Locomotion
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." Mehl based this description on its long limb bones and deep hip socket, two features which link it with bipedal dinosaurs. Since the 1970s however, Poposaurus has been considered a pseudosuchian archosaur more closely related to crocodilians than dinosaurs. Most of its close relatives (such as the large-bodied rauisuchids and ctenosauriscids) were obligate quadrupeds that could not walk on two legs. Although the entire skeleton was unknown, Poposaurus was expected to be similar in appearance to its relatives. In 2011, the nearly complete specimen YPM VP 057100 was described. The specimen confirmed Mehl's description, revealing that Poposaurus was indeed bipedal. The skeleton preserves both the fore and hind limbs, showing that Poposaurus had much shorter arms than legs.
Although Poposaurus and early dinosaurs were both bipedal, the method of locomotion evolved independently in each group. The independent origins are shown through several differences in the skeletons of Poposaurus and dinosaurs. Unlike dinosaurs, Poposaurus has the characteristic crurotarsal ankle of pseudosuchians, usually associated with quadrupedal locomotion. 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.
Although they evolved bipedal locomotion independently, Poposaurus and dinosaurs inherited a propensity for erect hind-limb driven movement from an early archosaur ancestor. The posture of this ancestral archosaur can be inferred from a method called extant phylogenetic bracketing. Archosauria is a crown group represented today by birds and crocodilians, meaning that the first archosaur was the last common ancestor of all birds and crocodilians. All birds have a fixed erect stance, and crocodilians have the ability to high walk with their limbs erect. If an erect stance is considered homologous in birds and crocodilians, phylogenetic bracketing implies that they inherited this trait from their common ancestor and that this ancestor also had an erect stance. With this reasoning, the first archosaurs are thought to have had the ability to high walk. 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).
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. 26 muscles, three ligaments, and two connective tissue structures were described. While the hypothesized muscles of Poposaurus share many aspects with those of birds, they are more similar to those of crocodilians. Poposaurus is thought to have had a puboischiotibialis muscle, but this muscle is absent in birds and probably non-avian dinosaurs as well. The extensor digitorum brevis was probably present on the foot of Poposaurus, but not in birds. The puboischiofemoralis externus muscle of Poposaurus is also similar to those of living crocodilians. Other aspects of the muscles of Poposaurus differ from those of crocodilians. For example, the puboischiofemoralis internus muscle originates on the spine in crocodilians and on the hip in Poposaurus. The hip origin for this muscle is considered to be the original condition for archosaurs, since it is also seen in birds and non-avian dinosaurs. Poposaurus is thought to have had adductor muscles that were even larger than dinosaurs, as their insertion site runs along the entire length of the femur.
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