Fossil Friday – horse metatarsal

DSCN0630 copyWhile the bulk of the Western Science Center’s paleontology collection comes from Diamond Valley Lake, we have significant collections from other localities. One of the most interesting collections comes from Southern California Edison’s El Casco Substation in northern Riverside County. This material, while probably still Pleistocene, is well over a million years old, about 4 to 6 times older than anything recovered from Diamond Valley Lake.

Horses are among the most common remains from El Casco. Shown at the top is a horse left metatarsal (foot bone), seen in anterior view. The bone is lying on its side, with the proximal end (closest to the ankle) on the right and the distal end (closest to the toe) on the left. The bone is somewhat deformed, so even though this is anterior view, part of the left side is visible (the edge closest to the scale bar). Notice that the bone looks a bit swollen at one point, adjacent to where Max the Mastodon is holding the scale bar.

Here’s the same bone in posterior view:

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Again, the proximal end of the bone is on the right. In this view we can see that three bones are actually present, one large bone in the middle and a slender bone (commonly called “splint bones”) on each side. In fact, all three of these bones are metatarsals.

All modern horses in the genus Equus (and fossil Equus like this one) have a single functional toe on each foot. Like all mammals, horse ancestors had five digits on each hand and foot, but over time most of those digits have been lost. The one remaining functional digit is the third one, the middle finger on the hand and middle toe on the foot. These are attached to the 3rd metacarpal (hand) and 3rd metatarsal (foot).

But what about those splint bones? Well, horses may only have one functional toe on each foot, but they still have remnants of the second and fourth metacarpals and metatarsals, which have not yet been completely lost. In the bone shown above, from the top you can see the 4th metatarsal, the 3rd metatarsal (by far the largest), and the 2nd metatarsal. The swollen area that is so visible in the top photo occurs near the distal tip of the 4th metatarsal, more clearly visible in lateral view:

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The distal tip of the 4th metatarsal is fused onto the side of the 3rd metatarsal. This is a pathological condition, is is likely a healed fracture of one or both of these bones. In medial view the 2nd metatarsal shows the normal condition, where it lies close against the 3rd metatarsal but with no fusion at the distal end:

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This injury was clearly not fatal, since it healed. There’s no telling what caused the injury, but some type of blow to the outside of the foot is possible; maybe it was kicked or stepped on by another horse. But it does give us a snapshot, if an incomplete one, into the life of this horse.

This specimen, along with many others in our collection, will be on display in our new exhibit Stories from Bones, which opens at the Western Science Center on October 31.


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