California shark teeth

Yesterday while doing fieldwork at one of my research localities, I spotted a beautiful shark tooth poking out of the cliff. Only a very tiny part of the serrated tooth blade could be seen, and initially I was unsure of how large it was. After a few minutes of chiseling, it was clear that this was a rather large specimen of the great white shark, Carcharodon carcharias, and upon removal, ended up being about 3 cm long - the second largest specimen I had collected from this locality. This was a good day, as I generally have only found about 3-4 of these teeth per year; they are not too hard to spot - in fact, they are fairly obvious due to the easy-to see enamel and serrations (unless a root lobe is all that is exposed). Altogether, I have collected perhaps 50 Carcharodon teeth from the Purisima Formation in total - they are relatively rare in comparison to marine mammal bones. And this is not for lack of trying: I've found that it generally takes about 3-4 trips to a given locality before I spot another specimen; whether this is due to examining exposures not covered by the previous trip, or erosion between trips, I'm not sure.

The Carcharodon carcharias tooth as found in the outcrop.

The prepared specimen.

I've found that shark teeth are nearly impossible to find during the summer months, and easy to find in the winter, when the cliffs are cleaned off by surf action. I rarely find teeth in the summer simply because I tend to pick the localities clean in the winter, and I have to wait until the erosion of the following winter to find anything. Shark teeth are so rare in west coast strata that it is not very fruitful to look for them on nearby beaches; indeed, I found one Carcharodon tooth in this manner, in 2006 - and I have not found another this way.

Certainly, the Purisima Formation is not the only shark-tooth yielding unit on the west coast. Two rock units that boast a healthy dose of shark teeth are the Sharktooth Hill Bonebed in the Round Mountain Silt near Bakersfield, one of the highest concentrations of fossil shark teeth in the world - and the Santa Margarita Sandstone near Santa Cruz. At both of these localities, one can find dozens of teeth with little work (Sharktooth Hill) or quite a bit of digging (Santa Margarita Sandstone). In high school, I played hooky one day and drove down to Santa Cruz, and spent 8 hours in a pit with a shovel and my screen, and collected 70 teeth - but only after ending up with enough screened sediment to fill a Volkswagen beetle.

Fossil mako teeth (Cosmopolitodus hastalis and Cosmopolitodus planus) from the Sharktooth Hill Bonebed, UCMP Collections (image borrowed from http://www.ucmp.berkeley.edu/).

In 2005, I was fortunate to contrast my west coast experience with that of Calvert Cliffs. I had three hours to check out the famous Brownie's Beach, where just looking through shelly debris on the beach, I found 80 shark teeth (mostly the reef shark Carcharinus), just on the surface of the beach, in an area smaller than one side of a tennis court. I've seen mason jars filled with shark teeth folks have scooped up from Florida beaches, and seen multitudes of shark teeth from the famous Lee Creek mine in North Carolina. Sure, there is plenty of lore and mysticism surrounding east coast shark teeth (and their collectors in particular!), but I have gotten the distinct impression that most shark-bearing strata on the west coast are depauperate compared to that on the east coast. Contrast nearly 100 teeth per 3 hours of work to 30 hours of work for one tooth, for example. Additionally, although I have not personally collected data on this, the obscene number of shark teeth from certain east coast units (i.e. 24,000 shark teeth from Calvert Cliffs from just 3 years of donations to the Calvert Marine Museum; Visaggi and Godfrey 2010) suggests that the ratio of sharks:marine mammals in the east coast is astronomically higher than in the Purisima Formation (which in my thesis, I determined it to be around 1:40) and other west coast units. How could this be?

A heap of shark teeth from Florida (image borrowed from www.sharkysshop.com).

A number of biological and taphonomic hypotheses can be made:
1) The fossil shark assemblages perhaps differ in their taxonomic composition, and perhaps there is some related preservation bias.
2) The western Atlantic had higher productivity and stronger upwelling in the Neogene, supporting a larger absolute population of sharks.
3) Sharks and marine mammals have a different preservation potential, which a large-scale taphonomic process could act upon.

Immediately dismissing out of hand absurd suggestions like fossil marine vertebrates were preserved differently along either coast or had different skeletal mineralogy between ocean basins, one can start to think about the above suggestions. For example, contrasting the Carcharodon carcharias-dominated shark assemblage of the Purisima Formation with that of the Calvert Formation which is dominated by carchariniforms like Carcharinus, Hemipristis, and Galeocerdo, this suggestion seems to have some merit. However, there is little difference in preservation potential between these different types of sharks, aside from differences in size. In general, marine vertebrate assemblages in bonebeds and the like are very poorly sorted, and all shark teeth are relatively small in comparison, to say, a whale jaw.


