Unexpected Patterns of Floral Diversity and Heterogeneity in the Earliest Paleocene of the Northern Rocky Mountains and Great Plains

The prevailing picture of the flora of the northern Rocky Mountains and Great Plains immediately following the Cretaceous-Tertiary boundary extinction was of a homogeneous and low diversity assemblage across the region. However, recent research in the Denver Basin has demonstrated that there are several high diversity floras in the earliest Paleocene, suggesting that inter-basinal diversity may be higher than previously supposed. To address this question of the diversity and heterogeneity of these plant communities, we assembled a floral comparison using published lists and our plant collections from four Laramide basins extending from western North Dakota and southwestern Saskatchewan to western Wyoming and south-central Montana: encompassing the Williston, Powder River, Hanna, and Bighorn Basins. We restricted our survey to a transect stretching between 42° N to 50° N to eliminate latitudinal climatic effects on diversity and floral composition. The earliest Paleocene flora of the area proved to be surprisingly diverse with 181 species and morphotypes tabulated. Furthermore, only 20% of these were cosmopolitan, i.e. found in three or more basins; 20% were shared between two basins; and 60% were endemic to a single basin. Because our floras were collected from similar depositional environments in a narrow latitudinal band, the effects of taphonomy and paleoecology were minimized. In contrast to the previous picture of a relatively homogeneous and undiverse flora, our data suggest that uplift of the Western Cordillera and the ensuing physical separation of depositional basins may have begun to stimulate floral diversification in the earliest Paleocene.