Fossil Friday – horse pelvis

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For this week’s Fossil Friday we have the pelvic bones from a horse, collected from the Pleistocene deposits at the east end of Diamond Valley Lake.

In most tetrapods (four-legged vertebrates that mostly live on land, and their descendants) the pelvic girdle is the primary structure for supporting the animal’s weight and allowing it to move. As such this is usually a large complex structure made up of six separate bones (three on each side) plus several specialized, fused vertebrae (the sacrals). The pelvic bones are the ilium, the ischium, and the pubis, and in mammals these are usually fused to each other at a young age; the fused group of three bones is called the innominate.

The field jacket shown above actually has portions of both the left and right innominates of a horse. The bone on the right side of the image is the left innominate, seen in dorsal view (from above), while the one on the left is the right innominate, seen in ventral view (from below). Below is a color-coded version of the same image:

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The bones outlined in blue are the ilia; he right ilium is damaged but the left one is largely intact. The yellow outlines mark the ischia, and the red is the pubis. Only the right pubis is visible; the left one is either hidden underneath the ilium and ischium or (more likely) not preserved at all.

Where the three bones come together they form a cavity called the acetabulum, outlined in green. This is the “socket” part of the hip’s “ball-and-socket” joint where the leg attaches to the hip. There is also a large gap between the pubis and the ischium called the obturator foramen, which serves as a passageway for nerves and blood vessels that run through the pelvis.

While there is a lot of variation is the details, all mammals that have back legs share this same basic hip structure, and even mammals that lack back legs (such as whales and sea cows) had ancestors with similar pelvic bones.

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