SHARK DIVERSITY AND WATER DEPTH ESTIMATION AT THE MIDDLE MIOCENE CARMEL CHURCH QUARRY ECOSYSTEM (CAROLINE COUNTY, VIRGINIA)

Fossil shark teeth are highly abundant along exposed outcrops of the middle Miocene Calvert Formation in northern Virginia and central Maryland, yet the vast majority is found as ‘float’, disassociated from their stratigraphic beds. In Caroline County, Virginia, the Calvert Formation (Bed 15) is broadly exposed at the Carmel Church Quarry fossil site. The fauna is nearly exclusively marine, including partial whale skeletons, sea turtles, bony fish, and rays. Decades of fossil collecting by the Virginia Museum of Natural History at this site has resulted in a large collection of specimens. A subsample of shark teeth collected from within the matrix was selected, after removing specimens that had been potentially reworked from the underlying Eocene Nanjemoy Formation.

In total, 776 reliably Miocene shark teeth were identified. Of these, the mako shark, Isurus, was the most common (38%), followed by tiger sharks (Galeocerdo & Physogaleus; 23%), bull shark (Carcharhinus; 18.3%), snaggle-tooth shark (Hemipristis; 9.4%), lemon shark (Negaprion; 7.1%), and cow shark (Notorhynchus; 1.5%). Rarities included the sawtooth shark (Pristis), mega-toothed sharks (Carcharocles megalodon, C. subauriculatus), angel shark (Squatina) and thresher shark (Alopias). The sand tiger shark Carcharias was common in the sample, but an unknown portion may be reworked from the Nanjemoy. Using the software Past, we calculated rarefaction, which showed a near-plateau curve within the Miocene sample. 95% confidence intervals indicated a likelihood of only 1–2 more potential species being recovered with continued sampling.

Comparing water depth records of the living relatives of these sharks results in an estimated past water depth between 10 and 92 meters, with a greater likelihood of 10–30 meters. These are largely due to the observations of Negaprion, Pristis, and Squatina in shallower habitats, but further supported by the shallower preferences of Notorhynchus, Alopias, Hemipristis, and Galeocerdo. Future directions for the site will be to conduct rare earth element analyses to better understand the relative abundance of Carcharias within the Miocene ecosystem. Studies such as these can lead to a better understanding of the environmental preferences of different sharks through time and possibly even aid in modern conservation.