Thursday, January 25, 2018

Hidden Anatomy: Researchers Make a Case that Modern Horses Have Five Toes--Even If We Can't See Them

One of the keys to the Solounias research is the leafy construction of the Eponychium hoof covering in the fetal horse. The researchers dissected fetal hooves and paid close attention to the construction of the Eponychium. From left: Medial view of fetal horse hoof;  Dorsal view of the fetal hoof, showing a smooth singular surface, representing the dominant digit III; Ventral view of a fetal horse specimen, showing four distinct infoldings that depict evidence of the Solounias paper's proposed digits I, II, IV and V.  (Detail from one of the many figures in the article.)

Scientists have long wondered how the horse evolved from an ancestor with five toes to the animal we know today. While it is largely believed that horses simply evolved with fewer digits, researchers at New York Institute of Technology College of Osteopathic Medicine (NYITCOM) pose a new theory suggesting that the remnants of all five toes are still present in the distal limb of the horse.

With this research, they propose the beginning of a rewrite of equine evolution and, by extension, anatomy. Along with it, they offer an explanation for the bars, frog and lamellae of the sole that will, if nothing else, challenge Hoof Blog readers to think about the way the foot is put together.

In a paper published in the January 24 edition of Royal Society Open Science, a team of researchers led by NYITCOM researcher, anatomy professor, and American Museum of Natural History paleontologist, Nikos Solounias, PhD, proposes that all five digits of the horse's limb have merged to form the compacted forelimb with a single hoof that we know today.

Detail from Figure 8 in the research paper shows  internal anatomy of a full-term Equus fetal hoof (NS 296). (a) Proximal-most section, where the frog is a large, double-sided structure. On either side of the section, there is a hoof cartilage surrounded by keratin distinct from the hoof proper. (b) Middle section through the hoof, in the region where the frog presents as two distinct ridges ventral to the distal phalanx. (c) Distal-most section, where the frog is merged centrally into a single structure. There is keratinous material surrounding the distal phalanx, comprising the hoof proper. Scale bar for (a–c) is 50 mm. (Detail from one of the many figures in the article.)

Frog dissection
The research includes both paleonotological interpretations as well as detailed dissections of the frog, bars and sole lamellae of fetal and adult horses. The researchers go into great detail on the orientation of sole lamellae as indicating the presence of "toes" that are no longer visible, but whose blood supply and nervous system survives. They refer to the bars as "keratinous under-folds" and contend that the leafy tulip-like outer folds of fetal hoof, sometimes call Eponychium, are divided into four sections.

The study would like the hoof to be appreciated for its display of "shades of pentadactyly" (five digits).

As widely taught, horses evolved to live on open grassland. Their anatomy required a more compact design to enable movement across the plains. Until now, scientists believed horses adapted to these conditions by gradually evolving with fewer digits than their five-toed ancestors, until the modern horse, which retains just the central digit.

Splint bones
Currently, scientists accept that the splints, small bones found along the outer sides of the metacarpal in modern horses, are partially-formed remnants of what were originally the 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 (with the surviving distal phalanx or coffin bone is regarded as the third digit). Arguing that the horse is not truly monodactyl, (one-toed), they contend that fragments of the “missing” first and fifth 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.

The five-toed hoofprint
Solounias first considered this theory in 1999 while studying fossil evidence from an eight-million-year-old horse known as Hipparion primigenium. The famous Laetoli footprints in Tanzania demonstrate that Hipparion walked alongside early humans, and was believed to have had three digits.

However, Solounias noticed that the bottom surface of Hipparion’s fossilized forelimb appeared to be divided in five sections, as though small toes had bonded together. He confirmed his finding in several of the impressions, and considered that Hipparion not only had five compacted toes, it also likely passed this trait on to its descendants.

“Interestingly, we not only find hints of the missing digits on the modern horse, but also its ancestors, such as Hipparion and Mesohippus, two species believed to have three toes,” said Solounias.

Neurovascular evidence in the fetal horse
The researchers have also discovered neurovascular evidence in support of the five-digit theory. Their dissections of modern equine fetus forelimbs reveal 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.”

Following the study, the researchers anticipate that geneticists will use cellular DNA samples to test the theory.

Material from New York Institute of Technology College of Osteopathic Medicine was helpful to this summary of the much longer paper. Illustrations used here are partial captures from extensive anatomical drawings and photographs included in the article. 

The full paper is published under an Open Access license with this citation:

Solounias N, Danowitz M, Stachtiaris E, Khurana A, Araim M, Sayegh M, Natale J. 2018 The evolution and anatomy of the horse manus with an emphasis on digit reduction. R. Soc. open sci. 5: 171782.

It may be downloaded at this link.

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