Estimation of locomotory forces and stresses in the limb bones of Recent and extinct equids

Abstract

The locomotory stresses acting at the midshaft of the third metacarpal of Equus are estimated from a geometrical model of the manus during peak loading, which is derived from in vivo data. The stress results compare favorably with those from in vivo strain gage experiments. The model is then modified to account for the anatomical and inferred functional differences in the feet of the two extinct, tridactyl equids Merychippus and Mesohippus, incorporating assumptions from earlier functional studies. Estimates of the stresses in the fossil metacarpals are derived, and fall within the range of values obtained in vivo for limb bones of a wide size range of mammals. The results indicate that the third metacarpal of Merychippus was the most highly stressed and that of Mesohippus the least, with stresses for Equus falling between. Thus the anatomical and functional changes in the manus between Mesohippus and Equus are not simply a result of the difference in body size.