Data from: 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 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.

趾数退化是马类演化的标志性主要趋势，但其成因与演化后果却鲜有定量验证。本研究通过对石化中央掌跖骨开展梁分析，探究了马类演化支系中，运动骨应力如何随趾数减少与体型增大而变化。研究采集了13个化石马属类群的骨内部几何结构数据，这些类群涵盖了马科（Equidae）系统发育的全谱系，以及从始马（Hyracotherium）到马属（Equus）的全部趾数退化与体型变化区间。为了考量侧趾的承重作用，本研究还建立了一种全新的趾数退化连续量化指标——趾退化指数（Toe Reduction Index, TRI）。研究结果显示，若不考虑侧趾的承重作用，即便晚至副马（Parahippus）时期的三趾马，其骨应力也会达到生理上无法承受的水平。反之，若将侧趾建模为承重结构，从马类辐射演化基部类群直至马属（Equus）的所有物种，其骨断裂安全系数大概率保持一致。本研究结论表明，中央掌跖骨通过内部结构出现显著的正异速生长，提升了抗弯曲性能，从而代偿了演化过程中的趾数减少与体重增加。本研究结果支持两项经典演化假说：其一，体重增加的选择压力促使动物演化出单根粗壮的掌跖骨，而非多根细小的掌跖骨；其二，随着马类四肢逐渐伸长，侧趾带来的惯性成本超过了其在稳定身体或承重方面的效用。