A new tiny eutherian from the Late Cretaceous of Alaska

A new eutherian, <i>Sikuomys mikros</i> 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 <i>Gypsonictops</i> 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 <i>S. mikros</i> as the sister taxon to <i>Gypsonictops</i> in the strict consensus tree. A regression equation for predicting body mass of insectivorans utilizing lower molar area estimates the mass of <i>S. mikros</i> 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 <i>S. mikros</i> did not hibernate, but rather was active year-round, akin to extant shrews (Soricidae). From a palaeobiogeographical standpoint, the occurrence of <i>S. mikros</i> 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