Broken teeth

After being inspired by Bobby Boessenecker’s blog post on “The Coastal Paleontologist” about a great white shark tooth fragment embedded in a sea otter skull, I leapt into action. There are some 1000 shark teeth from Carmel Church currently in my lab, waiting to be moved to collections; perhaps 300 of these teeth show no obvious signs of reworking. A 30-second survey this morning of the non-reworked subsample revealed the teeth shown above, each with a broken tip.

A few observations, based on this completely non-quantitative sample (remember, 30-second survey here, no claims of being a rigorous study): breakage seems to be most prevalent in odontaspids and in the genus Carcharhinus. In isurids, breakage seems to be more common in narrow teeth as opposed to broad teeth. In addition, there was a much lower rate of breakage than I expected; for all the taxa, broken teeth were a distinct minority (I’m excluding reworked and weathered teeth, and teeth in which the crown was broken at the base).

Bobby’s post also reminded me of another specimen from Carmel Church that I prepared years ago, but have never got around to writing up. Below is a cervical vertebrae neural spine (left lateral view) from the type specimen of the baleen whale Eobalaenoptera harrisoni:

Note in particular the two large, parallel grooves in the bone (arrows below). These are not typical post-burial fractures, because they don’t cut all the way across or through the bone:

The juvenile Carcharocles megalodon tooth (or Carcharodon megalodon, if you prefer) shown below was found lying flat against the neural spine:

A tiny tooth fragment was found in the sediment adhering to the bone, about a centimeter away from the large tooth:

The fragment turns out to be from the same tooth:

If we go back to the grooves in the neural spine, the tooth fits beautifully into them:

A few things should be noted here. Assuming this interpretation is correct, there is nothing to indicate that this bite occurred while the whale was alive. There is no obvious secondary bone growth, so this bite probably occurred during scavenging. The bites themselves are also interesting, in that the grooves cut completely through the cortical bone and into the cancellous bone, and there appears to be some fracturing and displacement of cancellous bone associated with the cut. This is different in appearance from most shark bite marks I’ve seen, in which the cortical bone is usually not penetrated.

Many shark bites on whales occur on the ribs, and as Brian Beatty and I talked about in our recent paper mysticete ribs seem to have relatively thick cortical bone; this might be why we don’t see more marks like this. However, if this specimen had not had the broken tooth right next to bone, I’m not sure I would have recognized this as being a shark bite. Had I seen this in a museum collection (rather than preparing it myself) without the associated tooth, if I noticed it at all I would have probably assumed it was the result of an accident that occurred during preparation. So perhaps bites like this often escape notice.