DINOSAUR DIVERSITY ACROSS THE JURASSIC–CRETACEOUS BOUNDARY, WESTERN INTERIOR, NORTH AMERICA

Not all of the Lower Cretaceous formations of the Western Interior are conformable with the sole underlying Upper Jurassic formation, the Morrison Formation. However, when all of the Lower Cretaceous formations are taken into account (Antlers, Cedar Mountain, Cloverly, Dakota, Frontier, Lakota, and Purgatoire Formations) the majority of the Early Cretaceous is preserved. The geographic extent of these Cretaceous units is very similar to that of the Morrison Formation. The Morrison Fm. represents at most 20 million years of deposition. The Early Cretaceous spans nearly 45 million years. The dinosaurs from both time periods inhabited and were preserved in alluvial plain environments including lacustrine and river deposits. If similar evolutionary tempo is assumed, then the Lower Cretaceous deposits should contain nearly twice as many dinosaurian taxa as the Morrison Fm.

Data on numbers of taxa and localities were collected from the Paleobiology Database in January, 2014. The Cretaceous deposits contain 20% more generic alpha diversity than the Jurassic Morrison Fm. This is far from the hypothetical 50% expected. One possible explanation for these results is differences in collection effort on the part of researchers. Collection effort was modeled using number of collection localities and the number of type species named from these units. The Morrison Fm has 15% more localities and 11% more type genera than the Lower Cretaceous units. Therefore, the apparent diversity difference cannot be explained by differences in collection effort.

There appears to be a low diversity of dinosaurs from the Early Cretaceous of the Western Interior. This can be explained by many things including taphonomy, phylogenetics, collection bias, and true measures of beta diversity. The vast majority (63%) of new taxa named from the Lower Cretaceous have been identified in the past 20 years. This suggests that a new interest in these deposits, possibly coupled with changes in collection techniques, could fill in this missing dinosaurian diversity. In addition, there is a larger specific diversity in the Morrison Formation dinosaurian fauna than in the Lower Cretaceous, which may be masking the true generic diversity of the Morrison Formation.