Crocodilia
Crocodilia is an order of semiaquatic, predatory reptiles that are known as crocodilians. They appeared 83.5 million years ago in the Late Cretaceous period and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, and diversified during the Mesozoic era. The order includes the true crocodiles, the alligators and caimans, and the gharial and false gharial. Although the term "crocodiles" is sometimes used to refer to all of these families, the term "crocodilians" is less ambiguous.
Extant crocodilians have flat heads with long snouts and tails that are compressed on the sides, with their eyes, ears, and nostrils at the top of the head. Alligators and caimans tend to have broader U-shaped jaws that, when closed, show only the upper teeth, whereas crocodiles usually have narrower V-shaped jaws with both rows of teeth visible when closed. Gharials have extremely slender, elongated jaws. The teeth are conical and peg-like, and the bite is powerful. All crocodilians are good swimmers and can move on land in a "high walk" position, traveling with their legs erect rather than sprawling. Crocodilians have thick skin covered in non-overlapping scales and, like birds, have a four-chambered heart and lungs with unidirectional airflow.
Like other reptiles, crocodilians are ectotherms or 'cold-blooded'. 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. Crocodilians have a largely carnivorous diet; some species like the gharial are specialized feeders while others, like the saltwater crocodile, have generalized diets. They are generally solitary and territorial, though they sometimes hunt in groups. During the breeding season, dominant males try to monopolize available females, who lay their eggs in holes or mounds and, like many birds, they care for their hatched young.
Some species of crocodilians, particularly the Nile crocodile, are known to have attacked humans, which through activities that include hunting, poaching, and habitat destruction are the greatest threat to crocodilian populations. Farming of crocodilians has greatly reduced unlawful trading in skins of wild-caught animals. Artistic and literary representations of crocodilians have appeared in human cultures around the world since at least Ancient Egypt.
Spelling and etymology
"Crocodilia" and "Crocodylia" have been used interchangeably for decades, starting with Karl Patterson Schmidt's re-description of the group from the formerly defunct term Loricata. Schmidt used the older term "Crocodilia", based on Richard Owen's original name for the group. Heinz Wermuth chose "Crocodylia" as the proper name, basing it on the type genus Crocodylus. Dundee, in a revision of many reptilian and amphibian names, argued strongly for "Crocodylia". Following the advent of cladistics and phylogenetic nomenclature, a more-solid justification for one spelling over the other was proposed.Prior to 1988, Crocodilia was a group that encompassed the modern-day animals, as well as their more-distant relatives that are now classified in the larger groups Crocodylomorpha and Pseudosuchia. Under its current definition as a crown group, rather than a stem-based group, Crocodylia is now restricted to the last common ancestor of today's crocodilians and all of its descendants, living or extinct.
Crocodilia appears to be a Latinism of the Greek word κροκόδειλος, which means both lizard and Nile crocodile. Crocodylia, as coined by Wermuth in regards to the genus Crocodylus, appears to be derived from the Ancient Greek κρόκη —meaning shingle or pebble—and δρîλος or δρεîλος, meaning worm. The name may refer to the animal's habit of resting on the pebbled shores of the Nile.
Phylogeny and evolution
Origins from pseudosuchians
Crocodilians and birds are members of the clade Archosauria. Archosaurs are distinguished from other reptiles particularly by two sets of extra openings in the skull; the antorbital fenestra located in front of the animal's eye socket and the mandibular fenestra on the jaw. Archosauria has two main groups: the Pseudosuchia and the Avemetatarsalia. The split between these two groups is assumed to have happened close to the Permian–Triassic extinction event, which is informally known as the Great Dying.Crocodylomorpha, the group that later gave rise to modern crocodilians, emerged in the Late Triassic. The most-basal crocodylomorphs were large, whereas the ones that gave rise to crocodilians were small, slender, and leggy. This evolutionary grade, the "sphenosuchians", first appeared around Carnian of the Late Triassic. They ate small, fast prey and survived into the Late Jurassic. As the Triassic ended, crocodylomorphs became the only surviving pseudosuchians.
