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Devonian Period (417-354 mya)
Intense reef building activity in shallow-water habitats indicates
that the Devonian climate is, on the whole, warm and stable. Early relatives of squids
called ammonoids first appear. Jawless fishes remain dominant among marine
vertebrates, though bony fishes and sharks diversify.
An explosion of plant life occurs during the Devonian. Early in the
period, land plants range in size from a few centimeters to no more than a meter tall.
By the end of the Devonian, complex branch and root systems produce trees 30 feet in
height. Arthropods, which now include early wingless insects, radiate. Before long,
the first four-legged animals emerge from the waters and join them on land.
The two prominent landmasses, Gondwana and Laurussia (sometimes
called Euramerica), are drawn together as a subduction zone forms where the tectonic
plates in which they ride meet. (Their collision won't happen until the early Permian.)
The Devonian period ends with a cataclysmic extinction event, particularly devastating
to warm-water marine communities. Nearly 70-80 percent of marine invertebrate species
are wiped out over two extinction pulses.
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400 mya: Oxygen nears present-day levels
Oxygen nears present-day levels of 21 percent by volume of the atmosphere.
375 mya: Land vertebrates
Amphibians are the first four-legged animals, or tetrapods, on
land. With, in most cases, well-defined limbs and feet, a supportive rib cage, and
a neck that enables its skull to rotate, amphibians are far better suited to moving
around and resting their bodies on land than their ancestors, fleshy-finned bony fish
called rhipidistians. Despite their modified bone structure, amphibians maintain a
strong connection to the water, as adults feed on fishes in shallower waters. They
also spawn in water, laying a number of smaller eggs that hatch into swimming larvae.
Early amphibians are large animals that do not much resemble living frogs or
salamanders.
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From water to land: Fish out of water (375 mya)
As it was for the first land plants and arthropods, several
key adaptations make the vertebrates' transition to life on land successful.
Amphibians evolve skeletal and muscular features that support body weight, enable
walking, and keep their heads up off the ground. But they are not the first
vertebrates to venture onto land; scientists think their ancestors, the
rhipidistian fish, used fleshy, lobed fins to shuffle ashore.
Many unmistakable physical similarities exist between
rhipidistians, which resemble modern lungfish, and the first true land vertebrates,
the amphibians. From fossil evidence, skulls, teeth, and vertebrae are nearly
identical between some "lobefins" like the rhipidistians and the earliest amphibians.
It's also clear that fleshy ventral fins, attached directly to lobefin skeletons
near the tail-end of the fishes, have moveable bones and muscles. Remarkably, the
bones within these fins are matched, one to one, with those in the legs of early
amphibians. Another shared trait is the ability to breathe air. Rhipidistians,
along with several other groups of early fishes, had nostrils and lungs.
It's important to understand that these physical adaptations,
while later recruited for use on land, originally evolved in the water. Lobefins
would have used ventral fins to improve swimming and steering, and to scuttle
through very shallow waters and stir up the silty bottom as they searched for
food. Rhipidistians probably evolved the capacity to breathe air to cope with
environments where oxygen levels in water were seasonally low, like in shallow
lagoons.
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Late Devonian extinction
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Date:
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364 mya
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Intensity:
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2
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Affected:
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Between 50-55 percent of
marine invertebrate genera, and 70-80 percent of marine invertebrate species
go extinct
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Hypotheses:
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Meteor impact, volcanism,
changes in ocean chemistry, oxygen depletion, glaciation
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Summary:
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During the Devonian period,
survivors from the late Ordovician extinction steadily recover. As the period
nears its end, however, two new extinction pulses occur, mainly affecting
marine populations. The first of these, at 364 mya, is more severe. Rugose corals and stromatoporoids, the primary reef-builders
of the period, are nearly wiped out. Among the other marine invertebrate victims
are brachiopods and trilobites. As for the marine
vertebrates, the enigmatic conodont animals, known only for their
widely scattered toothlike fossils, suffer; the jawless fishes are almost
entirely eliminated; and the jawed and heavily armored placoderms
go completely extinct. Interestingly, terrestrial plants and animals escape
largely untouched. While some populations may have been under stress from
changes in ocean chemistry and rapid cooling, a meteor strike and volcanism
have been suggested as possible triggers.
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Selective effect (364 mya)
The late Devonian extinction affects marine life far more than
life on land. Looking closely at the death toll, nearly all the jawless fish, as
well as every last placoderm, dies. Unlike these bottom-feeders, many
open-water swimmers, like bony fish and sharks, survive the extinction. Knowing
whether mass extinctions have random consequences, or whether risk varies from
species to species or group to group, would greatly enhance our understanding of
both extinction and evolution.
Selectivity is the idea that certain factors, such as body size,
geographic range, or feeding behavior, determine whether species, groups, or entire
families are more or less likely to die in a mass extinction. Data from some
extinction events seem to reinforce this idea. For example, many shallow-water and
reef-dwelling species probably died off in the Devonian because they (or their
habitats) were more sensitive to changes in ocean chemistry or temperature than
surviving animals that lived in deeper waters.
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360 mya: Seed plants
The earliest seed-bearing plants, or gymnosperms, are the seed
ferns. Unlike spore-producing plants, which need standing water nearby in which
swimming sperm can fertilize eggs, seed plants evolve pollen, which can be dispersed
by wind or animals from the male to the female reproductive organ. Hence, seed plants
can reproduce away from water and expand their range into drier habitats.
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