On the cutting edge: Otodus megalodon strengthened tooth edges through zinc incorporation in enameloid

Tooth structural integrity is critical for animals whose diets include hard substrates such as bone and shell.Because tooth material properties may be shaped by natural selection, they are important to consider in addition to morphological changes during studies of shark tooth evolution.We hypothesize that the megatooth shark Otodus megalodon evolved a structural adaptation by selectively incorporating zinc (Zn) into its fluorapatite enameloid to reinforce regions subjected to high stress.To our knowledge, this is the first study to examine zinc enrichment in mechanically stressed regions of Otodus spp.teeth.Micro X-ray fluorescence (µ-XRF) spectroscopy was used to spatially resolve zinc levels in the labial enameloid of 12 O. megalodon and five O. obliquus teeth.Statistical analyses show significantly lower Zn levels in basal regions of O. megalodon enameloid, near the root.Conversely, the highest Zn levels were observed along the outermost cutting edges and crown apex, regions associated with the highest stress levels during simulations of puncture and draw.In contrast, O. obliquus, a presumed primarily piscivorous ancestor of O. megalodon, showed less pronounced spatial variation of Zn overall, though O. obliquus also showed lower levels of Zn in the basal enameloid compared to other parts of the cusp.Notably, O. obliquus showed no consistent enrichment of Zn along the tooth edges compared to the interior regions.These results suggest that as otodontid sharks transitioned from a fish-based diet to preying on marine mammals, natural selection favored a chemical reinforcement of their teeth in response to increased functional demands.