PORTRAIT OF A LATE PALEOCENE (EARLY CLARKFORKIAN) TERRESTRIAL ECOSYSTEM - BIG MULTI QUARRY AND ASSOCIATED STRATA, WASHAKIE BASIN, SOUTHWESTERN WYOMING

In-depth understanding of past climatic and biotic change requires the study of ancient ecosystems. However, terrestrial plants and vertebra tes are preferentially preserved under very different taphonomic condi tions, and diverse fossil floras and faunas are rarely found in close association. Big Multi Quarry and associated strata in the uppermost F ort Union Formation of the Washakie Basin, southwestern Wyoming, provi de a uniquely detailed record of terrestrial fauna, flora, and climate during the early Clarkforkian. The Clarkforkian Land Mammal Age, appr oximately the last million years of the Paleocene, wars an interval of global warming that had profound biotic consequences. The mammalian f auna of Big Multi Quarry, consisting of 41 species, is the most divers e known from a single Clarkforkian locality. Unlike most other Clarkfo rkian faunas, this assemblage is not significantly biased against smal l forms. Lipotyphlan insectivores were dominant, and arboreally adapte d taxa were abundant and diverse. The closely associated and well-pres erved 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, sho wing that forest structure remained monotonous even as climate warned and mammals diversified in the Clarkforkian. The plant assemblage more closely resembles middle than early Clarkforkian floras of northern W yoming, suggesting northward migration, of the ranges of plant taxa co incident with warming. A great deal of research has focused on the unu sually warm interiors of continents in the terminal Paleocene and earl y Eocene. Multiple lines of evidence from our study, including sedimen tological indicators, analyses of the nearest living relatives and fun ctional analogues of the fossil plants and animals, size and margin an alysis of fossil leaves, and cenogram analysis of the mammalian fauna, indicate that southwestern Wyoming had a humid subtropical climate wi th little or no seasonal frost or marked dry season, well before the t erminal Paleocene.