Enameloid-bound δ N reveals large trophic separation among Late Cretaceous sharks in the northern Gulf of Mexico.

The nitrogen isotopic composition ( N/ N ratio, or δ N) of enameloid-bound organic matter (δ N ) in shark teeth was recently developed to investigate the biogeochemistry and trophic structures (i.e., food webs) of the ancient ocean. Using δ N , we present the first nitrogen isotopic evidence for trophic differences between shark taxa from a single fossil locality. We analyze the teeth of four taxa (Meristodonoides, Ptychodus, Scapanorhynchus, and Squalicorax) from the Late Cretaceous (83-84 Ma) Trussells Creek site in Alabama, USA, and compare the N isotopic findings with predictions from tooth morphology, the traditional method for inferring shark paleo-diets. Our δ N data indicate two distinct trophic groups, with averages separated by 6.1 ± 2.1‰. The lower group consists of Meristodonoides and Ptychodus, and the higher group consists of Scapanorhynchus and Squalicorax (i.e., lamniforms). This δ N difference indicates a 1.5 ± 0.5 trophic-level separation between the two groups, a finding that is in line with paleontological predictions of a higher trophic level for these lamniforms over Meristodonoides and Ptychodus. However, the δ N of Meristodonoides is lower than suggested by tooth morphology, although consistent with mechanical tests suggesting that higher trophic-level bony fishes were not a major component of their diet. Further, δ N indicates that the two sampled lamniform taxa fed at similar trophic levels despite their different inferred tooth functions. These two findings suggest that tooth morphology alone may not always be a sufficient indicator of dietary niche. The large trophic separation revealed by the δ N offset leaves open the possibility that higher trophic-level lamniforms, such as those measured here, preyed upon smaller, lower trophic-level sharks like Meristodonoides.