THE FOSSIL RECORD OF REPTILES


The fossil record of reptiles extends to the Carboniferous, and can be said to pass through three phases, in the late Palaeozoic, the Mesozoic (the 'age of dinosaurs), and the Tertiary to the present day.

Late Palaeozoic Reptiles

The fossil record of early reptiles shows the origins of the three major skull types - anapsid, diapsid, and synapsid, in the Carboniferous.

In the Permian, the synapsids ('mammal-like reptiles') dominated. These included the first herbivorous vertebrates, such as the pelycosaur Edaphosaurus that was preyed upon by large carnivores such as Dimetrodon. These reptiles make up 70% of the genera recovered from Early Permian localities. In the Late Permian, the pelycosaurs were replaced by a diverse group of more derived synapsids, the therapsids. There was a major geographic switch in the occurrence of Permian reptiles, from North America and Europe in the Early Permian to South Africa and Russia in the Late Permian. This probably has more to do with the chances of preservation than with any major geographic shift in where these synapsids lived.

The Dinosaurs

Amniote evolution followed a very new course during the Triassic. The devastation of the end-Permian mass extinction wiped out most of the dominant therapsid synapsids, and left an empty, bare world. Some therapsids survived, and re-radiated (notably the plant-eating dicynodonts and the flesh-eating cynodonts). But two new diapsid groups expanded to fill many niches: the rhynchosaurs as herbivores and the archosaurs mainly as carnivores. The basal archosaurs diversified, and gave rise to one group that would dominate for the rest of the Mesozoic era.

Dinosaurs appeared in the Late Triassic, and they dominated through the Jurassic and Cretaceous. The first dinosaurs were small flesh-eating bipeds, such as Eoraptor and Herrerasaurus from Argentina, and Coelophysis from North America. The plant-eating prosauropods appeared early, including Saturnalia from Brazil and the 5-metre long Plateosaurus from Germany. The first ornithischian also appeared at this time, Pisanosaurus from Argentina, but this group remained rare until the Jurassic.

Large marine reptiles also appeared in the Triassic: the placodonts, the nothosaurs, and the ichthyosaurs. They all have the euryapsid skull type which suggests that they probably evolved from terrestrial basal diapsids. In the air were the pterosaurs, close relatives of the dinosaurs.

The Jurassic and Cretaceous periods were the time of greatest reptile diversity. Dinosaur fossils have been uncovered all over the world. Pterosaurs of all sizes flapped about in the sky, and plesiosaurs and ichthyosaurs swam in the seas. This all came to an end with the mass extinction at the end of the Cretaceous, the KT event.

Tertiary and Modern Reptiles

With the extinction of the dinosaurs, pterosaurs, and great marine reptiles, the Earth was again an empty place. Reptiles never again ruled, and their dominant places on land were taken by the mammals. Birds took over as the major group of flying animals, and sharks and whales took over some of the roles of the ichthyosaurs and plesiosaurs.

Modern reptile groups had appeared mainly in the Triassic and Jurassic. The first turtles, the first crocodilians, and the first sphenodontids are known from the Late Triassic, the first lizards from the Mid Jurassic, and the first snakes from the Early Cretaceous. These groups all diversified under the noses of the dinosaurs and pterosaurs, but they never became hugely important. During the Tertiary, liards and snakes became much more diverse, but the turtles and crocodilians remained specialized and did not change their habits a great deal.


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