Early crocodyliform diversity
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. The earliest stage of crocodilian evolution was the protosuchians in the late Triassic and early Jurassic, which were followed by the mesosuchians that diversified widely during the Jurassic and the Tertiary. The eusuchians first appeared during the Early Cretaceous; this clade includes modern crocodilians.File:Suchodus durobrivense.jpg|thumb|left|Suchodus, a thalattosuchian highly adapted to a marine lifestyle
Protosuchians were small, mostly terrestrial animals with short snouts and long limbs. They had bony armor in the form of two rows of plates extending from head to tail; this armor would still be found in later species. Their vertebrae were convex on the two main articulating surfaces. The secondary palate was little developed; it consisted only of a maxilla. The mesosuchians underwent a fusion of the palatine bones to the secondary palate, and a great extension of the nasal passages behind the palatine and in front of the pterygoid bones. This adaptation allowed the animal to breathe through its nostrils while its mouth was open underwater. The eusuchians continued this process; the interior nostrils now opened through an aperture in the pterygoid bones. The vertebrae of eusuchians had one convex and one concave articulating surface. The oldest-known eusuchian is Hylaeochampsa vectiana from the Early Cretaceous whose remains occur on the Isle of Wight in the United Kingdom. It was followed by crocodilians such as the Planocraniidae, the hoofed crocodiles, in the Palaeogene. Spanning the Cretaceous and Palaeogene periods is the genus Borealosuchus of North America, with six species, though its phylogenetic position is not settled.
Diversification of modern crocodilians
The three primary branches of Crocodilia had diverged by the Late Cretaceous. The possible earliest-known members of the group may be Portugalosuchus and Zholsuchus from the Cenomanian-Turonian stages. Some researchers have disputed the classification of Portugalosuchus, claiming it may be outside the crown-group crocodilians. The morphology-based phylogenetic analyses, which are based on new neuroanatomical data obtained from its skull using micro-CT scans, suggest this taxon is a crown-group crocodilian and a member of the 'thoracosaurs' that was recovered as a sister taxon of Thoracosaurus within Gavialoidea, though it is uncertain whether 'thoracosaurs' were true gavialoids.Definitive alligatoroids first appeared during the Santonian-Campanian stages, while definitive longirostres first appeared during the Maastrichtian stage. The earliest-known alligatoroids and gavialoids include highly derived forms, which indicates the time of the divergence into the three lineages must have been a pre-Campanian event. 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.
Relationships
Crocodylia is cladistically defined as the last common ancestor of Gavialis gangeticus, Alligator mississippiensis, and Crocodylus rhombifer and all of its descendants. The phylogenetic relationships between crocodilians has been the subject of debate and conflicting results. Many studies and their resulting cladograms of crocodilians have found the "short-snouted" families of Crocodylidae and Alligatoridae to be close relatives, and the long-snouted Gavialidae is a divergent branch of the tree. The resulting group of short-snouted species, named Brevirostres, was mainly supported by morphological studies that analyzed only skeletal features.Recent molecular studies using DNA sequencing of living crocodilians have rejected the distinct group Brevirostres; the long-snouted gavialids are more closely related to crocodiles than to alligators, and the new grouping of gavialids and crocodiles is named Longirostres.
Below is a cladogram from 2021 showing the relationships of the major extant crocodilian groups. This analysis was based on mitochondrial DNA, including that of the recently extinct Voay robustus:
Anatomy and physiology
Though there is diversity in snout and tooth shape, all crocodilian species have essentially the same body morphology. They have solidly built, lizard-like bodies with wide, cylindrical torsos, flat heads, long snouts, short necks, and tails that are compressed from side to side. Their limbs are reduced in size; the front feet have five mostly non-webbed digits, and the hind feet have four webbed digits and an extra fifth. The pelvis and ribs of crocodilians are modified; the cartilaginous processes of the ribs allow the thorax to collapse when submerging and the structure of the pelvis can accommodate large amounts of food, or more air in the lungs. Both sexes have a cloaca, a single chamber and outlet near the tail into which the intestinal, urinary and genital tracts open. It houses the penis in males and the clitoris in females. The crocodilian penis is permanently erect; it relies on cloacal muscles to protrude it, and elastic ligaments and a tendon to retract it. The gonads are located near the kidneys.Crocodilians range in size from the dwarf caimans and African dwarf crocodiles, which reach, to the saltwater crocodile and Nile crocodile, which reach and weigh up to. Some prehistoric species such as the Miocene caiman Purussaurus were even larger, with some estimates putting it over. Crocodilians tend to be sexually dimorphic; males are much larger than females.