Although Berberis (sensu stricto) is virtually unknown in the Miocene megafossil record of western North America, the compound leaved barberries (Mahonia sensu stricto) have an interesting fossil history. About 15 years ago, Axelrod devoted much effort to taxonomically sorting out these leaflet impressions based on their morphology. Recent work by the present author has added overlooked material, newly discovered occurrences, and the renaming of an invalid taxon. Regardless of how many biologic entities the currently-recognized paleotaxa may actually represent, they do serve as a useful set of identifiable morphotypes. To that end, a simple dichotomous key to the Miocene megafossil Mahonia taxa of the western United States is presented. Application of this revision to the floras of western North America reveals the Neogene paleobiogeographic history of this group. There appears to be one primary center of Mahonia diversity, near the southwestern end of the Snake River Plain (centered on the region of the Succor Creek, ID/OR and Trout Creek, OR floras). One of two secondary centers is located about 280 km (180 mi) east on the southern edge of the central Snake River Plain (Trapper Creek, ID flora) and the other, about 400 km (250 mi) south in west-central Nevada. Later fossil occurrences appear to be restricted to the Columbia Plateau and intermountain regions east of the coastal ranges and are far less diverse taxonomically.

Key words: Berberis, key to morphospecies, Mahonia, megafossils, Miocene, western North America