The megaflora of the Hell Creek Formation in southwestern North Dakota is known from more than 100 quarries spanning 110 meters of stratigraphic section representing the final 1.5 to 3 million years of the Cretaceous. Megaforal change through the formation is extensive, allowing the delineation of a robust local zonation. In the interval between 30 and 60 meters below the top of the formation, and occurring in the top of paleomagnetic subchron C30N, five quarries have yielded floras dominated by unusually small leaves (length <3 cm) that are not particularly coriaceous. These quarries occur in fluvial channel deposits characterized by large-scale, high-angle lateral accretion beds of alternating fine sandstone, silty mudstone, and claystone. Leaf size is dramatically smaller than in stratigraphically adjacent floras and only a few taxa (Platanaceae, Cercidiphyllaceae, Taxodiaceae, and Cheirolepidiaceae) are in common with floras above and below. The remainder of the flora consists of more than 30 angiosperm taxa. These taxa not known elsewhere in the stratigraphic record and include leaves apparently belonging to the Rosaceae, Urticaceae, and Cannabaceae(?). This is the earliest, well-dated record of these three families. Many modern members of these families are either shrubby or herbaceous, growth forms poorly recorded in the fossil record due to their small stature, relatively low number of leaves and propensity (in the case of herbaceous plants) for the leaves to die and dry on the stem rather than dehisce. Unlike other Late Cretaceous herbaceous floras that are preserved in growth position by airfall- or slurry-deposited volcanic ash, these floras are found in fluvial channels. It is presently unclear whether these floras represent unusually cool or dry paleoclimates, special edaphic or sedimentological conditions, or taphonomic filtering. Whatever their genesis, it is clear that they provide a new and unusual view of the floristics of the Late Cretaceous.

Key words: Cretaceous, North Dakota, paleobotany, Rosaceae, Urticaceae