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. macrota body 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_PO4 values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminary ε_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. macrota modified 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 CO₂ produces warm conditions that also parsimoniously explain the observations.

中文翻译：

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用沙虎鲨探索始新世南大洋的生态和气候


对始新世气候变化的许多解释都集中在南大洋——那里的构造影响了海洋门户，海洋环流减少了热传输，温室气体减少促进了冰川作用。迄今为止，很少有研究关注高纬度的海洋脊椎动物来辨别这种气候转变对古生态和古环境的影响。第三纪始新世拉梅塞塔 (TELM) 地层拥有丰富的化石组合来表征这些影响；纹状体是一种已灭绝的 (†) 沙虎鲨，在拉梅塞塔地层中大量存在。通常通过追踪体型来表征和整合多个生态维度。 † S. macrota体型分布表明，在 TELM 2-5 期间，基于前牙冠高度的变化有限（ n = 450，平均值 = 19.6 ± 6.4 mm）。同样，根据牙齿釉质的 δ ¹⁸ O _PO4值（ n = 42；21.5 ± 1.6‰），在此期间环境条件保持稳定，对应的平均温度为 22.0 ± 4.0°C。我们的初步ε _Nd ( n = 4) 结果表明，基于单个单位变化和总体放射成因趋势，德雷克海峡在 TELM 2-3 (45-43 Ma) 期间随着太平洋输入而开放。解释这些观察结果的两个可能的假设是 (1) † S. macrota改变了其迁徙行为以改善与德雷克海峡开放相关的环境变化，或 (2) 当地气候变化较小，门户开放影响不大。 虽然我们不能排除生态解释，但与气候模型结果的比较表明，CO ₂增加会产生温暖的条件，这也可以简单地解释观测结果。