Thursday, October 29, 2009

Reconstructing a fossil walrus, part 1: the enigmatic Dusignathus santacruzensis

For a recent manuscript project I found myself in need of a cranial reconstruction of the strange pinniped Dusignathus santacruzensis. D. santacruzensis was named from the Purisima Formation in 1927 by Remington Kellogg, the father of modern marine mammal paleontology. Research and interviews conducted by F.A Perry have successfully relocated the type locality, which evaded Kellogg and later forays by E.D. Mitchell in the early 1960's. The holotype specimen consists of a few cranium fragments including a partial maxilla bearing a procumbent canine, a squamosal, and a fragment of the 'vertex' of the skull (a term usually relegated to cetaceans, but utilized for odobenids by Demere [1994]), as well as both dentaries. The 'exploded' nature of the cranium is actually fairly literal; interviews by F.A. Perry indicate the collector 'poked it with a stick and the skull exploded', and only some of the cranium fragments were recovered. The dentaries are thus far the most distinctive element of the taxon; they are very robust, also with a procumbent canine, curious postcanine teeth with anterior and posterior wear facets, and a sinuous ventral border of the dentary (the first and last are synapomorphies of the Dusignathinae - Demere [1994]). Mitchell (1975) depicted the first reconstruction of the cranium of D. santacruzensis.

Mitchell's 1975 reconstruction of Dusignathus santacruzensis.

Unfortunately, no more cranial remains of Dusignathus santacruzensis have been recovered from the Purisima Formation since. Trust me, this isn't for lack of trying - dozens of dedicated amateurs and professionals (myself included, for the last few years) have been scouring the coastal exposures of the Purisima Formation nonstop since the 1970's. Plenty of odobenid postcrania have been collected, in addition to an edentulous odobenine walrus cranium (Barnes and Perry, 1989).

Former display at the Santa Cruz Museum of Natural History of casts of the Dusignathus santacruzensis holotype (without squamosal) based on Mitchell's reconstruction. Done by my colleague, Frank Perry. I apologize for the reversed image.

A gigantic pinniped skeleton was unearthed in the late 1980's from the Capistrano Formation, and was named Gomphotaria pugnax (Barnes and Raschke, 1991). This very strange animal was initially perceived as being very different from Dusignathus in terms of its mandibular morphology. Gomphotaria, which had a 40cm+ skull, bore two stout, worn, procumbent tusks - in the cranium and in the mandible.

Gomphotaria pugnax, a pinniped straight from hell.

Collections from the late Pliocene San Diego Formation of southern California included two new species of walruses - the extremely bizarre toothless Valenictus chulavistensis (which deserves several posts by itself), and the 'bizarrer' Dusignathus seftoni, both described by Tom Demere (1994a), the curator of SDNHM. D. seftoni is known from several crania and jaws, a partial skeleton, and a handful of postcranial elements. This animal had a cranium generally similar to the well known Imagotaria, and very similar to the 'killer walrus' Pontolis, and Gomphotaria. A trait shared with Gomphotaria were the possession of upper and lower (but less procumbent) tusks, also highly worn. The dentary shared similarities with both taxa; the sinuous ventral border, but it had a gigantic lower canine.

Referred crania of Dusignathus seftoni, described by Demere (1994).

Although the holotype of D. santacruzensis does not have very large canines, Demere (1994b) suggested that the specimen represents a female. Indeed, the canines are relatively small for any male pinniped, although the skull fragments do preserve a sagittal crest. The root of the lower canine is also exceptionally long, suggesting some potential.

The holotype right dentary of Dusignathus santacruzensis, from
Repenning and Tedford (1977).

Sure enough, in March 2008 I collected (from near the type locality) a pair of associated tusks (one upper, one lower) - one is straight, and the other curved posteriorly and laterally; these are adult teeth (based on root pulp cavity closure), and lack the highly abraded and parasagittally oriented tusks of Gomphotaria, and instead have tusks more similar to D. seftoni (albeit unworn). Additionally, these tusks are substantially smaller than in Gomphotaria, which was a monster in comparison (although dwarfed by Pontolis magnus, another dusignathine, one of the largest carnivorans of all time, possibly only smaller than elephant seals). These are best identified as male tusks of D. santacruzensis. These tusks are larger than any male D. seftoni specimen, and indicate an animal that is pretty damn sexually dimorphic. One of the tusks exhibits curvature in a parasaggital plane, but also lateral curvature. Tom Demere has successfully convinced me that the curved tusk is the lower canine, very similar to D. seftoni.

