Meeting Abstract
Though extant horses consist of a single genus that is large, cursorial, and grazing, their fossil record shows considerable diversity and repeated evolution in these traits. Digit reduction is a key feature of surviving equids and has been hypothesized to be a result of increased body mass, emphasis on straight-line locomotion, and locomotor economy—linked to the spread of grasslands. To investigate potential mechanical drivers of digit reduction, we explored evolutionary changes in the beam mechanics of horse metacarpals. To assess internal geometry along the length of each bone, we used micro-CT scans of fossil horse and extant tapir metacarpal III. Taxa included Hyracotherium (tetradactyl), Parahippus (tridactyl), Equus (monodactyl), and extant tapir (tetradactyl). Stress in compression and AP and ML bending were analyzed assuming body-weight load, adjusted for the estimated percentage supported by metacarpal III. For most loading conditions, Equus experienced lower stresses than tetra- or tridactyl species. When assuming that the central metacarpal primarily carries the load, Equus experienced as little as 50% of the stress of others. Stress was more similar when loads were distributed more evenly among all digits in multi-toed species, but Equus metacarpals still often outperformed earlier horses. The relative gracility of tridactyl horses (e.g., Parahippus) or increased athletic performance in Equus may explain these results. Future work will incorporate anatomical and gait data from extant tapir and horse, providing more accurate values for moment arms, forces, and pressure distribution among toes. Data from additional fossil species, including those with large body size that retained side digits, will also shed more light on the biomechanical implications of evolutionary digit reduction in horses.
