A new tiny eutherian from the Late Cretaceous of Alaska

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 degrees 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 similar to 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.