Floristic composition and variation in late Paleocene to early Eocene floras in North America

The late Paleocene and early Eocene megafossil floras in North America are found primarily in the Williston, Green River, Powder River, Bighorn, and Alberta Basins of the northern Rocky Mountains and Western Interior. A few rare sites occur in the Mississippi Embayment of the Southeast. In contrast to the abrupt floristic changes seen at the K/T boundary, these floras document a gradual transition in species turnover, or, in the case of the Bighorn Basin, a long-term decrease in taxonomic diversity. This gradual transition is also in marked contrast to the rapid speciation among mammals of the early Eocene. Both preservation, and ability to place these floras within a temporal scale, determine how useful they are in assessing floristic changes across the Paleocene-Eocene transition. In some regions such as the Bighorn Basin of Wyoming precise stratigraphic control has allowed for documentation of paleoclimate change at a highly resolved temporal scale. At others, such as the Almont flora of the Williston Basin in North Dakota, exceptional preservation has provided the basis for describing individual taxa with the precision necessary to better understand their evolutionary and biogeographical histories. This study examines well-known plant taxa in the late Paleocene and early Eocene in the context of their depositional settings and temporal and spatial distribution. Integration of paleoecological and taxonomic studies is critical to understanding the evolutionary and depositional history of early Paleocene vegetation of North America