THE VERTEBRATE FAUNA OF THE UPPER PERMIAN OF NIGER-IX. THE APPENDICULAR SKELETON OF BUNOSTEGOS AKOKANENSIS (PARAREPTILIA: PAREIASAURIA)

Pareiasaurs were a group of herbivorous reptiles that lived during the middle to late Permian (265-252 Ma) in what is modern-day Europe, Asia, South America, and Africa. Field work in the Moradi Formation of northern Niger has produced multiple elements of the appendicular skeleton of the pareiasaur Bunostegos akokanensis. The considerable size disparity and morphological variation among the elements suggest that they represent ontogenetic stages ranging from relatively juvenile to adult. Here we present the first description of the scapulocoracoid, humerus, radius, ulna, pelvis, and femur of Bunostegos as well as some of the first ontogenetic data for postcranial osteology in pareiasaurs. As with the skull, numerous postcranial autapomorphies characterize Bunostegos, including laterally originating acromion process of the scapula; radius and ulna with continuous articular surface on humerus; paired crests on the olecranon process; ulna longer than humerus; pinched posterior margin of the acetabular rim; robust pelvic symphysis extending the length of the puboischiatic plate; lack of a distinct postaxial flange of the femur; and an elaborated femoral lateral condyle wrapping over the medial condyle. We incorporated data from the appendicular skeleton of Bunostegos into a revised phylogenetic analysis of pareiasaur relationships. The results of this analysis corroborate previous cranial analyses that place Bunostegos between Guadalupian taxa and the Lopingian velosaur subclade. Interestingly, several aspects of its postcranial anatomy suggest that Bunostegos possessed relatively upright forelimb posture, which would be unique among pareiasaurs and possibly Permian amniotes as a whole.