That strange string of bones

25 03 2010

Another view of the bone string found by Ray Bosse in Massachusetts

Well, not much response to these mystery bones from Ray Bosse’s beach. Ray himself submitted the photos with the thought that they likely came from a seal, and the SEANET blogger concurs.

For sure, this is a mammal’s spinal column, and in the photo shown here, the head end of the spine is toward the left, and the tail end to the right. All the roughly rectangular to triangular bones at the right side represent the tail. Many of you will object, indignantly shouting, “But seals have short tails! No way this is a seal!” Your SEANET blogger would like to point out the less than svelte nature of seals, meaning that much of the skeletal structure of the seal’s tail is effectively buried underneath a good deal of blubber.

In the photo, the seal’s rather slender pelvis is visible on either side of the spine. The pelvis does not particularly resemble a human pelvis, and this is a partial explanation for why seals look so absurd when they attempt to “walk” on land.

Half of a grey seal's pelvis

In humans, or dogs, or giraffes, the hind limbs must have a system of complex and robust musculature in order to run and jump and kick, etc etc. All those muscles need a wide, large pelvis to which they can attach. As seals use their hind limbs only to flap back and forth while swimming, their pelvii can be much reduced in size and complexity. The extreme of this evolutionary process can be seen in whales, which, of course, lack hind limbs altogether, but retain a very small, vestigial pelvis. Seals are somewhat intermediate between terrestrial mammals and whales and dolphins.

It occurs to the SEANET blogger that today’s post is a substantial departure from any seabird related news, but her affinity for dead things generally has overcome her today. Hopefully, you have enjoyed this brief lesson in the pelvic anatomy of various mammals. We will return to the usual seabird news next week.

A right whale pelvis and vestigial hind limbs: the extreme of marine mammal evolution

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