Dinosaur Evolution

Thecodonts: Crocodile-like Archosaurs:

Chasmatosaurus, a crocodile-like, meat-eater (a thecodont) from the Triassic period.

Thecodonts may have been ancestors of the dinosaurs. Thecodonts (like Chasmatosaurus) were socket-toothed reptiles that were the ancestors of dinosaurs, birds, pterosaurs, and crocodilians. These archosauriforms were low-slung, meat-eating quadrupeds had long jaws and a long tail (they looked a lot like crocodiles).

This is a cladogram of the clade Reptilia (reptiles), tracing the ancestry of these groups.

The Earliest Dinosaurs:
The first dinosaurs were small and lightly built, mostly about 10-15 feet long (3 to 4.5 m). They were bipedal carnivores or omnivores, and probably very agile and fast.

The world's oldest-known dinosaurs have been found on Madagascar, an island off the coast of SE Africa. These dinosaur fossils date from about 230 million years ago during the Triassic period.

Until recently, the earliest-known dinosaur was Eoraptor lunensis (meaning"dawn raptor") which lived about 228 million years ago. It was a small, primitive theropod (a bipedal meat-eater) about 3 feet (1 m) long. It lived in what is now Argentina, South America (fossils were found at the Ischigualasto Formation).