Reptiles are a group of cold-blooded tetrapod vertebrates that diverged from ancestral amphibians about 340 million years ago. There are two characteristics that distinguished early reptiles from amphibians and enabled them to colonize terrestrial habitats more extensively than their ancestors, scales and the ability to lay hard-shelled amniotic eggs. Scales protect reptiles from abrasion and loss of body moisture.
Crocodilians are a group of large reptiles that includes alligators, crocodiles, gharials, and caimans. Crocodilians are formidable predators with powerful jaws, muscular tail, large protective scales, streamlined body, and eyes and nostrils that are positioned on top of the head. Crocodilians first appeared about 84 million years ago during the Late Cretaceous and are the closest living relatives of the birds.