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, 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).
Alligator mississippiensis (Daudin, 1801) (captives) (above & below) - these juvenile American alligators differ from adults in the smaller size and by having light-colored crossbanding on the back, sides, tail, and limbs. Like all crocodilians, the alligator is a predator (carnivore), although it also known to be a fructivore (fruit eater).
Alligator mississippiensis juvenile.
In the above photo, note the presence of small black dots near the mouth of this alligator. All modern crocodilians have these strutures These are integumentary sense organs (ISOs). They provide the animal with sensations “beyond-the-five senses”. ISOs are connected to nerves that pass through canals in the jaw bones of the skull (see skull photo below). ISOs are traditionally inferred to only function when at the air-water interface. However, ISOs along the flanks of crocodiles are essentially always below water. It's been shown that ISOs don't detect electric fields or magnetic fields. Vibration detection seems the most likely function of crocodilian ISOs, especially upon consideration of effective nocturnal hunting by most modern forms.
Alligator mississippiensis juveniles (captives).
Alligator mississippiensis adult skull. Note the pits/canals in the jaw bones - they are especially evident below the teeth of the mandible. These canals (neurovascular foramina) originally housed nerves that connected to ISOs.
Classification: Animalia, Chordata, Vertebrata, Reptilia, Crocodilia, Alligatoridae
Most info. from Carroll (1988), Taylor & O'Shea (2006), Tait et al. (2006), and David Allen.