What are the Major Groups of Tetrapods?
Tetrapods are a monophyletic (descending from a common ancestor) group of land animals that evolved approximately 365 million years ago from lobe-finned fish called sarcopterygians. It is thought that tetrapods evolved incrementally from fish adapted to swim through weed-choked swamps. These fish evolved muscular lower fins, when they used to navigate these swamps, which eventually developed into full-fledged legs. Some of these early forms had numerous digits, unlike the five or fewer digits common to so many tetrapods today. The earliest tetrapods, called basal or stem tetrapods, such as the species Acanthostega, are more primitive than the common ancestor of all tetrapods alive today, and so do not fall into any of the main tetrapod groups. There are only a few dozen species in this classification, and they are all long extinct. Aside from the basal tetrapods, there are three major groups of tetrapods: amphibians, synapsids (meaning “fused arch,” mammals are the only synapsids alive
