Through a thorough examination of horse history, researchers have determined that the ancient ancestors of horses used to traverse the terrain with only a few hooved toes, gradually evolving into the single solid hoof that is observed in modern horses today.
This comprehensive analysis, titled "Hipparion Tracks and Horses' Toes: The Evolution of the Equid Single Hoof" published in Royal Society Open Science, dismisses a recent hypothesis proposing that the extra digits were preserved within the hoof and instead supports the earlier belief that evolution entirely eradicated them.
Horse Ancestors Had Hooved Toes
According to Science Alert, an international team of researchers from the United Kingdom, United States, and the Netherlands conducted a study using perissodactyl fossil records, modern horse hoof prints, and foot bone details to investigate the disappearance of extra toes in horses' evolutionary history.
The evolutionary progression of modern horses' foot changes has been a subject of research. The study revealed that the early ancestor of horses, Eohippus angustidens, had hooved feet with a pad similar to tapirs, featuring four toes on the front feet and three on the back.
In the present day, perissodactyls such as horses, asses, and zebras have evolved to possess a single toe surrounded by a durable and flexible keratinous hoof. Underneath the hoof, there is a V-shaped shock absorber called the frog.
The study authors explained in their paper that this anatomical change is seen as an adaptation for life in open habitats, accompanied by the development of high-crowned cheek teeth for grazing or incorporating grass into the diet, as well as an increase in body size.
Other perissodactyls like tapirs have managed to retain all of their toes, while rhinos have lost their fourth toe on the front feet, leaving them with three toes each. Vertebrate paleontologist Christine Janis from the University of Bristol in the UK said in a news release that in later fossil horses, only three toes were present on both the front and back feet.
These additional toes, known as side toes, were smaller than those of tapirs and likely did not touch the ground under normal circumstances, but they might have provided support in specific situations involving sliding or forceful impact, as suggested by vertebrate paleontologist Christine Janis from the University of Bristol.
How Did Modern Horses End Up With Only One Hoof?
The recent findings from the research team support the idea that horses' extra toes have indeed been lost over time and are not retained within the hoof. This confirms an earlier hypothesis proposed in a 2018 paper published in the Royal Society Open Science.
The theory in the previous paper suggested that the side toes in modern horses contribute to the foot's frog, which is partially based on an interpretation of hoof prints from an extinct three-toed horse called Hipparion.
However, Popular Science reported that the new study's co-author, biologist Alan Vincelette, states that while remnants of the upper portions of the side digits may be present in modern horses, the lower portions or toes have been lost entirely.
Hipparion's hoof prints, which lacked a frog, were considered evidence supporting the previous theory. In contrast, modern horse hoof prints with frogs do not consistently show the presence of the frog, whereas many prints made by three-toed horses display it.
The research team suggests that the development of the frog in the horse's hoof is independent of the side toes and likely evolved separately. The frog plays a crucial role in providing traction and shock absorption during the horse's movement.
Furthermore, the shape of the main toe in one-toed horses is round, while it is oval-shaped in three-toed horses. This difference in shape may be associated with ecological habitat or weight distribution factors.
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