DIET SELECTIVITY FOR LATE CRETACEOUS HYBODONTIFORMES AND LAMNIFORMES SHARK EXTINCTION IN NORTHERN GULF OF MEXICO

Late Cretaceous marine vertebrates from the northern Gulf of Mexico significantly declined during the Middle Campanian Crisis (~78 MYA), but the selectivity of this decline is unclear. Lamniform and hybodontiform sharks, two of the most abundant groups in Alabama, were particularly susceptible to extinction during this time. Paleoecological turnovers are often associated with trophic competition among different species for resources. Here, we investigated the selective nature of shark turnover by assessing diversity dynamics from the Santonian to Late Maastrichtian and using multivariate statistics to analyze tooth morphology. Data were collected from over 2,570 fossil occurrences from central and western Alabama containing 17 genera and 30 species. Multiple metrics of estimating extinction rates in six substage-based time-bins indicate an increasing trend in extinctions from the Early to Middle Campanian. By the beginning of the Early Maastrichtian, hybodontiform species were nearly extinct. Data from 10 different tooth dimensions measured from over 250 individual teeth were used to quantify size (e.g., small, medium, or large) and shape (e.g., triangular, narrow, or flattened). Principal component analysis showed morphologic similarities indicative of a common function (e.g., crushing, grasping, or tearing) and a tendency towards small to medium-sized teeth among species that become extinct. Similarities between morphologies are suggestive of similar dietary preferences. As the paleoenvironment of Late Cretaceous seas changed, large sharks with generalist diets were relatively unaffected. Small to medium-sized individuals with restrictive tooth morphology, suggesting a more specialist diet, were selectively targeted for extinction. These data support the coupling of tooth morphology with dietary selectivity as the primary selective parameter controlling hybodontiform and lamniform extinction during the Middle Campanian Crisis.