Postcranial Functional Morphology of Hyracotherium (Equidae, Perissodactyla) and Locomotion in the Earliest Horses

A nearly complete skeleton of Hyracotherium grangeri is described from the early Wasatchian (early Eocene) of the Clarks Fork Basin in northwestern Wyoming. This specimen includes a virtually complete, well-preserved pre-caudal vertebral column allowing the first thorough investigation of the locomotory abilities of these early Eocene horses. The posterior thorax (T) and lumbus (L) are divided into a facultatively dorsostable region (T15-L3) and an obligately dorsostable region (L3-L7). The facultatively dorsostable region is characterized by robust, cranially-oriented neural spines and well-developed attachment sites for epaxial musculature, whereas the obligately dorsostable region is characterized by embracing zygapophyses and less developed spines and attachment sites. Limb morphology exhibits a mixture of primitive characters and derived features associated with cursoriality. As described by previous authors, limb mobility is restricted at the elbow and ankle due to interlocking humero-radial and tibio-astragalar articulations, respectively. In contrast, the presence of a rounded femoral head and well-developed pelvic and femoral attachment sites for hindlimb adductor muscles suggests a highly mobile hip joint. Limited inversion of the manus and pes may have been possible via differential movement between the proximal and distal carpal elements and at the transverse tarsal joint, respectively. The obligately dorsostable region of the vertebral column likely resisted variably-oriented torque applied by hip musculature, thereby preventing dislocation of the pelvis and lumbus. The facultatively dorsostable region would have facilitated dynamic positioning of the center of mass during rapid acceleration. Brief comparisons with Orohippus and Mesohippus postcrania are used to discuss the selective pressures imposed by changes in body mass and habitat on the postcranial evolution of Paleogene horses.