It belongs to the clade Poposauroidea, an unusual group of Triassic pseudosuchians that includes sail-backed, beaked, and aquatic forms.

In 1907, paleontologist J. H. Lees described this fossil, an ilium (part of the hip) from the Popo Agie Formation, and identified it as that of the phytosaur Paleorhinus bransoni.

[2] In 1915, paleontologist M. G. Mehl named Poposaurus based on more complete material from the Popo Agie Formation, including vertebrae, hips, and limb bones.

[4] In 1961, American paleontologist Edwin Harris Colbert gave an extensive description of the known material of Poposaurus and classified it as a theropod dinosaur.

In 1915, Mehl described a "distal femur" in the holotype specimen of Poposaurus, but Galton interpreted this to be the fused end of the hip's pubis bones.

[5] In 1995, paleontologists Robert Long and Phillip Murry described new fossils of Poposaurus from the Placerias quarry in the Chinle Formation of Arizona.

They removed Postosuchus from Poposauridae, claiming that the material used in this assignment was a chimera, or a collection of bones belonging to different animals.

Long and Murry separated poposaurids like Poposaurus, Bromsgroveia, and the newly named Lythrosuchus from rauisuchians like Postosuchus, which they held in the family Rauisuchidae.

[8] In 2022, the partial remains (fragmentary thoracic vertebrae and part a right humerus) of an immature P. gracilis were described from the early Carnian-aged Doswell Formation of Virginia, marking the first occurrence of this genus from eastern North America.

[10] With the tail comprising about half the body length, Poposaurus was about 4–5 metres (13–16 ft) long and weighed 90 to 100 kilograms (200 to 220 lb) as an adult.

Below is the cladogram from Gauthier et al. (2011):[1] Avemetatarsalia Ornithosuchidae Gracilisuchus Turfanosuchus Revueltosaurus Aetosauria Ticinosuchus Qianosuchus Arizonasaurus Xilousuchus Poposaurus Lotosaurus Sillosuchus Shuvosaurus Effigia Prestosuchus Saurosuchus Batrachotomus Fasolasuchus Rauisuchus Polonosuchus Postosuchus Crocodylomorpha When M. G. Mehl first named Poposaurus in 1915, he described it as "a well-muscled creature light in weight, possibly bipedal in gait occasionally, and most assuredly swift in movement.

"[3] Mehl based this description on its long limb bones and deep hip socket, two features which link it with bipedal dinosaurs.

Poposaurus also has a "pillar erect" stance in which the acetabulum or hip socket faces downward and is positioned directly over the head of the femur.

In contrast, dinosaurs have "buttress erect" hip structures in which the acetabulum faces laterally and the head of the femur is angled to fit into it.

[12] Although they evolved bipedal locomotion independently, Poposaurus and dinosaurs inherited a propensity for erect hind-limb driven movement from an early archosaur ancestor.

Poposaurus and dinosaurs achieved a bipedal posture as their legs increased in size, their hips strengthened, and their spines adapted for dorsoventral flexion.

Other adaptations that may have facilitated bipedal locomotion include the development of a chambered heart and lungs with unidirectional airflow (both of which are assumed present in Poposaurus through phylogenetic bracketing).

[1][13] The leg musculature of Poposaurus was hypothesized in a 2011 study that examined muscle scars on the bones and made inferences based on phylogenetic bracketing.

The study hypothesized that Poposaurus had an ischiotruncus muscle running from the ischium at the back of the hip, across the pubis, and into the gastralia bones of the abdomen.