From ScienceDaily:
While it is largely believed that horses simply evolved with fewer digits, researchers pose a new theory that suggests remnants of all five toes are still present within the hooves of the horse.
For the first time, as published in the January 24 issue of Royal Society Open Science, NYITCOM researcher, Nikos Solounias, Ph.D., paleontologist and anatomy professor, and a team of researchers propose that the reduction in the number of digits is not a matter of simple attrition; instead, they believe that all five digits have merged to form the compacted forelimbs with hooves that we know today.
Currently, scientists accept that splints, small bones found along the outer sides of the metacarpal in modern horses, are partially formed remnants of second and fourth digits. Tapering mid-way down the metacarpal, these fragments were inherited from an earlier ancestor, but ceased to develop into fully formed digits in modern horses. While the NYITCOM researchers note that this explanation of the second and fourth digits is viable, they argue that it is incomplete and fails to account for the animal’s first and fifth digits. Arguing that the horse is not truly monodactyl, that is, one-toed, these researchers contend that fragments of the “missing” digits can be found in the form of ridges on the backside of the splints. According to the researchers, this demonstrates that the first and fifth digits were not simply lost to evolution, but attached to their neighboring second and fourth digits.
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The researchers have also discovered neurovascular evidence in support of the five-digit theory, with dissections of modern equine fetus forelimbs revealing a greater number of arteries and nerves than would be expected in a single digit.
“If today’s horse does indeed have one digit per forelimb, we would expect each forelimb to have a total of two veins, two arteries, and two nerve bundles,” said Danowitz. “However, our dissections found between five and seven neurovascular bundles per forelimb, suggesting that additional toes begin to develop, but do not become fully differentiated.”Paper. (public access) – Nikos Solounias, Melinda Danowitz, Elizabeth Stachtiaris, Abhilasha Khurana, Marwan Araim, Marc Sayegh, Jessica Natale. The evolution and anatomy of the horse manus with an emphasis on digit reduction. Royal Society Open Science, 2018; 5 (1): 171782 DOI: 10.1098/rsos.171782 More.
In short, the signature pentadactyl limb of the vertebrate is hard to just suppress.
See also: Can horses help solve a big evolution mystery?
Horse facial expressions similar to human ones?
Convergent evolution? Horses, humans see world the same way
and
Stasis: Life goes on but evolution does not happen