Plesiadapiforms are a group of mammals that most cladistic analyses support as stem primates (Silcox, 2001; Bloch et al., 2007; Chester et al., 2015, 2017; but see Ni et al., 2016) and that represent the oldest known members of the order (Van Valen and Sloan, 1965; Wilson Mantilla et al., 2021). They appear in the fossil record shortly after the Cretaceous/Paleogene (K/Pg) boundary (although the available evidence indicates that they likely originated in the Cretaceous; e.g., Clemens, 1974; Silcox and Gunnell, 2008; Fox and Scott, 2011; Wilson Mantilla et al., 2021) and subsequently radiated throughout the Paleocene into a taxonomically and ecomorphologically diverse group (e.g., Gidley, 1923; Gingerich, 1975; Krause, 1978; Gunnell, 1989; Beard, 1993; Van Valen, 1994; Silcox, 2001; Bloch et al., 2007, 2016; Silcox and Williamson, 2012; Chester and Bloch, 2013; Chester et al., 2015; Silcox et al., 2017; López-Torres et al., 2018; Wilson Mantilla et al., 2021). Thus, the spatiotemporal patterns of the plesiadapiform radiation are important for understanding the evolutionary history of Primates, other euarchontan mammals (i.e., Scandentia and Dermoptera) and the post-K/Pg recovery and diversification of placental mammals more broadly.

The earliest confirmed occurrences of plesiadapiforms come from deposits in northeastern Montana dated to the early Puercan (Pu1) North American Land Mammal ‘age’ (NALMA) (66.052–65.844 Ma; Wilson Mantilla et al., 2021), and all records of Puercan plesiadapiforms are taxonomically restricted to members of the Purgatoriidae and two species of the enigmatic genus Pandemonium (family incertae sedis; Van Valen and Sloan, 1965; Van Valen, 1994; Fox and Scott, 2011; Fox et al., 2014; Scott et al., 2016; Wilson Mantilla et al., 2021). Plesiadapiform diversity substantially increased in the following Torrejonian NALMA with the appearance of five families (but see Crowell et al. [2024], regarding whether picrodontids, which are included herein, are plesiadapiforms) that exhibit a wide range of dental morphologies (Bown and Rose, 1976; Silcox and Gunnell, 2008; Silcox and Williamson, 2012; Silcox et al., 2017; López-Torres et al., 2018). The record of early Torrejonian faunas between the Puercan and the well-known middle and late Torrejonian NALMAs, however, is comparatively sparse and has hampered our understanding of this interval of diversification in primate evolutionary history (Lofgren et al., 2004; Silcox and Williamson, 2012; Silcox et al., 2017).

The current record of Puercan plesiadapiforms includes the purgatoriid species Purgatorius unio, Purgatorius titusi (considered to be a junior synonym of P. unio by Silcox [2001], which we provisionally follow here), Purgatorius janisae, Purgatorius ceratops (but see Lofgren [1995] and Clemens [2004] for discussion of the age of this taxon and Buckley [1997] and Silcox et al. [2017] for discussion of its validity), Purgatorius coracis, Purgatorius pinecreeensis, Purgatorius mckeeveri and Ursolestes perpetior and Pandemonium dis and Pandemonium hibernalis (Van Valen and Sloan, 1965; Van Valen, 1994; Buckley, 1997; Fox and Scott, 2011; Fox et al., 2014; Fox et al., 2015; Scott et al., 2016; Wilson Mantilla et al., 2021). These taxa are known from the Tullock Member of the Fort Union Formation (P. unio, P. janisae, P. ceratops, P. mckeeveri, Purgatorius cf. coracis and Pa. dis; Van Valen and Sloan, 1965; Van Valen, 1994; Smith et al., 2018; Wilson Mantilla et al., 2021) and Bear Formation (P. unio and U. perpetior; Buckley, 1997; Fox et al., 2015) both in Montana (USA), the Ravenscrag Formation in Saskatchewan (Canada) (P. coracis and P. pinecreeensis; Fox and Scott, 2011; Scott et al., 2016) and the Scollard Formation in Alberta (Canada) (Pa. hibernalis; Fox et al., 2014). The record of early Torrejonian (To1) plesiadapiforms is depauperate compared with that of the Puercan and younger Torrejonian interval zones (To2 and To3); in Montana it is limited to the paromomyid species Paromomys farrandi and four specimens provisionally referred to ?Purgatoriidae from the Tullock Member of the Fort Union Formation (Clemens and Wilson, 2009). Elsewhere, the record includes Paromomys depressidens, the picrodontid species Draconodus apertus and possibly the palaechthonid species Plesiolestes nacimienti from the North Horn Formation in Utah (Tomida and Butler, 1980; Tomida, 1982; Gunnell, 1989), as well as the palaechthonid species Palaechthon woodi and specimens referred to Paromomys sp. from the Nacimiento Formation in New Mexico (Tomida, 1981; Silcox and Williamson, 2012). A number of taxa first appear in the middle (To2) and late (To3) Torrejonian—including members of the Palaechthonidae, Paromomyidae, Picrodontidae, Plesiadapidae and Carpolestidae (Matthew and Granger, 1921; Gidley, 1923; Simpson, 1937; Gazin, 1968, 1971; Wilson and Szalay, 1972; Gingerich, 1975; Kay and Cartmill, 1977; Rigby, 1980; Gunnell, 1989; Van Valen, 1994; Scott and Fox, 2005; Fox et al., 2010; Silcox and Williamson, 2012)—which represents a time of substantial taxonomic and morphological radiation among plesiadapiforms (Silcox and Gunnell, 2008; Silcox et al., 2017; López-Torres et al., 2018). They may have also experienced modest increases in body size, although the fossil record suggests that it is not until the Tiffanian and Clarkforkian NALMAs that a considerably large range of body sizes among plesiadapiforms began to emerge (with the exception of the purgatoriid Ursolestes from the late Puercan, which is comparatively larger than other known contemporaneous taxa; Gunnell, 1989; Silcox and Gunnell, 2008; Fox et al., 2015; Silcox et al., 2017). Therefore, an improved record of To1 plesiadapiforms may help illuminate the early stages of this Torrejonian diversification.

