Poposauroidea
Poposauroidea is a clade of advanced pseudosuchians (archosaurs closer to crocodilians than to dinosaurs). It includes poposaurids, shuvosaurids, ctenosauriscids, and other unusual pseudosuchians such as Qianosuchus and Lotosaurus. However, it excludes most large predatory quadrupedal "rauisuchians" such as rauisuchids and "prestosuchids". Those reptiles are now allied with crocodylomorphs (crocodile ancestors) in a clade known as Loricata, which is the sister taxon to the poposauroids in the clade Paracrocodylomorpha. Although it was first formally defined in 2007, the name "Poposauroidea" has been used for many years. The group has been referred to as Poposauridae by some authors, although this name is often used more narrowly to refer to the family that includes Poposaurus and its close relatives.
Poposauroids went extinct at the end of the Triassic period along with other non-crocodylomorph pseudosuchians. However, they were among the most diverse and long-lasting members of non-crocodylomorph Pseudosuchia, with Xilousuchus (a ctenosauriscid) living near the very beginning of the Triassic and Effigia (a shuvosaurid) surviving up until near the end of the Triassic. Despite the high level of diversity and anatomical disparity within Poposauroidea, certain features of the clade can be determined, particularly in the structure of the snout and pelvis (hip). Many of these features are examples of convergent evolution with dinosaurs, with bipedal poposauroids such as Poposaurus and shuvosaurids having been mistaken for theropod dinosaurs in the past.[1]
Poposauroidea was a diverse group of pseudosuchians, containing genera with many different ecological adaptations. Some (Poposaurus and shuvosaurids) were short-armed bipeds, while others (ctenosauriscids and Lotosaurus) were robust quadrupeds with elongated neural spines, creating a 'sail' like that of certain "pelycosaurs" (like Dimetrodon) and spinosaurids. Lotosaurus and shuvosaurids were toothless and presumably beaked herbivores while Qianosuchus, Poposaurusand ctenosauriscids were sharp-toothed predators. The ecological disparity of many members of this clade means that it is difficult to assess what the ancestral poposauroid would have looked like.
Poposauroids can be differentiated from other pseudosuchians by the structure of the tip of the snout, particularly the premaxillary bone which lies in front of the nares (nostril holes). This bone possesses two bony extensions ("processes") which wrap around the nares. The anterodorsal process, which wraps above the nares to contact the nasal bones on the top edge of the snout, is typically quite short in pseudosuchians. However, poposauroids have elongated anterodorsal processes, longer than the main premaxillary body. The posterodorsal process, which wraps below the nares to contact the maxilla on the side of the snout, possesses the opposite state. It is much shorter in poposauroids (compared to other pseudosuchians), restricted to only a portion of the lower edge of the nares. This has the added effect of allowing the maxilla to form the rest of the hole's lower and rear edge, with the front edge of the maxilla becoming concave as well. Although these snout features are rare among pseudosuchians, they are much more common in certain avemetatarsalians (bird-line archosaurs) such as pterosaurs and saurischian dinosaurs. The rear branch of the maxilla tapers in most poposauroids, with the exception of Qianosuchus. This contrasts with loricatans, in which this branch is rectangular in shape.[1]
