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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.

Late Devonian extinction

Date:

364 mya

Intensity:

2

Affected:

Between 50-55 percent of marine invertebrate genera, and 70-80 percent of marine invertebrate species go extinct

Hypotheses:

Meteor impact, volcanism, changes in ocean chemistry, oxygen depletion, glaciation

Summary:

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.