Fossil Friday – mastodon caudal vertebra

Even in huge animals, not every bone is large. This small bone, less than 4 cm in length, comes from one of the largest Pleistocene animals at Diamond Valley Lake — a mastodon.

This particular specimen is a caudal, or tail, vertebra. Its simple shape is not due to broken and missing pieces; in fact the bone is almost complete. The prominent spines and processes found sticking out on most vertebrae tend to be reduced or absent on caudal vertebrae in most animals. This is unsurprising from a functional standpoint. Those projecting spines are not simply decoration; they provide anchor points for muscles that control movement of the head, the legs, or other parts of the body. For the most part the tail doesn’t serve as the anchor point for any muscles, and so the vertebrae don’t generally have large processes. (There are a few exceptions: the first few caudal vertebrae have large processes that anchor the muscles that control the movement of the tail itself. There are also some animals such as alligators that have flattened tails, generally for swimming, which have enlarged caudal spines.) Their simple shape can sometimes make caudal vertebrae tricky to identify. They are sometimes mistaken for toe bones, but in most terrestrial animals even toe bones will have complex articulations and muscle attachment points that caudal vertebrae generally lack.

The complete lack of processes indicates that this vertebra is from fairly close to the tip of the tail. To be that far back and still have a length of almost 4 cm indicates that it actually came from a big animal, and in fact other vertebrae associated with this bone indicate that it came from a mastodon.

Looking at the bone end-on (below) we can see that the end is fairly rough:


This is because the bone is missing the vertebral epiphyses, the caps of bone that fit onto each end of the vertebra. The epiphyses remain essentially as separate bones attached to the vertebra with cartilage while the animal is still growing, but eventually fuse onto the vertebra. The fusion of all the various epiphyses indicates that the animal has reached physical maturity; in elephants (and people) this is generally sometime around age 20-25. The lack of fused epiphyses in this vertebra (and, in fact, none of the epiphyses are fused and the other bones associated with this specimen) indicates that it came from a young animal, likely not more than a few years old.

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