EARLIEST EOCENE MIACIDAE (MAMMALIA: CARNIVORA) FROM NORTHWESTERN WYOMING

Journal of Paleontology, Jan 2008 by Heinrich, Ronald E, Strait, Suzanne G, Houde, Peter

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 more generalized than U. rudis and closer to that of the ancestral miacid condition (Rose, 1981). A phylogenetic analysis of the basalmost species of the North American early Eocene genera Uintacyon, Miacis, Vassacyon Matthew, 1909a, Vulpavus Marsh, 1871, and O�dectes Wortman, 1901 using viverravid camivorans as the outgroup (Heinrich, 1997), however, did little to clarify the basal miacid morphology, and the geographical origin of the family has yet to be resolved.

We describe here recently collected fossils including new species of Uintacyon, Miacis, and Vassacyon from Wa-0 localities in the southern Bighorn Basin, and the previously all but unknown upper dentition of Miacis deutschi Gingerich, 1983 from the Clarks Fork Basin. The new species are the smallest known, and possibly basalmost members of tiieir respective genera. Previous studies on mammals from Wa-0 localities have found that many species from this faunal zone are of smaller size than closelyrelated taxa from earlier and later sediments (Gingerich, 1989; Strait, 2004), and it has been suggested that this smaller size is a response to climactic change that occurred in the earliest Eocene (Gingerich, 2003; Strait, 2004). The new species are considered in light of this evolutionary dwarfing hypothesis. The new material considerably increases our knowledge of miacid species diversity in the earliest Eocene and, along with reconsideration of the early Eocene Asian miacid, suggests that much of the initial diversity within the family may considerably predate the Paleocene-Eocene boundary. Although specimens of the new Miacis and Vassacyon species are rare and of limited use for phylogenetic analysis, comparisons of the upper dentitions of Miacis deutschi and the new species of Uintacyon shed some light on functional differences in mastication that may have contributed to the evolutionary divergence of Miacis and Uintacyon, and provide some additional circumstantial evidence that the morphology of early Miacis is primitive for Miacidae.

ABBREVIATIONS

Institutions.-IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing; UCMP, University of California Museum of Paleontology, Berkeley; UM, University of Michigan Museum of Paleontology, Ann Arbor; USGS, United States Geological Survey, Denver, Colorado; USNM, National Museum of Natural History, Washington DC.

Measurements.-CL, maximum length of the upper canine, similarly defined for P1-P3; CW, maximum width of the upper canine, similarly defined for P1-P3; P4L, length of labial aspect of P4 between anterior border of parastyle and posterior border of metacone blade; P4Ln, length of lingual aspect of fourth upper premolar between anterior border of protocone and posterior border of metacone blade; P4W, width of P4 between labial border of parastylar shelf and lingual border of protocone; MIL, length of first upper molar between anterior border of parastyle and posteriormost margin of tooth perpendicular to the transversely oriented long axis of the tooth, similarly defined for M2; MlWa, anterior width of M1 between labial border of parastylar shelf and lingual border of protocone parallel to the transversely oriented long axis of the tooth, similarly defined for M2; MlWp, posterior width of M1 between labial border of metastylar shelf and lingual border of protocone parallel to the transversely oriented long axis of the tooth, similarly defined for M2; m1L, maximum length of lower first molar between anterior border of trigonid and posterior border of talonid, similarly defined for m2 and m3; m1W, maximum width of m1 between labial and lingual borders of trigonid perpendicular to the anteroposterior axis of the tooth, similarly defined for m2 and m3.