A tale of two tail bones

The photo above shows two of the bones collected at Carmel Church last summer, which have recently been prepared. In spite of their dissimilar appearance, both are chevron bones, and both apparently come from the same whale.

To orient you, both bones above are shown in left lateral (side view). The bone on the left has some damage, but it’s mostly hidden in this view, while the bone on the right is complete. Here are the same two bones in anterior view:

The damage to the left bone is more obvious in this view, but the big difference in shape is still apparent.

So, what are chevrons and why do these two look so different from one another if they come from the same whale?

Chevrons, or haemal arches, are basically small, usually “V”-shaped, bones that hang below the tail vertebrae and articulate with those vertebrae. The open part of the “V” corresponds to a groove on the bottom of the vertebra; when the bones are articulated, the “V” and groove together form a channel called the haemal canal. The caudal artery and caudal vein pass through this canal. Here’s an articulated blue whale skeleton, showing the articulated tail with the chevrons attached (specimen at the North Carolina Museum of Natural Sciences):

Notice that as you move back along the tail, the chevrons not only get smaller, but their shapes change as well. Comparing this image to our two Carmel Church specimens, it looks like one of the chevrons is from near the front of the tail, while the other is pretty far back on the tail, suggesting that we have recovered a large part of this whale’s tail (we haven’t opened the jackets with the vertebrae yet). This is of interest for Carmel Church specifically because none of our other articulated whale skeletons, including Diorocetus and Eobalaenoptera, included any tail vertebrae.

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5 Responses to A tale of two tail bones

  1. Doug says:

    Yeah, i have been wondering about that myself. I noticed that whenever you talk about these whale skeletons they lack tails. And that’s why i love Carmel Church. It keeps throwing us curve balls. If i ever get my museum off the ground, i want to put in a Carmel Church exhibit in the marine mammal hall.

  2. Chevrons indeed are V-shaped, normally. There are exceptions of course, just last summer I collected a double chevron. I think I’ll post a picture sometime this week, it just looks very unusual! I think someone did told me it has been previously observed in cetacea, but not for sirenia.

  3. Alton Dooley says:

    According to Flower (1885) in “An Introduction to the Osteology of the Mammalia”, the chevron grows from two ossification centers, one on each side. During growth these two ossifications eventually fuse to form the complete chevron. But apparently the ossifications don’t always fuse, leaving a kind of “split” chevron. Could this be what you have in your sirenian?

  4. Alton Dooley says:

    Oh, and Doug, I’d love to see a Carmel Church hall!

  5. I wrote about it, including pics, go to my blog and you’ll see what I meant by double.

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