TROPHIC SEPARATION OF LATE CRETACEOUS SHARKS IN THE NORTHERN GULF OF MEXICO: EVIDENCE FROM THE NITROGEN ISOTOPES OF TOOTH ENAMELOID

Nitrogen isotopes of enameloid-bound organic matter (δ¹⁵N_EB) in fossil shark teeth offer unique insight into their trophic ecology. Traditionally, tooth morphology and preserved evidence of predator-prey interactions are used to infer the feeding behavior of sharks in the fossil record. Here, we apply the δ¹⁵N_EB proxy to two species of Lamniformes (Squalicorax and Scapanorhynchus) and two species of Hybodontiformes (Meristodonoides and Ptychodus) from a single Late Cretaceous fossil locality (Trussells Creek, AL, USA) to test existing morphology-based hypotheses about the feeding ecology of these taxa. Each taxon has a distinct tooth type (e.g., grinding/crushing for Ptychodus, clutching for Meristodonoides, piercing for Scapanorhynchus, and gouging/slicing for Squalicorax) and thus a different inferred diet. Based on these differences, we hypothesized that lamniforms would occupy a higher trophic level (i.e., have a higher δ¹⁵N­_EB value) than hybodontiforms. Here we show that δ¹⁵N_EB is 6.1 ± 2.1 ‰ higher in lamniform than hybodontiform sharks. We estimate that this offset translates to an approximately 1.5 ± 0.5 higher trophic level for lamniforms than co-occurring hybodontiforms, consistent with morphological interpretations. A trophic level offset greater than one is robust for the full range of literature trophic discrimination factors. However, despite clear morphological differences between the two analyzed lamniforms (i.e., Squalicorax and Scapanorhynchus), δ¹⁵N_EB suggests that the two taxa fed at similar trophic levels. Thus, tooth morphology alone may not always accurately reflect a shark’s position in the paleo-food web, and δ¹⁵N_EB analyses provide useful additional information for paleo-dietary reconstruction.