Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period (Campanian stage). They are the closest living relatives of birds (the only known living dinosaurs), as the two groups are the only known survivors of the Archosauria. Members of the crocodilian stem group, the clade Crurotarsi, appeared about 220 million years ago in the Triassic Period and exhibited a wide diversity of forms during the Mesozoic Era.

The correct vernacular term for this group is "crocodilians" and it includes the alligator, crocodile and gharial and caiman families. The term 'crocodiles' is sometimes incorrectly used to refer to [[alligators and caiman, or even their distant prehistoric relatives, "marine crocodiles".

Spelling

The group is often spelled 'Crocodylia' for consistency with the genus Crocodylus (Laurenti, 1768). However, Richard Owen used the -i- spelling when he published the name in 1842, so it is generally preferred in the scientific literature fact: date=April 2008. The -i- spelling is also a more accurate Latinization of the Greek κροκόδειλος (crocodeilos, literally "pebble-worm", referring to the texture and shape of the animal).

Description

Crocodilians have a flexible semi-erect (semi-sprawled) posture. They can walk in low, sprawled "belly walk," or hold their legs more directly underneath them to perform the "high walk." Most other reptiles can only walk in a sprawled position, and chameleons are the only modern reptiles with a more erect posture than crocodilians.Fact: date=April 2007 The semi-erect posture makes it possible for some species to gallop on land if necessary. An Australian species can reach a speed of over 16 km/h while galloping on an irregular forest floor.Fact: date=April 2007 Crocodilian ancestors, fast-moving terrestrial predators like the rauisuchians, actually had a fully erect posture, indicating that the sprawling and semi-erect posture of crocodilians evolved after they adapted to as semi-aquatic ambush predators. Their ankle bones, or tarsi are highly modified. Modern crocodilian locomotion is not a primitive trait, but a specialization for their semi-aquatic lifestyle.

Teeth and jaws

All crocodilians have, like Homo sapiens (humans), thecodont dentition (teeth set in bony sockets) but unlike mammals, they replace their teeth throughout life (though not in 'extreme' old-age). Juvenile crocodilians replace teeth with larger ones at a rate as high as 1 new tooth per socket every month. After reaching adult size in a few years, however, tooth replacement rates can slow to two years and even longer. Very old members of some species have been seen in an almost "edentulous" (toothless) state, after teeth have been broken and replacement slowed or ceased. The result of this is that a single crocodile can go through at least 3,000 teeth in its lifetime. Each tooth is hollow, and the new one is growing inside the old. In this way, a new tooth is ready once the old is lost.