Bisonalveus gracilis n. sp. (Pentacodontidae, Cimolesta): novel dental adaptations and their paleobiological implications in a small Paleocene mammal

A new species of shrew-sized late Paleocene (middle Tiffanian) eutherian mammal, Bisonalveus gracilis (Pentacodontidae, Cimolesta), is described from the DW-2 locality and correlative sites in the Paskapoo Formation, central Alberta, Canada. B. gracilis differs from the genotypic species, B. brown GAZIN, in having a less robust cheek dentition and a reduced, single-rooted p2, and from B. holtzmani GINGERICH in having substantially smaller molars. Previously published evidence implies that B. gracilis possessed a venomous bite, indicated by vertically emplaced, dagger-like upper canines (C) containing a well-defined channel for conveying venomous saliva, a Venom Delivery System (VDS) analogous to that of the extant Caribbean eulipotyphlan mammal Solenodon BRANDT. Further research has revealed that the third upper incisor (I3) of B. gracilis closely resembles the upper canine in these novel features, strengthening the hypothesis that parrs of the dentition of the species were adapted for this specialized function. Moreover, the molars of B. gracilis exhibit sharply pointed major cusps and well-developed, vertical shearing crests, some of which are neomorphic, while aspects of M2 are convergent on the shearing M1 of Carnivora. Collectively, the unique dental features of B. gracilis appear to be adaptations for consuming animal tissue and imply that B. gracilis was a tiny predator, employing a venomous bite in securing its energy requirements from animal protein. Recent alternatives to this interpretation of the paleobiology of B. gracilis are discussed but are shown to be lacking in their factual support, from either the fossil evidence or from modern analogues.