It was phylogenetically defined in 2011 by Sterling Nesbitt as Poposaurus gracilis and all taxa more closely related to it than to Postosuchus kirkpatricki, Crocodylus niloticus (the Nile crocodile), Ornithosuchus woodwardi, or Aetosaurus ferratus.

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).

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.

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).

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.

The posterodorsal process, which wraps below the nares to contact the maxilla on the side of the snout, possesses the opposite state.

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.

For example, in most archosaurs each side of the braincase possesses a pit from where the internal carotid arteries may exit the brain.

In early poposauroids, these pits migrated to the underside of the braincase, thereby resembling the primitive condition seen in archosaur relatives such as Euparkeria and proterochampsians.

This second neck trait contrasts with the condition in other pseudosuchians, phytosaurs, and pterosaurs, which have short and stout cervical ribs.

This differs compared to the vertebrae of most other early pseudosuchians (as well as Euparkeria and phytosaurs), which have neural spines that expand outward to form a flat, rectangular surface when seen from above.

Poposauroids had first primordial sacral ribs with additional forward branches, which lie on the inner edge of the ilium's preacetabular blade.

The only exception to this is Qianosuchus, which possessed numerous tiny osteoderms, lying in a row extending down the neck and body.

[7] In all poposauroids, the tip of the fibula (outer shin bone) is symmetrical and straight when seen from the side, rather than slanted as in other non-crocodylomorph pseudosuchians.

Some poposauroids had very short arms compared to the length of their legs, although disarticulation in Qianosuchus and a lack of limb material in ctenosauriscids means that it is unknown whether this trait was basal to the group as a whole.

Missing data for ctenosauriscids also obscures when certain traits of the caudal vertebrae and ankle bones were gained or lost within Poposauroidea.

Postosuchus was widely considered to be a poposauroid for the next ten years and was included in many phylogenetic analyses of Triassic archosaurs.

[9] In 2005, Sterling Nesbitt noted that "ctenosauriscids" such as Arizonasaurus, Bromsgroveia, and Lotosaurus shared many similarities with "poposaurids" such as Poposaurus, Sillosuchus, and "Chatterjeea" (now known as Shuvosaurus).

Unlike many recent studies, they found Rauisuchia to be monophyletic, consisting of two major clades: Rauisuchoidea and Poposauroidea.

[12] In his massive revision of archosaurs which included a large cladistic analysis, Sterling J. Nesbitt (2011) found Xilousuchus to be a poposauroid most closely related to Arizonasaurus.