In-depth understanding of past climatic and biotic change requires the study of ancient ecosystems. However, terrestrial plants and vertebrates are preferentially preserved under very different taphonomic conditions, and diverse fossil floras and faunas are rarely found in close association. Big Multi Quarry and associated strata in the uppermost Fort Union Formation of the Washakie Basin, southwestern Wyoming, provide a uniquely detailed record of terrestrial fauna, flora, and climate during the early Clarkforkian. The Clarkforkian Land Mammal Age, approximately the last million years of the Paleocene, was an interval of global warming that had profound biotic consequences. The mammalian fauna of Big Multi Quarry, consisting of 41 species, is the most diverse known from a single Clark-forkian locality. Unlike most other Clarkforkian faunas, this assemblage is not significantly biased against small forms. Lipotyphlan insectivores were dominant, and arboreally adapted taxa were abundant and diverse. The closely associated and well-preserved fossil plant assemblage was overwhelmingly dominated by a single species belonging to the birch family. Floral richness, heterogeneity, and evenness were as low as in the Tiffanian of the same region, showing that forest structure remained monotonous even as climate warmed and mammals diversified in the Clarkforkian. The plant assemblage more closely resembles middle than early Clarkforkian floras of northern Wyoming, suggesting northward migration of the ranges of plant taxa coincident with warming. A great deal of research has focused on the unusually warm interiors of continents in the terminal Paleocene and early Eocene. Multiple lines of evidence from our study, including sedimentological indicators, analyses of the nearest living relatives and functional analogues of the fossil plants and animals, size and margin analysis of fossil leaves, and cenogram analysis of the mammalian fauna, indicate that southwestern Wyoming had a humid subtropical climate with little or no seasonal frost or marked dry season, well before the terminal Paleocene.