The second suggestion has some merit: the old skeletal supply v. concentration idea discussed by Susan Kidwell back in her 1985 paper (although she was talking about mollusks). She argued that skeletal concentrations are usually caused by changes in sedimentation rate - a slowdown in the rate of sedimentation eliminates the diluting effect on bioclasts, allowing them to form a shell or bonebed (or even just a zone where they are slightly more abundant). Kidwell also argued that computer modelling she used demonstrated that her concentration model still worked even when the skeletal supply rate changed. Is it reasonable to assume that the populations of organisms have not changed through time? Of course not. Can we, from a uniformitarian standpoint, work with this in mind? Using Kidwell's model, perhaps - perhaps not. This being said, I'm not sure that more teeth = more sharks. This is a relatively simplistic view of the fossil record, and in the past, interpreting the paleoecology of fossil organisms has been fraught with problems (usually of the taphonomic kind). Sharks aren't the only organisms who would enjoy higher productivity - you'd expect marine mammals to be going bat-shit crazy with all the extra fish, krill, etc. to feed on as well (and eventually dying, shedding their hard parts onto the seafloor as well along with shark teeth).

The sedimentologic fossil concentration model proposed by Kidwell (1985).

What about the third suggestion? Sharks and marine mammals clearly have different skeletal anatomy, and although some elasmobranchs have been found with preserved skeletal elements (including skates from the Purisima Formation - stay tuned!), all that typically preserve are teeth, and occasionally, dermal denticles. Marine mammals have teeth, skulls, earbones, and postcrania that get preserved frequently. Teeth are harder than bones, and probably have a higher preservation potential with respect to purely physical taphonomic processes (i.e. abrasion from winnowing, erosion, and transport). In fact, my data from my master's thesis indicate that shark teeth are less commonly abraded or fragmented relative to marine mammal bones, and therefore it is probably kosher to say they have a higher preservation potential.

Okay, so what? There is clearly some differential preservation potential. But the Atlantic and Pacific oceans are big places: any specific sedimentological process (i.e. bonebed formation) is going to vary along hundreds of miles of the continental shelf, and through geologic time, and it may be difficult to pinpoint one single phenomenon responsible. In fact, to really examine this, we have to zoom way far out, to the basin level. What is the single most obvious difference between Neogene strata of the east coast and the west coast? Again we turn back to the work of Susan Kidwell (1993). In general, because the east coast is a passive continental margin, most of the strata (i.e. Calvert Fm., Pungo Ls., Yorktown Fm., Bone Valley Fm.) are deposited in low subsidence settings ('low' sensu Kidwell, 1993). Most of the strata on the west coast, including the Purisima Fm., are deposited in smaller basins undergoing "wrench" tectonics (i.e. pull-apart basins) or even rifting (Imperial Group, southern California), which Kidwell (1993) classified as medium-high subsidence basins; most basins in California probably qualify under this category (and could be determined via deposition 'rates' and basin thickness/depth).

The concept of supply versus concentration in Kidwell's model can be extrapolated to an entire basin: basin subsidence controls the sedimentation rate, and instead of looking at the microanatomy of a single shellbed, the types of fossil concentrations and their thicknesses and lateral extent can be compared between formations and basins. Just like a longer pause in sedimentation may form thicker shell bed, a formation deposited under lower rates of sedimentation will result in more numerous, thicker, and more architecturally complex shell concentrations. The Calvert Formation in particular was Kidwell's example of a low-subsidence setting. In general, Neogene marine strata of the west coast in general have less numerous and thinner shell concentrations, a general proxy for the sedimentation rate.

A taphonomic process of this magnitude can then be imparted upon a given example of differential preservation: at the end of this, the generally lower sedimentation rate of the western Atlantic resulted in higher amounts of taphonomic damage, more widespread and longer-duration hiatuses/bonebeds, and could have effectively acted as a taphonomic "wedge". By wedge, I am referring to the eventual difference in the preserved abundances of shark teeth and marine mammal bones. Marine mammal bones, with their relatively lower preservation potential, perhaps lost out due to their greater susceptibility to damage by abrasion and fragmentation.

References Cited

Kidwell, S. M. 1985. Paleobiological and sedimentological implications of fossil
concentrations. Nature 318:457-460.

Kidwell, S. M. 1993. Influence of subsidence on the anatomy of marine siliciclastic
sequences and on the distribution of shell and bone beds. Journal of the Geological
Society, London 150:165-167.

