Friday, November 12, 2010

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

16 comments:

Morgan said...

wow...that is a beautiful skull

J. Velez-Juarbe said...

Hi Robert,

That is a nice looking skull! Doesn't Albireo have those bosses too?
I've been wanting to ask you, have you found any monodontid (skulls)? As far as I know there is only Denebola brachycephala from the Late Miocene of Isla Cedros and an undescribed Denebola-like thing from CA. Most other pre-Pleistocene monodontids seem to be based on periotics or very incomplete material.

Robert Boessenecker said...

Morgan - thanks dude! Yes, I'm pretty proud of it. It's definitely one of the prettier skulls I've collected.

Jorge - Albireo has huge premaxillary eminences, and I've heard of some monodontids as having them (Monodon, maybe; Delphinapterus doesn't seem to, and I can't recall about Denebola). If delphinids and phocoenids are that closely related, than perhaps it might not be too surprising to have mild premaxillary eminences in a basal delphinid (or, on the other hand, this relatively high degree of cranial asymmetry in a basal phocoenid).

Denebola is known from the Purisima Formation as well; there are a couple skulls, and I know of at least a dozen periotics from the Purisima. I'm not sure if its brachycephala or not, because the skull still needs to be prepared (a probable project for me next year). There is some kind of delphinapterine that Larry Barnes has from the San Mateo Formation, and I'd put good money down that its Denebola (although I must confess I haven't seen it, and probably won't be able to for some time). There's also some periotics and a pair of partial dentaries from the San Diego Formation of some kind of delphinapterine- although after seeing my poster at SVP 2009, Tom Demere thought it might be some kind of globicephaline (although the dentery to me appeared to be too straight, then again - I have one horrible photo of it in its specimen tray, and not in occlusal aspect). Just describing San Diego monodontids would make a nice short paper on its own.

J. Velez-Juarbe said...

Yes, Monodon and a monodontid from the Early Pliocene of Belgium have the eminences (Lambert & Gigase, 2007); looking at Barnes (1984) Denebola doesn't seem to have it, just like Delphinapterus.
Its good to know that there are more skulls referable to Denebola, otherwise monodontids seem to have a pretty scanty fossil record. The thing from the San Mateo Fm is the one reported in Barnes & Deméré (1991), they do mention that it is closest to Denebola. As for over here in the east coast, there's some material from the Yorktown Fm reported in Withmore & Kantenbach (2008).

Robert Boessenecker said...

Yeah, monodontids do appear to be pretty rare - last year I tallied up odontocete periotics (in private and museum collections) from the Purisima Formation, and monodontids (all Denebola) comprise 8% of the assemblage (N=120). I'm genuinely surprised that monodontids haven't been reported from the Pliocene of Florida; I guess that's what part of Whitmore's (1994) hypothesis about the range of Pliocene Delphinapterus was based on (showing up in North Carolina, but not in Florida). There is also some Pleistocene material from Canada, including a nearly complete skeleton called "Felix". From what I remember, there is also some material from Baja California listed by Barnes (1998).

Cpowell said...

So Bobby,

Please post a picture of the shark tooth you mention early on in this post and I was wondering if you ever see mollusks associated with any of the skeletal elements from the Purisima. Frank mentioned he had and I'd like to document them if we can find a site with them again. Thanks. BTW - good work.

Robert Boessenecker said...

Hey Chuck,

That tooth is from the very base of the Purisima Formation - I'll send you a picture later today or tonight; It's huge.

Regarding mollusk-vertebrate associations, I know of two from the same locality. One is the whale-fall assemblage that Frank discovered in the 1990's, and I found a large concretion a couple years ago (which I got a few chunks of) that was packed with Naticid snails and fish bone fragments, and since it was only ~60 feet away from Frank's whale fall assemblage, I'd suspect they belong to the same assemblage.

Howard Garrett said...

