Scientists Confirm Modern Horses Have Lost Their Extra Toes

Plantar (underneath) view of feet of a four-toed tapir (left) and a one-toed horse (right) (by Nuria Melisa Morales-García. In the middle, a reconstruction of the extinct three-toed horse Hipparion, by Karolina Suchan-Okulska. Overall design by Morales-García. Credit: University of Bristol

The distant ancestors of modern horses were equipped with hooved toes rather than a single hoof, a feature that faded away over time, according to recent research.

The ancient equids, such as the Eocene Hyracotherium, had feet similar to those of modern tapirs: four toes in front and three behind, each with an individual hoof and an underlying foot pad.

In contrast, modern equids like horses, asses, and zebras possess only a single toe, which is actually the vestigial third toe on each foot. This toe is covered by a thick-walled keratinous hoof and has a triangular frog on the sole, serving as a shock absorber.

A team of scientists from the UK, US, and the Netherlands conducted an analysis of hoof prints and foot bones from modern horses as well as fossil records to uncover the fate of these lost digits.

Professor Christine Janis from the University of Bristol’s School of Earth Sciences explained, “The upper portions of the additional hand and foot bones remain fused with the remaining central one, but where are the fingers and toes?”

She further added, “In later fossil horses, there were only three toes in the front and back. These side toes were smaller and shorter than those of a tapir, and they likely did not touch the ground under normal circumstances. However, they may have provided support in exceptional situations, such as sliding or forceful impact.”

In a study published today in the journal Royal Society Open Science, the researchers confirm the previous hypothesis that these toes have completely disappeared through evolution, rather than being retained within the hoof as suggested in a 2018 paper published in the same journal.

Lead author Professor Alan Vincelette, from St. John’s Seminary, Camarillo, California, commented, “Although it does seem that remnants of the side digits have been retained within the hoof of modern horses, as previously claimed, the distal lower portions or toes have simply been lost.”

The 2018 paper proposed that the side toes of modern horses are still present within the hoof, contributing to the frog, despite the absence of actual bones. This proposition was based on the interpretation of hoof prints of an extinct three-toed horse called Hipparion. However, the new observations cast doubt on this notion.

According to Professor Christine Janis, “While the idea that modern horses have retained all their original toes as within-hoof remnants is intriguing, it can be proven incorrect.”

Alan Vincelette added, “The frog of the horse’s hoof evolved independently of the side toes as a unique structure providing shock absorption and traction during locomotion.”

The team’s research also revealed that the shape of the feet of one-toed horses differs from that of the main toe of three-toed horses. One-toed horses have round-shaped feet, while three-toed horses have oval-shaped feet. This difference could be related to variations in weight distribution and ecological habitat.

More information:
Alan Vincelette et al, ‘Hipparion tracks and horses’ toes: the evolution of the equid single hoof’, Royal Society Open Science (2023). DOI: 10.1098/rsos.230358

Provided by University of Bristol


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Modern horses have lost their additional toes, scientists confirm (2023, June 20)
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