Sharks as apex predators in the Late Cretaceous Western Interior Seaway empirically revealed by zinc isotope analyses

• Enamel(oid) δ 66 Zn values are well preserved in Late Cretaceous marine vertebrates. • Zinc isotopes reveal significant resource partitioning among Cretaceous marine taxa. • Late Cretaceous sharks occupied top trophic positions comparable to today. Food web structures and trophic interactions among Late Cretaceous marine taxa remain largely ambiguous due to the challenges in reconstructing ecological interactions in the deep-time fossil record. Here we analyse enamel(oid) δ 66 Zn values, a trophic-level proxy, across 16 marine vertebrate taxa from two Turonian–Coniacian boundary localities in New Mexico and Kansas, USA, deposited in what was then the Western Interior Seaway (WIS). Our results demonstrate well-preserved enamel(oid) δ 66 Zn values in both localities, but locality-specific differences in the diagenetic modification of dentine δ 66 Zn values. We highlight significant resource partitioning among the analysed taxa within the WIS, including sharks. For example, Archaeolamna , Cretodus and Cretoxyrhina occupied very high trophic positions, whereas Cretalamna was likely foraging opportunistically across several trophic levels. Our study expands the use of enamel(oid) δ 66 Zn analysis to Mesozoic fossils for the first time and demonstrates that robust reconstructions of food web dynamics and trophic interactions are possible in deep time as far back as 90 million years ago, providing new avenues for palaeobiological and evolutionary research.