Visaggi, C.C. and S.J. Godfrey. 2010. Variation in Composition and Abundance of Miocene Shark Teeth from Calvert Cliffs, Maryland. Journal of Vertebrate Paleontology 30:1:26-35.

Getting material curated at UCMP

On my last visit to UCMP, I spent an hour with a student volunteer and my good friend Ash Poust placing some material I had donated last year into specimen trays. These are all specimens collected from a locality in the Purisima Formation I studied as an undergraduate student. Most of this material was collected in 2005 and 2006, and I slowly curated and prepared it during my undergraduate career (and during grad school). I've now gotten about 1/2 of this collection to UCMP, and already it fills a drawer and a half.
Which is bad (and good). It's bad because the entire collection from this locality will take up half a cabinet by itself, not including the large oversize material. What's worse is that this material is roughly 1/3 of my entire collection - I have material I've collected from other Purisima localities, as well as the Santa Margarita Sandstone. All in all, my collection will probably require two-three cabinets at UCMP. Keep in mind I've already donated about 1/10 of my collection to the Santa Cruz Museum of Natural History.

Shark teeth and bird bones from the Purisima Formation.

That being said, it was a very satisfying experience to see all this material finally looking like it was part of a museum collection. I'm more motivated than ever to get the rest of it curated (and out of my house!). Now that I'm writing up the marine mammal assemblage from this locality, I can finally get rid of it all and get it into a proper museum setting.

Help preserve the Sharktooth Hill Bonebed

Hey Folks,

This web page was posted several months ago, but to anyone who is interested, the Natural History Museum of Los Angeles County is trying to raise money to purchase the Sharktooth Hill locality for posterity.

To those unaware, the venerated Bob Ernst (who formerly owned the property) passed away several years ago. Unbeknownst to many, the vast collections at the Buena Vista Museum of Natural History made from Sharktooth Hill were not an actual museum collection, but Ernst's private collection on display, and passed on to his widow. When she began having financial problems, she began to sell and auction off the collection, including many specimens which have now been published in peer-reviewed articles (a big, big, BIG screw up on the part of certain researchers). This is really bad, because unless these specimens are donated to collections of real museums - that work is effectively rendered untestable.

That being said, here is the page, posted by Don Prothero and Teresa LaVelle:

http://www.skeptic.com/eskeptic/11-05-11/

A personal tour of the Sharktooth Hill locality will be given by the museum director, John Long, for donations of over 2000$.

Unfortunately, I'm still relatively poor, as a student, and anything I could scrounge together would barely amount to a drop in the bucket - however, I can help out by posting it here, and hoping that this message can get spread a little further. This is probably the most spectacular marine vertebrate fossil site in western North America, and so many problems could be solved for Mrs. Ernst and marine mammal paleontologists if this money were raised.

A new specimen of Parapontoporia

Last fall I made a couple posts detailing an excavation (here and here) of a new odontocete skull from a relatively young (middle-late Pliocene) horizon in the Purisima Formation. This specimen was collected over a six hour period, and the excavation was pulled off and completed just before sunset. The specimen is still not completely prepared, but it does include a complete braincase, the posterior half of the rostrum, both petrotympanics (articulated petrosal and tympanics), and part of one of the lower jaws.

The new skull of Parapontoporia sternbergi in oblique dorsolateral view; bottom photo is labeled.

Three species of Parapontoporia have been described: Parapontoporia pacifica, from the late Miocene Almejas Formation in Baja California, Parapontoporia sternbergi, from the Pliocene San Diego Formation, the Mio-Pliocene San Mateo Formation, and possibly from the Pliocene Wilson Grove Formation, and of course, Parapontoporia wilsoni from the Mio-Pliocene Purisima Formation. Most of the Purisima material is referable to P. wilsoni, which is characterized by a very deep "basin" at the base of the rostrum (shown poorly in the photo below) and by a facial region that is longer than it is wide. P. sternbergi has a shallower basin, and has a facial region wider than it is long (and also appears to me to be smaller in general; P. wilsoni crania can be up to 20% larger than those of P. sternbergi). P. pacifica is not known from a complete braincase, and it has a flat base of the rostrum (i.e. no rostral basin). The new specimen exhibits a shallow rostral basin, and has a braincase that is wider than long, and is damn tiny - all suggesting that it is assignable to Parapontoporia sternbergi rather than P. wilsoni. "But the Purisima Fm. species is Parapontoporia wilsoni!", you might say. Just like modern cetaceans, fossil cetaceans likely had a cosmopolitan distribution - and Parapontoporia sternbergi is primarily known from late Pliocene rocks (specimens identified as P. sternbergi from Miocene strata should probably instead be called Parapontoporia sp.). The type locality of P. wilsoni is about 5.3 Ma, slightly older than the San Diego Fm. Likewise, the new specimen from the Purisima is from a horizon about 10 meters below an ash bed dated at 3.35 Ma, and therefore probably late Pliocene also.

