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

巨齿鲨是地球上现存的最大的鲨鱼物种，其生物学特征和其他已灭绝的鲨鱼基本上仍不为人所知。例如，目前还不清楚“巨齿鲨”是“冷血”还是“温血”，以及其他化石鲨鱼物种的比较情况。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上新世晚期（约260万年前）导致“巨齿鲨”灭绝的原因，从而有助于更好地了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”来重建过去1500万年来“巨齿鲨”和其他鲨鱼物种的体温，以及饮食行为和海水化学。此外，这项工作支持博士生，在西班牙裔服务机构的博士后研究员，并为代表性不足的少数民族（URM）学生的本科研究经验。 鲨鱼牙齿是最丰富的脊椎动物化石，其对成岩蚀变的抵抗力为生物化学分析提供了基础。本计画以古生态学与地球化学为架构，探讨板状鲨鱼内温性动物的系统发育史。具体来说，这项工作包括四个目标：（1）使用“聚集”同位素测温法（即，13 C和18 O在碳酸盐矿物晶格中形成键或“团”的热力学偏好），并使用钙同位素评估觅食行为（2）利用鲨鱼牙齿磷酸盐和碳酸盐的氧同位素（18 O）重建和比较鲨鱼栖息地;（3）从生物磷灰石推断中新世中期气候适宜期（距今1700 - 1500万年）和上新世（距今约530 - 260万年）之间海水Sr/Ca的变化;（4）建立一个少数民族学生的研究社区。将“聚集”同位素测温法（一种相对较新的确定体温的方法）与其他稳定同位素和地球化学代用指标结合起来，探索地质时期的鲨鱼古生态，这是同类中的第一个。此外，这个项目恰逢围绕“巨齿鲨”的兴起和灭绝的辩论的兴趣高涨。因此，这项研究将古生物学问题与地球化学技术相结合，使科学家们更接近解决“巨齿鲨灭绝假说”。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

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.