American Alligator (Alligator mississippiensis). Alligators are members of a group of animals called archosaurs that include the crocodilians, dinosaurs, and pterosaurs and began to evolve during the Triassic Period of the Mesozoic Era, or approximately 245 million years ago. Modern alligators first appeared during the Paleogene Period of the Cenozoic Era, or about 39 million years ago. Crocodilians and birds are the only representatives of archosaurs alive today, and, as such, alligators are more closely related to birds than they are to lizards. Male American alligators typically reach lengths of 4m (13 feet) and weigh 360kg (790 pounds); females seldom reach lengths of more than 3.05m (10 feet). American alligators inhabit freshwater marshes and swamps of the southeastern United States and feed on insects, turtles, fish, crustaceans, birds, and mammals. American alligators are most prevalent in the states of Louisiana and Florida, with each state hosting over a million animals. Photographed in Eastern Atchafalaya Basin, Louisiana, USA.