Friday, March 13, 2015

There are sharks in those hills!

It was late January, when, alongside some of my colleagues from Vertebrate Paleontology, we headed north towards California's Central Valley, leaving behind the heavy traffic and skyscrapers of LA. More specifically, we were heading to Bakersfield to spend part of the weekend excavating at the world-renowned paleontological site of Sharktooth Hill.
Left: shark teeth are abundant in this area, hence the name. Right: the Sharktooth Hill National Natural Landmark, some of the earlier localities are around this hill, like the pits on the middle of the hillside towards the right. These photos were taken in the spring of 2014.
The hills northeast of Bakersfield where Sharktooth Hill is located; this photo was taken earlier this year (my personal favorite time to go there). 
A brief introduction to STH
Ancient marine deposits known as the Round Mountain Silt are exposed throughout the hills northeast of Bakersfield. Within this deposit there is a particularly dense accumulation of bones of (mainly) marine organisms, known as the Sharktooth Hill (STH) Bonebed. Since the early 1900's, many fossils have been discovered and described from this site, including cetaceans, birds, pinnipeds, and turtles (e.g. Kellogg, 1931; Howard, 1966; Mitchell, 1966; Barnes, 1988; Lynch and Parham, 2003). The bonebed is regarded as one of the most densest accumulation of marine organisms, and is one of the densest accumulation of fossil whales, rivaled only by Cerro Ballena in Chile (Pyenson et al., 2009; Pyenson et al., 2014)!
The Sharktooth Hill Bonebed, here we can see some large ribs of a mysticete being cleaned and you can get an idea of the high density of bones. 
This accumulation was deposited in the seafloor between 15.9-15.2 million years ago, and for many years after its discovery, there were a variety of hypotheses as to what caused such a high concentration of bones. It wasn't until 2009, when Nick Pyenson and his colleagues published the results of a very thorough study detailing how it came to be. The major finding was that the bonebed formed during a period of low sediment deposition that lasted about 700 thousand years. Because of the slow sedimentation, the bones of the dead organisms were not buried soon after death and were easily scavenged and scatters throughout the ocean floor, resulting in a mélange of bones. The paper also highlights that the assemblage may not represent a single snapshot ancient ocean life in California, but that researchers have to keep in mind that these organisms died over a period of several hundred thousand years. This makes the STH bonebed different from other dense marine tetrapod assemblages such as Cerro Ballena where the bonebeds were formed over much shorter spans and can be considered as more precise snapshots in time.

The ongoing NHM dig
The Natural History Museum of LA has had a long history of digging at STH, going back more than 50 years, and we probably hold the largest collection of material from that site. Recently, after a long hiatus, we've had the opportunity to start an excavation at a new site in the area*. At this new site the bonebed is close to the surface, which reduces the amount of time we spend removing overburden.
Setting up our quarry. Vanessa (middle) and Sam (far right) prepare the grid that marks our site. Lisa (far left) picks to tools of the trade she'll be using this day.
For now, a lot of what we do during our digs is carefully removing overburden and exposing the bonebed square meter by square meter. It may sound a bit slow, but we do it this way as part of our plan is to eventually be able to reconstruct the bonebed digitally and have a really good record of how the different bones at our excavation were related to each other before removal. So as you may guess, once a square meter(s) of the bonebed are exposed, we make sure we take a lot of photos before removing the fossils from the ground. The work is methodical and can be tedious at some times (i.e. when you're not finding bones) but in the end it is very rewarding.
Vanessa and Sam work on the northeast corner of our quarry. To the left are a bunch of mysticete ribs, and a few vertebrae and skull fragment all jumbled up.
What's next?
Even after so many years of work at the STH Bonebed, there are still mysteries to solve, new species to be described and/or redescribed based on new finding. So, as we continue our dig at STH, stay tuned for more updates as well as upcoming publication on fossils from this amazing and unique deposit!

*Our dig is possible thanks to the generosity of the landowners who are really supportive of our work out there, and also very enthusiastic, often participating in the dig as well; and to people who have donated money to cover for expenses.

References

Barnes, L. G. 1988. A new fossil pinniped (Mammalia: Otariidae) from the middle Miocene Sharktooth Hill Bonebed, California. Contributions in Science 396:1-11.

Howard, H. 1966. Additional avian records from the Miocene of Sharktooth Hill, California. Contributions in Science 114:1-11.

Kellogg, R. 1931. Pelagic mammals from the temblor Formation of the Kern River region, California. Proceedings of the California Academy of Sciences 19:217-397.

Lynch, S. C., and J. F. Parham. 2003. The first report of hard-shelled sea turtles (Cheloniidae sensu lato) from the Miocene of California, including a new species (Euclastes hutchisoni) with unusually plesiomorphic characters. PaleoBios 23:21-35.

Mitchell, E. D. 1966. The Miocene pinniped Allodesmus. University of California Publications in Geological Sciences 61:1-46.

Pyenson, N. D., C. S. Gutstein, J. F. Parham, J. P. Le Roux, C. Carreño Chavarría, A. Metallo, V. Rossi, H. Little, A. M. Valenzuela-Toro, J. Velez-Juarbe, C. M. Santelli, D. Rubilar Rogers, M. A. Cozzuol, and M. E. Suárez. 2014. Repeated mass stranding of Miocene marine mammals from the Atacama of Chile point to sudden death at sea. Proceedings of the Royal Society B 281:20133316.

Pyenson, N. D., R. B. Irmis, L. B. Barnes, E. D. Mitchell Jr., S. A. McLeod, and J. H. Lipps. 2009. Origin of a widespread marine bonebed deposited during the middle Miocene Climatic Optimum. Geology 37:519-522.

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