Fossil Friday – horse toe

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Some of the most common animals in the Pleistocene deposits around Diamond Valley Lake are horses. While we don’t have any complete individual horse skeleton in the WSC collections, across all our different specimens we probably have just about every bone in the skeleton represented. Shown here is an isolated horse hoof.

Horses have highly specialized feet, with only a single toe on each foot. That toe is Digit III, equivalent to our middle fingers (on the front feet) and our middle toes (on the back feet). Modern horses actually also have non-functional remnants of the 2nd and 4th toes, a relict of their multi-toed ancestors.

Fingers and toes are made up of a series of bones called phalanges (singular: phalanx) that articulate end-to-end with each other; the articulations are the joints in our fingers and toes. In horses, the last phalanx, or ungal, is enlarged into an arc-shaped wedge of bone to support the weight of the animal. In life, the ungal is covered with a keratinized sheath, the hoof itself, which doesn’t usually preserve in fossils.

The image at the top shows a horse ungal in dorsal view, with anterior at the top. The scalloped area at the back of the bone is the articulation with the next phalanx in the toe.

Here’ same ventral view of the same bone:

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The lateral view, showing the wedge shape (the articulation with the next phalanx is at the upper right):

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Note the how symmetrical this bone is in dorsal and ventral view. Horses are the only hoofed mammals in the valley that have a single toe, and as such are the only ones with a more-or-less symmetrical hoof-shaped ungal. Other hoofed mammals from these deposits, such as bison, have two toes with much more asymmetrical ungals.

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