Temporal range: Pennsylvanian–Present, 312–0Ma
|Row 1: emerald tree boa and tuatara
Row 2: green sea turtle and Nile crocodile
Row 3: Southern cassowary and Atlantic puffin
Sauropsids are a group of land vertebrates which includes all existing reptiles and birds and their fossil ancestors. They are one of the two groups which evolved from egg-laying amniotes in the first part of the Carboniferous period.
Origin of tetrapods
More or less, the evolution of tetrapods has taken this course:
Nine genera of Devonian tetrapods have been described. These earliest tetrapods were not terrestrial. They lived in swampy habitats like shallow wetlands, coastal lagoons, brackish river deltas, and even shallow marine sediments.
Between the lobe-finned fish tetrapods and the first amphibia and amniotes in the Middle Carboniferous lies a gap of 30 million years, with few satisfactory tetrapod fossils. This, noted in 1950, is Romer's Gap. Some new fossils were found in the 1990s, such as Pederpes, right in the middle of the Romer Gap. The gap still obscures the details of the tetrapod transition.
Sometime, in the later Devonian or earliest Carboniferous, the fishapods became mainly land-based. One group of them kept their link to the water, and always laid their eggs in water. They became the amphibians. The others evolved a way of laying eggs on land. They were the amniotes, whose key innovation was the cleidoic egg.
Sometime in the middle or lower Carboniferous, the amniotes split into two lines. One line lead to the reptiles of all kinds, and we call that the Sauropsida. The other line led eventually to the mammals, and we call that the Synapsida. It is not right to say "mammals evolved from reptiles" because both groups derived from early amniotes. In any event, modern reptiles are vastly different from modern mammals. Both groups have evolved for over 300 million years from the early amniotes.
The cladogram presented here illustrates the "family tree" of sauropsids, and follows a simplified version of the relationships found by M.S. Lee, in 2013. All genetic studies have supported the hypothesis that turtles are diapsid reptiles; some have placed turtles within archosauromorphs, though a few have recovered turtles as lepidosauromorphs instead. The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.
Images for kids
Benjamin Waterhouse Hawkins' 1855 reconstruction of a Dicynodon as a turtle-like creature.
Sauropsid Facts for Kids. Kiddle Encyclopedia.