Collaborative Research: Biogeochemical Fingerprinting of the Megatoothed ("Megalodon") Shark: A Dual Study in Thermophysiological Evolution and Seawater Chemistry

标志性的巨齿鲨--地球上现存的最大鲨鱼物种--和其他已灭绝的鲨鱼的生物学在很大程度上仍不为人所知。例如，目前还不清楚巨齿鲨是冷血还是温血，以及其他鲨鱼化石物种的比较情况。了解这些生物学细节，再加上环境重建，可以帮助研究人员了解是什么导致巨齿鲨在上新世晚期(约260万年前)灭绝，从而有助于更好地理解大型鲨鱼物种对生态系统变化的敏感性。该项目使用牙齿的同位素指纹不仅重建了体温，还重建了巨齿鲨和其他鲨鱼物种在过去1500万年的饮食行为和海水化学。此外，这项工作还支持一名博士生、一家西班牙裔服务机构的博士后研究员，以及为少数族裔(URM)学生提供本科生研究经验。鲨鱼牙齿是最丰富的脊椎动物化石，它们对成岩改造的抵抗力为地球化学分析提供了底物。该项目利用一个耦合的古生态和地球化学框架，探索了板鲨内吸热的系统发育历史。具体地说，这项工作包括四个目标：(1)使用‘块状’同位素测温法估计现代和祖先海洋脊椎动物的体温(即13C和18O在碳酸盐矿物晶格中形成键或‘块’的热力学偏好)，并使用钙同位素(44Ca/42Ca)和生物磷灰石中的3D微磨损分析来评估觅食行为；(2)利用鲨鱼牙磷酸盐和碳酸盐的氧同位素(18O)重建和比较鲨鱼的栖息地；(3)从生物磷灰石中推断中新世气候最佳时期(距今1700万~1500万年)至上新世(距今约530万~260万年)海水锶/钙的变化；(4)建立少数民族学生研究共同体。与其他稳定的同位素和生物地球化学指标相结合，探索地质年代中鲨鱼的古生态，这是第一次。此外，这个项目恰逢人们对围绕巨齿的兴起和灭绝的争论兴趣高涨之际。因此，这项研究将古生物学问题与地球化学技术相结合，使科学家更接近于解决“巨齿鲨灭绝假说”。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The biology of iconic megatoothed 'megalodon' shark-the largest shark species to have existed on Earth-and other extinct sharks remain largely unknown. For example, it is unclear whether 'megalodon' was 'cold blooded' or 'warm blooded' and how other fossil shark species compare. Understanding these biological details coupled with environmental reconstructions could help researchers understand what led to the demise of 'megalodon' during the late Pliocene (about 2.6 million years ago), and thus help to better understand the sensitivity of large shark species to ecosystem changes. This project uses isotopic 'fingerprinting' of teeth to reconstruct not only the body temperatures, but also dietary behavior and seawater chemistry of 'megalodon' and other shark species during the past 15 million years. In addition, this work supports a PhD student, a postdoctoral research fellow at a Hispanic Serving Institution, and undergraduate research experiences for under-represented minority (URM) students. Shark teeth are the most abundant vertebrate fossil and their resistance to diagenetic alteration provides a substrate for gechemical analysis. This project explores the phylogenetic history of endothermy within lamniform sharks using a coupled paleoecological and geochemical framework. Specifically, this work consists of four objectives: (1) estimate body temperatures of modern and ancestral marine vertebrates using 'clumped' isotope thermometry (i.e., thermodynamic preference of 13C and 18O to form bonds, or 'clump', in the carbonate mineral lattice), and assess foraging behavior using calcium isotopes (44Ca/42Ca) and 3D microwear analysis in bioapatite; (2) reconstruct and compare shark habitats using oxygen isotopes ( 18O) of shark tooth phosphate and carbonate; (3) infer changes in seawater Sr/Ca between the mid-Miocene Climate Optimum (17-15 million years before present) and Pliocene (~5.3-2.6 million years before present) from bioapatite; (4) build a research community of under-represented minority students. Coupling 'clumped' isotope thermometry-a relatively new method to determine body temperature-with other stable isotope and biogeochemical proxies to explore shark paleoecology through geologic time is the first of its kind. In addition, this project coincides with an upsurge of interest in the debate surrounding the rise and extinction of 'megalodon.' This research, therefore, combines paleontological questions with geochemical techniques to bring scientists one step closer to tackling the 'megalodon extinction hypothesis'.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.