Osteology of the late Triassic bipedal archosaur Poposaurus gracilis (Archosauria: Pseudosuchia) from Western North America

Poposaurus gracilis is a bipedal pseudosuchian archosaur that has
been poorly understood since the discovery of the holotype fragmentary
partial postcranial skeleton in
1915. Poposaurus. gracilis is a member of
Poposauroidea, an unusually morphologically divergent clade of
pseudosuchians containing taxa that are bipedal, quadrupedal, toothed,
edentulous, and some individuals with elongated thoracic neural spines
(i.e., sails). In 2003, a well preserved, fully articulated, and nearly
complete postcranial skeleton
of P. gracilis was discovered with some
fragmentary cranial elements from the Upper Triassic Chinle Formation of
Grand Staircase‐Escalante National Monument of southern Utah, USA. The aim
of this work is to describe the osteology of this specimen in detail and
compare P. gracilis to other closely related
pseudosuchian archosaurs. The open neurocentral sutures throughout the
majority of the vertebral column, the small size of this individual, and
the presence of seven evenly spaced cyclic growth marks in the
histologically sectioned femur indicate that this specimen was a
skeletally immature juvenile, or subadult when it died. The pes
of P. gracilis contains multiple skeletal
adaptations and osteological correlates for soft tissue structures that
support a hypothesis of digitigrady for this taxon. When coupled with the
numerous postcranial characters associated with cursoriality, and the many
anatomical traits convergent with theropod dinosaurs, this animal likely
occupied a similar ecological niche with contemporaneous theropods during
the Late Triassic Period.