Crown primates first appear in the fossil record of North America at the beginning of the Eocene as part of a larger group of immigrant taxa dispersing from eastern Asia during the Paleocene Eocene Thermal Maximum (Rose et al., 2012; Fleagle, 2013). These earliest North American primates include members of two larger clades: omomyoids and adapoids (Szalay and Delson, 1979; Hartwig, 2002). During the Wasatchian North American Land Mammal Age (NALMA), which spans approximately the first five million years of the Eocene (Alroy, 2000; Robinson et al., 2004; Gunnell et al., 2009), omomyids1 in the subfamily Anaptomorphinae exhibited greater species richness and were much more abundant than omomyids in the subfamily Omomyinae (Gunnell, 1997). Wasatchian omomyids in both subfamilies were uniformly small-bodied (i.e., less than 500 g in body mass; Conroy, 1987; Gunnell, 1997; Fleagle, 2013). In the subsequent Bridgerian NALMA (∼50.3–46.2 Ma),2 omomyines diversified to match anaptomorphines in species richness and became far more abundant (Gunnell, 1997). Nevertheless, Bridgerian omomyids of both subfamilies remained relatively small with few exceptions (Conroy, 1987). By comparison, North American Wasatchian and Bridgerian adapoids in the subfamily Notharctinae represent a radiation of primates that was exclusively greater than 500 g in body mass (Covert, 1985; Conroy, 1987; Fleagle, 2013). Among living primates, 500 g in body mass (i.e., ‘Kay's threshold’) bears an important relationship with diet (Kay and Covert, 1984). Primate species with body masses below Kay's threshold have primarily faunivorous, exudativorous, or frugivorous diets, whereas species with body masses above Kay's threshold have primarily folivorous or frugivorous diets. Among extant primate frugivores, species with body masses below and above Kay's threshold tend to rely on fauna or leaves (respectively) as a protein source (Kay and Covert, 1984). Accordingly, for more than nine million years during the early and early middle Eocene, North American primate faunas were characterized by omomyids occupying small-bodied faunivorous, frugivorous, and possibly exudativorous niches and notharctines occupying large-bodied folivorous and frugivorous niches (Covert, 1985; Strait, 2001; Gilbert, 2005).

In contrast to the patterns established during the Wasatchian and Bridgerian, North American primate faunas during the Uintan NALMA (∼46.2–42.0 Ma) underwent a fundamental reorganization. Omomyine species richness remained essentially stable compared to the preceding Bridgerian, but anaptomorphine species richness declined precipitously (Williams and Kirk, 2008). Additionally, notharctines became extinct by the middle Uintan (Ui2; Gunnell et al., 2009). In their absence, larger-bodied primate niches in North America were mainly filled by a diverse radiation of Uintan omomyines with body masses above Kay's threshold (Conroy, 1987; Fleagle, 2013; Atwater and Kirk, 2018). In Trans-Pecos3 Texas, immigrant adapoids from Asia arrive after the local extinction of notharctines and persist at least from the late Uintan to the subsequent Duchesnean NALMA (∼42.0–38.0 Ma; Wilson and Szalay, 1976; Kirk and Williams, 2011). These shifts in the composition and structure of Uintan primate faunas occurred against a backdrop of substantial changes in climate and habitats and associated increases in biotic provincialism (Lillegraven, 1979; Wing, 1987; Frederiksen, 1991; Stucky, 1992; Walsh, 1996; Devore and Pigg, 2010; Townsend et al., 2010; Westerhold et al., 2020).

Uintan primate faunas are best known from three areas in North America: Trans-Pecos Texas, Southern California, and Rocky Mountain basins in Utah, Wyoming, and Colorado4 (Robinson et al., 2004; Gunnell et al., 2009). Restudy of Uintan primates from Southern California in recent years has led to the recognition of several new genera and species (Atwater and Kirk, 2018; López-Torres et al., 2018). Similarly, analyses of newly discovered Uintan primates from Texas and Wyoming in the last 16 years have increased the number of known taxa (Williams and Kirk, 2008; Murphey and Dunn, 2009; Kirk and Williams, 2011). Here, we describe new samples of Uintan omomyoids from two intermontane basins formed during the Laramide orogeny: the Uinta Basin of northeastern Utah (Dickinson et al., 1988; Lawton, 2008) and the Tornillo Basin of Trans-Pecos Texas (Lehman, 1991; Bataille et al., 2016; Kelson et al., 2018). This effort focuses on previously undescribed specimens of Ourayia uintensis, Mytonius hopsoni, and Diablomomys dalquesti recovered over several decades of field collecting, as well as new species of Ourayia and Mytonius from the Tornillo Basin. In doing so, we seek to provide greater clarity regarding the validity of the genera Ourayia, Mytonius, and Diablomomys and to offer a refined assessment of omomyid species richness within each basin. Our larger goals are to provide a clearer understanding of the changes in the primate faunas of the Uinta and Tornillo basins over time and to provide an improved basis for the comparison of Uintan primates between regions.

