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The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a node-based clade , a group with a common ancestry descended from a single "node" (the node being the nearest common ancestor of living species).
Though stem-tetrapods originated in the preceding Devonian, it was in the earliest Carboniferous that the first crown tetrapods appeared, with full scaleless skin and five digits. During this time, amphibians (including many extinct groups unrelated to modern forms, referred to as "basal tetrapods") were the predominant tetrapods, and included ...
Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only. Fossil Illuminates Evolution of Limbs from Fins Scientific American, 2 2 April 2004.
The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water. [18] Panderichthys is a 90–130 cm (35–50 in) long fish from the Late Devonian period (380 Mya). It has a large tetrapod-like head.
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Acanthostega is a partially aquatic tetrapod with developed limbs that shares features common with the earlier tetrapods, Panderichthys and Eusthenopteron. [3] Like Panderichthys , the humerus of Acanthostega is flattened dorso-ventrally, the intermedium terminates level with the radius, and the endoskeleton can be divided into stylopodium ...