Sharks eating whales

2013-10-08bThere’s a skull from a juvenile baleen whale sitting in my lab. This whale, which was collected from Westmoreland State Park, has proven to be particularly difficult to prepare, and it’s been taking up table space for years. While waiting for digitization equipment to arrive, Christina passed the time by trying to make some headway on cleaning this specimen.Last week while working on a section of the maxilla from this whale she found what appeared to be a small rock embedded in the bone. While this is common at Carmel Church, it’s unusual in the fine-grained, diatom-rich sediments at Westmoreland. Removing the rock revealed a small, elongate hole in the bone (above).

But once it was pulled out of the bone, it turned out that the “rock” wasn’t a rock at all:



It was the tip of a shark tooth, just 3 mm long. The tooth is clearly serrated, and looks like the genus Carcharhinus. We haven’t found the rest of the tooth, and it appears that it wasn’t present, suggesting that the tooth broke when the shark bit into the bone. This is fairly significant. It’s often assumed that broken shark teeth were broken by geological processes during or after burial, but I’ve come to suspect that many of the broken shark teeth we find in various deposits were broken during feeding.

This isn’t quite as obvious as it sounds, because of the nature of tooth replacement in sharks. Sharks constantly replace their teeth, with the older teeth falling out as new teeth grow in. That means that the teeth are not strongly attached to jaws; there’s nothing comparable to a tooth socket in a shark. Given this, if a shark bites into something hard like a bone, wouldn’t it simply fall out rather than break?

Apparently the tooth can break, at least sometimes, as this example demonstrates. There are actually other examples, including some cases in which there’s no doubt at all that the tooth was broken during biting. And that makes me wonder about the high frequency of non-reworked, broken shark teeth in some deposits, especially those that, like Carmel Church, have abundant whales or other marine tetrapods. How many of those teeth were broken during feeding?

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