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 P4, but less derived than its Bridgerian relative Mysops Leidy, 1871, in having less lingually hypsodont upper cheek teeth and a less trenchant protoloph on P4. 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 cylindrodontids, 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.
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