Three-Dimensional Data on Limb Extremities of Ungulates with Pads Provide Insights into Morphological Changes Associated with the Evolution of Unguligrady

Camelidae and Hippopotamidae (Artiodactyla) and Rhinocerotidae and Tapiridae (Perissodactyla) are unguligrades with pads, which support their body weight with their hooves and volar pads on their feet. In order to investigate the evolution of unguligrady from a functional morphological point of view, it is necessary to examine the limbs of living unguligrades with pads. In this study, we focused on the extremities of limbs of camelids and rhinoceroses and analyzed the differences between them and those of bovids, which are unguligrade without pads, using CT to simulate three-dimensional movements. The results showed that metapodials and proximal phalanges were interlocked by pulleys in unguligrades, whereas a large gap was observed between metapodials and proximal phalanges in unguligrades with pads. However, the angle changes of proximal phalanges of unguligrades with pads were smaller than those observed in unguligrades. This suggests that the pads inhibit wide spreading of the digits in these taxa. Whereas unguligrades, such as bovids, have adapted to various terrains by modifying the mobility of their digits, our results suggest that volar pads have limited the ability of unguligrades with pads to adapt to a variety of terrains.