Shark-bitten dolphin skull

In 2008 I spent the day before Christmas Eve shivered on a cold, wind-blasted California beach prospecting for vertebrate fossils in the Purisima Formation. I was home on winter break, and although it is far more cold where I go to graduate school in Montana (as I write this I'm looking out at the results of our first winter snow), nothing is worse than being wet and miserably cold out on the foggy, windy coast of the golden state (except perhaps being wet and miserable on the Oregon coast, which I've done).

The thrill (or promise) of discovery is more than enough to keep me fueled in the field during the winter. Indeed, when the birds start singing and the snow melts in the spring, most paleontologists start to get field fever - the field season for most vertebrate paleontologists is during the summer months. Anyone who's ever tried to do coastal fieldwork during the summer, on the other hand, is in for a rude awakening. No erosion takes place during the summer, and many of the outcrops are totally buried. The exposures that are above the beach sand level (which is higher during the summer) are typically covered with dust, sand, and grime, which obscures fossils. The storms in the winter months clean this nasty coating off, and transport beach sand into offshore bars, often exposing strata below the beach (I see new fossil localities every winter this way). Winter is my field season.

Historically, I've had really good luck the day before Christmas Eve. It's my last day before Christmas to make it out in the field. The prior year, I found a humongous Carcharocles megalodon tooth (the only specimen known from the Purisima Formation), and discovered a partially articulated fur seal skeleton.

The Christmas Eve dolphin.

At 4pm, the tide was beginning to come back in, and with little over an hour of daylight, it was looking like I was going to come home empty-handed. I went to one last cove before I turned around to head back to the beach. I walked for a few minutes and spotted something in a boulder I had not seen on my way out: a pair of flat bones joined along an articulation that looked suspiciously (even from 20 feet away) like the palate of a dolphin skull. Upon closer examination, yes indeed! It was a dolphin skull in a mollusk shell bed; the width and flatness of the palate suggested it was not Parapontoporia, the most common odontocete in the Purisima Formation. I set about chopping into the boulder; fortunately, most of it was relatively soft. However, an extremely hard calcium-carbonate cemented concretion the size of a basketball had formed over the dorsal surface of the braincase and rostrum, and this slowed digging down. By dusk, the concretion didn't budge. After another half hour, it finally popped out of the boulder, and I lugged the 45 pound block back to my car. Exhausted, I drove home, drank a couple of hard-earned beers with dinner, and passed out.

View of the facial region of the skull.

When it came time to go back to Montana, I decided I would rather take the fossil as a carry-on than risk checking it and picking up a broken fossil that I had paid 25 bucks for thanks to baggage fees. After arriving in Bozeman (with a very sore back and neck from lugging 65 pounds

of luggage through the Denver airport), I almost immediately began preparation (starting, of course, with acetic acid baths for several weeks to soften the concretionary matrix). It took about two months to prepare, and as you can see from the above photos, it is damn beautiful. I initally identified it as something like Haborophocoena - it bears numerous similarities. However, after showing them photos of the specimen at SVP 2009 in Bristol, UK, Olivier Lambert and Giovanni Bianucci both think this represents a basal delphinid rather than a basal phocoenid. I'm inclined to agree with them, although part of my original ID was based on the presence of premaxillary eminences, which this specimen has (a phocoenid character). However, the ascending process of the right premaxilla is in contact with the nasals while the left is not (a delphinid character). Whatever it is, it will require preparation of the ventral aspect, and more careful analysis of the morphology than what I've been able to do thus far. Whatever it is, it appears to represent a new genus and species, and will make a beautiful holotype specimen in the future. During preparation, one curious thing I noticed was a notch in one of the premaxillary eminences (the large pads/bumps in front of the bony nares). I initially dismissed it as a pathology.

The left premaxillary eminence showing linear gouges (red lines) and missing bone.

Upon closer examination (which admittedly did not occur until yesterday, almost two years after collection) it became apparent that the abnormal area had two distinct, paralell linear gouges, and a short, less distinct third one in the middle (this one is still partly filled with matrix). Around these gouges is an area of exposed cancellous bone, where the bone has been removed.

Additional gouges present near the base of the rostrum.

I also found four more gouges present: two long ones, and two short ones; all but one are parallel. In fact, aside from the one gouge seen above trending towards the upper left corner of the photo, all the gouges are parallel. This is a textbook set of shark-inflicted bite marks. There are a lot of papers on this in the literature, documenting shark bites on dolphins, baleen whales, pinnipeds, sea turtles, other shark teeth, mosasaurs, plesiosaurs, dinosaur bones, sea stars, and probably other marine critters as well.

In fact, the first record of these types of trace fossils were actually first documented in the modern environment: on predated and scavenged sea-otter carcasses from Monterey Bay, and reported by Ames and Morejohn (1980). The reported linear gouges, subparallel wavy small gouges, and a specimen including a shark tooth embedded in a sea otter skull. The morphology of the traces along with the tooth identified the culprit as the Great White Shark, Carcharodon carcharias. Two years later, these exact types of traces were identified by Tom Demere and Richard Cerutti (1982) on a baleen whale dentary (of my favorite whale, Herpetocetus!), and identified as "Carcharodon sulcidens" (a taxon now just considered to be fossil Carcharodon carcharias).

It's not clear what type of shark fed on my poor little dolphin, or if it was a case of predation or scavening; from what I've read, the majority of carcasses that exhibit bites have bite marks on the posterior portion of the body, which is just about as far as you can get from the face. This makes total sense, given how a shark would have to bite into a fleeing dolphin during pursuit. Furthermore, it's interesting to note that this bite would have had to go clean through the dolphin's melon (if it had not already decomposed). Anyway, I interpret these traces as drag marks from the apices of the shark's teeth; I suppose later on I can figure out the relative motion of the shark's mouth during the bite (most likely lateral shake feeding). It'll make for a nice short paper some day...

Ames, J. A., and Morejohn, G.V., 1980, Evidence of white shark, Carcharodon carcharius, attacks on sea otters, Enhydra lutris: California Fish and Game, v. 66, p. 196-209.

Deméré, T.A., and Cerutti, R.A., 1982, A Pliocene shark attack on a cetotheriid whale: Journal of Paleontology, v. 56, p. 1480-1482