Probing the ecology and climate of the Eocene Southern Ocean with sand tiger sharks Striatolamia macrota

During the Eocene, the Earth climate system transitioned from greenhouse to icehouse conditions. Central to many explanations is the Southern Ocean—where tectonic configurations influenced oceanic gateways, ocean circulation reduced heat transport, and/or greenhouse gas declines prompted glaciation. To date, few studies have explored the implications of this climate transition on high latitude, marine vertebrates. Seymour Island near the Antarctic Peninsula preserves a rich, diverse fossil assemblage in the Tertiary Eocene La Meseta (TELM) Formation (Fm). We examine teeth of Striatolamia macrota, an extinct (†) sand tiger shark that is abundant throughout the La Meseta Fm. to discern ecological and environmental characteristics. Body size is an integrative ecological trait often tracked through time and space. †Striatolamia macrota body size distributions indicate no shifts during TELMs 2-5 based on anterior tooth crown height (n=450, mean=19.6 ± 6.4 mm, total body length 88-389 cm). Similarly, environmental conditions seem to remain stable through this period based on †S. macrota enameloid with mean d18OPO4 value of 21.5 ± 1.6‰ (n=39) and mean temperature estimates of 22.2 ± 5.4°C. This ecological and environmental stasis is intriguing given changes in ocean circulation as tracked by neodymium isotope composition. Our preliminary eNd (n=4) results indicate an early Drake Passage opening with Pacific inputs as early as TELM 2 based on 1 unit variation with an overall radiogenic trend, similar to previous results from deep ocean localities. We hypothesize that †S. macrota modified its migration behavior to preclude environmental changes related to the Drake Passage opening. Our results shed important light on paleoceanographic conditions and indicate paleoecological responses to climate change.

Methods
see methods in Kim, S. L., Zeichner, S. S., Colman, A. S., Scher, H. D., Kriwet, J., Mörs, T., & Huber, M. (2020). Probing the ecology and climate of the Eocene Southern Ocean with sand tiger sharks Striatolamia macrota. Paleoceanography and Paleoclimatology, 35, e2020PA003997. https://doi.org/10.1029/2020PA003997