A new Late Cretaceous metatherian from the Williams Fork Formation, Colorado

Abstract

Heleocola piceanus, a new, relatively large metatherian from Upper Cretaceous ('Edmontonian') strata of the Williams Fork Formation in northwestern Colorado is described, based on a recently discovered jaw fragment (MWC 9744), in addition to three isolated teeth initially referred by other studies to Aquiladelphis incus and Glasbius piceanus. Although sharing several morphologic characters with the Lancian genus Glasbius, H. piceanus lower molars are considerably larger than those of Glasbius and differ from the latter in lacking a buccal cingulid, possessing carnassiform notches on the cristid obliqua and entocristid, and bearing an entoconulid on m3. To examine the relationship of Heleocola piceanus to other metatherians, H. piceanus was scored into a previously existing taxon-character matrix. Our phylogenetic analysis recovers H. piceanus as the sister taxon to Glasbius, which is consistent with our morphologic comparisons. H. piceanus represents the oldest member of the Glasbiidae. A regression equation for predicting body mass of dentally conservative metatherians that utilizes the length of m1 estimates the mass of H. piceanus at 855-1170 g, which is comparable in mass to today's muskrat (Ondatra zibethicus) and large relative to other Late Cretaceous pediomyoids. Based upon its molar morphology, specifically the low inflated cusps, low height differential between the trigonid and talonid, and near-bunodont morphology, H. piceanus is interpreted as an omnivore with a plant-dominated diet.

Fig 1. Geographic and stratigraphic location of the Williams Fork Formation study area and the new metatherian.

(A) General location in western United States, with study area in northwestern Colorado marked by yellow box. (B) Enlargement showing exposures of Mesaverde Group (including Williams Fork Formation) in the Douglas Creek Arch (green) between the Uinta Basin to the west, Piceance Basin to the east, and Blue Mountain uplift to the north; smaller black rectangle identifies study area of the new metatherian; outcrop distribution from Tweto [16]. (C) Locations of the four localities that yielded the new metatherian (red stars) southwest of Rangely, Colorado; strikes and dips from Noll [19] [Unpublished]. (D) Regional stratigraphy in the study area showing the thickness, age, and neighboring units of the Williams Fork Formation and Mesaverde Group. (E) Stratigraphic section of the Williams Fork Formation showing Sandstones A–H and stratigraphic occurrences of the four specimens of the new metatherian (by locality). D and E based on data in Noll [19] [Unpublished].

Fig 2. Heleocola piceanus from the Williams Fork Formation, Colorado.

(A–C), Holotype UCM 57354, in occlusal (A), buccal (B), and lingual (C) aspects. (D–F) UCM 57491, in occlusal (D), buccal (E), and lingual (F) aspects.

Fig 3. Left dentary fragment of Heleocola piceanus from J&M Site (MWC Loc. L-2012-13), Williams Fork Formation, Colorado.

(A–C), MWC 9744 in occlusal (A), buccal (B), and lingual (C) aspects.

Fig 4. Aquiladelphis incus from Milk River Formation, Alberta, Canada and Glasbius intricatus from Hell Creek Formation, Montana, USA.

(A–C) UALVP 29111, left mx in occlusal (A), buccal (B), and lingual (C) aspects. (D–G) UCM 53915, right dentary fragment with m2–m4 of Glasbius intricatus in buccal (D), occlusal (E, G), and lingual (F) aspects. Dentary has been digitally removed on (G).

Fig 5. Simplified phylogeny of Metatheria based upon strict consensus of 491 trees of 534 steps (consistency index 0.197; retention index 0.700) in S2 Fig.

Numbers at each node correspond to Bremer branch supports calculated from a pool of 30,000 suboptimal trees of up to 10 steps longer than the shortest trees. Heleocola piceanus (in red) is recovered as the sister taxon to Glasbius; this clade along with early Paleocene Bolivian polydolopimorph Roberthoffstetteria nationalgeographica form the sister group to the Pediomyidae.