Fossil Friday – mastodon pelvis

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When the permanent exhibits were being installed at WSC, several high-profile specimens were molded and cast for the displays, including the large mastodon nicknamed Max. It turned out that Max’s pelvis was too fragile to withstand molding, so while the pelvis is on exhibit it was the only recovered element of Max’s skeleton that wasn’t reproduced.

The mammalian pelvic girdle is a complex structure made up of multiple bones that are generally separate in young animals but mostly fuse with age. All of the various pelvic bones are preserved in Max, although some of them are incomplete.

The image above shows the pelvis in close to dorsal view, maybe somewhat posterodorsal (above and behind), with a Max scale bar for scale. It is rotated so that anterior is toward the top of the image. Below is a marked-up version of the same photo:

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The largest bone, outlined in blue, is the ilium (only the bones on the right side and the midline have been color-coded). The ischium is outlined in red, and the pubis, partially hidden under the overlying bones, is yellow. The bulge on the side where the ilium and ischium meet is the hip socket, or acetabulum. The actual socket opens to the side and down, so it’s hidden in this view. The pubis also comes up to the bottom of the acetabulum, but that is also hidden in dorsal view.

The left and right ilia are separated by, and fused to, a structure called the sacrum, which is made up of a series of fused sacral vertebrae, outlined in purple above. Max appears to have four sacral vertebrae; the number can vary on an individual basis, and some mastodons have five. The first two tail vertebrae (caudal vertebrae) are also present, outlined in dark blue. It’s not clear if they are actually fused to the sacrum, or simply interlocked with it. Excluding the tail vertebrae, Max’s pelvis is therefore made up of 10 bones: the left and right ilia, left and right ischia, left and right pubes, and four sacral vertebrae.

Besides serving as the attachment points connecting the legs to the body, the pelvic girdle supports the back of the abdominal cavity. The digestive tract exits the abdominal cavity through the urethra and anus, so the pelvic bones are arranged in a tube to allow for this, as well providing a path for the birth canal in females. The sacrum forms the top of this tube, while the ilia and ischia make up the sides and pubes make up the bottom. This is more obvious is we look at the pelvis from below and behind (posteroventrally) – exactly where you don’t want to stand with an elephant that has recently eaten or that’s about to give birth:

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Below are two more views of Max’s pelvis, to give a better idea of the arrangement of the various bones. First, an oblique view from above the right side (anterior is to the lower right):

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And above the left side (anterior is to the lower left):

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Notice in each of these views, and especially in the marked-up image, that the boundaries between the various bones are not immediately obvious. Max, while not elderly, was a fully mature mastodon (probably over 30 years old), so these bones were solidly fused to each other. In a younger mastodon the boundaries between the bones would be more obvious.

Max’s pelvis only has a partial support cradle, so unfortunately we can’t flip it over to see the ventral side. Even so, we can see quite a lot in the views that are available.

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