Morphological indicators of sprawling and non-sprawling limb posture in tetrapods

BECK, A.L.*; BLOB, R.W.; HOPSON, J.A.: Morphological indicators of sprawling and non-sprawling limb posture in tetrapods.

Determination of locomotor postures in fossil tetrapods is difficult because their behavior cannot be observed directly. To evaluate morphological indicators of limb posture that could be used to interpret stance in extinct tetrapods, we have used multivariate statistical analyses to explore osteological correlates of locomotor posture in extant species. Our sample of extant taxa includes generalized mammals, lizards, and crocodilians spanning a large size range. Morphological measurements and observations relevant to locomotor function were taken from the forelimb, hindlimb and girdle skeletons of museum specimens. Principal components analyses as well as bivariate comparisons of limb and girdle elements indicate strong differences between sprawling and non-sprawling taxa. For instance, highly sprawling taxa are characterized by limbs with high angles of femoral torsion, short metatarsals, and reduced articular surfaces on the distal humerus. In contrast, non-sprawling taxa show significantly smaller surface areas than sprawling taxa for the hip, knee, and ankle joints. These analyses suggest several quantifiable morphological features that could aid the evaluation of limb postures in fossil taxa, thereby facilitating the evaluation of changes in limb posture over the evolution of mammalian locomotion.

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