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2-3. VERTEBRAE OF ICHTHYOSAURS

Fish-shaped ichthyosaurs had strange trunk vertebrae shaped like ashtrays, or hockey pucks, as Sarah at Scientific American suggested (good comparison, don't you think?). This shape is very rare among reptiles, which made scientists wonder what had been going on in the backbones of ichthyosaurs.

When you look at lizard-shaped ichthyosaurs, you don't see any hockey-pucks. Instead, their vertebrae are quite normal in being elongated and cylindrical, as in the containers for camera films. The only thing that is unusual about the backbone of lizard-shaped ichthyosaurs is that there are too many vertebrae compared to average reptiles. Then, what does this all mean?

The hint was hidden in sharks. Some sharks have hockey-puck vertebrae in their trunks, while others have film-case vertebrae. And what is more, hockey-puck vertebrae are known only in thick-bodied sharks like the great white, whereas film-case vertebrae are present in slender sharks, such as catsharks (see figure below). If you make a graph, you can see that the thicker-bodied sharks have thicker vertebral column and flatter (or more disk-like) vertebrae. A similar trend is observed in ichthyosaurs.

So, here is the current interpretation. When ichthyosaurs first appeared in lizard shape, the number of trunk vertebrae drastically increased (see figure below). These ichthyosaurs probably swam like eels, by undulating the entire body. Then, as fish-shaped ichthyosaurs evolved from among lizard-shaped ones, the body thickened together with the vertebral column. Since there were many vertebrae already in the trunk, each of them ended up becoming flat and hockey-puck shaped when the column thickened. Whales do not have hockey-puck vertebrae because there was no drastic increase in the number of trunk vertebrae.

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Last updated on November 15, 2000