Evolutionary historyWhen their extinct species and stem group are examined, the crocodylian lineage (clade Pseudosuchia, formerly Crurotarsi) proves to have been a very diverse and adaptive group of reptiles. Not only are they an ancient group of animals – at least as old as the dinosaurs – they also evolved into a great variety of forms. The earliest forms, the sphenosuchians, evolved during the Late Triassic, and were highly Gracility, gracile terrestrial forms built like greyhounds. During the Jurassic and the Cretaceous, marine forms in the family Metriorhynchidae, such as ''Metriorhynchus'', evolved forelimbs that were paddle-like and had a tail similar to modern fish. ''Dakosaurus andiniensis'', a species closely related to ''Metriorhynchus'', had a skull that was adapted to eat large marine reptiles. Several terrestrial species during the Cretaceous were herbivorous, such as ''Simosuchus clarki'' and ''Chimaerasuchus paradoxus''. A number of lineages during the Cenozoic became wholly terrestrial predators.
Taxonomy and phylogenyHistorically, all known living and extinct crocodiles were indiscriminately lumped into the order (biology), order Crocodilia. However, beginning in the late 1980s, many scientists began restricting the order Crocodilia to the living species and close extinct relatives such as ''Mekosuchus''. The various other groups that had previously been known as Crocodilia were moved to Crocodylomorpha and the slightly more restrictive Crocodyliformes. Crocodylomorpha has been given the rank of superorder in some 20th and 21st century studies. The old Crocodilia was subdivided into the suborders: * Eusuchia: true crocodiles (which includes crown-group Crocodylia) * Mesosuchia: 'middle' crocodiles * Thalattosuchia: sea crocodiles * Protosuchia: first crocodiles Mesosuchia is a paraphyletic group as it does not include eusuchians (which nest within Mesosuchia). Mesoeucrocodylia was the name given to the clade that contains mesosuchians and eusuchians (Whetstone and Whybrow, 1983).
PhylogenyBelow is a cladogram modified from Nesbitt (2011) and Bronzati (2012). The previous definitions of Crocodilia and Eusuchia did not accurately convey evolutionary relationships within the group. The only order-level taxon that is currently considered valid is Crocodilia in its present definition. Prehistoric crocodiles are represented by many taxa, but since few major groups of the ancient forms are distinguishable, a conclusion on how to define new order-level clades is not yet possible. (Benson & Clark, 1988).
BiologyThe Crocodylomorpha comprise a variety of forms, shapes, and sizes, which occupied a range of habitats. As with most amniotes, Crocodylomorphs were and are oviparous, laying eggs in a nest or mound, known from strata as old as the Late Jurassic. Adult size varies widely, from about 55 cm long in ''Knoetschkesuchus'' to much larger dimensions, as in ''Sarcosuchus''. Most crocodylomorphs were carnivores, but many lineages evolved to be obligate piscivores, such as the extant gharials.
Sources* Michael J. Benton, Benton, M. J. (2004), ''Vertebrate Palaeontology (Benton), Vertebrate Palaeontology'', 3rd ed. Blackwell's, Blackwell Science Ltd * Hay, O. P. 1930 (1929–1930). Second Bibliography and Catalogue of the Fossil Vertebrata of North America. ''Carnegie Institution Publications'', Washington, 1, 990 pp.