The Crocodilia (also Crocodylia) of today are the only living members of the great archosaurian group that ruled the world throughout the Mesozoic era (251–65 million years ago (mya)). As such, these animals, together with birds, represent the only relatives of extinct dinosaurs, thecodonts, and pterosaurs. Biologists recognize approximately 21 extant species, divided into three major lineages: the gavials, alligators, and crocodiles. For many people, the Crocodilia are familiar amphibious reptiles that occupy most of the tropical and subtropical climates throughout the world. These animals have essentially remained unchanged since the Late Cretaceous (65.5 mya) and retain a remarkable body armor. It is interesting to think how animals that are so heavily armored are able to be sensitive to the environment. Another consideration when examining their sensory systems is the nature of their environment. Crocodilians live, mate, and hunt prey both on land and in water. Animals that thrive on both land and underwater are faced with the task of interpreting stimuli in different media.