Crocodylomorpha

The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.

During Mesozoic and Early Tertiary times, the Crocodylomorpha were far more diverse than they are 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 Tertiary saw a wide diversity of terrestrial and semi-aquatic lineages. "Modern" crocodilians do not appear until the Late Cretaceous.

Evolutionary history
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 Tertiary and Pleistocene became wholly terrestrial predators.

Phylogenetic definition
The Crocodylomorpha are defined phylogenetically by Sereno 2005 as "The most inclusive clade containing Crocodylus niloticus (Laurenti 1768) but not Poposaurus gracilis Mehl 1915, Gracilisuchus stipanicicorum Romer 1972, Prestosuchus chiniquensis Huene 1942, Aetosaurus ferratus Fraas 1877."

This a stem-based definition and therefore includes all taxa closer to extant crocodilians than to other crurotarsan clades.

Taxonomy and phylogeny
Historically, all known living and extinct crocodiles were indiscriminately lumped into the order Crocodilia. But it is now known that this is erroneous, based on the uniqueness of the crocodilian morphology. Thus, the order Crocodilia is nowadays restricted to the living species and close extinct relatives such as Mekosuchus.

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).

Phylogeny
The following classification is from the Bristol University webpage on Crocodylomorpha. All these taxa are extinct except for the three Eusuchian families shown here. There are in addition many other, less well-known groups that are not included in this current listing.


 * Subdivision Crurotarsi
 * Superorder Crocodylomorpha
 * Suborder Sphenosuchia
 * Clade Crocodyliformes (Order Crocodilia in the broad sense)
 * Family Gobiosuchidae (extinct)
 * Family Hsisosuchidae (extinct)
 * Family Shartegosuchidae (extinct)
 * Suborder Protosuchia (extinct)
 * Suborder Thalattosuchia (extinct)
 * Suborder Metasuchia* (extinct)
 * Order Crocodilia (strict sense)
 * Superfamily Gavialoidea
 * Family Gavialidae
 * Superfamily Crocodyloidea
 * Family Mekosuchidae (extinct)
 * Family Crocodylidae
 * Superfamily Alligatoroidea
 * Family Nettosuchidae (extinct)
 * Family Alligatoridae

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).