Dietary niche partitioning among large sharks in the Late Cretaceous Western Interior Seaway of North America documented by zinc isotopes

The Western Interior Seaway (WIS) was a major epicontinental sea that divided North America during the Late Cretaceous with a rich ecosystem that hosted a wide variety of marine life. The seaway was home to a diverse range of species, from microscopic planktonic organisms to giant reptiles and sharks. However, food web structures and trophic interactions among Late Cretaceous marine taxa remain largely ambiguous due to the challenges in reconstructing ecological interactions in the fossil record. Fossil evidence of predator-prey interactions such as preserved bite marks, stomach content or faeces is limited. For sharks, trophic interactions can be inferred from morphological comparisons of teeth with modern counterparts. Yet, none of these methods alone can decisively identify the overall diet nor can they quantify a species’ trophic position. Thanks to recent methodological advancements, an animal’s trophic position can now be reconstructed on geologic timescales by analysing geochemical proxies preserved in dental enamel(oid). Among these novel proxies are zinc isotope ratios (⁶⁶Zn/⁶⁴Zn), reported as δ⁶⁶Zn value, a trophic-level proxy that is increasingly applied to address archaeological and palaeobiological research questions.

Here we use enamel(oid) δ⁶⁶Zn values to investigate the food web structures and trophic positions among lamniform sharks within the WIS. We focus on specimen from two Upper Cretaceous localities in the U.S., the Tocito Sandstone-Mulatto Tongue of the Mancos Shale in New Mexico and the Codell Sandstone Member of the Carlile Shale in Kansas. The fossil assemblages are dated to the Turonian-Coniacian transition, just prior to the radiation of Mosasauroidea (extinct marine lizards) to becoming the dominant marine predator of the WIS. Our results demonstrate well-preserved enamel(oid) δ⁶⁶Zn values in both localities, but locality-specific differences in the diagenetic modification of dentine δ⁶⁶Zn values. We highlight significant resource partitioning among the 16 analysed taxa within the WIS. Archaeolamna cf. A. kopingensis, Cretodus sp. and Cretoxyrhina mantelli occupied very high trophic positions, whereas Cretalamna cf. C appendiculata was likely foraging opportunistically across several trophic levels. We expand the use of enamel(oid) δ⁶⁶Zn analyses to Mesozoic fossils and demonstrate that the analyses of enameloid δ⁶⁶Zn values of multiple taxa within fossil assemblages enables robust reconstructions of food web dynamics and trophic interactions, providing new avenues for palaeobiological and evolutionary research in deep time.