The Tullock Member of the Fort Union Formation in northeastern Montana (Garfield and McCone counties; Fig. 1) provides an excellent window into the record of earliest Paleocene mammalian faunas, from the Puercan (Pu1 and Pu3 interval zones; localities considered Pu2 in age are unknown in this region) to early Torrejonian (To1 interval zone) NALMAs (Archibald, 1982; Dingus, 1983; Fastovsky, 1987; Swisher et al., 1993; Clemens, 2002, 2015; LeCain et al., 2014; Wilson, 2014; Sprain et al., 2015, 2018; Weaver et al., 2022). The local faunas in this region are tied into a high-precision chronostratigraphic framework that spans the first ca. 1.2 Ma after the K/Pg boundary (66.052–64.866 Ma; Sprain et al., 2018). More than 10,000 mammalian specimens from over 100 Pu1 and Pu3 localities have been recovered from this study system; in contrast, previous collecting efforts from localities higher in the local section (the Farrand Channel and Horsethief Canyon local faunas; FC and HT local faunas, hereafter) have produced significantly fewer specimens and many remain undescribed (Wilson, 2014). Clemens and Wilson (2009) authored the first systematic analysis of mammal fossils from the FC and HT local faunas, wherein they described 78 specimens that included four upper molars they provisionally referred to ?Purgatoriidae and 33 specimens for which they erected a new species within the paromomyid genus Paromomys (Par. farrandi). Clemens and Wilson (2009) assigned a To1 age to these two local faunas based on the presence of Paromomys and other taxa (e.g., the arctocyonid archaic ungulates Mimotricentes and Oxytomodon) characteristic of To1, as well as their stratigraphic position and associated geochronological data; it should be noted, however, that the index taxon for To1—the archaic ungulate Periptychus carinidens—remains unknown in this region (Lofgren et al., 2004). The FC and HT local faunas have been correlated to magnetic polarity chron 28r (but see discussion in Clemens and Wilson [2009]) and are bracketed by coals that have radioisotopic ages of 65.118 ± 0.024/0.048 Ma and 65.041 ± 0.023/0.048 Ma (FC) and 65.118 ± 0.024/0.048 Ma and 64.866 ± 0.023/0.047 Ma (HT) (Sprain et al., 2015, 2018; for a complete description of the geological setting of FC and HT localities see Archibald [1982] and Clemens and Wilson [2009]). Thus, chronostratigraphic data indicate a younger age for the FC and HT local faunas than known Pu3 localities and an older age than other known To1 localities, and if their correlation to the To1 interval zone is correct they would represent the oldest and most northerly To1 local faunas known (Swisher et al., 1993; Lofgren et al., 2004; Clemens and Wilson, 2009; Sprain et al., 2018).

Here we report a large assemblage of plesiadapiform dental fossils recovered from the FC and HT local faunas. These new fossils include 118 identifiable isolated teeth and dentigerous (tooth bearing) dentary fragments and, to our knowledge, represent the largest published sample of plesiadapiforms from this time interval. We describe these new records and reassess the specimens from the FC and HT localities provisionally attributed to ?Purgatoriidae by Clemens and Wilson (2009). Further, to contextualize what these new specimens add to the record of plesiadapiforms and to compare trends in the diversity of the group through time, we evaluate plesiadapiform species richness, mean body mass and body-mass disparity in each NALMA zone (acme zone, interval zone and lineage zone; see Lofgren et al. [2004]) of the Paleocene. Taken together, these new specimens provide a substantial contribution to our understanding of this important interval in plesiadapiform evolutionary history and their radiation in the early Cenozoic more broadly.