FIELDS, PATRICK F. Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824-1312, fieldspa@msu.edu. - A review of the middle Miocene Mahonias of western North America and a key to their species.
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