Abstract
The largest and most iconic extinct shark to have ever lived is Otodus megalodon (Lamniformes). Fossil teeth and vertebrae indicate this highly successful apex predator achieved lengths up to nearly 20 m,weights exceeding 20,000 kg, and global distribution throughout the Miocene and Pliocene (23-2.5 MYA). A general consensus indicates that the ability to thermo regulate in O. megalodon acted as a key driver for the evolution of gigantism that impacted its ecological role and success in surviving environmental changes.There is little agreement as to the primary cause for O. megalodon's disappearance but models suggest predator-prey dynamics or environmental change influenced its extinction. To address these uncertainties, we propose to constrain body temperatures of both living and extinct marine vertebrates across the mid-Miocene and Pliocene using 'clumped' isotope thermometry (CIT) in mid-latitude carcharhiniform and lamniform shark teeth from the Pacific and Atlantic Oceans. Preliminary results from several clumped isotope measurements conducted on aquarium-reared and wild-caught shark teeth provide encouraging results that do not deviate from previously published calibrations of bioapatite. Applying these same calibration equations to the fossil record provides encouraging results. Indeed, comparison of extant Carcharodon carcharias (white shark) with O. megalodon from Pliocene deposits of Japan, reveal that the much larger O. megalodon maintained a higher body temperature compared with its modern lamniform equivalent, C. carcharias, owing to its much larger body size. While still preliminary, these results may provide clues as to what may have led to the demise of O. megalodon during the Pliocene. For example, one hypothesis is that O. megalodon consumed large quantities of prey in order to maintain such a high body temperature. However, cooling of ocean temperatures during the Pliocene would have constrained the species to lower latitudes where ocean temperatures were warmer, whilst its preferred prey (e.g., whales) evolved traits to adapt to cooler temperatures of the higher latitudes. Therefore, large climatic shifts combined with evolutionary limitations may provide the "smoking gun" for the extinction of the largest shark species to ever roam the planet.