The reptiles are generally classified into three groups, the turtles, the lepidosaurs (snakes and lizards) and the archosaurs (dinosaurs, pterosaurs, and crocodiles). The archosaurs were the dominant land animals during the Mesozoic Era (230 to 65 million years ago), their members were the largest and most populous vertebrates of the time. The only animals of the archosaur group to survive to modern times are the crocodilians.
The crocodiles probably first arose in the mid-Triassic Era (230 to 195 million years ago). Their form has changed little from the time of their first development to modern times.
The crocodiles probably arose from early reptiles that were capable of running upright on long, slim hindlegs. Although our modern crocodilians generally move about slowly, on all four of their short, sprawling limbs, they are capable of moving quickly, and even rising up to run on only their hind legs for short bursts. Due to their being ectothermic, however, they can only sustain such extreme activity for short periods of time. Their sprawling gait also makes it difficult for them to intake sufficient oxygen into their lungs to breathe while maintaining such activity for a long period.
The modern crocodilians are known as the subOrder Eusuchia, and are made up of the crocodiles, alligators, caimans, and gavials. The Eusuchians first appear in the fossil record during the Upper Cretaceous Period. The alligators and caimans arose during the Paleocene Epoch (65 to 55 million years ago), while the gavials did not appear until the Eocene Epoch (55 to 36 million years ago) or early Oligocene Epoch (36 to 22.5 million years ago).
One group of animals that is often mistaken for the crocodiles is the Order Choristodera. This group of extinct reptiles diverged or branched of from the main group of diapsid reptiles during the early Cretaceous Period. They lived in the freshwaters of North America, Europe, and Asia until they became extinct during the Eocene Epoch. The most famous of the Choristodera is Champsosaurus.
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