A GRAPHIC APPROACH TO UNDERSTANDING ANGIOSPERM DIVERSITY IN SPECIES-RICH MEGAFLORAL QUARRIES IN THE UPPERMOST CRETACEOUS HELL CREEK FORMATION OF MONTANA, NORTH DAKOTA, AND SOUTH DAKOTA

The megaflora from the Hell Creek Formation at the southern end of the Cedar Creek anticline in eastern Montana, southwestern North Dakota, and northwestern South Dakota is now known from 132 individual quarries with vouchered specimens located at the Denver Museum of Nature & Science, the Yale Peabody Museum, and the National Museum of Natural History. While these quarries contain ferns, Ginkgo, the cycad Nilssoniocladus, and cupressaceous and araucarian conifers; nearly all are dominated (>90%) by angiosperms, both in terms of number of specimens and species. The megaflora of the 100-meter-thick formation is divided into three zones: the basal HCI, the middle HCII, and the upper HCIII. Most of the Hell Creek angiosperm species do not appear to have survived the Cretaceous-Paleogene extinction event and most of them have not been described or named. Using a morphotype system based on leaf architectural characters, we have defined more than 350 angiosperm taxa from the Hell Creek Formation. This diversity has been challenging to document, so we have turned to graphic illustration to characterize the megaflora of individual quarries as a path toward documenting the megaflora of the entire formation. The best quarries are ones that yield complete and well-preserved leaves and we have focused on four species-rich quarries to refine our approach: two from HCII (Skunk Hunt-DMNH loc. 4301 & Licking Leaves DMNH loc. 2307) and two from HCIII (Mud Buttes-DMNH loc. 428 & Battleship DMNH loc. 900). Each leaf drawing is based on a specific, vouchered specimen, thus allowing a visual overview of each quarry. Collectively, these four quarries have yielded more than 200 angiosperm leaf morphotypes. These floras show that, even for single quarries, Late Cretaceous angiosperm diversity is unusually high. Our future work will focus on the angiosperms of the basal HCI megaflora and the post K-Pg FUI megaflora with the goal of better characterizing floral change throughout the final 1.5Ma of the Cretaceous and first 1Ma of the Paleocene.