Applying zinc isotopes to investigate the trophic positions of extinct marine vertebrates, including the megatooth shark Otodus megalodon, in ancient marine ecosystems

Reconstructing ancient food web structures and trophic interactions of extinct taxa is challenging and typically relies on rare fossil evidence such as preserved bite marks, stomach content or faeces. In addition, trophic and diet inferences may be drawn from an anatomical approach, i.e., functional morphology. Yet none of these methods alone can decisively identify the overall diet nor can they quantify a species’ trophic position. To address these uncertainties we utilize new geochemical methods to identify an animal’s trophic position on timescales of millions of years from the analyses of dental enamel(oid). Here we focus on zinc isotope ratios (⁶⁶Zn/⁶⁴Zn), reported as δ⁶⁶Zn value, a trophic-level proxy that is increasingly being applied in archaeological and palaeobiological research. Specifically, we use enamel(oid) δ⁶⁶Zn values to investigate ancient marine food web structures and the roles of their inhabitants in them, including gigantic extinct predators. Extinct gigantic Mesozoic and Cenozoic marine predators reaching sizes beyond any macropredatory animal existing today have no apparent modern counterparts. Indeed their size alone may imply a very different resource use for these animals or ecosystem structures vastly different from present ones.

The diet and trophic position of the megatooth shark Otodus megalodon is still subject of debate and is often linked to its evolution towards gigantism and its extinction. Here we present novel Early Miocene megatoothed shark δ⁶⁶Zn values from the enameloid of O. megalodon and Otodus chubutensis teeth from two time-equivalent localities in the Paratethys in comparison to the isotopic composition of 20 other sympatric marine vertebrate species with variable trophic ecology and dietary preferences. The inclusion of a larger number of taxa permits a more robust reconstruction of food web dynamics amongst its inhabitants, along with greater insight into the ecological roles of dietary-ambiguous extinct taxa and related length of extinct food chains. We thus contextualise the trophic position of O. megalodon/chubutensis within its food web in greater detail than previously possible. In general, our results support the notion that O. megalodon/chubutensis occupied a high trophic position, though possibly not as high as previously estimated. Perhaps more importantly in terms of its evolution, our findings suggest that Otodus possessed a higher dietary flexibility on a population level than previously understood.