Crocodiles, Alligators or Gharials?
One of the most commonly asked questions is - "What is the difference between a crocodile and an alligator?"
Firstly, it is worth noting that the term “crocodilians” refers to all members of the Family Crocodylidae (“true” crocodiles), Family Alligatoridae (alligators and caimans) and Family Gavialidae (gharial, Tomistoma). In contrast, the term “crocodiles” refers only to the "true" crocodiles.
The main criteria used to distinguish members of the three crocodilian Families are associated with the head, particularly the jaws and skull. The skull and jaws of all crocodilians function identically, and are composed of the same suite of bones. But there is variation in the extent to which different bones compose certain structures. Fortunately, there are some external characteristics of the head that allow members of the three Families to be distinguished.
Alligators and caimans
Alligatorids tend to have broad snouts, which are often referred to as being "shovel-shaped". The upper jaw is so broad that when the jaws are closed many of the teeth of the lower jaw fit into sockets along the edge of the expanded upper jaw. In all crocodilians the 4th tooth back from the front, on the lower jaw, is greatly enlarged. In the alligators and caimans, this tooth fits into a socket in the upper jaw when the jaws are closed, such that its tip is hidden. There are seven extant alligatorid species, divided amongst four genera (Alligator, Caiman, Melanosuchus, Palpebrosus). When people talk about "alligators", they are referring to the American Alligator (Alligator mississippiensis).
The upper jaw of "true" crocodiles is not as broad as that of alligators and caimans. Furthermore, it is sharply constricted or notched on the snout. In contrast to alligators and caimans, when "true" crocodiles close their jaws the enlarged 4th tooth on the lower jaw rests in that notch, and its tip is clearly visible. This is a major distinction between "true" crocodiles and alligators and caimans. There are 14 species of "true" crocodiles, in three genera; Crocodylus, Osteolaemus, Mecistops).
The Gharial and Tomistoma have greatly elongated snouts. This elongation has been achieved more by compacting the cranial part of the skull, at the rear, than by elongation of the whole head. Thus, the head length of a 3 m long Gharial is not very different from the head length of a Saltwater Crocodile of the same total length - Gharials simply have a far greater proportion of the head allocated to snout.Sources:
G. Webb and C. Manolis (1989). Crocodiles of Australia. Reed Books: Sydney;
K. Richardson, G. Webb and C. Manolis (2000). Crocodiles: Inside and Out. Surrey Beatty and Sons: Sydney.