Crocodiles (subfamily Crocodylinae) or true crocodiles are large aquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. Crocodylinae, all of whose members are considered true crocodiles, is classified as a biological subfamily. A broader sense of the term crocodile, Crocodylidae that includes Tomistoma, is not used in this article. The term crocodile here applies only to the species within the subfamily of Crocodylinae. The term is sometimes used even more loosely to include all extant members of the order Crocodilia, which includes Tomistoma, the alligators and caimans (family Alligatoridae), the gharials(family Gavialidae), and all other living and fossil Crocodylomorpha.
Although they appear to be similar to the untrained eye, crocodiles, alligators and the gharial belong to separate biological families. The gharial having a narrow snout is easier to distinguish, while morphologicaldifferences are more difficult to spot in crocodiles and alligators. The most obvious external differences are visible in the head with crocodiles having narrower and longer heads, with a more V-shaped than a U-shapedsnout compared to alligators and caimans. Another obvious trait is that the upper and lower jaws of the crocodiles are the same width, and the teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed; therefore, all teeth are visible unlike an alligator; which possesses small depressions in the upper jaw, into which the lower teeth fit. Also, when the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. For hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define the family that the species belongs to. Crocodiles have morewebbing on the toes of the hind feet and can better tolerate saltwater due to specialized salt glands for filtering out salt, which are present but non-functioning in alligators. Another trait that separates crocodiles from other crocodilians is their much higher levels of aggression.
Crocodile size, morphology, behavior and ecology somewhat differs between species. However, they have many similarities in these areas as well. All crocodiles are semiaquatic and tend to congregate in freshwaterhabitats such as rivers, lakes, wetlands and sometimes in brackish water and saltwater. They are carnivorous animals, feeding mostly on vertebrates such as fish, reptiles, birds and mammals, and sometimes oninvertebrates such as molluscs and crustaceans, depending on species and age. All crocodiles are tropical species that, unlike alligators, are very sensitive to cold. They separated from other crocodilians during theEocene epoch, about 55 million years ago. Many species are at the risk of extinction, some being classified as critically endangered.