Horses ‘used to have toes’, new study shows
Human beings are familiar with horses that have a single hoof, but their distant ancestors actually had five toes.
Researchers say that the hooved toes vanished over time, thanks to evolution.
Ancient animals, such as the Eocene Hyracotherium, had feet like those of a modern tapir: four toes in front and three behind, each individually hooved with an underlying foot pad.
Modern equids such as horses, asses and zebras, have only a single toe, the left-over original third toe on each foot, encased in a thick-walled keratinous hoof, with an underlying triangular ‘frog’ on the sole that acts as a shock absorber.
Researchers came to the conclusion after examining modern hoofprints alongside fossils.
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Author Professor Christine Janis, from the University of Bristol's School of Earth Sciences, explained, "The upper portions – the remains of the additional hand and foot bones – remain as 'splint bones' fused with the remaining central one, but where are the fingers and toes?"
"In later fossil horses there were only three toes front and back. The extra toes, known as side toes, in these horses were smaller and shorter than in a tapir, and likely did not touch the ground under normal circumstances, but they may have provided support in exceptional situations, such as sliding or forceful impact."
The findings were published today in Royal Society Open Science.
They confirm the older notion that these toes really have been completely lost in evolution, not somehow retained within the hoof, as proposed in another recent paper published in the same journal in 2018.
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Lead author Professor Alan Vincelette, of St John's Seminary, Camarillo, California, said, "Although it does seem that remainders of the proximal (upper portions) of the side digits have been retained in modern horses, as the earlier 2018 paper claimed, the distal (lower portions, or toes) have simply been lost.
The 2018 paper proposed that in modern horses these side toes are retained within the hoof of the central toe, in part contributing to the frog—although there are no actual bones within the frog.
Professor Janis said, "While the notion that modern horses have retained all of their original toes as within-hoof remnants is a novel one, and so rather appealing, it can be shown to be incorrect."
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