There are 23 widely recognized species of crocodiles, alligators, caimans, and gharials, all members of the order Crocodylia. Superficially they resemble reptiles, yet their closest cousins are birds and extinct Dinosauria, a group known as archosaurs ("ruling reptiles"). Modern Crocodylia are the latest iteration of the Crocodylomorpha, a major group whose evolutionary heritage spans almost 240 million years. Crocodylia are often described as "living fossils," unchanged in millions of years, but this description is inaccurate. The Croc-odylomorpha were a diverse and successful group occupying terrestrial, freshwater, and marine ecosystems, and their modern counterparts are barely less fantastic. This group is informally referred to as "crocodilians," although the term "crocodylians" technically refers to members of the order.
Every successful group has a beginning. The earliest croc-odyliforms were terrestrial hunters that shared an ankle with modern Crocodylia, but little else, yet they were dominant predators whose legacy diversified throughout the Jurassic and Cretaceous periods. Their success is evident in excavations, as more crocodyliform material than dinosaur bones often turns up. Discoveries have been remarkable, such as curious peglike teeth and spiked protective plates from Desmatosuchus that indicate a defensive, vegetarian lifestyle. Despite dabbling in herbivory, it was in carnivory that croc-odyliforms excelled. The awe-inspiring skulls of Sarcosuchus and Deinosuchus paint a picture of massive killers over 35 ft (10.7 m) in length. From terrestrial beginnings, crocodyli-forms branched out into freshwater and marine habitats. In extreme thalattosuchians (marine crocodiles), limbs were replaced with paddles and a fluked tail to compete with sharks and ichthyosaurs for dominance of the sea. However, these marine forms were an evolutionary dead end, and freshwater species with limited marine ability proliferated from the Juras sic onward. Modern Crocodylia first appeared over 100 million years ago, and despite experiments with various and occasionally bizarre forms, the semiaquatic predator has become their signature role.
Scientists disagree about crocodyliform classification and evolutionary relationships. In 2003, 23 species of Crocodylia are widely recognized, divided into three families: Alligatori-dae (alligators and caimans; eight species), Crocodylidae (crocodiles; 14 species), and Gavialidae (gharial; one species). The Crocodylidae are further divided into two subfamilies, Crocodylinae and Tomistominae. Some taxonomists also divide Alligatoridae into two subfamilies: Alligatorinae and Caimaninae.
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