Pseudosuchia

Clade of reptiles

Automatic taxobox
| name = Pseudosuchians
| fossil range = Early Triassic–Present,
| image = HMNS Paleo Hall Postosuchus and Desmatosuchus. jpg
| image caption = Postosuchus (Rauisuchidae) and Desmatosuchus (Aetosauria)
| taxon = Pseudosuchia
| authority = Zittel, 1887
| subdivision ranks = Subgroups
| subdivision =
Dasygnathoides
Phytosauria? Ornithosuchidae
Suchia
| synonyms = *Crocodylotarsi Benton & Clark, 1988 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. 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. Sterling Nesbitt's 2011 analysis places 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.

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.

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. The end Triassic extinction caused the extinction of all the pseudosuchians with the exception of Sphenosuchia and Crocodyliformes (both Crocodylomorpha), the latter being the ancestors of modern day crocodiles. 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. This allowed the dinosaurs to succeed them as the dominant terrestrial carnivores and herbivores. 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. Interestingly, the Mesozoic range of cranial disparity is higher than the Triassic one, suggesting crocodylomorphs attained a high degree of diversification compared to Triassic pseudosuchians.

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 non crocodilians, but when defined as a clade, Pseudosuchia came to include 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. Bold terminal taxa are collapsed. Several results of the study, such as the retention of a monophyletic Rauisuchia, the retention of phytosaurs within Pseudosuchia, and a close relation between aetosaurs and crocodylomorphs, replicate the results of older studies. However, the findings of Nesbitt (2011) have been more widely supported by pseudosuchian focused analyses published since 2011.