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.