Tetrapods evolved from lobe-finned fish and developed legs from muscular lower fins to navigate algae-choked swamps. There are three main groups of tetrapods: amphibians, synapsids, and sauropsids. Synapsids and sauropsids are amniotes and evolved from amphibians around 320 million years ago.
Tetrapods are a monophyletic (descended from a common ancestor) group of land animals that evolved about 365 million years ago from lobe-finned fish called sarcopterygians. Tetrapods are thought to have evolved incrementally from fish adapted to swim through algae-choked swamps. These fish developed 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, as opposed to the five or fewer digits common to 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 living today, and thus do not fall into any major tetrapod group. There are only a few dozen species in this classification and they are all long extinct.
Aside from the basal tetrapods, there are three main groups of tetrapods: amphibians, synapsids (meaning “arc spindle”, mammals are the only synapsids alive today), and sauropsids (meaning “lizard face”). These groups are all monophyletic with the possible exception of amphibians; some researchers suspect that salamanders may have evolved from an ancient amphibian more primitive than the common ancestor of all other living amphibians. Another name for synapsids is theropsid, meaning “beast face,” and another name for sauropsids is “reptile.”
Both synapsids and sauropsids are amniotes, meaning that the fetal forms are nourished by a complex array of nutrients and membranes until they mature enough to breathe air and walk the world independently. For synapsids, this involves either a uterus (placental mammals) or a pouch (marsupials). Sauropsids lay eggs. Synapsids and sauropsids are thought to have evolved from amphibians at about the same time, about 320 million years ago, during the Carboniferous period. Hylonomus is the earliest confirmed reptile as of this writing, while Archaeothyris is the earliest synapsid. Both resembled small lizards and were dwarfed by the large amphibians that dominated terrestrial ecosystems at the time. It wasn’t until after an ice age near the end of the Carboniferous that some of the large amphibians began to go extinct, and synapsids and sauropsids increased in size to fill empty niches.
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