title: Exceptionally preserved North American Paleogene metatherians: adaptations and discovery of a major gap in the opossum fossil record.
abstract: A major gap in our knowledge of the evolution of marsupial mammals concerns the Paleogene of the northern continents, a critical time and place to link the early history of metatherians in Asia and North America with the more recent diversification in South America and Australia. We studied new exceptionally well-preserved partial skeletons of the Early Oligocene fossil Herpetotherium from the White River Formation in Wyoming, which allowed us to test the relationships of this taxon and examine its adaptations. Herpetotheriidae, with a fossil record extending from the Cretaceous to the Miocene, has traditionally been allied with opossums (Didelphidae) based on fragmentary material, mainly dentitions. Analysis of the new material reveals that several aspects of the cranial and postcranial anatomy, some of which suggests a terrestrial lifestyle, distinguish Herpetotherium from opossums. We found that Herpetotherium is the sister group to the crown group Marsupialia and is not a stem didelphid. Combination of the new palaeontological data with molecular divergence estimates, suggests the presence of a long undocumented gap in the fossil record of opossums extending some 45Myr from the Early Miocene to the Cretaceous.
authors.affiliation: Palaeontologisches Institut und Museum, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland. m.sanchez@pim.uzh.ch
authors.forename: Marcelo
authors.initials: M
authors.lastname: Sánchez-Villagra
authors.forename: Sandrine
authors.initials: S
authors.lastname: Ladevèze
authors.forename: Inés
authors.initials: I
authors.lastname: Horovitz
authors.forename: Christine
authors.initials: C
authors.lastname: Argot
authors.forename: Jeremy J
authors.initials: JJ
authors.lastname: Hooker
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Macrini
authors.forename: Thomas
authors.initials: T
authors.lastname: Martin
authors.forename: Scott
authors.initials: S
authors.lastname: Moore-Fay
authors.forename: Christian
authors.initials: C
authors.lastname: de Muizon
authors.forename: Thomas
authors.initials: T
authors.lastname: Schmelzle
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Asher
mesh_terms: D000220:Adaptation, Biological; D000818:Animals; D001842:Bone and Bones / Q000033:anatomy & histology; D017753:Ecosystem; D005580:Fossils; D009893:Opossums / Q000033:anatomy & histology* / Q000145:classification; D010802:Phylogeny; D014070:Tooth / Q000033:anatomy & histology; D014959:Wyoming