It’s an epiphysis! Answer to Freak of the Week #13

Several of you had a good idea of what this was, and Juli knew it exactly! This creepy Valentine’s gift is the epiphysis, or growth cap, of a vertebra. This particular one is from a black bear but I’ve seen this same shape in some deer, elk, and cattle vertebrae. So what is an epiphysis!? Many of the bones in the body, such as long bones, vertebrae, ribs, and some bones of the feet and hands have separate ends of the bones – distinct and unattached growth centers from the main shaft of the bone called epiphyses. As the animal ages and the bones increase in length, the bony epiphyses move outward and allow this growth in length to occur. Once maturity is reached and the bone stops growing in length, the epiphyses finally fuse to the main body of the bone.

This pattern of growth is really useful in studying bones because you can tell the relative age of the animal by the fusion of the epiphyses to the bones. If the epiphyses are completely separate from the bone, then the animal is very young or a sub-adult. Once the epiphyses start to fuse then the animal has reached skeletal maturity. Full adulthood and older age are when the epiphyses fully fuse to the rest of the bone and the line where they attach (epiphyseal line) becomes completely obliterated.

Of course the fusion of epiphyses only provides a relative age of the individual and is more of an indication of skeletal maturity than chronological age. And to make it more confusing, epiphyses on different bones fuse at different ages in one individual! But it does give you a pretty good idea of the approximate age of the animal. In the case of this bear, the epiphysis is unattached from the body of the vertebra (top photo) but I put it in place on the bottom photo so you can see where it fits. Sure enough, when I check the records, this bear was listed as a “young male” who was hit by a car. Skeletally, we would call it a sub-adult since it reached adult size but the skeleton wasn’t completely mature.

By the way, there are different types of vertebrae in the body, and each type has different characteristics. This particular vertebra is a lumbar vertebra, or vertebra of the lower back. The heart-shaped epiphyses were only found on some of the lumbar vertebra of this bear.

Posted March 12, 2014
Topics: Natural History

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