Meeting Abstract

74.5  Tuesday, Jan. 6  The Evolution of Unguligrady and Forefoot Mechanics in Even-Toed Ungulates CLIFFORD, Andrew/B; Brown University andrew_clifford@brown.edu

Most extant even-toed ungulates adopt an unguligrade foot posture whereby body weight is supported through hoofed distal phalanges. This foot posture is derived from a petadactyl and digitigrade foot comparable to extant canids. The digitigrade-unguligrade transition is marked by variable digit loss, except for the emphasis of digits III and IV, and ligamentous replacement of interosseus muscles. In order to test the hypothesis that interosseus ligaments (IL) resist torque at the metacarpophalangeal (MCP) joint during stance, I compared data collected in vivo with data collected through in vitro preparations. Extant suids possess characteristics similar taxa that first adopt an unguligrade stance, so minipigs were run in a trackway connected to a forceplate and filmed using bi-planar cinefluoroscopy to determine the total joint torque-by-angle relationship at the MCP joint during stance. Reduced preparations of the IL permitted calculation of the torque-by-angle relationship of the IL alone. These two relationships yield the proportion of joint torque taken up by a novel ligamentous structure present only in unguligrade species. Results indicate that IL in forefeet perform a majority of work during stance at the MCP and that this proportion increases with MCP extension. IL increase step length by permitting a functionally longer foot that does not require more metabolic energy through muscle contraction to maintain an elevated stance. Unguligrady may have first evolved to permit longer leg length without incurring additional metabolic cost, since longer legs generally permit cheaper locomotion. The extremely modified and highly cursorial limbs of most extant even-toed ungulates may be an exaptation of a limb originally modified to reduce the cost of locomotion.