Fieldwork to collect vertebrate fossils began in the Uinta Basin (Fig. 1) in 1870 through the efforts of O.C. Marsh et al. at the Yale Peabody Museum (Rasmussen et al., 1999). The first crown primate collected from Uinta Formation exposures within the basin was described by Osborn (1895).5 By the 1980s, the entire published sample of crown primates from the Uinta Basin consisted of 6 specimens, all of which represent large omomyids (Gazin, 1958; Simons, 1961; Robinson, 1968; Krishtalka, 1978). At present, there is little consensus in the published literature as to whether these specimens should be accommodated within one genus (Ourayia; Simons, 1972; Szalay, 1976; Szalay and Delson, 1979), two genera (Macrotarsius and Ourayia; Gunnell, 1995a; Gunnell and Rose, 2002; Tornow, 2008), or three genera (Macrotarsius, Mytonius, and Ourayia; Krishtalka, 1978).

Beginning in 1993, regular fieldwork resumed in the Uinta Basin through the efforts of D. Tab Rasmussen after a hiatus of nearly 3 decades (Rasmussen et al., 1999). This fieldwork has continued to the present under the direction of the fourth author. Systematic collecting of fossiliferous horizons in the Uinta Formation within a well resolved stratigraphic context (Townsend et al., 2006; Gunnell et al., 2009) has led to a considerable expansion of the number of fossil crown primate specimens known from the Uinta Basin. These newer specimens are all middle to late Uintan (Ui2–Ui3) in age (Townsend et al., 2006; Gunnell et al., 2009) and include multiple omomyid genera. With the exception of the Uinta Basin endemic taxon Chipetaia lamporea (Rasmussen, 1996), the majority of this expanded primate sample has not previously been described in detail. Here, we focus on describing newer specimens of Ou. uintensis and My. hopsoni and document the validity of both taxa as originally named (Gazin, 1958; Robinson, 1968).

The presence of terrestrial vertebrate fossils in the Tornillo Basin (see Fig. 2 in Lehman, 1991) was first noted by Udden (1907). However, early systematic collecting of fossil vertebrates in the Tornillo Basin focused on Mesozoic taxa (e.g., Brown, 1941; Colbert and Bird, 1954), and it was not until 1952 that John A. Wilson first documented the presence of Paleogene mammals within the basin (Wilson et al., 1952). Subsequent collecting by Wilson et al. in the 1960s–1980s focused on fossiliferous exposures in three main areas: Tornillo Flat in the southeastern part of the basin, the Agua Fria/Green Valley area in the south-central part of the basin, and the Sierra Vieja in the northwestern part of the basin (see Fig. 1 in Williams and Kirk, 2008). Fieldwork in the Sierra Vieja led to the recovery of only two primate specimens: a maxilla of Macrotarsius sp. indet. (late Uintan; West, 1982) and the holotype cranium of Rooneyia viejaensis (Duchesnean; Wilson, 1966). Similarly, Runkel (1988) described a small sample6 of crown primates from Tornillo Flat and other localities within Big Bend National Park. However, the largest sample of crown primates from the Tornillo Basin was recovered from exposures of the Devil's Graveyard Formation in the Agua Fria/Green Valley area (West, 1982; Wilson, 1986). Initial fieldwork in this area was undertaken by Wilson et al. between 1970 and 1979.7 Based on fossils recovered in the course of this early fieldwork, Wilson and Szalay (1976) described the Duchesnean adapoid Mahgarita stevensi. West (1982) described additional crown primate specimens from the Devil's Graveyard Formation, including an early Uintan (Ui1a) sample of 35 Omomys carteri teeth (Campisano et al., 2014).8 Wilson (1986:372) “tentatively identified” an additional sample of late Uintan (Ui3) Om. carteri from the Serendipity locality (TMM 41745) but did not describe these specimens in detail.

Fieldwork to collect additional Eocene mammals from the Devil's Graveyard Formation was undertaken sporadically by various research groups in the 1980s and 1990s. Systematic paleontological fieldwork focused primarily on the lower and middle members of the Devil's Graveyard Formation (Stevens et al., 1984) resumed in 2005 under the direction of the first author and has continued to the present. Accordingly, the sample of crown primates from the Tornillo Basin has been substantially expanded since the time of the initial summaries that were provided by West (1982) and Wilson (1986). Williams and Kirk (2008) described the omomyid D. dalquesti on the basis of a partial maxilla collected by a field crew from Duke University in 1996. Kirk and Williams (2011) subsequently described the adapoid Mescalerolemur horneri, which has close phylogenetic affinities with Mahgarita stevensi. Both D. dalquesti and Me. horneri are endemic taxa known thus far only from the late Uintan of the Devil's Graveyard Formation. Here, we focus on describing new species of the omomyids Ourayia and Mytonius from the Tornillo Basin, as well as new specimens of D. dalquesti that provide much more information about the dental morphology of this taxon.