Mammut pacificus is a recently described species of mastodon from the Pleistocene of California and Idaho. We report the easternmost occurrence of this taxon based upon the palate with right and left M3 of an adult male from the Irvingtonian of eastern Montana. The undamaged right M3 exhibits the extreme narrowness that characterizes M. pacificus rather than M. americanum. The Montana specimen dates to an interglacial interval between pre-Illinoian and Illinoian glaciation, perhaps indicating that M. pacificus was extirpated in the region due to habitat shifts associated with glacial encroachment.

Purple Bench is a middle Eocene fossil locality in the Devil's Graveyard Formation of the Trans-Pecos region of West Texas. In addition to yielding a range of taxa characteristic of the Uintan North American Land Mammal Age, the Purple Bench locality is noteworthy in documenting a number of endemic species that are known only from the site. Despite the Uintan character of the mammalian fauna, the absolute age of Purple Bench is a matter of debate. This uncertainty stems from the wide interval of time encompassed by current radiometric dates bracketing the Purple Bench locality and from conflicting magnetostratigraphic correlations in the Devil's Graveyard Formation. This study constrains the absolute age of the Purple Bench locality through detrital zircon U-Pb geochronological analyses. For these analyses, 147 new detrital zircon U-Pb ages were collected from five tuffaceous sandstones and reworked tuff horizons and analyzed via Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). These new detrital zircon U-Pb geochronological analyses suggest a maximum depositional age of 43.7 +0.8 /-0.2 Ma for the Purple Bench tuff, a significant marker horizon immediately below the Purple Bench locality. These new maximum depositional age dates presented here provide constraints on the true depositional age of the lower and middle members of the Devil's Graveyard Formation, bringing clarity to the previously ambiguous age of the fossil-bearing Purple Bench locality. The age constraints presented here also aid the characterization of the temporally and spatially variable Uintan North American Land Mammal Age.

The Friars Formation of San Diego County, California, has yielded a middle Eocene mammalian fauna from the early part of the Uintan North American Land Mammal Age. Prior research on the primate fauna from the Friars Formation provides evidence of one notharctine and multiple omomyine species, but many specimens collected since the early 1980s remain unstudied. Here we describe three new omomyine genera from the Friars Formation. These new taxa range in estimated body mass from about 119 g to 757 g, and substantially expand the diversity of middle Eocene omomyoids known from Southern California. Resolution of the phylogenetic relationships of the new Friars Formation omomyines is complicated by the fact that different character-taxon matrices and tree building methods produce different results. Nevertheless, all preliminary phylogenetic analyses are congruent in recovering a close relationship between the three new genera and the omomyines Macrotarsius, Omomys, Ourayia, and Utahia. Prior research has documented a shift in omomyoid diversity in North America from the anantomophine-rich Bridgerian to the omomyine-rich Uintan. Our description of three new Uintan omomyine taxa from the Friars Formation further emphasizes these opposite trends in anaptomorphine and omomyine species richness during the middle Eocene. All three of the new taxa are currently known from only the Friars Formation in San Diego County, California. Four of the previously known omomyoid genera from Southern California (Dyseolemur, Chumashius, Yaquius, and Stockia) are also endemic to the region, further highlighting the provincial character of primate faunas in Utah, Southern California, and West Texas during the Uintan.