Using extant patterns of dental variation to identify species in the primate fossil record: a case study of middle Eocene Omomys from the Bridger Basin, southwestern Wyoming

New specimens of middle Eocene omomyines (Primates, Omomyoidea) from the Uinta Basin of Utah and the Tornillo Basin of Texas, with clarification of the generic status of Ourayia, Mytonius, and Diablomomys

In the middle Eocene, multiple lineages of North American omomyoids independently evolved body masses greater than 500 g. Most of these large-bodied omomyoids are known from small sample sizes, which has contributed to a lack of consensus regarding their alpha taxonomy. Here, we describe new Uintan omomyine specimens from the Uinta Basin of Utah and the Tornillo Basin of Texas. These new samples expand the hypodigms of Diablomomys dalquesti, Mytonius hopsoni, and Ourayia uintensis, and favor the recognition of new species of Mytonius and Ourayia based on specimens from the Tornillo Basin. These samples support the recognition of Diablomomys as a valid genus distinct from Omomys, Ourayia as a valid genus distinct from Macrotarsius, and Mytonius as a valid genus distinct from Ourayia. Although Diablomomys and Omomys co-occur in the late Uintan of the Tornillo Basin, Ourayia and Mytonius are time-successive taxa with a wide distribution across multiple Laramide basins. The data presented here reinforce the conclusion that the Uintan was a time period in which omomyines diversified to include a large number of taxa with body masses above Kay's threshold and frugivorous-folivorous diets. These data also provide evidence that North American primate faunas exhibited a shifting pattern of regional endemism during the middle Eocene. By the early Uintan, primate faunas from Southern California were already quite distinct from primate faunas of the central Rocky Mountains or Trans-Pecos Texas. By the late Uintan, primate faunas in all three regions demonstrated greater provincialism, with Trans-Pecos Texas and Southern California both exhibiting a large number of endemic primate taxa and sharing only a single primate genus (Macrotarsius) in common. This increase in primate endemism across the Uintan may be tied to changes in paleohabitats associated with the larger trend toward decreasing temperatures from the Early Eocene Climatic Optimum to the Eocene/Oligocene transition.

Euarchontans from Fantasia, an upland middle Eocene locality at the western margin of the Bighorn Basin

The fossil record of North American Eocene mammals is best known from relatively low-elevation 'basin center' fossil localities in intermontane depositional basins of the Western Interior. This sampling bias, largely drawn from preservational bias, has limited our understanding of fauna from higher elevation Eocene fossil localities. Here we describe new specimens of crown primates and microsyopid plesiadapiforms from a middle Eocene (Bridgerian) locality ('Fantasia') from the western margin of the Bighorn Basin in Wyoming. Fantasia has been considered a 'basin-margin' site and geological evidence suggests that it was already at a high elevation relative to the basin center at the time of deposition. New specimens were described and identified using comparisons across museum collections and published faunal descriptions. Linear measurements were used to characterize the patterns of variation in dental size. Contrary to expectations derived from other Eocene basin-margin sites in the Rocky Mountains, Fantasia has low anaptomorphine omomyid diversity and lacks evidence for the co-occurrence of ancestor-descendant pairs. Fantasia also differs from other Bridgerian sites in having low abundance of Omomys and unusual body sizes of several euarchontan taxa. Some specimens of Anaptomorphus and cf. Omomys are larger than those found in coeval sites, while specimens of Notharctus and Microsyops are intermediate in size between middle and late Bridgerian samples of these genera from basin-center sites. These findings suggest that high elevation fossil localities like Fantasia may record atypical faunal samples that should be more thoroughly explored to understand faunal dynamics during the periods of significant regional uplift like that represented by the middle Eocene record of the Rocky Mountains. Furthermore, modern faunal data indicate that species body mass may be influenced by elevation, which may further complicate the use of body mass to determine species identity in the fossil record in the regions of high topographic relief.

New middle Eocene omomyines (Primates, Haplorhini) from San Diego County, California

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.

Early Eocene primates from Gujarat, India

The oldest euprimates known from India come from the Early Eocene Cambay Formation at Vastan Mine in Gujarat. An Ypresian (early Cuisian) age of approximately 53Ma (based on foraminifera) indicates that these primates were roughly contemporary with, or perhaps predated, the India-Asia collision. Here we present new euprimate fossils from Vastan Mine, including teeth, jaws, and referred postcrania of the adapoids Marcgodinotius indicus and Asiadapis cambayensis. They are placed in the new subfamily Asiadapinae (family Notharctidae), which is most similar to primitive European Cercamoniinae such as Donrussellia and Protoadapis. Asiadapines were small primates in the size range of extant smaller bushbabies. Despite their generally very plesiomorphic morphology, asiadapines also share a few derived dental traits with sivaladapids, suggesting a possible relationship to these endemic Asian adapoids. In addition to the adapoids, a new species of the omomyid Vastanomys is described. Euprimate postcrania described include humeri, radii, femora, calcanei, and tali, most of which show typical notharctid features and are probably attributable to asiadapines. Anatomical features of the limb elements indicate that they represent active arboreal quadrupedal primates. At least one calcaneus is proximally shorter and distally longer than the others, resembling eosimiids in this regard, a relationship that, if confirmed, would also suggest an Asian or southeast Asian faunal connection. Isolated teeth from Vastan Mine recently attributed to a new eosimiid, Anthrasimias gujaratensis, appear to provide that confirmation. However, their attribution to Eosimiidae is equivocal. They are similar to teeth here tentatively referred to Marcgodinotius, hence A. gujaratensis may be a junior synonym of M. indicus. Corroboration of eosimiids at Vastan requires more compelling evidence. Although definitive conclusions are premature, available evidence suggests that the Vastan adapoids, at least, were derived from western European stock that reached India near the Paleocene-Eocene boundary.

Relief index of second mandibular molars is a correlate of diet among prosimian primates and other euarchontan mammals

This study describes and tests a new method of calculating a shape metric known as the relief index (RFI) on lower second molars of extant euarchontan mammals, including scandentians (treeshrews), dermopterans (flying lemurs), and prosimian primates (strepsirhines and tarsiers). RFI is the ratio of the tooth crown three-dimensional area to two-dimensional planar area. It essentially expresses hypsodonty and complexity of tooth shape. Like other measurements of complexity, RFI ignores taxon-specific features, such as certain cusps and crests, which are usually considered in more traditional studies of tooth function. Traditional statistical analyses of the study sample show that RFI distinguishes taxa with differing amounts of structural carbohydrates in their diets, with frugivore/gramnivores being significantly lower in RFI than omnivores, and insectivores/folivores being significantly higher in RFI than the other two. Information on absolute size, or body mass, is needed to reliably parse out insectivores and folivores; however, if the study sample is limited to Primates, RFI alone distinguishes many folivores (lower) from insectivores (higher). Finally, phylogenetically independent contrasts of RFI and dietary preference are strongly correlated with one another, indicating that variance in RFI is better explained by dietary diversity than phylogenetic affinity in this sample. Because of the accuracy and phylogenetic insensitivity of the RFI among Euarchonta, this method can be applied to fossil primates and stem-primates (plesiadapiforms) and used to elucidate and compare their dietary preferences. Such comparisons are important for developing a more detailed view of primate evolution.