INTRODUCTION

Early Paleocene metatherians from North America are poorly known. Recent studies (e.g., Sánchez-Villagra et al. 2007; Horovitz et al. 2009) argued that crown clade marsupialia originated near the beginning of the Paleocene while the closest sister clade, Herpetotheriidae, has a Late Cretaceous origin. This conclusion did not address problems related to the generic identity and relationships of earliest Paleocene (Puercan) metatheria which remain contentious (Krishtalka and Stucky 1983; Johanson 1996a, 1996b; Clemens 2006). In addition, the conclusions regarding the relationship of purported Cretaceous herpetotheriids remain untested.

Until recently, herpetotheriids were not recognized from earlier than the basal Eocene (e.g., Krishtalka and Stucky 1983). Johanson (1996a) described a taxon from the early Paleocene (Torrejonian) of Wyoming, Swaindelphys cifellii, which is generally regarded as the oldest herpetotheriid from the Cenozoic of North America (Case et al. 2005; Martin et al. 2005; Korth 2007).

Here we report on three new metatherian taxa from the early Paleocene (Torrejonian) of the Nacimiento Formation, New Mexico. The first two of these represent two new species of Swaindelphys. This is the first report of Swaindelphys outside of the Wyoming. The third represents a new species of Peradectes and represents the only report of Peradectes from the Torrejonian of North America.

Geologic Setting

The majority of metatherian mammal specimens described here were recovered from microvertebrate fossil localities using screenwashing methods (e.g., Cifelli et al. 1996). These localities are widely distributed within the San Juan Basin (Figure 1) and span localities assigned to the Pantolambda cavirictum – Mixodectes pungens zone and Mixodectes pungens zone (Williamson 1996). The sites are middle to late Torrejonian in age (Lofgren et al. 2004; Figure 2).

Methods. Tooth nomenclature follows Davis (2007) All measurements are in mm and were made to the nearest 0.05 mm using a Wild^TM measuring reticule and a Leica^TM MZ 6 microscope.

Abbreviations for Descriptive Statistics. DW = distal width; L = length; MW = mesial width; W = width.

Institutional abbreviations. AMNH, American Museum of Natural History, New York; NMMNH, New Mexico Museum of Natural History and Science, Albuquerque; UALP, University of Arizona Laboratory of Paleontology, Tucson.

Specimens that Taylor (1984) collected were originally deposited at the UALP. However, that collection was transferred to the NMMNH in 2005 (Williamson et al. 2006). All specimens from the UALP collection were assigned NMMNH specimen numbers (Table 1) and will be referred to by the NMMNH numbers in this report. All detailed locality information is on file at the NMMNH.