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Plantar (below) image of the legs of a four-toed tapir (left) and a one-toed horse (right) (by Nuria Melisa Morales-García. In the center, a reproduction 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 had hooves instead of a single hoof, which disappeared over time, scientists say.

The animals, like the Eocene Hyracotherium, had feet like those of a modern tapir: four toes at the front and three at the back, each with hooves with underlying pads.

In contrast, modern equids such as horses, donkeys and zebras have only one toe, the original third toe on each foot, encased in a thick-walled keratin layer, with an underlying triangular frog on the sole that acts as a shock absorber.

An international team of scientists, from the UK, the US and the Netherlands, analyzed hoof prints and foot bones from modern horses and the fossil record to find out what happened to the missing digits.

Author Professor Christine Janis from the University of Bristol’s School of Earth Sciences explained: “The upper parts – the remains of additional hand and foot bones – remain as ‘braced bones’ joined to the rest in the middle, but where are the fingers and toes?”

“Later fossil horses had only three toes on the front and back. The extra toes, known as lateral toes, in these horses were smaller and shorter than in tapirs, and probably did not touch the ground under normal circumstances, but they may have provided support under special conditions, such as when slipping or a powerful blow.”

In the results, which were published today in Royal Society Open Sciencethey confirm the older idea that these toes were actually completely lost in evolution, not somehow retained within the hooves, as suggested by another recent paper published in the same journal in 2018.

Lead author Professor Alan Vincelette, from St. John’s Seminary, Camarillo, California, said: “Although it appears that remnants of the proximal (upper part) lateral canines have been retained in modern horses, as the previous 2018 paper claimed, the distal parts (lower parts or toes) have simply been lost.”

The 2018 paper suggested that in modern horses, these lateral tears remain within the hoof of the middle toe, contributing in part to the frog—even though there are no bones in the frog.

This was based in part on the interpretation of the hoof prints of an extinct three-toed horse, Hipparion (not in a direct line to modern horses) from Laetoli, Tanzania 3.7 million years ago, the same site that yielded the famous footprints of the hominid Australopithecus. These hoof prints clearly lacked a frog, adding weight to the idea that the lateral tears of horses like Hipparion now contribute to the frog of modern horses.

Although not all hoofprints of modern horses with a frog record its presence, an unmistakable frog can be seen in many hoofprints known to have been made by three-toed horses. These observations cast doubt on the idea that modern horse-hoofed frogs evolved from the lateral toes of tridactyl equids.

Author Professor Christine Janis of the University of Bristol’s School of Earth Sciences said: “While the idea that modern horses have retained all of their original toes as remnants within the hoof is novel and rather attractive, it can be shown to be wrong.”

Alan Vincelette added: “The frog in the horse’s hoof evolved independently of the lateral hooves as a unique structure that provides shock absorption and traction during movement.”

The team also shows that the feet of single-toed horses have a different shape than the big toes of three-toed horses, being round rather than oval, a difference that may be related to different weight distribution and/or ecological habitat.

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

Diary information:
Royal Society Open Science

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