Ancient horses such as Hyracotherium leporinum, a tiny horse relative from the Eocene of England, had feet like those of a modern tapir: four toes in front and three behind, each individually hooved with an underlying foot pad. In contrast, 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. In new research, paleontologists analyzed hoof prints and foot bones from modern horses and fossil records to discover what happened to the lost digits.
“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?” said University of Bristol’s Professor Christine Janis.
“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.”
In their research, Professor Janis and her colleagues confirmed the older notion that these toes really have been completely lost in evolution, not somehow retained within the hoof, as proposed in a 2018 study.
“Although it does seem that remainders of the proximal (upper portions) of the side digits have been retained in modern horses, as the 2018 paper claimed, the distal (lower portions, or toes) have simply been lost,” said St. John’s Seminary Professor Alan Vincelette.
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.
This was partially based on an interpretation of the hoof prints of an extinct three-toed horse,
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