THE EARLY EOCENE RODENT TUSCAHOMYS (CYLINDRODONTIDAE) FROM THE GREAT DIVIDE BASIN, WYOMING: PHYLOGENY, BIOGEOGRAPHY, AND PALEOECOLOGY RobeRt L. Anemone [Research Associate, Section of Vertebrate Paleontology, Carnegie Museum of Natural History] Department of Anthropology, Western Michigan University, Kalamazoo, Michigan 49008 robert.anemone@wmich.edu mARy R. DAwson Curator Emeritus, Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 15213 dawsonm@carnegiemnh.org K. ChRistopheR beARD Curator, Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 15213 beardc@carnegiemnh.org ANNALS OF CARNEGIE MUSEUM VoL. 80, numbeR 3, pp. 187–205 15 June 2012 The early Eocene rodent Tuscahomys Dawson and Beard, 2007, is a dominant component of earliest Wasatchian mammal faunas from the United States, where its geo- graphic range is known to have extended from the Gulf Coastal Plain of eastern Mississippi to the Bighorn Ba- sin of northwestern Wyoming (Dawson and Beard 2007; Beard and Dawson 2009; Rose et al. 2012). These earliest Wasatchian faunas containing Tuscahomys play a pivotal role in documenting how North American mammalian communities responded to the dramatic, yet short-lived episode of global warming known as the Paleocene-Eo- cene thermal maximum (or PETM) (Gingerich and Smith 2006; Yans et al. 2006; Beard 2008; Rose et al. 2012). Further information on the distribution and paleobiology of Tuscahomys may therefore yield broader insights into the climate-mediated biotic change that transpired during the PETM. Tuscahomys is also the oldest known repre- sentative of the extinct rodent family Cylindrodontidae. Like many other fossil rodent taxa, the phylogenetic af- inities of cylindrodontids have been debated through the years (Wood 1980a, 1980b, 1984; Emry and Korth 1996; Averianov 1996; Dashzeveg and Meng 1998). As such, Tuscahomys potentially illuminates how cylindrodontids are related to other early Cenozoic rodent taxa and how the highly derived cheek tooth morphology of later cylin- drodontids evolved. In July 2009 a ield party under the leadership of the senior author discovered abundant remains of Wasatchian mammals at a new site in the southern part of the Great Divide Basin, Sweetwater County, Wyoming (Anemone et al. 2010). The mammalian fauna from this site, designated here as the Smiley Draw local fauna, is dominated by two “condylarths” (the meniscotheriid Meniscotherium Cope, 1874, and the hyopsodontid Haplomylus Matthew, 1915) and the new species of Tuscahomys described below. To date, 400 identiiable specimens representing at least 63 individuals of the new species of Tuscahomys have been recovered from the Smiley Draw local fauna. To our knowledge, this is the largest sample pertaining to a single species of rodent ever collected from an early Cenozoic locality in North America. The Smiley Draw local fauna in the Great Divide Basin can be correlated with the lower part of the Wa-4 faunal zone on the basis of the biostrati- graphic record of the omomyid primate Tetonius Matthew, ABSTRACT A new species of the cylindrodontid rodent Tuscahomys Dawson and Beard, 2007, is very well represented by maxillary, mandibular, and dental remains from the late early Wasatchian Smiley Draw local fauna in the Great Divide Basin of Sweetwater County, Wyoming. Tuscahomys walshi, new species, is more derived than earlier Wasatchian members of the genus in showing greater lingual hypsodonty on its upper cheek teeth and a more discrete hypocone on P 4 , but less derived than its Bridgerian relative Mysops Leidy, 1871, in having less lingually hypsodont upper cheek teeth and a less trenchant protoloph on P 4 . Tuscahomys walshi thus partly fills a previously existing gap in the early fossil record of cylindrodontids. Tuscahomys shows an unusual pattern of faunal association and relative abundance. It typically occurs as a surprisingly common element of some, but by no means all, early Wasatchian faunas, and it frequently co-occurs with the “condylarths” Meniscotherium Cope, 1874, and Haplomylus Matthew, 1915. This highly selective pattern of relative abundance and faunal association suggests some form of niche specificity for early cylin- drodontids, the details of which remain unknown. Phylogenetic analysis supports a relatively basal position for Tuscahomys (and Cylindrodontidae) within the Rodentia, rendering any direct derivation of earliest Wasatchian Tuscahomys from older North American paramyids suspect. An Asian origin for Cylindrodontidae, with subsequent dispersal of Tuscahomys into North America at or near the Clarkforkian-Wasatchian boundary, cannot be dismissed. Key woRDs: dispersal, Meniscotherium, PETM, Rodentia, Smiley Draw local fauna INTRODUCTION