Parapontoporia sternbergi, the most completely known species within Parapontoporia, exhibits an extremely elongate rostrum, filled with about 80 teeth per quadrant; that's a total of 320 teeth. I don't know the specifics for other toothy odontocetes (such as Eurhinodelphinids), but this strikes me as being a terrifyingly large number of teeth for a mammal, and I would not be surprised if Parapontoporia was the toothiest of all mammals. Additionally, the skull of parapontoporia is asymmetrical, unlike Pontoporia and more like the now extinct chinese river dolphin Lipotes. Parapontoporia was obviously named because it is closely related to the La Plata River Dolphin, Pontoporia, right? Right? Believe it or not, this new specimen weighs in on the phylogenetic relationships of Parapontoporia.

A reconstruction of a nearly complete skull from the San Diego Formation referred to Parapontoporia sternbergi by Barnes (1985).

The holotype of Parapontoporia wilsoni from the Purisima Formation.

When Larry Barnes published is major study of Parapontoporia in 1985 (the year I was born...), there were no skulls of Parapontoporia with associated earbones. Because of the similarity of Parapontoporia to modern river dolphins like Pontoporia and Lipotes, he looked through museum collections and tried to identify possible petrosals that could be referred to this new taxon. Oddly enough, one set of six petrosals from the San Diego Formation appeared very similar to modern Lipotes, although Barnes felt that the skull was more similar to Pontoporia.

Isolated petrosals from the San Diego Formation referred to Parapontoporia sternbergi by Barnes (1985).

Because of this discrepancy, there has been some disagreement (not in the published literature - in discussion only) about what the real petrosal of Parapontoporia looks like. At a UCMP visit once, Nick Pyenson showed me another petrosal which he felt might be a better match than the ones Barnes referred. This is a fairly serious matter, because these isolated petrosals have been used in various cladistic analyses to fill in the data matrix - and given the key phylogenetic position of Parapontoporia as a sister taxon to Delphinoidea - means that if the petrosals are not in fact from Parapontporia, there could be some serious errors in previously phylogenetic studies.

The petrotympanic complexes of the new Purisima Formation specimen of Parapontoporia sternbergi in dorsal/cerebral aspect.

The petrotympanic complexes in (near) ventral aspect.

This specimen is the first skull of Parapontoporia with well-preserved petrotympanic complexes associated with it. These petrosals are identical to those referred to this taxon by Barnes (1985), and confirm his referral. Interestingly, this also confirms the Lipotes-like morphology of the earbones. Previous cladistic analyses (e.g. Muizon 1988) have resulted in Parapontoporia being the sister taxon to Lipotes rather than Pontoporia, and several authors have suggested such a relationship, despite the hand-drawn cladogram and assertions of Barnes (1985). This new find indicates the Lipotes-like ear morphology does belong to Parapontoporia.

Petrosals of A) Inia, B) Pontoporia, and C) Lipotes.

Another Purisima Fm. skull of Parapontoporia wilsoni with a petrotympanic complex, in a private collection.

I'd be lying if I said that this were the first specimen ever found with petrosals and tympanics; two other specimens are known. One is a specimen of Parapontoporia pacifica from the Capistrano Formation at the San Diego Natural History Museum, which has a crushed tympanic and petrosal. The other specimen is also from the Purisima Formation, shown above, but has been unavailable for study. The petrosal is barely exposed and stuck within the concretion. This specimen remains in a private collection. This no longer bothers me, as the new specimen has both petrotympanics fully freed from the skull, and one of which has already been CT-scanned at UT Austin for a study by Manuel Martinez and Jonathan Geisler (among other authors, which I am dead last for this minor contribution). Look out for it at SVP!

References and further reading:

BARNES, L. G. 1984. Fossil odontocetes (Mammalia: Cetacea) from the Almejas Formation,
Isla Cedros, Mexico. Paleobios 42:1–46.

BARNES, L. G. 1985. Fossil pontoporiid dolphins (Mammalia: Cetacea) from the Pacific coast
of North America. Contributions in Science, Natural History Museum of Los Angeles
County 363:1–34.

