Mechanics of evolutionary digit reduction in fossil horses (Equidae)

Digit reduction is a major trend that characterizes horse evolution, but its causes and consequences have rarely been quantitatively tested. Using beam analysis on fossilized centre metapodials, we tested how locomotor bone stresses changed with digit reduction and increasing body size across the horse lineage. Internal bone geometry was captured from 13 fossil horse genera that covered the breadth of the equid phylogeny and the spectrum of digit reduction and body sizes, from Hyracotherium to Equus. To account for the load-bearing role of side digits, a novel, continuous measure of digit reduction was also established—toe reduction index (TRI). Our results show that without accounting for side digits, three-toed horses as late as Parahippus would have experienced physiologically untenable bone stresses. Conversely, when side digits are modelled as load-bearing, species at the base of the horse radiation through Equus probably maintained a similar safety factor to fracture stress. We co...

趾数减少是马类演化的核心特征趋势，但其成因与后果鲜有定量验证。通过对化石中心跖骨进行梁分析（beam analysis），我们测试了马类谱系中运动骨骼应力随趾数减少及体型增大的变化规律。我们采集了13个马类化石属的内部骨骼几何结构，这些属覆盖了马科系统发育的全范围，以及从始马属（Hyracotherium）到马属（Equus）的趾数减少与体型变化谱。为考量侧趾的承重作用，我们建立了一种新颖的连续趾数减少度量指标——趾减少指数（toe reduction index, TRI）。结果显示，若不考虑侧趾，即使晚至副马属（Parahippus）的三趾马也会承受生理上无法维系的骨骼应力；相反，当侧趾被建模为承重结构时，从马类辐射演化起点到马属的物种，其骨折应力安全系数或保持相似水平。我们共...