Abstract
BackgroundThe early evolution of living marsupials is poorly understood in part because the early offshoots of this group are known almost exclusively from jaws and teeth. Filling this gap is essential for a better understanding of the phylogenetic relationships among living marsupials, the biogeographic pathways that led to their current distribution as well as the successive evolutionary steps that led to their current diversity, habits and various specializations that distinguish them from placental mammals.Methodology/Principal FindingsHere we report the first skull of a 55 million year old peradectid marsupial from the early Eocene of North America and exceptionally preserved skeletons of an Oligocene herpetotheriid, both representing critical groups to understand early marsupial evolution. A comprehensive phylogenetic cladistic analysis of Marsupialia including the new findings and close relatives of marsupials show that peradectids are the sister group of living opossums and herpetotheriids are the sister group of all living marsupials.Conclusions/SignificanceThe results imply that North America played an important role in early Cenozoic marsupial evolutionary history and may have even been the center of origin of living marsupials and opossums. New data from the herpetotheriid postcranium support the view that the ancestral morphotype of Marsupialia was more terrestrial than opossums are. The resolution of the phylogenetic position of peradectids reveals an older calibration point for molecular estimates of divergence times among living marsupials than those currently used.
Highlights
Extant marsupials are limited mostly to Australia and South America whereas the few Central and North American representatives are relatively recent immigrants from South America through the Panamanian Isthmus circa 3 million years ago
Metatheria includes the common ancestor of all extant marsupials plus all extinct mammals that are more closely related to living marsupials than to extant placentals [3]
Other derived characters supporting a clade composed of Herpetotherium, peradectids and other marsupials include: (1) a reduced to absent metaconule on the upper molars, (2) a cavum epiptericum that is primarily or exclusively floored by the alisphenoid, (3) a rostral tympanic process of the petrosal forming a distinct crest, and (4) a tympanic sinus that is almost entirely formed by the alisphenoid
Summary
Extant marsupials are limited mostly to Australia and South America whereas the few Central and North American representatives are relatively recent immigrants from South America through the Panamanian Isthmus circa 3 million years ago. This is possible based on new fossils of this group, such as Mimoperadectes labrus, from the earliest Eocene of the Clark Fork Basin, Wyoming [11]. Etymology Named for Dr Peter Houde from New Mexico State University who, in the process of studying limestone avifauna from the Clark Fork Basin, has discovered and prepared many spectacular early Eocene mammal specimens including the type specimen of Mimoperadectes houdei.
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