Horses Have Five Toes on Each Foot – #365papers – 2018 – 33

Solounias, Danowitz, Stachtiaris, Khurana, Araim, Sayegh, and Natale, 2018, The evolution and anatomy of the horse manus with an emphasis on digit reduction: Royal Society Open Science, v 5, 171782.

What’s it about?

Modern horses have a single obvious hoof on each lef, which is representative of the third digit – equivalent to our middle finger. The fossil record shows that horses evolved from three-toed ancestors, which themselves evolved from four-toed ancestors, which even further back, came from five-toed ancestors.

In modern horses, the remnants of digits 2 and 4 (our index and ring fingers) are evident as tiny splint bones fused to the cannon bone (third metacarpal or metatarsal) of the leg. The authors use study of limbs of adult, full-term fetal, and early fetal horses. They trace the positions of nerves and blood vessels in the legs of horses, as well as consider the articulation between the horse’s toe bones and carpal-metacarpal joints to show that modern horses retain parts of all five of the original toes that their deep ancestors possessed.

Why does it matter?

Aside from the coolness factor, this means that the transition from an animal with five toes to one with only one dominant toe is really not as complex as originally thought.

Why did I read this?

My deep interest in science, especially evolution, started with a handful of horse books when I was a child. I can’t help but want to learn everything that I possibly can about horses. Imagine my delight that a lot of my research focuses on fossil horses!

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