The first evidence of Heosminthus from North America and the phylogenetics of Sminthidae (Mammalia, Rodentia, Dipodoidea): biogeographical implications

Abstract

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.

https://zoobank.org/urn:lsid:zoobank.org:pub:B7D4245C-992F-4956-96AD-10D7CFA8E5C8

Keywords:

• locomotion
• morphometrics
• Plesiosminthus
• Sicista
• variation

Supplemental material

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2022.2111232.

Acknowledgements

This work would not have been possible without the help and field work of Donald Rasmussen who collected the type series and first recognized the existence of an unusual sminthid in Montana. Gregory Wilson Mantilla provided space, equipment, and mentorship during the initial stage of this research. Yuri Kimura shared data and insights into sminthid phylogenetic relationships. Joshua Samuels shared his thoughts on the material from Montana and data about Plesiosminthus fremdi. Bill Simpson (FMNH), Joshua Samuels and Chris Schierup (John Day Fossil Beds National Monument), Desui Miao and David Burnham (KUVP), Kallie Moore and George Stanley (UMPC), Ron Eng, Meredith Rivin, Regan Dunn and Christian Sidor (UWBM) and Donald Rasmussen provided access to specimens. Sharlene Santana, Abby Vander Linden and Katie Stanchak provided access to μCT scanning facilities and technical assistance. Numerous students and colleagues assisted with field work. Nasser Alrashdi assisted with initial data collection. Landowners and ranchers in Granite and Powell counties, in particular the Radtke, Dingwall, Dutton and Morse ranches, allowed access to their land. Many students helped recover, identify and curate specimens. Samantha Hopkins, Gregory Wilson Mantilla, Winifred Kehl, and members of the Wilson lab engaged in fruitful discussions about this project. Funding for this project was provided by the American Philosophical Society Lewis and Clark Fund, Sigma-Xi, the Geological Society of America, the Evolving Earth Foundation, the Northwest Association, the Paleontological Society Caster award and Norman Newell early career grant, the Tobacco Root Geological Society, the UWBM, the University of Washington Department of Biology, and the Ohio State University at Marion.

Associate Editor: Adrian Lister