of Byronosaurus jaffei. The study

benefited greatly from conversations with

Amy Balanoff and Alan Turner and the

thoughtful reviews of Octaveo Mateus and

Larry Witmer. The collection and study of

these specimens was supported by NSF

Grants DEB-9300700 and ATOL 0228693

and by an AMNH Lerner-Gray postdoctoral

fellowship to GSB.

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APPENDIX 1

ANATOMICAL ABBREVIATIONS

alvf

alveolar foramina

alvp

alveolar process of maxilla

an

angular

aof

antorbital fenestra

aofo

antorbital fossa

apm

ascending process of maxilla

ar

acoustic recess

asc

anterior semicircular canal

atr

anterior tympanic recess

bof

facet for basioccipital on parabasi-

sphenoid

bp

basipterygoid process

car

cerebral carotid canal

cc

cavum cranii

cfma

caudal fenestra of maxillary antrum

ci

crista interfenestralis

CN VI

abducens foramen (for cranial nerve

VI)

CN VII

facial foramen (for cranial nerve VII)

CN VIIIc

vestibulocochlear foramen (for co-

chlear branch of cranial nerve VIII)

CN VIIIv

vestibulocochlear foramen (for ves-

tibular branch of cranial nerve VIII)

CN X, XI

jugular foramen (for cranial nerves X

and XI and jugular vein)

CN XII

hypoglossal foramen (for cranial

nerve XII)

CN V

trigeminal fenestra (for cranial nerve

V)

cp

clinoid process

cr

cochlear recess

cv

cavum vestibulare

d

dentary

dg

dentary groove

dmf

upper division of cavum metoticum

ds

dorsum sellae

dss

suture between right and left ossifi-

cations of the dorsum sellae

dt

dentary tooth

dtr

dorsal tympanic recess

egg

eggshell

emf

external mandibular fenestra

ep

ectopterygoid

ex/op

exoccipital/opisthotic

exn

external naris

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AMERICAN MUSEUM NOVITATES

NO. 3657

exop

exoccipital/opisthotic suture

f

frontal

ff

facial fossa

fm

foramen magnum

fmo

medial aperture of cavum metoticum

fov

fenestra ovalis

fpl

foramen perilymphatica

fpr

fenestra pseudorotunda

fr

floccular recess

hf

hypophyseal fossa

idp

interdental plate

ifb

interfenestral bar

ifc

interfenestral canal

inb

internarial bar

inn

internal naris (choana)

j

jugal

jpp

jugal process of palatine

lac

lacrimal

lb

lacrimal boss

lbd

labial margin of dentary

lcp

laterosphenoid contact of prootic

lsf

surface of contact with laterosphenoids

man

maxillary antrum

mg

maxillary groove

mpm

maxillary process of premaxilla

mpp

maxillary process of palatine

ms

metotic (prevagal) strut

mt

maxillary teeth

mx

maxilla

mxf

maxillary fenestra

n

nasal

nb

nasal boss

nc

nasal capsule

nlc

nasolacrimal canal

npm

nasal process of premaxilla

occ

occipital condyle

or

orbit

orm

orbital margin

os

osseous shaft buttressing cerebral carot-

id canals to rostral face of dorsum sellae

otc

otosphenoidal crest

pal

palatine

par

prearticular

pas

postantral strut

pb

parabasisphenoid

pbl

preorbital bar of lacrimal

pc

parasphenoid capsule

pfp

pneumatic foramen of palatine

pmx

premaxilla

po

postorbital

pop

paroccipital process

pp

parasphenoid process

pr

parasphenoid rostrum (5 cultriform

process)

prl

pneumatic recess of lacrimal

pro

prootic

prp

paroccipital ramus of prootic

prpa

pneumatic recess of palatine

psc

posterior semicircular canal

psm

palatal shelf of maxilla

pt

pterygoid

ptr

posterior tympanic recess

pvc

pneumatic fossa within acoustic recess

qf

surface for articulation with quadrate

rpp

rostral process of prootic

s

splenial

sa

surangular

sal

supraantorbital process of lacrimal

sf

surangular foramen

sol

supraorbital process of lacrimal

sp

supradentary

sqf

surface for articulation with squamosal

v

vomer

vcf

vestibulocochlear fossa

ve

vestibular eminence

vp

vestibular pyramid

vpp

vomeropterygoid process of palatine

vr

vomerine ridge

?

unknown or questionable

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BEVER AND NORELL: PERINATE SKULL OF BYRONOSAURUS

51

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