ASSESSING THE EFFECTS OF LATITUDE AND ENVIRONMENT ON SIZE DISTRIBUTIONS OF EOCENE SAND TIGER SHARKS

The early Eocene marks a warm period in Earth’s history with greenhouse climate conditions at high latitudes. Fossils from the Eocene Arctic and Antarctic include temperate flora and fauna from marine and terrestrial ecosystems. These regions were warmer than today, but had dark winters with limited productivity, which likely affected physiology and growth. The most abundant marine vertebrate fossils in Eocene high latitude sites are sand tiger shark teeth, including Striatolamia macrota. These teeth yield body size and environmental data in their tooth size and δ¹⁸O values, respectively. Here, we compare size distributions from estuarine/deltaic and marine localities at high and mid-latitudes to assess effects of latitude and environment. Based on prior studies, the fossil shark-bearing Eocene strata of the Eureka Sound Fm. (ca. 52-53 Ma) on Banks Island in Canada’s western Arctic is inferred as a brackish, deltaic environment, whereas La Meseta Fm. (44-56 Ma) on Seymour Island, Antarctica is an open marine environment. We also chose two Gulf of Mexico sites: the early Eocene Tuscahoma Fm. at Red Hot Truck Stop (MS) and middle Eocene Stone Creek Fm. at Whiskey Creek Bridge (TX). We identified and measured anterior tooth crown height from S. macrota as a body length proxy (n > 140 per locality) and analyzed δ¹⁸O_PO4 values from a subset of teeth (n≥5 per locality). There are differences in the mid-latitude sand tiger teeth size distribution (mean ± 1s), with smaller teeth in the estuarine Tuscahoma Fm. (11.6±3.4) than the marine Stone Creek Fm. (22.5±4.6); however, δ¹⁸O values suggest minimal temperature and salinity differences. In contrast, the two high latitude sites have overlapping size distributions; the Arctic Eureka Sound Fm. has a smaller range of tooth size (13.7±3.4) and low salinity whereas La Meseta Fm. teeth span a wider and larger range (17.1±6.6) and marine salinity. This size discrepancy could be due to segregated populations with juvenile nurseries or smaller individuals in estuarine/deltaic environments. Alternative explanations may be taphonomic or collecting bias. It is interesting to note the largest teeth were from the Antarctic La Meseta Fm. but the Stone Creek Fm. had larger mean tooth size suggesting a complex interaction between latitudinal and environmental effects.