[N] 2006 Nearly complete skeleton of Poposaurus (archosaur)from the Chinle Fm. Utah
Joyce, W. & Gauthier, J. (2006) A nearly complete skeleton of Poposaurus (archosaur) from the Late Triassic of Utah JVP 26(3) Abstracts pp.83
The Yale Peabody Museum has been conducting fieldwork in the Chinle Formation of southern Utah since 2002. In the course of four field seasons, a significant amount of vertebrate fossil material was recovered, that consists primarily of fragmentary basal archosaur and temnospondyl remains of limited diagnostic value. This situation contrasts with the find of a skeleton in 2003 along the western rim of Circle Cliffs in the newly established Grand Staircase-Escalante National Monument in Garfield County. That find appears to represent one of the most complete bipedal stem-crocodilian found to date worldwide.
The specimen likely once consisted of a single, fully preserved individual that eroded headfirst down to the base of the neck. The rest of the specimen is largely preserved in full articulation with little post-mortem distortion. Comparison with the holotype of Poposaurus gracilis, which consists of a partial pelvis and femur from the Upper Triassic Popo Agie Formation of Wyoming, reveals only minor differences in size and degree of ossification at muscle attachment sites and the specimens is thus interpreted as a less mature individual of that species.
The postcranium reveals clear evidence that corroborates the hypothesis that Poposaurus gracilis possessed a bipedal stance and gait. Anatomical traits that favor this interpretation include a forelimb to hindlimb ratio of 1:2.2, a hind limb to presacral column ratio of approximately 1:1, the presence of a long tail (approximately 65 percent of the total vertebral column length) that placed the center of gravity at the hip joint, restriction of lateral movement in the thoracic vertebrae, a narrow pelvic girdle, a modest lesser trochanter and prominently overhanging supracetebular buttress, and the presence of a prominent anterior iliac spine. Although the pes exhibits four well-developed digits, Poposaurus gracilis walked digitigrade on the median three, and may have left only tridactyl imprints. Overall, the postcranial skeleton of Poposaurus gracilis resembles that of the contemporaneous Coelophysis bauri in gestalt, although the body is estimated to somewhat larger.