Two new species of Swaindelphys, S. encinensis and S. johansoni and one new species of Peradectes, P. coprexeches, represent the first metatherians to be identified and described from the late early Paleocene (Torrejonian) of the Nacimiento Formation, San Juan Basin, New Mexico. These taxa significantly increase the taxonomic and morphological diversity of Paleocene metatherians Metatherians have previously been generally poorly known, constituting minor components of Paleogene faunas of western North America (see
Krishtalka and Stucky 1983;
Korth 2007). However, data presented here indicate they were at least locally abundant members of the mammalian fauna in the late early Paleocene of the San Juan Basin of northwestern New Mexico. At one locality, L-7583, for example, metatherian specimens referable to Swaindelphys johansoni and Peradectes coprexeches constitute over 25% (n = 50) of the generically identifiable mammal specimens (n = 189). This approaches the total number of identifiable multituberculate specimens (n = 68) from the same locality and exceeds that of any other taxon of therian mammal. Of the metatherians, P. coprexeches predominates (n = 41). Preliminary calculations of the minimum number of individuals (MNI) indicates that P. coprexeches [MNI = 10] constituted over 30% of the total therian mammal abundance [MNI ~ 31]. S. johansoni is relatively more rare [MNI = 2] and didn't exceed about 6%.
Swaindelphys was previously represented by a single species, S. cifellii, from the middle Torrejonian (To2) of Swain Quarry, "Fort Union Formation," northern Wyoming. The new species described here are significantly larger than S. cifelli and add new morphological information to this taxon. One of these new taxa, S. johansoni, includes a dP3 that has not previously been described for any early Paleocene metatherian. It differs significantly from those described for other Paleogene metatherians including Peradectes and Herpetotherium.
Swaindelphys has been considered a basal herpetotheriid by numerous workers (e.g.,
Hooker et al. 2008). However, some workers have proposed that certain Cretaceous taxa represent older representatives of Herpetotheriidae (Case et al. 2005;
Martin et al. 2005). These putative Cretaceous herpetotheriids formed the basis for extending the origin of Herpetotheriidae to the Late Cretaceous by recent studies on the origins and relationships of Herpetotheriidae and crown-clade marsupials (Sánchez-Villagra et al. 2007;
Horovitz et al. 2008). However, we note that neither Swaindelphys nor putative Cretaceous herpetotheriids have been included in the phylogenetic analyses accompanying these studies that would test these conclusions.
The morphology of Swaindelphys has important relevance to questions regarding the taxonomy of earliest Paleocene (Puercan) metatherians.
Johanson (1996a) noted that the lower dentition of Thylacodon pusillus or taxa referred to T. cf. T. pusillus, closely resemble that of Swaindelphys cifellii. This might imply a possible close relationship between Thylacodon and Swaindelphys and would perhaps nullify the validity of Thylacodon pusillus as the holotype consists of a partial dentary with lower molars (see
Clemens 2006). Others have suggested that Thylacodon is a synonym of Peradectes (Clemens 1979;
Clemens 2006). The taxonomic validity of Thylacodon remains unresolved (e.g.,
Clemens 2006), and examining this issue is beyond the scope of this study. However, based on the new specimens described here, we are able to confirm that while the teeth of S. encinensis overlaps in size with those of T. pusillus, they differ significantly, especially in features of the molar talonid cusps. We find that the lower teeth of these taxa can be readily distinguished. This finding lends support to the suggestion that Thylacodon is a valid taxon (e.g.,
Krishtalka and Stucky 1983) rather than a synonym of Swaindelphys.
Peradectes coprexeches is the first peradectid metatherian to be described from Torrejonian age strata. However, a similar unidentified metatherian is described from the Wagonroad locality of the North Horn Formation, central Utah (Tomida and Butler 1980) based on an isolated lower molar. This tooth is slightly larger than lower molars referred P. coprexeches. An undescribed metatherian is present from the Gidley Quarry of the Fort Union Formation, Crazy Mountain Field, Montana, based on a partial dentary with p3-m2 (AMNH 35956). Metatherians have not been reported in previous faunal summaries of the Gidley Quarry fauna (Rose 1981, table 39;
Williamson 1996, table 7). This new occurrence suggests that peradectids were present and widespread throughout North America for at least part of the Torrejonian. We further suggest that their absence from many faunas is largely due to sampling biases due to their small size.
Based on the results reported here, all Nacimiento Formation early Paleocene (Torrejonian) metatherian taxa are restricted to the latter part of the Torrejonian (Figure 2). Peradectes coprexeches is limited to fossil horizons G and H of Williamson (Williamson 1996) and therefore is present only within the Pantolambda cavirictum – Mixodectes pungens and Mixodectes pungens zones of the Nacimiento Formation (latter part of To2-To3;
Lofgren et al. 2004). Swaindelphys johansoni has been documented only in the Pantolambda cavirictum – Mixodectes pungens Zone and S. encinensis is reported only from the Mixodectes pungens Zone. Few microvertebrate localities are known for earlier Torrejonian sites of the Nacimiento Formation, and therefore the absence of small mammals such as metatherians from strata below fossil horizon G is likely due to collecting biases.