Mechanics of evolutionary digit reduction in fossil horses (Equidae)
Mechanics of evolutionary digit reduction in fossil horses (Equidae)
复制标题
DOI：
10.1098/rspb.2017.1174
复制
发表时间：
2017-08-30
影响因子：
4.7
通讯作者：
Pierce, Stephanie E.
中科院分区：
文献类型：
--
作者：
McHorse, Brianna K.;Biewener, Andrew A.;Pierce, Stephanie E.
数字减少是马进化的一个主要趋势，但其原因和后果很少得到定量检验。通过对中心后足化石进行束流分析，我们测试了整个马系中运动骨应力如何随着趾的减少和体型的增加而变化。内部骨骼几何形状是从 13 个马属化石中捕获的，这些化石涵盖了从始祖马到马属的马科动物系统发育以及指趾减少和体型范围的范围。为了解释侧面手指的承重作用，还建立了一种新颖的、连续的手指减少测量方法——脚趾减少指数（TRI）。我们的结果表明，如果不考虑侧趾，晚至副马的三趾马将经历生理上难以维持的骨骼应力。相反，当侧趾被建模为承重时，马基部通过马属辐射的物种可能保持与断裂应力相似的安全系数。我们得出的结论是，中心后足通过内部几何形状中的大量正异速生长变得更能抵抗弯曲，从而补偿了进化性手指的减少和体重的增加。这些结果支持了两个历史假设：体重的增加选择了单个、健壮的后足而不是几个较小的后足；而且，随着马的四肢变长，侧脚趾的惯性成本超过了其稳定或承重的效用。
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 conclude that the centre metapodial compensated for evolutionary digit reduction and body mass increases by becoming more resistant to bending through substantial positive allometry in internal geometry. These results lend support to two historical hypotheses: that increasing body mass selected for a single, robust metapodial rather than several smaller ones; and that, as horse limbs became elongated, the cost of inertia from the side toes outweighed their utility for stabilization or load-bearing.