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DEVONIAN PERIOD

416-359 million years ago

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amphibians in marsh

The first and most primtive amphibians evolved in the Late Devonian and are called labyrinthodonts. They retained a number of primitive traits from their fish ancestors and many adaptations to land would not occur until later groups of amphibians.

Labyrinthodonts could reach more than 1 m in length (similar to rhipidistian fish were also large). The notochord was still a major structural element for the support. It wasn't much constricted by the vertebrae and extended from the tail into the brain. The components of vertebrae were not attached to each other (the pleurocentra and intercentra were still not attached to neural arch). The skulls contained almost all the bones present in rhipidistian skulls, unlike the skulls of modern amphibians.
Labyrinthodonts were a successful group through the Triassic Period and most became extinct during the mass extinctions at the end of that Period. They had continued to thrive after the evolution of reptiles in the Carboniferous and, in many areas, they were the most common fossil group in the vertebrate fauna. Unlike the Paleozoic forms, the Triassic labyrinthodonts were aquatic. Most were more than 2 meters in length, some with 1 meter long skulls and large fangs in their lower jaws. Members of five families survived the end-Triassic extinctions and existed in the Jurassic and some into the Cretaceous as well (Milner, 1989; Colbert, 1956).