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ABSTRACT-
Fossil carnivorans are described from earliest Eocene localities in the Clarks Fork and southern Bighorn basins of Wyoming. Three new species, Miacis rosei, Uintacyon gingerichi, and Vassacyon bowni, collected from the base of the Wasatchian North American Land Mammal Age (Wa-0), are the smallest and possibly most basal members of their respective genera, and increase from one to four the number of miacids known from this faunal zone. An upper dentition of Miacis deutschi from slightly younger (Wa-2) deposits is also described. Previously known only from lower teeth and a single M1, the specimen of M. deutschi includes the left P3-M2, alveoli for the canine, first two premolars and the last molar, as well as most of the maxilla. The new material helps fill gaps in our knowledge of the dental morphology of basal Miacidae and provides insight into the functional differences of the carnassial teeth in the diverging Uintacyon and Miacis lineages. It also provides an opportunity to further assess the hypothesis that climactic warming in the earliest Eocene resulted in evolutionary dwarfing of mammalian species; based on three criteria for identifying dwarfed species at least one of the new taxa, U. gingerichi, is consistent with this hypothesis.
INTRODUCTION
THERE is increasing evidence that all living members of the order Carnivora trace their origin to the family Miacidae (Wesley and Flynn, 2005; Polly et al., 2006), a paraphyletic taxon of early camivorans that are best known from die Eocene of North America. The earliest North American miacids are all described from the Clarks Fork and Bighorn basins of northwestern Wyoming where mammalian biostratigraphy over the Paleocene-Eocene boundary is well documented and reasonably well correlated between the two basins (Bown et al., 1994; Gingerich, 2001; Gingerich and Clyde, 2001). Uintacyon rudis Matthew, 1915 is the oldest miacid, ranging from the middle Clarkforkian (latest Paleocene) to the middle Wasatchian North American Land Mammal Ages, with Miacis winkleri Gingerich, 1983 appearing next in the earliest Eocene at the very base of the Wasatchian North American Land Mammal Age (Gingerich, 1983). Unlike Uintacyon Leidy, 1872, specimens attributed to the genus Miacis Cope, 1872 are also known from the early Eocene of Europe and possibly Asia, supporting a contention that the morphology exhibited by early Miacis is...