FORDYCE, E., AND C. DE MUIZON. 2001. Evolutionary history of cetaceans: a review. Pages
169–233 in J. -M. Mazin and V. de Buffrenil, eds. Secondary adaptation of tetrapods to
life in water. Verlag Dr. Friedrich Pfeil, Munich, Germany.

GEISLER, J. H., AND A. E. SANDERS. 2003. Morphological evidence for the phylogeny of
Cetacea. Journal of Mammalian Evolution 10:23–129.

MUIZON, C. de. 1988. Les relations phylog`en´etiques des Delphinida (Cetacea, Mammalia).
Annales de Paleontologie 74:159–227.

PYENSON, N. D. 2009. Requiem for Lipotes. Marine Mammal Science 25:714-724.

Recent fieldwork in the Purisima Formation, Part 3: mysticete earbones and wildlife

Hey Folks,

Sorry for yet another delay - I've been pretty busy, working on several manuscripts (a thesis-length paper on the Purisima Formation marine mammals from my undergraduate field area in Halfmoon Bay, a new manuscript on shark bitten cetacean bones, and my contribution to a paper of the mollusk and vertebrate assemblage of a late Miocene marine locality in Sonoma County), applying to the University of Otago Doctoral Program to work with R. Ewan Fordyce (in New Zealand), and digging up a new right whale fossil from the Purisima Formation. There are plenty of topics I have thought of to write about on here, but not enough time!


Continuing on with my series of posts about recent Purisima Fm. fieldwork with Dick Hilton, I've written a little about a new mysticete earbone. There are lots of mysticete earbones from the Purisima Fm., from number of different taxa, including cetotheriids, many balaenopterids, and balaenids. Dick had originally spotted this specimen during a field trip earlier in the spring, but was unable to collect it. On our first day of our expedition back in late May, we spotted it easily, and given the easier tides, we were able to quickly excavate it. I was immediately struck with the size of the specimen, and in particular a large knob called the dorsal posterior prominence. This very distinctive earbone morphology is characteristic of the extinct rorqual "Megaptera" miocaena, which Remington Kellogg described in the 1920's from the late Miocene Sisquoc Formation of southern California. Several authors including Deméré et al. (2005) and Dooley et al. (2004) have suggested that it does not belong in Megaptera at all, and that it requires a new genus to be erected. However, it has appeared in some phylogenetic analyses (Bisconti, 2008; Marx, 2010) as a sister taxon to modern Megaptera novaeangliae (the modern Humpback Whale for the uninitiated). It really needs to be reanalyzed and probably redescribed.

Fossil tympanics of "Megaptera" miocaena from the upper Miocene San Mateo Formation (left) and Purisima Formation (right).

Fossils of this taxon are now known from Tortonian and Messinian (6-11 Million Years Old) strata in California, including the Sisquoc Formation, Purisima Formation (two localities), and the Santa Margarita Sandstone (a new specimen of which will soon be donated to UCMP). Other vertebrates from this time period include the dusignathine walruses Pontolis and Gomphotaria, the odontocetes Denebola, Parapontoporia, Albireo, and Piscolithax, as well as other mysticetes such as Nannocetus and Herpetocetus; all of these taxa are now known from multiple strata of this age, suggesting a distinct, and well-represented late Miocene marine mammal fauna from the eastern North Pacific.

Lastly, we spotted some wildlife during the trip, the photos of which are below.

Northern elephant seals spotted from the point.

A peregrine falcon near a nest at the edge of a cliff.

A coyote that ran along the beach while we stopped for a snack.

References:

Bisconti, M. 2008. Morphology and phylogenetic relationships of a new eschrichtiid genus (Cetacea: Mysticeti) from the Early Pliocene of northern Italy. Biol J Linn Soc 153: 161–186.

Deméré, T.A., A. Berta, and M.R. McGowan. 2005. The taxonomic and evolutionary history of fossil and modern balaenopteroid mysticetes. Journal of Mammalian Evolution 12:99–143.

Dooley, A. C., Jr., Fraser, N. C., and Luo, Z.-X. 2004. The earliest known member of the rorqual-gray whale clade (Mammalia, Cetacea). J. Vertebr. Paleontol. 24: 453–463.

Kellogg, R. 1922. Description of the skull of Megaptera miocaena, a fossil humpback whale from the Miocene diatomaceous earth of Lompoc, California. Proc. US Natl. Mus. 61: 1–18.

Marx, F. G. 2010. The more the merrier? A large cladistic analysis of mysticetes,
and comments on the transition from teeth to baleen. J Mammal Evol 18:
77–100.