Crocodilians are a type of crocodylomorph pseudosuchian, a subset of archosaurs that appeared about 235 million years ago and were the only survivors of the Triassic–Jurassic extinction event.

They are found mainly in the warm and tropical areas of the Americas, Africa, Asia, and Oceania, usually occupying freshwater habitats, though some can live in saline environments and even swim out to sea.

During the breeding season, dominant males try to monopolize available females, which lay their eggs in holes or mounds and, like many birds, they care for their hatched young.

[7] Crocodylia, as coined by Wermuth[3] in regards to the genus Crocodylus, appears to be derived from the ancient Greek[8] κρόκη (kroke)—meaning shingle or pebble—and δρîλος or δρεîλος (dr(e)ilos), meaning worm.

[16] During the early Jurassic period, dinosaurs became dominant on land and the crocodylomorphs underwent major adaptive diversifications to fill ecological niches vacated by recently extinguished groups.

Mesozoic crocodylomorphs had a much greater diversity of forms than modern crocodilians; they became small, fast-moving insectivores, specialist fish-eaters, marine and terrestrial carnivores, and herbivores.

[6] Additionally, scientists conclude environmental factors played a major role in the evolution of crocodilians and their ancestors; warmer climate is associated with high evolutionary rates and large body sizes.

This analysis was based on mitochondrial DNA, including that of the recently extinct Voay robustus:[27] Caiman Melanosuchus Paleosuchus Alligator Crocodylus †Voay Mecistops Osteolaemus Gavialis Tomistoma Though there is diversity in snout and tooth shape, all crocodilian species have essentially the same body morphology.

[48] When in bright light, the pupils of a crocodilian contract into narrow slits, whereas in darkness they become large circles, as is typical for animals that hunt at night.

[65] Crocodilians appear to have undergone a "nocturnal bottleneck" early in their history, during which their eyes lost traits like sclerotic rings, an annular pad of the lens and coloured cone oil droplets, giving them dichromatic vision (red-green colourblindness).

[38] Research also suggests alkaline ions released into the blood from the calcium and magnesium in the dermal bones act as a buffer during prolonged submersion when increasing levels of carbon dioxide would otherwise cause acidosis.

[89] Other possible reasons for the peculiar circulatory system include assistance with thermoregulatory needs, prevention of pulmonary oedema, and quick recovery from metabolic acidosis.

[38] All species have a palatal valve, a membranous flap of skin at the back of the oral cavity (mouth) that protects the oesophagus and trachea when the animal is underwater.

The main means of warming is sun's heat, while immersion in water may either raise its temperature via thermal conduction or cool the animal in hot weather.

Temperature-sensing probes implanted in wild American alligators have found their core body temperatures can fall to around 5 °C (41 °F), but as long as they remain able to breathe, they show no ill effects when the weather warms.

[116] Crocodilians are typically creatures of the tropics; the main exceptions are the American and Chinese alligators, whose ranges are the southeastern United States and the Yangtze River, respectively.

[121] West African crocodiles in the deserts of Mauritania mainly live in gueltas and floodplains but they retreat underground and to rocky shelters, and enter aestivation during the driest periods.

Species with sharp teeth and long, slender snouts, like the Indian gharial and Australian freshwater crocodile, are specialized for snapping fish, insects, and crustaceans.

Species whose snouts and teeth are intermediate between these two forms, such as the saltwater crocodile and American alligator, have generalized diets and opportunistically feed on invertebrates, fish, amphibians, reptiles, birds and mammals.

[129] To tear a chunk of tissue from a large carcass, a crocodilian continuously spins its body while holding prey with its jaws, a manoeuvrer that is known as the death roll.

[48] Depending on the species, female crocodilians may construct either holes or mounds as nests,[48] the latter made from vegetation, litter, sand or soil.

For American alligators, groups of young associate with adults for one-to-two years while juvenile saltwater and Nile crocodiles become independent in a few months.

[135] They can communicate with sounds, including barks, bellows, chirps, coughs, growls, grunts, hisses, moos, roars, toots and whines.

[135] Dominant individuals intimidate rivals by swimming at the surface and displaying their large body size, and subordinates submit by holding their head forward above water with the jaws open and then flee below.

[48] Flooding is a major cause of failure of crocodilians to successfully breed; nests are submerged, developing embryos are deprived of oxygen and juveniles are swept away.

As they grow older, pet crocodilians are often abandoned by their owners, and feral populations of spectacled caimans exist in the United States and Cuba.

[165] The Chinese alligator was historically widespread in the eastern Yangtze River system but is currently restricted to some areas in south-eastern Anhui due to habitat fragmentation and degradation.

[170] In Ancient Egyptian religion, both Ammit the devourer of unworthy souls and Sobek the god of power, protection and fertility are represented as having crocodile heads.

[178] The crocodile was described in the late-13th century Rochester Bestiary, which is based on classical sources, including Pliny's Historia naturalis (c. 79 AD)[179] and Isidore of Seville's Etymologies.

[180][181] Isidore said the crocodile is named for its saffron colour (Latin croceus, 'saffron') and may be killed by fish with serrated crests sawing into its soft underbelly.