Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Despite Pseudosuchia meaning "false crocodiles", the name is a misnomer as true crocodilians are now defined as a subset of the group.
The clade Pseudosuchia is potentially equivalent to Crurotarsi even though the latter has a different, node-based definition: "all taxa the least inclusive clade containing Rutiodon carolinensis Emmons, 1856, and Crocodylus niloticus Laurenti, 1768." However, a major 2011 study of Triassic archosaur relations proposed that Rutiodon's group, Phytosauria, was not closely related to other traditional "crurotarsans", at least compared to "bird-line archosaurs" (Avemetatarsalians) such as pterosaurs and dinosaurs. As a result, Crurotarsi could be a much broader clade than Pseudosuchia. Other recent studies support a more traditional phylogeny.
Contrary to popular belief, crocodilians differ significantly from their ancestors and distant relatives, as Pseudosuchia contains a staggering diversity of reptiles with many different lifestyles. Early pseudosuchians were successful in the Triassic period. They included giant, quadrupedal apex predators such as Saurosuchus, Prestosuchus, and Fasolasuchus. Ornithosuchids were large scavengers, while erpetosuchids and gracilisuchids were small, light-footed predators. A few groups acquired herbivorous diets, such as the heavily armored aetosaurs, and several were bipedal, such as Poposaurus and Postosuchus. The bizarre, ornithomimid-like shuvosaurids were both bipedal and herbivorous, with toothless beaks.
Many of these Triassic pseudosuchian groups went extinct at or before the Triassic–Jurassic extinction event. However, one group, the crocodylomorphs, survived the major extinction. Crocodylomorphs themselves evolved a diverse array of lifestyles during the Jurassic and Cretaceous periods, although only a single subset of crocodylomorphs, the Crocodilia, survive to the present day. Living crocodilians include crocodiles, alligators, caimans, and gavialids.
Origin, decline, and re-use of the clade name Pseudosuchia
The name Pseudosuchia 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.
In 2011 paleontologist Sterling Nesbitt proposed that Crurotarsi, as it was then 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.