Arikareean (Oligocene Miocene) Herpetotherium (Marsupialia, Didelphidae) from Nebraska and Florida

Abstract

North American Herpetotherium (Marsupialia, Didelphidae) is a small mouse-sized (<80g) opossum that ranged from the middle Eocene into the early Miocene (Uintan-Hemingfordian) of the Great Plains, Pacific Northwest, and Atlantic Southeast. New Arikareean (30-18.8Ma) Herpetotherium samples are described from the lower Arikaree Group of Nebraska (Dw-121 or Ridgeview Local Fauna, a locality south of Chadron, from localities of the Wildcat Ridge region, and the McCann Canyon Local Fauna) as well as from 4 localities in Florida (I-75, Brooksville 2, SB-1A, and White Springs). The samples from Dw-121 and Brooksville 2 are the first large (>25) populations of Herpetotherium described from the Arikareean age. Three species are recognized in the Arikareean: H. fugax, H. youngi, and H. merriami. Most early Arikareean samples are referred to H. fugax and are slightly smaller on average than older H. fugax. Later Arikareean Herpetotherium samples are referable to H. youngi on the basis of possessing a single central stylar cusp. A large species, H. merriami, from the early Arikareean John Day Formation in Oregon is similar to Herpetotherium molars described from the early Arikareean or late Whitneyan Florida I-75 locality. Taxonomy of Herpetotherium has previously been based on tooth size and upper molar stylar cusp morphology. H. fugax exhibits several configurations of stylar cusp morphology, separate to fused central stylar cusps, and a wide range of dental size variation. These ranges of variation are similar to those exhibited by the Chadronian species, H. valens.