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.

Phylogenetic Relationships and Classification of Didelphid Marsupials, an Extant Radiation of New World Metatherian Mammals

“…Because our analyses did not include any nonmarsupial outgroup, the position of the marsupial root node was not determined, and the phylogenetic interpretation of characterstate transformations on the branch separating didelphids from nondidelphid marsupials is correspondingly equivocal. If, as most recent analyses suggest, didelphids are the basalmost branch of Marsupialia (Nilsson et al, 2004;Amrine-Madsen et al, 2003;Horovitz and Sá nchez-Villagra, 2003;Asher et al, 2004;Sá nchez-Villagra et al, 2007;Meredith et al, 2008;Beck, 2008), then such transformations might be didelphid synapomorphies, or they could be synapomorphies of the unnamed clade that includes caenolestids and Australidelphia.…”

“…Some stem metatherians (e.g., herpetotheriids) that are strikingly similar to didelphids in most respects (including ear morphology; Gabbert, 1998) differ from didelphids by having a distinct posterior cingulid on the lower molars in addition to the diagnostic endocranial and postcranial features described by Sá nchez-Villagra et al (2007). 23 REMARKS: A wide range of fossil taxa have at one time or another been regarded as didelphids (e.g., by Simpson, 1935Simpson, , 1945Clemens, 1979;Marshall, 1981;McKenna and Bell, 1997), but phylogenetic analyses suggest that most of the extinct forms once thought to be closely related to Recent opossums (e.g., {Alphadon, {Andinodelphys, {Glasbius, {Herpetotherium, {Jaskhadelphys, {Pediomys) are stem metatherians and not members of the crown group Marsupialia (Rougier et al, 1998;Wible et al, 2001;Luo et al, 2003;Sá nchez-Villagra et al, 2007). Herein we explicitly restrict Didelphidae to living didelphimorphians, their most recent common ancestor, and all of its descendants.…”

“…Comparisons with stem metatherians that are believed to be close outgroups to Marsupialia, including {Mayulestes (see Muizon, 1998), {Pucadelphys (see Marshall et al, 1995), and {Herpetotherium (see Gabbert, 1998;Sá nchez-Villagra et al, 2007) suggest that most of the craniodental traits by which didelphids differ from other marsupials are plesiomorphic, but one exception merits comment. Bone homologies on the posterodorsal braincase have received little attention in the literature, but they appear to be phylogenetically informative at many taxonomic levels within Metatheria.…”

Comparative Morphology of the Internal Nasal Skeleton of Adult Marsupials Based on X-ray Computed Tomography

“…1). Other analyses found Dromiciops nested within the Australiasian radiation, either as the sister taxon to Diprotodontia (e.g., Horovitz and Sá nchez-Villagra, 2003;Cardillo et al, 2004;Sá nchez-Villagra et al, 2007; fig. 2), or as the sister taxon to a clade that is more inclusive than Diprotodontia (e.g., Springer et al, 1997;Asher et al, 2004).…”

“…Although testing relevant hypotheses in many of these fields requires an understanding of evolutionary relationships, many aspects of marsupial phylogeny remain enigmatic despite recent advances involving analyses of both morphological and molecular data (e.g., Springer et al, 1997;Burk et al, 1999;Colgan, 1999;Jansa and Voss, 2000;Amrine-Madsen et al, 2003;Horovitz and Sá nchez-Villagra, 2003;Jansa, 2003, 2009;Asher et al, 2004;Cardillo et al, 2004;Nilsson, 2004;Jansa et al, 2006;Sá nchez-Villagra et al, 2007;Beck, 2008;Meredith et al, 2008). The major areas of conflict in marsupial phylogeny involve: the placement of Dromiciops gliroides, the only extant member of the South American group Microbiotheriidae, within Australiadelphia (the least inclusive clade containing Dromiciops and all extant Australiasian groups); relationships between Notoryctes, Dasyuromorphia, and Peramelemorphia; and higher relationships within Diprotodontia.…”