Mystery bone revealed!/?

5 07 2011

Human pelvis and sacrum. The acetabulum is the site of the hip joint, where the femur attaches.

Well, here is what I can provide by way of explanation for the mysterious bone Anne Hess found in Maine. The things I know for sure: this is a bone from the sacral (base of the spine) area. The sacrum in mammals is composed of variable numbers of vertebrae that fuse together during development to form a solid base of attachment for the pelvis on either side. To orient you, I have provided an image of a human sacrum with the pelvic bones still attached. You can see that the human sacrum has four holes on each side. These are the remnants of the spaces between the individual vertebrae when they fused together. The human sacrum is about 4 inches in length, and is about as wide. Most mammals have a much narrower sacrum. The reason for this is the habit exhibited by most humans of walking upright. A wide, almost triangular sacrum is needed to withstand the downward forces of an erect spine.

The specimen Anne found is only half a sacrum (it was split lengthwise), and it is a distinct relief to your blogger that this half sacrum appears far too narrow to be human. But in terms of size, at about four inches long, this sacrum must once have belonged to a rather large animal. So, what’s my guess? I would venture to say that this may be a deer sacrum. The size would be about right, and the shape resembles that of both bison and reindeer sacra (yep, busting out the Latin plurals, ladies and gentlemen) that I found on the all-knowing internet. Here’s a fantastic side by side of a human versus a deer sacrum:

Northern Fulmar spine and pelvis--note the extensive fusing of vertebrae and also the very small overall size.

Finally, because I cannot write a post that is entirely devoid of seabirds, here is a bit of comparative anatomy for you. How can we tell that this half sacrum found by Anne is mammalian at all, and not avian? Take a look at this photo of a Northern Fulmar spine and pelvis. Birds exhibit a general evolutionary theme of fusing bones together in the pursuit of both weight economy, and a rigid skeleton that can withstand the forces of flight. To that end, birds have not only a few fused vertebrae forming a sacrum, but a structure known as the synsacrum, which incorporates a whopping fourteen fused vertebrae, and in some species, is also fused to part of the pelvis. At the top of the photo of the fulmar skeleton, you can see what appears to be a smooth, continuous ridge of bone. This continuous ridge actually represents several fused vertebrae.

Beyond the substantial anatomical differences, note also the massive size difference here. While Anne’s specimen was about 4 inches long, a similar segment of fulmar synsacrum would be only about 3/4″ long. And a fulmar is no wispy little songbird either. Thus, Anne’s find, were it possible that it were avian, would apparently have belonged to an enormous prehistoric emu.

Thanks to Anne for providing the fodder for this little exercise. I certainly enjoyed my research on the subject, though it has now consumed an absurd part of what might have been a productive morning. Nonetheless, please do send in any and all photos of mysterious dead things, Seanetters!

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