Eocene Shark Teeth From Peninsular Antarctica: Windows to Habitat Use and Paleoceanography

Eocene climate cooling, driven by the falling <i>p</i>CO₂ and tectonic changes in the Southern Ocean, impacted marine ecosystems. Sharks in high-latitude oceans, sensitive to these changes, offer insights into both environmental shifts and biological responses, yet few paleoecological studies exist. The Middle-to-Late Eocene units on Seymour Island, Antarctica, provide a rich, diverse fossil record, including sharks. We analyzed the oxygen isotope composition of phosphate from shark tooth bioapatite (δ¹⁸O_p) and compared our results to co-occurring bivalves and predictions from an isotope-enabled global climate model to investigate habitat use and environmental conditions. Bulk δ¹⁸O_p values (mean 22.0 ± 1.3%o) show no significant changes through the Eocene. Furthermore, the variation in bulk δ¹⁸O_p values often exceeds that in simulated seasonal and regional values. Pelagic and benthic sharks exhibit similar δ¹⁸O_p values across units but are offset relative to bivalve and modeled values. Some taxa suggest movements into warmer or more brackish waters (e.g., <i>Striatolamia, Carcharias</i>) or deeper, colder waters (e.g., <i>Pristiophorus</i>). Taxa like <i>Raja</i> and <i>Squalus</i> display no shift, tracking local conditions in Seymour Island. The lack of difference in δ¹⁸O_p values between pelagic and benthic sharks in the Late Eocene could suggest a poorly stratified water column, inconsistent with a fully opened Drake Passage. Our findings demonstrate that shark tooth bioapatite tracks the preferred habitat conditions for individual taxa rather than recording environmental conditions where they are found. A lack of secular variation in δ¹⁸O_p values says more about species ecology than the absence of regional or global environmental changes.