Macroevolution and geochemistry of Cenozoic giant sharks

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ABSTRACTFossil sharks of the Family Lamnidae have a rich Cenozoic global history and are represented today by species such as the great white (Carcharodon carcharias). During the Miocene, the extinct lamnid shark C. megalodon attained truly enormous proportions. With a mean estimated body size of ~40 tons, this extinct species rivals some dinosaurs and whales as one of the largest animals that ever lived. The fundamental question that we seek to answer is .how did large body size evolve within lamnid sharks like C. megalodon?. Relative to its smaller ancestor, individuals of C. megalodon could have attained very large body size by growing faster (increased rate), for a longer time (increased longevity), or a combination of these two. These growth modes will be determined within the lamnid clade using seven extinct and modern species of Carcharodon and its sister-taxon Isurus compared to the extinct outgroup Otodus obliquus. Macroevolution is the study of evolutionary patterns and processes observed within closely related groups of species (clades). In order to understand lamnid body size macroevolution, two key features need to be determined for any given individual: (1) How many years old was it when it died? This will be done by studying chemical signatures preserved in vertebral centra; (2) How large was it? This will be determined from associated teeth, whose dimensions are highly correlated to body size. Shark skeletons are composed mostly of cartilage, which is not prone to fossilization. Nevertheless, the two elements needed for this study are exceptions to this rule: (1) vertebral centra, which during life replace cartilage with bone; and (2) durable teeth. Although incremental growth rings preserved in centra are oftentimes annular, they sometimes are not. A simple counting of the rings, therefore, could potentially lead to erroneous individual age determinations. To circumvent this problem, proxy seasonal and annual cycles represented by the oxygen isotopic signatures preserved in centra will be used to calibrate the individual age of each specimen. This will be done after centra are analyzed geochemically to determine, relative modern lamnids, the extent of diagenesis (alteration during fossilization) and how this process affected oxygen isotopic signatures archived in fossil centra. These analyses will include = determination of physical and chemical characteristics such as crystallinity, carbonate content, trace, minor, and rare earth elemental concentrations, and oxygen isotopes. Our pilot studies indicate that: (1) oxygen isotopic signatures can remain in centra even when highly altered; and (2) certain elements and compounds known to occur in unaltered bone can be used as tracers to model diagenesis in fossil bone. Intellectual Merit.--This project will integrate concepts, methods, and data from paleobiology and geochemistry. Body size macroevolution is of fundamental interest to scientists, e.g., how it might relate to the concept of Cope's "Law" (evolution of increased body size within clades). This study will provide a case study using a clade that evolved what may be the largest body size ever. The related geochemical part of this project will quantify diagenesis in bone and calibrate incremental growth in fossil sharks. These results will facilitate more informed interpretations of the isotopic signatures used forpaleobiological interpretations in other extinct vertebrates.Broader Impacts. One graduate student will use part of this project for her Ph.D. research. An undergraduate student research assistant will be recruited. This research will be presented to UF students in graduate seminars and to other colleagues/students at national meetings and invited seminars. K-12 teachers will be involved in summer research experiences. Shark teeth are highly prized by fossil collectors and inmegalodonlt evokes much public interest. This research will be disseminated to the public through general lectures, fossil festivals, field trips, newsletters, and exhibits at the Florida Museum of Natural History. The public outreach will be promoted on our website (www.flmnh.ufl.edu), which receives 7.2 million cybervisits (60 million iihitsls) per year.