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The Eurasian fossil record is rich with the remains of sminthid rodents, a clade today represented by a single genus, Sicista. In contrast, the North American record of the family is sparse and exclusively composed of species of Plesiosminthus, an animal bearing a grooved incisor, during the Oligocene. This fossil record is the product of dispersal events from Eurasia. We here present the first occurrence of a smooth-incisor sminthid from North America. The new species, Heosminthus teredens from the Arikareean-aged Cabbage Patch beds of Montana, represents a rare occurrence of cranial material showing the clear association between incisors and cheek teeth, and the first postcranial remains for fossil sminthids. Our phylogenetic analysis, expanding on prior work, shows that Heosminthus teredens is most closely related to Heosminthus borrae, from Mongolia. In combination with published radioisotopic dating, our phylogenetic framework of sminthids provides evidence for an additional dispersal of sminthid rodents from Eurasia to North America 30 million years ago via Beringia. Our morphometric analysis of the tibia and metatarsal supports a terrestrial ecology for the new species; it is possible the animal had limited scansorial and/or saltatorial abilities. The large sample size of teeth available for the new taxon enables an analysis of morphological variation and indicates a high degree of polymorphism as well as a large size variation, even when controlling for stratigraphical and geographical range. [ABSTRACT FROM AUTHOR]