Allometry of the Thoracolumbar Region in Running Mammals

Mammals use flexion and extension of the back to increase their stride length and assist with breathing during running. The degree to which vertebral column bending increases stride length varies between dorsomobile (e.g., cheetah) and dorsostable (e.g., horse) taxa. It has been suggested that stability of the thoracolumbar region may correlate with body size because dorsomobile gaits are energetically expensive at large size. This dissertation investigates allometry of the thoracolumbar region and asks: How is vertebral structure influenced by increasing body size, and does this vary among families with different running styles? It presents new data on the influence of size and locomotion on the axial skeleton, an understudied anatomical region. To address these questions, three families of running mammals with a large size range were sampled: Felidae (cats, dorsomobile), Bovidae and Equidae (bovids and horses, dorsostable). Vertebral material was examined from 57 species (n=216) of felids and bovids, and five extant species and eight fossil genera (n=77) of equids. Vertebral data (linear measures, 2D and 3D landmarks) were compared to body size estimated from limb dimensions. Scaling of the ventral column (centra and discs) is consistent with its perceived role in body support as a dorsal compressive element, becoming craniocaudally shorter and dorsoventrally deeper with increasing body size. Morphological features of the lumbar region associated with stability are also correlated with size, but size explains a greater proportion of shape variation in bovids than felids. This suggests passive stabilization of the lumbar region in dorsostable groups may be a size-dependent response to cursoriality. Preliminary data suggest that the shape of the intervertebral joint complex reflects its range of motion, highlighting the utility of joint shape for understanding axial function. Specifically, craniocaudal patterns of lumbar morphology reflect differences in craniocaudal mobility between dorsomobile and dorsostable runners, which isolate mobility to the lumbosacral joint. Based on joint shape, small-bodied fossil equids (e.g., Hyracotherium) likely had more flexibility of the anterior lumbar region than modern horses, and therefore may have used more diverse gaits. However, specialization of the lumbosacral joint evolved early in equids, predating unguligrady and extreme digit loss