Unfortunately, dentaries of female D. seftoni are not yet known, and male dentaries of D. santacruzensis are not yet known. I guess I just have to keep looking in the Purisima Formation!

Coming up - the actual reconstruction process I used, now that the essential (albeit convoluted) backstory is done.

T. A. Demere. 1994a. Two new species of fossil walruses (Pinnipedia: Odobenidae) from the Upper Pliocene San Diego Formation, California. Proceedings of the San Diego Society of Natural History 29:77-98

Demere, T.A. 1994b. The family Odobenidae : a phylogenetic analysis of fossil and living taxa. Proceedings of the San Diego Society of Natural History 10:99-123.

R. Kellogg. 1927. Fossil Pinnipeds from California. Contributions to Palaeontology from the Carnegie Institution of Washington 27-37

Mitchell, E.D., jr. 1975. Parallelism and convergence in the evolution of the otariidae and phocidae. In Biology of the Seal, p. 12-26.

Repenning, C. and Tedford, T., 1977. Otarioid seals of the Neogene. USGS Professional Paper 992.

News flash - Pachycephalosaurus=Stygimoloch=Dracorex

This doesn't really have anything to do with marine vertebrates, but this is a neat study that was conducted in part here at MSU, and I think it is one of Horner's neater studies. This Monday saw the publication of Horner and Goodwin, 2009: Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus, in the journal PLoS One (which is viewable free here).

Basically, the rundown is this: Dracorex, Stygimoloch, and Pachycephalosaurus are all postulated to be very closely related. As most of you already know, these are bone headed dinosaurs. The smallest (Dracorex) has no dome, and big squamosal spikes. Pachycephalosaurus (the largest) has a huge dome, with blunt spikes. And Stygimoloch , well, is sort of in the middle. Unfortunately, pachycephalosaurid fossils are extremely rare, and the record typically consists of fairly crappy material, including a lot of isolated, reworked fronto-parietal domes. Stygimoloch is the least known, originally named by Galton and Sues (1983) off of a squamosal.
Ontogeny of Pachycephalosaurus wyomingensis: juvenile, upper left ("Dracorex"), subadult, upper right ("Stygimoloch"), and adult (Pachycephalosaurus).

Histological analysis indicated that the smallest (Dracorex) was a 'juvenile', and that Stygimoloch and Pachycephalosaurus are 'subadult' and 'adult' (or, are at least in that relative ontogenetic order). The histology shows that in Stygimoloch and Pachycephalosaurus, the ornamental horns are undergoing resorption, and actually shrinking in size through ontogeny, and the dome is growing. Previously these features were intepreted to represent apomorphic conditions, should each fossil be mistaken (or assumed) for adults of different taxa.

Jack gave the talk version of this at the SVP meeting in 2007, and it wasn't very well received (which made it all the more entertaining). Unfortunately, there wasn't any time for questions. However, this year Jack gave a talk on metaplastic bone in dinosaurs, and he again described the histological evidence for the synonymy, and there was time for several questions. Inane questions like "why would these animals go through all the trouble of changing their head during growth - its expensive!", which received the answer "well, I don't know - but the histology shows that the dome was growing bigger, and the horns were getting littler." A brief comment here - besides the obvious option for intraspecific display (i.e. being able to tell another individuals age within a population), modern mammals do something far stranger - cervids (deer) grow out huge, heavy antlers every year, and then shed them. Just imagine a ten year old animal, and how much bone (by volume) that is, and how many calories that took to produce, in bulk.

I personally think this is a really neat paper; not everyone may agree with Horner on a lot of issues, but he constantly hounds graduate students at MSU to think in a transformational context, and not a typological context. Typologists either haven't trained themselves to think in terms of transformations (be they ontogenetic or phylogenetic), or are busy naming new taxa when they shouldn't be, and won't allow anyone to kill their 'baby'. Often in the world of dinosaur paleontology typology goes hand-in-hand with neo-nazi cladism. Fossil organisms change through time in two ways - ontogenetically in a single individual, and on evolutionary timescales - the full examination of both ranges of variation can lead to additional synonymization, and a more accurate (and hence, better) understanding of the fossil record and evolution. And a few pissed off, bitter dinosaur fanboys. You SVP-ers and vertpaleo list subscribers know who I'm talking about.

