Pseudosuchia

Pseudosuchia ("false crocodiles") is one of two major divisions of Archosauria and includes living crocodilians and all archosaurs more closely related to crocodilians than to birds (what are often called "crocodilian-line archosaurs"). The name was originally given to a group of superficially crocodile-like prehistoric reptiles from the Triassic period, but fell out of use in the late 20th century, especially after the name Crurotarsi was established in 1990 to label the clade (evolutionary grouping) of archosaurs encompassing most reptiles previously identified as pseudosuchians. By this time, Pseudosuchia had also been defined as a clade, but it was not widely embraced until 2011, when paleontologist Sterling Nesbitt proposed that Crurotarsi as it was originally defined must include not only crocodilian-line archosaurs, but all other archosaurs including birds, non-avian dinosaurs, and pterosaurs. The clade Pseudosuchia as originally defined could still be used to identify crocodilian-line archosaurs, and since many recent studies support Nesbitt's findings, Pseudosuchia is now commonly used.

Taxonomic history
The name Pseudosuchia was coined by Karl Alfred von Zittel in 1887-1890 to include three taxa (two aetosaurs and Dyoplax) that were superficially crocodilian-like, but were not actually crocodilian. Hence the name "false crocodiles".

In mid-20th century textbooks like Alfred Sherwood Romer's Vertebrate Paleontology and Edwin H. Colbert's Evolution of the Vertebrates, Pseudosuchia constitutes one of the suborders of the now-abandoned order Thecodontia. Zittel's aetosaurs were placed in their own suborder, Aetosauria. Colbert considered small lightly built archosaurs such as Ornithosuchus and Hesperosuchus —both of which were at the time reconstructed as theropod dinosaur-like bipeds — to be typical pseudosuchians. These small forms were assumed to be the ancestors of all later archosaurs. The name Pseudosuchia became a wastebasket taxon into which all thecodonts that did not fit in the other three suborders could be placed. Even Sharovipteryx and Longisquama, two enigmatic Triassic reptiles that bear little resemblance to archosaurs, have been regarded as pseudosuchians.

Gauthier and Padian (1985) and Gauthier (1986) became the first to establish the name Pseudosuchia in a phylogenetic context, using it as a branch-based taxon for all archosaurs more closely related to crocodilians than to birds. This made the name Pseudosuchia somewhat ironic, because true crocodiles (i.e. members of Crocodylia) were now included in the group. Phylogenetic definitions of Pseudosuchia include "Crocodiles and all archosaurs closer to crocodiles than to birds" (Gauthier and Padian), "Extant crocodiles and all extinct archosaurs that are closer to crocodiles than they are to birds" (Gauthier 1986), and more recently "the most inclusive clade within Archosauria that includes Crocodylia but not Aves" (Senter 2005). As a branch-based clade, Pseudosuchia is the sister taxon of another branch-based clade, the Avemetatarsalia. Avemetatarsalians are bird-line archosaurs, including pterosaurs and dinosaurs (the latter including birds).

A different definition was suggested by Benton and Clark, 1988: the node-based taxon including the last common ancestor of Rauisuchidae and aetosaurs and all of its descendants. Benton and Clark also named a group called Crocodylotarsi, which includes most taxa now considered pseudosuchians.

In 1990. Paul Sereno erected the clade Crurotarsi to supplant Pseudosuchia. However, Sereno defined Crurotarsi as a node-based clade, relying on the inclusion of groups such as Phytosauria, Aetosauria, and Crocodylomorpha. It is not equivalent to Pseudosuchia, which by definition must include all crocodilian-line archosaurs. For many years, Pseudosuchia and Crurotarsi have been considered partial synonyms because the latter clade encompasses all crocodilian-line archosaurs in most phylogenetic analyses. However, many recent analyses (starting with Sterling Nesbitt's 2011 analysis) place one crurotarsan group, Phytosauria, outside Pseudosuchia. Since the definition of Crurotarsi relies on phytosaurs, their placement outside Pseudosuchia (and thus Archosauria) means that the clade Crurotarsi includes both pseudosuchians and avemetatarsalians.

Description
Pseudosuchia is one of the two primary "daughter" clades of the Archosauria. The skull is often massively built, especially in contrast to ornithodires; the snout is narrow and tends to be elongated, the neck is short and strong, and the limb posture ranges from a typical reptilian sprawl to an erect stance like dinosaurs or mammals although achieving it a different way. The body is often protected by two or more rows of armored plates. Many crurotarsans reached lengths of three meters or more.

Evolution
Pseudosuchians appeared during the late Olenekian (early Triassic); by the Ladinian (late Middle Triassic) they dominated the terrestrial carnivore niches. Their heyday was the Late Triassic, during which time their ranks included erect-limbed rauisuchians, herbivorous armored aetosaurs, the large predatory poposaurs, the small agile sphenosuchian crocodilians, and a few other assorted groups.

At the end Triassic extinction, all of the large pseudosuchians died out. A study published in 2010 postulates that there is significant evidence that volcanic eruptions changed the climate, causing a mass extinction that wiped out the dinosaurs' main competitors. Furthermore, this allowed the dinosaurs to succeed them as the dominant terrestrial carnivores and herbivores. Only the Sphenosuchia and the Protosuchia (Crocodylomorpha) survived.

As the Mesozoic progressed, the Protosuchia gave rise to more typically crocodile-like forms. While dinosaurs were the dominant animals on land, the crocodiles flourished in rivers, swamps, and the oceans, with far greater diversity than they have today. With the end Cretaceous extinction the dinosaurs became extinct, with the exception of the birds, while the crocodilians continued with little change. Today, the crocodiles, alligators, and gharials are the surviving representatives of this lineage.

Phylogeny
Pseudosuchia was defined as a stem-based clade in 1985. It includes crocodiles and all archosaurs more closely related to crocodiles than to birds. A second clade with a similar definition, Crocodylotarsi, was named in 1988, possibly as a replacement for Pseudosuchia. The name Pseudosuchia, meaning "false crocodiles", has been used for over a century, and traditionally included only aetosaurs, but when defined as a clade, Pseudosuchia came to includes the group Eusuchia ("true crocodiles") as well. Crocodylotarsi may have been named to remove confusion, but as a stem-based clade it is synonymous with Pseudosuchia. Because Pseudosuchia was named first, it has precedence. A third group, Crurotarsi, traditionally included the same archosaurs as Pseudosuchia, but as a node-based clade it is not synonymous. The scope of Crurotarsi has recently been changed by the phylogenetic placement of phytosaurs. In 2011, Sterling J. Nesbitt found phytosaurs to be the sister taxon of Archosauria, and therefore not crocodile-line archosaurs. Because phytosaurs are included in the definition of Crurotarsi, crurotarsans are not solely crocodile-line archosaurs but also bird-line archosaurs and phytosaurs. Under this phylogeny, Crurotarsi includes phytosaurs, crocodiles, pterosaurs and dinosaurs, while Pseudosuchia still contains only crocodile-line archosaurs. Below is a cladogram modified from Nesbitt (2011) showing the new changes (bold terminal taxa are collapsed).

Cladogram after Brusatte, Benton, Desojo and Langer (2010), bold terminal taxa are collapsed.