Sauropsid

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.

The synapsids are the other group which evolved from amniotes. They gave rise, eventually, to the mammals.

Living sauropsids include lizards, snakes, turtles, crocodiles, and birds. Extinct sauropsids include non-avian dinosaurs, pterosaurs, plesiosaurs, ichthyosaurs, and many others.

Origin of tetrapods

More or less, the evolution of tetrapods has taken this course:

• Sarcopterygii
• Coelacanthimorpha (Coelacanths)
• Dipnoi (lungfish)
• Tetrapodomorpha The earliest tetrapods lived in water. Clear fossil tetrapod tracks from the mid-Devonian predate previous tetrapod records by 18 million years.

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.

Romer’s gap

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 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.

Tetrapods

• Land-dwelling tetrapods
• Amphibians > amniota and present-day amphibians
• Amniota
• Sauropsida > present-day reptiles and birds
• Synapsida > present-day mammals