McHorse notes that the early ancestors of horses had four toes on each front limb and three toes on each back limb. At this time in horse history, roughly 55 million years ago, such animals like those in the genus Hyracotherium were about the size of a small dog and lived in forests that covered much of North America.
It has long been known that changing climatic conditions allowed grasslands to expand. Selective pressures resulting from the new open terrains then drove increases in the body mass of horse ancestors and caused them to lose all but one toe per limb. Many questions have remained, however, such as what the underlying mechanical consequences were of standing on just a single toe.
To help answer these questions, McHorse, Pierce, and Biewener performed micro-CT scans of 12 fossil species in the horse family tree. The scientists then used an engineered “beam bending” analysis to calculate how much stress each species’ lower leg bones were experiencing during regular movement and high-speed running. The stress data were then compared to the fracture stress of bone.
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The researchers now believe that, as horses evolved, they soon lost their fourth toes on their front limbs, leaving them with three toes on each limb. Each was “not quite a hoof, but not quite a claw either,” McHorse said, adding that the toes were “more like what a living hyrax has — sort of thick, modified nails.”
Hyracotherium probably had some sort of pad under each foot, and would have had a less upright foot posture than living horses, which essentially stand on tip-toe all of the time.
“As body mass increased, and side toes shrunk, the middle digit compensated by changing its internal geometry, allowing ever-bigger horse species to eventually stand and move on one toe,” Pierce explained.
“The bone within the load-bearing digit of later horses was distributed farther away from the center of its cross-section, allowing it to better resist bending,” she continued. “The total amount of bone also increased, allowing it to better resist compression as well as bending, which are of critical importance for animals with large body sizes.”