Anyway, I still need to read the entire paper; much of this post is based on my recollection of various presentations. I also apologize for the 1.5 week hiatus; I've got some good posts planned, and will come out shortly after this.

Galton PM, Sues H-D (1983) New data on pachycephalosaurid dinosaurs (Reptilia: Ornithischia from North America. Can J Earth Sci 20: 462–472.

Horner JR, Goodwin MB (2009) Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus. PLoS One

Horner JR, Goodwin MB (2006) Major cranial changes during Triceratops ontogeny. Proc R Soc Lond Biol 273: 2757–2761.

Horner JR, Goodwin MB, Woodward H (2007) Synonymy consequences of dinosaur cranial ontogeny. J Vert Paleont 27: 92A.

Friday, October 16, 2009

Summer Adventures Part 7: Sea otter cranium with embedded shark tooth

On the friday of my last week of summer, I picked my girlfriend up from the SFO airport and we booked it to Golden Gate Park, where the prestigious California Academy of Sciences resides. Over the last five years their collections have been at their temporary facility near the Metreon. I visited several years ago (2007?) to look at northern fur seal skeletons. For my SVP presentation, I needed to compare my partial fossil Globicephala cranium with modern Globicephala, just to make sure my ID was correct. After I finished making my comparisons (and confirming my ID), I started photographing other marine mammal parts, and came across a box marked "Enhydra lutris nereis, with shark tooth!" E. L. nereis is the southern sea otter, which used to live from British Columbia south to Mexico; they were declared extinct, and have since repopulated, but only along Central California, and very slowly.
I pulled this skull out, and sure as s*** there was a tiny piece of a Carcharodon carcharias tooth embedded in the skull near the temporal/occipital contact or lambdoidal crest.
This skull was actually described and figured by Ames and Morejohn (1980), who described multiple cases of sea otter carcasses bearing tell tale bite marks in soft tissue, as well as two different bite morphologies: linear gouges, and parallel scrapes, originating from the serrations being dragged across the bone surface. They also figured about a dozen tooth fragments that had broken off and were embedded in soft tissue, and in some cases, skeletal tissue like this specimen.

Here's a closeup picture, and you can clearly see the serrations that diagnose Carcharodon carcharias.

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

Sunday, October 11, 2009

Summer Adventures Part 6: Fieldwork with Richard Hilton - the "North Coast Project" continued

So now comes part 6 of a 435 part series, better know a... sorry, I thought of Colbert when typing this first sentence. NO, I assure you, my summer wasn't that long. Shortly after I returned home for the summer, my family and I retired to Lake Tahoe for the 4th of July weekend. From there I took a beautiful drive up through the eastern side of the northern Sierras, skirting Mt. Lassen and Mt. Shasta, took I-5 up to Grants Pass, and then down to Crescent City, and further on up the coast from there to meet Richard Hilton, a paleontologist at Sierra College in Rocklin, and the writer of the lavishly illustrated Dinosaurs and other mesozoic reptiles of California (If I recall correctly, Richard is doing some fieldwork with fellow blogger/faithful visitor Neil Kelley, who is researching Triassic marine reptiles). Richard also brought along his close friend Paul Goldsmith, a cinematographer.

The beach here is safe enough to drive a regular 4x4 on, without modified low pressure tires.

Hilton typically goes prospecting for mesozoic marine tetrapods and dinosaurs in California, and Neogene mammal bearing localities in Nevada. However, he did some fieldwork in the Purisima Formation at Point Reyes in the 1960's, and thus he has a soft spot for marine mammal remains.

Sea lion femur in calcareous nodule.

After a few hours of looking, I finally spotted a concretion that looked like a fossil. Sure enough, there was some bone poking out of the sides. After a split second I was able to identify it as a sea lion (Eumetopias) femur. I got this puppy prepared just before SVP, and it is covered in fossil barnacles.

Richard Hilton fifty feet up a cliff of predominantly unconsolidated sand (read: not sandstone). This photo was taken nearly straight down.

