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Plesiadapiforms (putative stem primates) appear in the fossil record shortly after the Cretaceous/Paleogene boundary and subsequently radiated throughout the Paleocene into a taxonomically and ecomorphologically diverse group. The oldest known plesiadapiforms come from early Puercan (the oldest North American Land Mammal 'age' [NALMA] of the Cenozoic) deposits in northeastern Montana, and all records of Puercan plesiadapiforms are taxonomically restricted to members of the Purgatoriidae and the enigmatic genus Pandemonium. Plesiadapiform diversity substantially increased in the following Torrejonian NALMA, but the sparse record of faunas between the Puercan and the well-known middle and late Torrejonian has hampered our understanding of this important interval in early primate evolution. Here we report new plesiadapiform dental fossils from early Torrejonian (To1) deposits from the Tullock Member of the Fort Union Formation in northeastern Montana that record several poorly known taxa including members of the Purgatoriidae, Paromomyidae and Pandemonium , and that document the largest and most diverse assemblage of To1 plesiadapiforms known. We describe a new species of the purgatoriid Ursolestes (Ursolestes blissorum , sp. nov.) that represents the largest plesiadapiform known from the early Paleocene and, among other taxa, provides additional evidence that the temporal range of purgatoriids extended into the Torrejonian. Large sample sizes of the oldest known paromomyid, Paromomys farrandi , allowed us to document intraspecific variability and one undescribed tooth locus. Our observations illuminate changes in dental morphology of some taxa that occurred in To1 and may inform the acquisition of certain diagnostic plesiadapiform dental characters. We evaluate plesiadapiform species richness, mean body mass and body-mass disparity through the Paleocene and reveal unrecognized levels of richness in To1 and a general trend of stable body mass and body-mass disparity. Our findings contribute to documented patterns of plesiadapiform provincialism in the early Paleocene and shed light on the early stages of their Torrejonian radiation. [ABSTRACT FROM AUTHOR]

Paleogene microsyopid plesiadapiforms are among the oldest euarchontans known from relatively complete crania. While cranial endocasts are known for larger-bodied Eocene microsyopine microsyopids, this study documents the first virtual endocast for the more diminutive uintasoricine microsyopids, derived from a specimen of Niptomomys cf. Niptomomys doreenae (USNM 530198) from the late Paleocene of Wyoming. Size estimates of smaller-bodied uintasoricines are similar to those inferred for the common ancestor of Primates, so the virtual endocast of Niptomomys may provide a useful model to study early primate brain evolution. Due to the broken and telescoped nature of the neurocranium of USNM 530198, a μCT scan of the specimen was used to create a 3D model of multiple bone fragments that were then independently isolated, repositioned, and merged to form a cranial reconstruction from which a virtual endocast was extracted. The virtual endocast of Niptomomys has visible caudal colliculi, suggesting less caudal expansion of the cerebrum compared to that of euprimates, but similar to that of several other plesiadapiforms. The part of the endocast representing the olfactory bulbs is larger relative to overall endocast volume in Niptomomys (8.61%) than that of other known plesiadapiforms (∼5%) or euprimates (<3.5%). The petrosal lobules (associated with visual stabilization) are relatively large for a Paleocene placental mammal (1.66%). The encephalization quotient of Niptomomys is relatively high (range = 0.35–0.85) compared to that of Microsyops (range = 0.32–0.52), with the upper estimates in the range of values calculated for early euprimates. However, this contrast likely relates in part to the small size of the taxon, and is not associated with evidence of neocortical expansion. These findings are consistent with a model of shifting emphasis in primate evolution toward functions of the cerebrum and away from olfaction with the origin of euprimates. [ABSTRACT FROM AUTHOR]

Abstract: New specimens of micromomyid plesiadapiforms recovered from the late Paleocene and early Eocene of the Clarks Fork and Powder River Basins, Wyoming, include previously unknown tooth positions of Chalicomomys antelucanus, the earliest record and first substantial Paleocene sample of Tinimomys graybulliensis, and additional specimens of early Eocene T. graybulliensis, forming the largest known sample (n = 84, MNI = 14) of a micromomyid species from a single fossil locality. These specimens and newly documented intraspecific variability, coupled with the first detailed descriptions of the dentition of Dryomomys szalayi, allow for a systematic revision of the family. Cladistic analysis of the 11 known micromomyid species using 28 morphological characters produced three most-parsimonious cladograms. Results suggest that several Tiffanian taxa previously classified in the genus Micromomys (excluding the type species Micromomys silvercouleei) are more primitive and are referred to a new genus Foxomomys (Foxomomys fremdi, Foxomomys vossae, and Foxomomys gunnelli). Two other Paleocene and early Eocene species previously classified in Micromomys are instead found to share a special relationship with Dryomomys (Dryomomys millennius and Dryomomys willwoodensis) based primarily on the relative size and shape of the premolars. Results further suggest that early Eocene Chalicomomys (monotypic: Chalicomomys antelucanus) is the sister taxon to a clade that includes Dryomomys and Tinimomys, which diverged from each other by the late Tiffanian. The shape of P4 and the relative size of P3 have distinct patterns of change through the evolution of the group. Additionally, there is a gradual reduction of P2, with Foxomomys having a double-rooted P2, Micromomys, Chalicomomys, and Dryomomys having a single-rooted P2, and Tinimomys lacking a P2. Body size increases from more primitive micromomyids (Foxomomys and Chalicomomys) to more derived genera (Dryomomys and Tinimomys), and size also increases from the older and/or more primitive species within the Dryomomys and Tinimomys lineages. [Copyright &y& Elsevier]