A new tiny eutherian from the Late Cretaceous of Alaska

Abstract

A new eutherian, Sikuomys mikros gen. et. sp. nov., is described from Upper Cretaceous (upper Campanian) strata of the Prince Creek Formation cropping out along the lower Colville River in northern Alaska, USA. The taxon represents the northernmost occurrence of a Mesozoic eutherian (palaeolatitude 80–85°N). The Alaskan taxon differs morphologically from Gypsonictops in having: weak pre- and postcingula on P5, upper molars with small conules, narrower pre- and postcingula, and a postcingulum that extends lingually past the protocone, p5 lacking a paraconid, lower molars with a metaconid that is taller than, or subequal in height to, the protoconid, and a less anteroposteriorly compressed trigonid. Phylogenetic analysis recovers S. mikros as the sister taxon to Gypsonictops in the strict consensus tree. A regression equation for predicting body mass of insectivorans utilizing lower molar area estimates the mass of S. mikros at ∼10.8 grams, approximately one-third to one-fifth that of other gypsonictopids. The occurrence of this lilliputian eutherian – the smallest terrestrial vertebrate known from the high palaeolatitude Prince Creek Formation – provides insight into its overwintering strategies. The pattern in which the smallest species of a lineage occurs at the highest latitudes suggests that S. mikros did not hibernate, but rather was active year-round, akin to extant shrews (Soricidae). From a palaeobiogeographical standpoint, the occurrence of S. mikros is evidence that some leptictidans lived year-round in the Arctic, a probable prerequisite for dispersal between North America and Asia, hypothesized by others to be the place of origin for Leptictida.

http://zoobank.org/urn:lsid:zoobank.org:pub:8D9F78CE-F34D-4D31-A9F0-2121D6E2B23A

Keywords:

• Arctic
• Gypsonictopidae
• Leptictida
• Prince Creek Formation
• phylogeny

Acknowledgements

Fossils were collected by field parties from UAMES and UCMP over multiple field seasons (1988–2018). We thank Kevin May, Zack Perry, Dustin Stewart, Katherine Anderson, Camile Heninger and the numerous staff, students, and volunteers at the UCMP, UCM, UAMES, Florida State University and Oklahoma State University for their efforts in the field and lab collecting and picking fossils from sediment. We acknowledge the community of Nuiqsut and the Inupiat on whose ancestral lands our fieldwork occurs. The specimens described here are on loan to JE from UAMES. Research was supported by funding from the United States Geological Survey, UCMP, University of Alaska Museum, CU-Boulder, the National Science Foundation Office of Polar Programs (DPP 8619283 and DPP 8821816 to WAC) and the National Science Foundation Division of Earth Sciences (EAR 1226730 to GME and PSD, and EAR 1736515 to PSD, GME, and JJE). We kindly acknowledge the logistical and administrative support over many years by the State of Alaska Office of History and Archaeology and the Bureau of Land Management (US Department of the Interior) and particularly R. VanderHoek, M. Kunz, B. King, J. Keeney and B. Breithaupt. Fossils collected by joint UCMP and UAMES field parties were collected under BLM permit numbers AA-55836 and AA-061366, and the UAMES under permits AA-091193 and AA-93310. P. Holroyd and L. Vietti loaned specimens and casts to JE from UCMP and the University of Wyoming Geological Museum, respectively. The micro-CT scans were done on a Zeiss Xradia Versa XRM-520^TM (funding source NSF MRI 1726864 to V. Ferguson, CU-Boulder). XRM imaging and image reconstruction were conducted by A. Gestos in the CU Biomechanics and Biomimetics (MIMIC) Laboratory (V. Ferguson lab) in the School of Engineering at CU-Boulder. Dental measurements were obtained using an Ehrenreich Photo Optical Shopscope^TM on loan to JE from the University of Wyoming (J. Lillegraven). J. Wible provided the data matrix and advice to JE on the cladistic analysis. C. Lynch and C. Schwarz of the Laboratory for Interdisciplinary Statistical Analysis (LISA) at CU-Boulder provided advice and assistance on the estimation of body mass. We thank J. Wible and an anonymous reviewer whose careful reviews considerably improved the manuscript. The manuscript also benefited from the advice of Editor in Chief Z. Johanson, Associate Editor T. Halliday and Senior Production Editor A. Meier.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental material

Supplemental material for this article can be accessed here: https://doi.org/10.1080/14772019.2023.2232359.

Associate Editor: Thomas Halliday