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Abstract Eocene climate cooling, driven by the fallingpCO₂and 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_pvalues (mean 22.0 ± 1.3‰) show no significant changes through the Eocene. Furthermore, the variation in bulk δ¹⁸O_pvalues often exceeds that in simulated seasonal and regional values. Pelagic and benthic sharks exhibit similar δ¹⁸O_pvalues across units but are offset relative to bivalve and modeled values. Some taxa suggest movements into warmer or more brackish waters (e.g.,Striatolamia,Carcharias) or deeper, colder waters (e.g.,Pristiophorus). Taxa likeRajaandSqualusdisplay no shift, tracking local conditions in Seymour Island. The lack of difference in δ¹⁸O_pvalues 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_pvalues says more about species ecology than the absence of regional or global environmental changes. 

Abstract Many explanations for Eocene climate change focus on the Southern Ocean—where tectonics influenced oceanic gateways, ocean circulation reduced heat transport, and greenhouse gas declines prompted glaciation. To date, few studies focus on marine vertebrates at high latitudes to discern paleoecological and paleoenvironmental impacts of this climate transition. The Tertiary Eocene La Meseta (TELM) Formation has a rich fossil assemblage to characterize these impacts;Striatolamia macrota, an extinct (†) sand tiger shark, is abundant throughout the La Meseta Formation. Body size is often tracked to characterize and integrate across multiple ecological dimensions. †S. macrotabody size distributions indicate limited changes during TELMs 2–5 based on anterior tooth crown height (n = 450, mean = 19.6 ± 6.4 mm). Similarly, environmental conditions remained stable through this period based on δ¹⁸O_PO4values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminaryε_Nd(n = 4) results indicate an early Drake Passage opening with Pacific inputs during TELM 2–3 (45–43 Ma) based on single unit variation with an overall radiogenic trend. Two possible hypotheses to explain these observations are (1) †S. macrotamodified its migration behavior to ameliorate environmental changes related to the Drake Passage opening, or (2) the local climate change was small and gateway opening had little impact. While we cannot rule out an ecological explanation, a comparison with climate model results suggests that increased CO₂produces warm conditions that also parsimoniously explain the observations.