Wednesday, February 2, 2011

Mammal bite marks on fur seal bones, part 2

A few months after I collected the radius, I was invited to go examine Frank Perry's private collection. He's donated the majority of his material to UCMP, LACM, and the Santa Cruz Museum, but there was some remaining material. Several specimens he allowed me to borrow and prepare, including a partial juvenile Parapontoporia cranium, a walrus vertebra, and several fur seal bones. One of these was a very small humerus from a fur seal pup, roughly the same size individual as the radius I mentioned earlier. This specimen also happened to have a circular depression with a ring fracture, although it is much more shallow, and larger. This may be attributable to a larger, blunter tooth. Both of these specimens are probably attributable to the species Thalassoleon macnallyae, although in the article the bones are only identified to the family otariidae.
The right radius of a juvenile fur seal in anterior (left) and lateral (middle) views, and a closeup of the bite mark (right).

So, what caused these? Many, many, many studies have been published on shark tooth inflicted bite marks, which are typically linear gouges. These gouges are sometimes associated with removed 'chunks' of bone, but never have any fractures. These are obviously not linear gouges, and instead appear to be the result of the bone surface being pushed in. Circular holes can be caused by boring clams (pholad clams), but these are eroded, and do not result in fracturing. As it turns out, many similar tooth marks have been reported for conical mammalian teeth, of terrestrial mammalian predators and scavengers. One single similar tooth mark has been reported for a marine mammal: a skull of a juvenile sea lion (Eumetopias) from the Pleistocene of British Columbia (see the paper for more comments on this article). In fact, this is only the second reported occurrence of probable mammalian bite marks on fossil marine mammal bones.

Figure 2 from Boessenecker and Perry (2011) showing the bones and bone modifications.

The next question is, what type of mammal has the dental equipment capable of inflicting this sort of damage? Several pinnipeds, including the bizarre walrus Dusignathus santacruzensis, have teeth small enough to inflict these punctures. Most dolphins have teeth that are too small, and too closely spaced to make these punctures. Larger odontocetes, including the beluga relative Denebola, have larger teeth which are spaced far enough apart to form the punctures. Recently, Jonathan Geisler, Frank Perry, and I presented a poster on a pilot whale-like delphinid, and something the size of this cetacean could easily have produced the bite marks. The possibility remains that a terrestrial carnivore, like a canid, felid, or ursid; modern mammalian carnivores often prey upon or scavenge upon pinnipeds on shorelines. Lastly, the fur seal Thalassoleon has teeth that could produce the punctures. But Thalassoleon is the same species, you say! Well, oddly enough, extant fur seals and sea lions frequently commit infanticide - killing juveniles of their own species, sometimes in order to feed, other times as a part of aberrant sexual behavior where juveniles are mistaken for females.

Table of biogenic bone modifications from Boessenecker and Perry (2011) reported from marine vertebrate bones.

Unfortunately, it isn't possible to narrow the possibilities down any further. I'm getting tired, so stay tuned for part 3.


Alton Dooley said...

Just some idle thoughts...

I'm wondering if, in the crushed depressions, there's a consistent relationship between the size of the tooth and the size of the depression (I'm thinking about the type of break in Figure 2G). I wonder if the tooth could cause a circular break that's significantly larger in diameter than the tooth itself.

This is complicated by the fact that the diameter of the conical tooth changes with distance from the apex. But, since in these breaks the tooth didn't completely penetrate the bone, I wonder if there's a relationship between the depression diameter and the maximum diameter of the tooth?

What I'm getting it is can you determine tooth diameter even if the tooth didn't completely penetrate the bone?

Robert Boessenecker said...

Yes, this is something someone else brought up when I discussed this MS before submitting it. With the bite mark on the humerus, that could potentially have been caused by a small tooth - or something a little bit bigger. All I think we can say is that whatever bit into that spot had a tooth with an apical 'diameter' less than or equal to the width of the puncture.