Here I triumphantly show off the gigantic mysticete scapula fragment.

Later in the day, I found a huge chunk of bone (as float) about 80 feet up a cliff. However, after spending an hour and a half looking, the three of us couldn't find the rest of the bone - we found two more pieces which attach well, but nothing else in the loose talus, or in the exposure. Perhaps it was the last remnant of something big that had eroded out of the cliff, and the rest of it had already continued down to the river below. It is a very small chunk of a very large scapula (best guess); it is difficult to say exactly what it is because of its incompleteness. However, there are two important aspects to this fossil: 1) Mysticete bones are extremely rare at this outcrop, and rare from Pleistocene deposits in general (at least in North America); 2) there are two very large shark tooth bite marks on this bone, which are subparallel (which means that tooth spacing can be determined) ; one of these marks is nearly 5 inches long! That'll make a good short paper some day.

Richard Hilton ascends the cliff, looking for more fragments of the mysticete scapula.

Very strange trace fossils from this unit.
Closeup of the strange trace fossils.

One prominent feature of this unit I found on my first visit here over a year ago were the presence of bizarre, spiky trace fossils. I've seen cross-sections of these in the Purisima Formation, but this is the only other unit I've seen these in (which also happens to be a marine, Neogene, blue sandstone deposited in the lower shoreface). F.A. Perry hypothesized in his UCSC senior thesis that these were larval chambers for some invertebrate, potentially calianassid shrimp, and that the little spikes were larval escape structures. Here at this Oregon locality they are preserved in 3D, so study of them from this locality may prove much easier than in the Purisima.
Richard searching tirelessly for bones.

Displaying the mysticete scapula fragment at the end of the day.
Wisps of dust from the overlying late Pleistocene terrace pour down as veils over the shelly early Pleistocene deposits.

Other stops on our trip were the Miocene St. George Formation near Crescent City, the Pleistocene Moonstone Beach Formation, and the Plio-Pleistocene Centerville Beach section of the Wildcat Group. We returned empty handed from these other localities.

Monday, October 5, 2009

Summer Adventures Part 5: new Herpetocetus cranium

Hey gang, I've finally returned from SVP and London, and finally beaten (nearly) this bad cold I've had (several of my office mates keep insinuating that I got swine flu; some other MSU students did - I was not so unlucky). So, in keeping with the attitude of my most recent posts, I will continue on with my summer adventures.

Earlier this summer I made a quiz type post, and never bothered following up on it (except in the comments). Well... for those of you too lazy to figure out what it was, now I'll show you.
I've shown the dorsal surface of the unprepared fossil adjacent to a photo of the well preserved cranium of Herpetocetus bramblei that I presented on at SVP in 200, with certain features lettered. A=zygomatic process of the squamosal; B= vertex; C = occipital condyles; D = exoccipital 'crest'. Herpetocetus, while exhibiting some decidedly 'primitive' features (primitive isn't politically correct, but given the nature of the cetacean cranium, its really convenient, and since this isn't peer reviewed, I don't care about 'PC'), also has a radically telescoped cranium. The radical telescoping isn't very obvious from the posterior cranium, though, and instead the primitive features are more salient. These include the extremely narrow and triangular supraoccipital shield, and the very anteriorly oriented zygomatic processes. Not terribly obvious from this specimen (because they are worn off), but very obvious on the H. bramblei cranium I have are vertically oriented, very well developed lambdoidal crests; these are typically laterally oriented in mysticetes, overhanging the temporal fossa. Now I'm just yapping; maybe I should really focus my chi into writing up that description...

In any event, the new fossil as exposed shows several diagnostic features of Herpetocetus (outlined above[ish]), and some features (such as the narrow supraoccipital) that are unique as far as Pliocene (and latest Miocene) mysticetes go. This last photo shows the slab (very, ver, very) slowly dissolving away in an acetic acid bath. I let it go while I was gone at SVP... and it didn't look any different when I got back. Acetic acid doesn't really degrade Purisima Formation nodules; it pretty much just softens them up to make airchisel prepping easier.

Stay tuned, folks - I still have tales to tell of fieldwork with Dick Hilton in Oregon, shark teeth lodged in sea otter skulls, and some other less interesting stuff I can't seem to remember at the moment.

Oh, and I got to be the 6,000th visitor to this blog - I rule.