Scientists Confirm Modern Horses Have Lost Their Additional Toes

The ancestors of modern horses possessed hooved toes rather than a single hoof, which gradually disappeared over time, according to a team of researchers. These ancient animals, like the Eocene Hyracotherium, had feet similar to those of modern tapirs, with four toes in front and three behind, each individually hooved and having an underlying foot pad.

In contrast, modern equids, such as horses, asses, and zebras, only have a single toe – the last remaining third toe on each foot. This toe is covered by a thick-walled keratinous hoof and includes an underlying triangular frog on the sole that serves as a shock absorber.

The scientists conducted a comprehensive analysis involving hoof prints, foot bones from modern horses, and fossil records to uncover the fate of the lost toes. Professor Christine Janis from the University of Bristol’s School of Earth Sciences explained that while the additional hand and foot bones remain as “splint bones” fused with the central one, the fingers and toes themselves have vanished.

Later fossil horses only displayed three toes both in front and back, with smaller and shorter extra toes known as side toes. These side toes were most likely not in contact with the ground under normal circumstances but may have provided support in exceptional situations, such as sliding or forceful impact.

In their findings published in Royal Society Open Science, the team confirms that the toes have been completely lost in evolution and are not retained within the hoof, as proposed in a 2018 paper published in the same journal. Lead author Professor Alan Vincelette stated that while remnants of the proximal portions of the side digits are present in modern horses, the distal portions have simply been lost.

The 2018 paper suggested that these side toes are retained within the hoof of the central toe, contributing to the frog, despite the absence of actual bones within the frog. However, the team’s observations of hoof prints from three-toed horses cast doubt on this theory. They found that while not all hoof prints with frogs indicate the presence of side toes, many prints known to be made by three-toed horses clearly show the presence of a frog. This challenges the notion that the frog in modern horse hooves evolved from the side toes of tridactyl equids.

Professor Christine Janis emphasizes that the idea of modern horses preserving all their original toes as within-hoof remnants may be appealing but is proven to be incorrect. Alan Vincelette adds that the frog of the horse’s hoof evolved independently of the side toes and serves as a unique structure for shock absorption and traction during locomotion.

The team’s research also demonstrates that the feet of one-toed horses have a different shape from the main toe of three-toed horses, being round instead of oval. This difference in shape may be related to variations in weight distribution and ecological habitat.

 

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