Temporal range: Late Triassic–Recent, 225–0 Ma
|Hesperosuchus an early crocodylomorph|
During Mesozoic and early Cenozoic times, Crocodylomorpha was far more diverse than it is now. Triassic forms were small, lightly built, active terrestrial animals. These were supplanted during the early Jurassic by various aquatic and marine forms. The Later Jurassic, Cretaceous, and Cenozoic saw a wide diversity of terrestrial and semi-aquatic lineages. "Modern" crocodilians do not appear until the Late Cretaceous. Among the largest crocodylomorphs are: the 8.2 metre long Crocodylus thorbjarnarsoni, the 8.9 metre long Deinosuchus riograndensis, the 9.6 metre long Machimosaurus hugii, and the 9.7 metre long Sarcosuchus imperator.
When their extinct species and stem group are examined, the crocodylian lineage (clade 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 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 evolved herbivory, such as Simosuchus clarki and Chimaerasuchus paradoxus. A number of lineages during the Cenozoic became wholly terrestrial predators.
Taxonomy and phylogeny
Historically, all known living and extinct crocodiles were indiscriminately lumped into the 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 renamed Crocodylomorpha and the slightly more restricted 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 crocodilies (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).
The previous definitions of Crocodilia and Eusuchia do not accurately resemble the evolution of the group. The only order-level taxon that is currently considered valid is Crocodilia in the present definition. Prehistoric crocodiles are represented by many taxa, but since few major groups of the ancient forms are recognizable, a decision where to delimit new order-level clades is not yet possible. (Benson & Clark, 1988).
The Crocodylomorpha comprise a variety of forms, shapes, sizes, that occupied a range of habitats, therefore also a variety in their biology. As most reptiles, Crocodylomorpha were oviparous, laying the eggs in the nest or mound, known from strata as old as Late Jurassic. The sizes in adults vary between about 20 cm long as in Knoetschkesuchus to giant sizes as In Sarcosuchus. Most crocodylomorphs were meat-eaters, but many lineages evolved into a mandatory piscivory, such as the Gharials.
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