Alligator mississippiensis (Daudin, 1801) - American alligator (captives, Newport Aquarium, Newport, Kentucky, USA).

The crocodilians have been around since the Triassic (early Mesozoic). Crocodilians include four groups: eusuchians (the only living group), mesosuchians (extinct), protosuchians (extinct), and sphenosuchians (extinct). Some fossil crocodilians reached such immense sizes that they preyed on dinosaurs. Another fossil group of reptiles, the phytosaurs (upload.wikimedia.org/wikipedia/commons/d/d2/Rutiodon_BW.jpg), closely resemble crocodilians, but are unrelated (phytosaurs are crurotarsan archosaurs).

All modern and fossil crocodilians have the same overall skeletal structure, although some fossils forms did depart somewhat from the stereotypical crocodile body plan (quadrupedal, dorso-ventrally compressed bodies, elongated skull & snout with eyes atop the head). So, the crocodilians are, in relative terms, evolutionarily conservative.

Living crocodilians are tropical to temperate, predatory semiaquatic reptiles, but fossil representatives include fully marine forms and inferred terrestrial forms.

Only two living crocodilians are native to America - the American alligator and the American crocodile. Both occur in the far-southeastern parts of the USA (Florida).

These are juvenile American alligators, which differ from adults in having smaller body sizes and by having light-colored crossbanding on the back, sides, tail, and limbs. As are all crocodilians, the alligator is a predator (carnivore), although it also known to be a frugivore (fruit eater).