Abstract
Triassic archosaurs and stem-archosaurs show a remarkable disparity in their ankle and pelvis morphologies. However, the implications of these different morphologies for specific functions are still poorly understood. Here, we present the first quantitative analysis into the locomotor abilities of a stem-archosaur applying 3D modelling techniques. μCT scans of multiple specimens of Euparkeria capensis enabled the reconstruction and three-dimensional articulation of the hindlimb. The joint mobility of the hindlimb was quantified in 3D to address previous qualitative hypotheses regarding the stance of Euparkeria. Our range of motion analysis implies the potential for an erect posture, consistent with the hip morphology, allowing the femur to be fully adducted to position the feet beneath the body. A fully sprawling pose appears unlikely but a wide range of hip abduction remained feasible—the hip appears quite mobile. The oblique mesotarsal ankle joint in Euparkeria implies, however, a more abducted hindlimb. This is consistent with a mosaic of ancestral and derived osteological characters in the hindlimb, and might suggest a moderately adducted posture for Euparkeria. Our results support a single origin of a pillar-erect hip morphology, ancestral to Eucrocopoda that preceded later development of a hinge-like ankle joint and a more erect hindlimb posture.
Highlights
Triassic archosaurs and stem-archosaurs show a remarkable disparity in their ankle and pelvis morphologies
SAM PK 5867 is still partially covered in matrix and the femur is overlaying most of the right ilium, obscuring most of the pelvic bones
We showed that in Euparkeria the osteology permitted the adduction of the femur into a fully vertical position (FE = 90°, ABAD = 0°), and it was even feasible to further adduct the femur medially and overstep the other limb (ABAD = − 30°; Supplementary Video S1)
Summary
Triassic archosaurs and stem-archosaurs show a remarkable disparity in their ankle and pelvis morphologies. The oblique mesotarsal ankle joint in Euparkeria implies, a more abducted hindlimb This is consistent with a mosaic of ancestral and derived osteological characters in the hindlimb, and might suggest a moderately adducted posture for Euparkeria. Archosaurs were the predominant group of large terrestrial and aerial vertebrates in the Mesozoic era and included pterosaurs, the familiar dinosaurs (including birds), crocodylomorphs and an intriguing variety of Triassic forms Their well-documented fossil record allows retracing the evolution of the dramatic differences in morphology and locomotion in this group. Quantitative biomechanical approaches have overwhelmingly focused on non-avian dinosaurs (e.g.15–27); to comprehend the evolution of the different morphologies and the associated locomotor specialisations within archosaurs, it is essential to assess what the ancestral condition for Archosauria was. None of these hypotheses have been quantitatively assessed; and essentially all are based on almost two-dimensional visual interpretations of specimens still encased in matrix, rather than 3D manipulation of prepared or digital specimens
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