My friend Bobby Boessenecker (who blogs at The Coastal Paleontologist) published a paper today with Frank Perry in the journal Palaios (abstract). In the paper they describe circular bite marks found on several fur seal bones from the Miocene-Pliocene Purisima Formation in California. Circular bite marks are a bit unusual in marine mammals. Parallel linear bite marks caused by sharks are much more common, such as these massive cuts in a whale jaw from Peru:
Circular bites are much more common in terrestrial mammals. They’re essentially punctures caused by the canines (and occasionally premolars) of mammalian carnivores, which are circular in cross section. Sometimes the punctures go all the way through, and sometimes they cause a circular crushed area in the bone. A good example of this is on display at the University of Nebraska State Museum, where you can see a Moropus bone that was apparently bitten by the entelodont Dinohyus:
Looking at the pictures in the Palaios paper, I was reminded of a partial proboscidean femur in the VMNH collection that we briefly placed on exhibit in 2009 (top of page). We don’t have a lot of information about this specimen; it came to the museum decades ago as a donation. I’m pretty sure it’s from the Pleistocene of Kentucky, and it’s most likely from a mastodon (Mammut americanum), although I haven’t yet ruled out a mammoth (Mammathus sp.). The image at the top of the page shows nothing remarkable in posterior view. But if we look at the anterior side, there’s a large circular depression that’s very similar to the ones in the fur seal and Moropus bones:
This is pretty clearly the same type of feature described by Boessenecker and Perry, even though its diameter is five times larger (note the scale bars). What could have caused such a puncture? Certainly the most obvious possibility is a large carnivore, particularly something like a bear as they are known to bite into bones. The giant short-faced bear, Arctodus simus (below, from Mammoth Site), is a Pleistocene species that has already been implicated in the scavenging of mammoth bones from Saltville, Virginia (Schubert and Wallace, 2009).
However, the location of this puncture on the anterior side of the femur does allow for another possibility. Research presented by Daniel Fisher at past SVP meetings indicates that, like modern elephants, mammoths and mastodonts engaged in male-male combat that resulted in horrific, and sometimes fatal, injuries from tusk stabs. That at least opens the possibility that the puncture in the VMNH femur was caused by the tusk of another mastodont.
This provides another point of similarity with the Palaios paper. Boessenecker and Perry suggest several equally likely candidates for inflicting the bites on the fur seal bones, including toothed whales, other pinniped species, and terrestrial carnivores. However, given that the bones are from a juvenile fur seal, they also mention the possibility that the injury could have resulted from a case of intraspecific infanticide, a behavior that is known in modern fur seals.
Congratulations to Bobby and Frank on the publication of this interesting and thought-provoking paper!