Cycads from the Cenozoic of western North America are not well known. Only three taxa have been published, two in 1962 and one in an abstract in 1936. We have evaluated newly discovered fossils cycads from twelve localities in the Paleocene Denver, Dawson, Hanna, and Fort Union Formations in Colorado and Wyoming, the middle Eocene Klondike Mountain Formation in northeastern Washington, the middle Eocene Clarno Formation from eastern Oregon, and have re-evaluated the previously described cycads from Wyoming, Colorado, and Oregon. These specimens allow us to begin to establish patterns of Cenozoic cycad diversity. Analysis of the specimens included the description of the leaves, pinnules, and cuticle. Based on pinnule attachment, shape, and venation, two general groups of cycad leaves are recognized. Pinnules of the first type are characterized by having widely decurrent attachment, entire margins, lanceolate shape, and anastomosing venation. Similar characters are shared by the Jurassic and Cretaceous genus Ctenis and the Paleocene genus Dioonopsis. No extant cycad family shares all of the above characters. The second type of pinnule is characterized by narrow decurrent attachment, lack of a midrib, entire margins, linear shape, and open parallel veins that occasionally dichotomize. These characters are shared by extant genera in the Zamiaceae such as Dioon, Zamia, and Lepidozamia. Wide variation within these two pinnule types suggests that they probably each represent more than one species. Despite extensive sampling of megafloras in the western United States, cycads are presently unknown in rocks younger than the middle Eocene (ca. 43 mya). These new Paleogene cycads offer an opportunity to better understand the evolution and extinction of the Cycadales in the western United States during the Cenozoic.

Key words: Cycadales, North America, Paleogene