Presenter: Michael Griffiths (William Paterson University)

Description:
The evolution and extinction of megatooth sharks, most notably Otodus megalodon (Lamniformes: Otodontidae), remain enigmatic. It has been proposed that the ability of megatooth sharks to thermoregulate acted as a key driver for the evolution of gigantism that impacted their ecological role and success in surviving environmental changes. Yet, despite advances in paleohistology, it is still unclear if, for example, O. megalodon was ectothermic or endothermic, and whether its thermophysiology could help to explain the iconic shark’s demise during the Pliocene. Here, we present novel geochemical constraints on the influence of ocean temperatures on modern shark tooth bioapatite ‘clumped’ isotope (Δ₄₇) signatures, as well as evidence for thermoregulation in O. megalodon from both Δ₄₇ paleothermometry and phosphate oxygen isotopes. Our results show that O. megalodon maintained an overall warmer body temperature than ambient seawater, providing new quantitative support of recent biophysical modeling studies that suggest endothermy was a key driver for gigantism.  Trophic position is also fundamental characteristic of animals, yet is unknown in many extinct species. Here we also reconstruct the trophic level of extinct megatooth sharks through the Cenozoic by measuring various biogeochemical tracers of food web dynamics, including the ¹⁵N/¹⁴N of enamel-bound organic matter (δ¹⁵NEB), and zinc (δ⁶⁶Zn) and calcium (δ⁴⁴Ca) isotope ratios. Very high (low) δ¹⁵NEB (δ⁶⁶Zn, δ⁴⁴Ca) values in O. megalodon and chronospecies O. chubutensis indicate that they occupied a higher trophic level than is known for any marine species, extinct or extant. Collectively, these results help to contextualize current working hypotheses for the extinction of O. megalodon—e.g., sea-level fluctuations during the Pliocene resulted in global habitat loss and/or a drop in the diversity of potential prey or appearance of new competitors (e.g., Carcharodon carcharias).