Robert, I found your blog via a search for llanocetus. I'm working on a slide show here on Whidbey Island, on "Resident and Transient orcas - How did that happen?"
For context I'm giving the evolutionary history of cetacea, with emphasis on the way species radiated into ever more obscure niches in the relatively stable marine environment and made amazing adaptations to specialize on a narrow range of prey.
I haven't been able to find any reference to when Orcinus orca originated. I'm assuming the essentially modern orcas arrived around 6-10 mya, but other than the advent of modern dolphins 12-15 mya (is that right?), and one hearsay report of an orca tooth from 6 mya, I can't find anything definitive on when modern orcas arrived. Do you have any clues or theories on that?
Thanks.
Howard
Orca Network

Robert Boessenecker said...

Hi Howard,

There aren't any bona fide records of Orcinus older than 2-3 Ma; there is a possible early Pliocene tooth reported from Japan (~3-5 Ma), but who knows, odontocete teeth aren't terribly diagnostic anyway. The only bona fide pre-Pleistocene record is Orcinus citoniensis, from the late Pliocene of Italy. There is a single earbone of this animal from the Early Pleistocene Red Crag of England. The reference is included below.

As far as radiating into ever more specialized niches - I disagree. The cetacean fossil record shows a number of different successive cetacean faunas through time, and through a very UNstable marine environment (End-Eocene climate crash, opening/closure of the Panama seaway, and Pleistocene changes in sea level (and the Messinian Salinity Crisis in the Mediterranean). Furthermore, there are many extinct bizarre, highly specialized cetaceans and other marine mammals that lived during the Pliocene, which have subsequently gone extinct, presumably in favor of more generalized extant cetaceans.

Bianucci. 1996. The Odontoceti (Mammalia, Cetacea) from Italian Pliocene systematics and phylogenesis of Delphinidae. Palaeontologrphia Italia 83:73-167

Robert Boessenecker said...

Nevermind, I misread your comment about specialized niches. Whoops!

Howard Garrett said...

Thanks Robert, that's very helpful.
I'm drawing from Frederick Grassle's theories on predictable ecosystems leading to greater diversity, where very generally, long-term stability of marine ecosystems vs. terrestrial ecosystems lead to greater species diversity. Ref: J. Frederick Grassle and Howard L. Sanders 1973
Woods Hole Oceanographic Institute
Life histories and the role of disturbance.
While looking up Bianucci, I also just found Whales, whaling, and ocean ecosystems, ed. by James Estes, which should be very informative.
Thanks again, and I look forward to more visits to your informative blog.
Howard

Pavel said...

Dear Robert,

Thank you very much for your posts and photos.

Could this small dolphin be a monodontid? Or saying in other words, why it could not be a monodontid?

Pavel

Robert Boessenecker said...

Pavel,

It lacks several features of monodontids, including maxillae exposed along the anterior margin of the bony nares, deeply entrenched sulci on the premaxillae, a wide rostrum, an elongate temporal fossa, and among other features, it's very small (i.e. 6-7" wide at the orbits) and has a rostrum that is way too long and narrow.

Pavel said...

The first of these traits is the most important. However, in extant narwhals proximal parts of premaxillae can be totally fused with maxillae, so the sutures anterior to nares can be not visible.

I do not insist it is a monodontid but it can be interesting to compare them (if you have access to Denebola skulls), as well as with basal phocoenids.

Thank you.

Robert Boessenecker said...

Those are all important characteristics of Monodontids. As for Monodon - Monodon practically lacks a fossil record, and every other known monodontid (modern or fossil) exhibits well defined maxillary exposure anterior to the nares. I've already compared it with monodontids - there aren't many; none of them look like this, and they're all much larger.

Otherwise, it does share some features in common with phocoenids, and in fact, I had originally identified it as a phocoenid; it has features of both Phocoenidae and Delphinidae.

Anonymous said...

Those are really cool!
Good luck finding more!