According to researchers, the ancient ancestors of contemporary horses possessed multiple hooved toes instead of a single hoof, which gradually disappeared over time.
Ancestors such as Eocene Hyracotherium exhibited foot structures resembling those of present-day tapirs, featuring four toes at the front and three at the back. Each toe was equipped with its own hoof and supported by an underlying foot pad.
In contrast, contemporary equids such as horses, asses, and zebras possess a solitary toe, which is a remnant of the original third toe on each foot. This lone toe is protected by a sturdy keratinous hoof, while the underside of the hoof features a triangular frog that functions as a shock absorber.
To unravel the mystery behind the lost digits, a team of international scientists hailing from the United Kingdom, the United States, and the Netherlands conducted a thorough analysis. They examined both hoof prints and foot bones from contemporary horses as well as fossil records.
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 team’s findings, which have been published today in the journal Royal Society Open Science, solidify the longstanding understanding that these toes have indeed been entirely lost during the course of evolution. This contradicts the suggestion made in a separate paper published in the same journal in 2018, which proposed that the toes might have been somehow retained within the hoof.
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.”
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