PALEOBOTANY OF AN EXTRAORDINARY POND DEPOSIT FROM THE MIDDLE HELL CREEK FORMATION (UPPER MAASTRICHTIAN; UPPER CRETACEOUS) OF HARDING COUNTY, NORTHWESTERN SOUTH DAKOTA

The Hell Creek Formation of Montana and the Dakotas has yielded the best sampled and most diverse Maastrichtian megaflora. Distribution of 328 floral morphotypes over a hundred quarries from many sedimentological facies in North Dakota shows a pattern of floral heterogeneity across the landscape and floral change through the 1.4 Ma duration of the formation. The Hell Creek flora is overwhelmingly dominated by angiosperms which comprise more than 89% of all leaf species. Several sites preserve herbaceous vegetation and even these are dominated by angiosperms, a dramatic change from early Maastrichtian herbaceous floras from Wyoming, where ferns and cycads were more abundant. A dramatic floral change occurs in the uppermost 20 m of the formation and leaf margin analysis suggests that this change is driven by a warming climate. High species-level extinction at the K-T boundary effectively terminates this flora and none of the floral dominants from Hell Creek landscapes are dominant on the subsequent Paleocene landscapes, which are dominated by floral elements that inhabited Cretaceous mires. Recent discovery of an unusual new site in Harding County, South Dakota adds information about the nature of aquatic vegetation in the Hell Creek landscape. This site is a series of stacked pond deposits composed of claystone that has preserved exceptionally detailed fossils, including insects, whole and partial fish, and a staggering diversity of leaves seeds, and fruits. More than 100 species of plants have been recovered from this single quarry site, making it the single most diverse known Cretaceous quarry. The site includes a highly diverse array of aquatic and arboreal angiosperms and presents a particularly clear view of a latest Cretaceous pond and pond-margin flora. Many taxa from this site are not known from Paleocene pond deposits suggesting that even though many species that lived in Cretaceous mires survived and flourished in the Paleocene, the species that lived in Cretaceous ponds did not.