An anachronistic Clarkforkian mammal fauna from the Paleocene Fort Union Formation (Great Divide Basin, Wyoming, USA)

The Clarkforkian (latest Paleocene) North American Land Mammal Age (NALMA) remains a relatively poorly sampled biostratigraphic interval at the close of the Paleocene epoch that is best known from the Bighorn Basin of northwestern Wyoming. A period of global warming between the cooler early and middle Paleocene and the extreme warming of the early Eocene, the Clarkforkian witnessed significant floral and faunal turnover with important ramifications for the development of Cenozoic biotas. The combination of warming global climates with mammalian turnover (including likely intercontinental dispersals) marks the Clarkforkian and the succeeding Wasatchian (Earliest Eocene) NALMAs as periods of intense interest to paleobiologists and other earth scientists concerned with aspects of biostratigraphy and with the biotic effects of climate change in the past. In this paper we describe a new Clarkforkian mammalian fauna from the Great Divide Basin of southwestern Wyoming with some surprising faunal elements that differ from the typical suite of taxic associations found in Clarkforkian assemblages of the Bighorn Basin. Several different scenarios are explored to explain this “anachronistic” assemblage of mammals from southern Wyoming in relation to the typical patterns found in northern Wyoming, including the concepts of basin-margin faunas, latitudinal and climatic gradients, and a chronologically transitional fauna. We suggest that the observed faunal and biostratigraphic differences between southern and northern Wyoming faunas most likely result from latitudinal and associated climatic differences, with floral and faunal changes being reflected somewhat earlier in the south during this period of marked climate change.