Six fossil leaf species are described from impression fossils collected from the Paleocene Fort Union Formation in the Williston Basin in southwestern North Dakota, USA. They are Meliosma vandaelium sp. nov., Meliosma thriviensis sp. nov., Ternstromites paucimissouriensis sp. nov., Macginitiea nascens sp. nov., Dicotylophyllum horsecreekium sp. nov. and Dicotylophyllum hansonium sp. nov. These species represent some of the elements of the Fort Union Formation that are biostratigraphically important megafloral zone taxa or are species that demonstrate an evolutionary relationship to floras from the Eocene Golden Valley Formation. Some of the species described here suggest that new species endemic to the Williston Basin evolved through the Paleocene and into the Eocene. If the pattern of the origination of endemic daughter species seen in the Williston Basin is consistent across the Western Interior basins of North America, it might have driven up gamma diversity through the Paleogene. This provides a possible explanation for the relatively steep vegetation diversity gradients seen in mid- and high-latitude pollen floras in North America during the late Paleocene and early Eocene.