Abstract:
Abundant fossil occurrences of Late Cretaceous sharks (Chondrichthyes, Elasmobranchii), especially Hybodontiformes and Lamniformes, in the northern Gulf of Mexico offer an ideal opportunity to study shark diversity patterns leading up to the end-Cretaceous mass extinction. Here, we present a dataset representing 16 genera and 25 species (five hybodontiforms and 20 lamniforms). Species counts and extinction and origination rates are quantified across the Coniacian to Danian. Both groups display an overall increasing trend of extinctions that peak in the Campanian (e.g., the ‘Middle Campanian Crisis’: MCC) but show noticeable differences concerning the magnitude and relative timings. Hybodontiforms show increased diversity losses from the Santonian to middle Campanian before regionally disappearing during the entire Maastrichtian. Lamniforms declined in diversity in the early to middle Campanian (about half the magnitude of the hybodontiform losses) and the end-Maastrichtian. For both taxa, originations declined from the Santonian to middle Campanian and remained near zero toward the end-Cretaceous. Statistical analyses (correlation tests, principal component analysis) were used to determine the potential importance of marine environmental changes, such as sea level, sea surface temperature, marine productivity on the two different extinction trends of hybodontiforms and lamniforms. Lamniform diversity loss is negatively correlated with globally decreasing sea levels. Hybodontiform diversity loss is positively correlated with globally increasing sea levels. Sustained fluctuations in marine productivity and sea surface temperature in the Gulf of Mexico may have contributed to low origination rates for lamniforms following the MCC. Previously published occurrences in upper Campanian to upper Maastrichtian strata in the Northern Atlantic coast indicate that few species did not become extinct globally. This study suggests that changing marine paleoenvironments (esp., sea level changes) could have had a big impact on Cretaceous sharks (particularly benthic durophagous forms that lived in near-shore environments) due to newly emerged unsuitable habitats.
