Crocodile

Classically, the order Crocodilia is divided into six suborders: Protosuchia, Archaeosuchia, Mesosuchia, Thalattosuchia, Sebecosuchia, and the Eusuchia. Today, Crocodilia are familiar inhabitants of rivers, swamps, and lakes, distributed in the tropical and subtropical regions of North and South America, the Caribbean, Africa, Asia, and Australia. However, crocodilians did not begin their evolution as amphibious reptiles. The earliest recognizable crocodilians arose in the Upper Triassic and appear to have been terrestrial in nature. Protosuchians are small, averaging below two meters in length. They have terminal nostrils, a single row of dermal armor on either side of the vertebral column, and elongated carpal bones and hindlimbs. They show other classic morphological features of crocodilians in that the pubic bone does not contribute to the formation of the hip socket as in all other reptiles. Before apparently giving rise to the Mesosuchia, the first amphibious crocodilians, the protosuchids, gave rise to another side group, the Notosuchia. The Mesosuchia appear to be the ancestors of the living Eusuchia. The Thalattosuchia are Mesozoic marine crocodilians, most of which have longirostrine (having long, thin jaws) mandibles adapted for fish eating. Some Thalattosuchia, the geosaurs, apparently were well adapted to the marine environment, having reevolved a tail fin for aquatic locomotion. The living Crocodilia are all placed within the suborder Eusuchia. They are divided into three families, the Gavailidae, Alligatoridae, and Crocodylidae, the latter with its two subfamilies, the Crocodylinae, and the Tomistominae. The gharials or gavials of Nepal and India comprise the family Gavialidae. They are longirostrine crocodilians specialized for fish eating. They attain large size, growing to nearly nine meters. Their limbs are reduced more than in most crocodilians and they are unable to climb over obstacles of more than a half meter high. The Crocodylidae are divided into alligatorines, which include the American and Chinese alligator and the caimans, and the Crocodylinae, which include members of the genus Crocodilus, Osteolaemis, and Tomistoma. The placement of the fourth mandibular tooth in crocodilians has often been cited as the difference between these families, in that in alligatorines the fourth mandibular tooth resides in a socket in the upper jaw when the mouth is closed. Within the crocodiles this tooth fits in a groove in the upper jaw and thus is visible when the jaw is closed. The longirostrine crocodilians, such as gharials, tomistomines, and Crocodilus johnstoni, have longirostrine jaws and have interdigitating teeth. Thus, the tooth character is equivocal in these taxa. Another method of dividing the living forms into the subfamilies has to do with salt tolerance. With one exception alligatorines are salt intolerant. Crocodiles are often found in brackish waters and some, such as Crocodilus porosus, have been found hundreds of miles off the coast of Australia. Unlike alligatorines, crocodiles have well-developed salt glands in the tongue. Gharials, although not marine, have salt glands in the tongue as well as an orbital salt gland. This trait may indicate an oceanic origin for gharials.

Aquatic Adaptations
Eusuchians are better adapted to the aquatic environment than their mesosuchian ancestors. The secondary palate is well developed, extending the internal nostrils back to the pharynx. The external nostrils face dorsally so the crocodiles can stay hidden in the water. Here a fleshy valve separates off the mouth from the pharynx and the internal nostril openings. Eusuchians also have an earflap or Ohr Klappe that closes off the external ear from the water during dives. The Eusuchia have enhanced the dermal armor and have a biserial or double row of osteoderms running on either side of the vertebral column. This dermal armor was once thought to be evolved for protection but this system is tied to back muscles that aid the crocodilians in bending the vertebral column for locomotion. Dermal armor was widespread in early archosaurs but is bestdeveloped in eusuchian crocodilians. All crocodilians have a strong bite. Although crocodilians have jaw adductor muscles in the temporal region of the skull, the largest jaw adductor originates on the palate. Except for fish-eating species that have sharp, curved, conical teeth, most crocodilians have more squatshaped teeth in the rear of the jaw for crushing. Crocodilians will often position turtles in the posterior region of the jaw in order to crush the shells of those they cannot swallow whole.