Meeting Abstract
Mammals are a morphologically and ecologically diverse clade. They inhabit environments around the world from oceans to treetops and display a range of locomotory behaviors that includes running, climbing, swimming, and digging. In contrast to living mammals, the ancestors of mammals, non-mammalian “pelycosaurs”, are traditionally reconstructed as simple terrestrial quadrupeds with limited ecological scope. The transition from “pelycosaurs” to mammals is characterized by a postural shift that greatly impacted locomotor behavior. The limbs of “pelycosaurs” were abducted to the side, and they moved with a sprawling gait, analogous to lizards and salamanders. The limbs of mammals, however, are positioned underneath the body and operate primarily in a parasagittal plane of motion. To understand how reorientation of the limbs during mammalian evolution impacted musculoskeletal function, we built virtual hindlimb models of two taxa, Dimetrodon milleri and Monodelphis domestica, to represent “pelycosaurs” and mammals, respectively. For each model, we determined hip joint range of movement in three orthogonal planes and estimated moment arms for all major muscles of the proximal hindlimb. Three functional differences were found in mammals compared to “pelycosaurs”: increased flexion-extension range of motion, reduction of abduction-adduction moment arms, and acquisition of joint-stabilizing flexion-extension muscles. These functional changes are underpinned by a relative decrease in the width of the pelvis, bringing muscle origins closer to the hip center of rotation. Our findings demonstrate a structural change associated with multiple functions and illustrate how musculoskeletal reorganization led to dramatic shifts in locomotory behavior during mammalian evolution.
