Publications and Presentations

Abstract: Background- Anosteira pulchra is one of two species of the obligately-aquatic freshwater clade Carettochelyidae (pig-nosed turtles) from the Eocene of North America. Anosteira pulchra is typically rare in collections, and their distribution is poorly documented. The Uinta Formation [Fm.] contains a diverse assemblage of turtles from the Uintan North American Land Mammal Age. Whereas turtles are abundantly preserved in the Uinta Fm., A. pulchra has been reported only from a few specimens in the Uinta C Member.

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Abstract: We present new fossil records of the geoemydid turtle Bridgeremys pusilla from the Uinta Formation of Utah. Turtles are abundant throughout the unit, and known taxa are similar to those from the older strata in the Upper Green River Basin in Wyoming from the Bridger and Washakie Formations. B. pusilla is known from Bridgerian deposits but was not previously known from after the Turtle Bluff Member of the Bridger Formation. The taxon was coveal with two species of the geoemydid Echmatemys (E. callopyge and E. wyomingensis), a common genus of extinct pond turtles known primarily from lacustrine and fluvial deposits in western North America, including the Uinta Basin. In addition to previously documented morphological differences, our geometric morphometric analyses revealed significant differences in epiplastral morphology between B. pusilla and the two coeval Echmatemys species. Bridgeremys pusilla shared several morphological characters with Testudinidae. However, our anatomical network analysis suggests that the carapace of B. pusilla distributed stress forces in a manner more similar to emydids (basal and derived) than to derived testudinoids (Testudinidae and Emydidae), including Echmatemys species. This finding changes our understanding of the ecology of the species and sheds light onto how geoemydid turtles of the Uinta Formation may have partitioned the available ecospace. These new Uintan records extend the geographic range of B. pusilla into the Uinta Basin and stratigraphically through the top of the Uinta Formation, extending the temporal range of the taxon by more than 4 million years through the Uintan North American Land Mammal Age to the base of the Duchesne River Formation.

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Abstract: The middle Eocene of North American was an important time in carnivoramorphan evolution, because it is when the order showed increases in diversity taxonomically and ecomorphologically. We describe new fossils of “miacoids” from the Uinta Formation of northeastern Utah, and assess their phylogenetic and paleobiological significance. A new species of Miocyon (Matthew, 1909), is described, Miocyon vallisrubrae, which, in several aspects of lower molar morphology, is intermediate between the smaller Miocyon major (Matthew, 1909) and the larger Miocyon scotti (Wortman and Matthew, 1899). In addition, new well-preserved dental and gnathic specimens of M. scotti are described, along with fragmentary cranial and postcranial remains. The taxonomy of Miocyon is reviewed in light of the new material, which highlights the divergent specializations of the genus. These include an m1 with a relatively broad talonid and an m2 that is large, broad and relatively flat with little difference in height between the trigonid and talonid. These features suggest a shift from a shearing-dominated diet to a greater reliance on grinding. An ecological analogue for the radiation of this lineage among living carnivorans may be the sympatric African species of jackals. The distribution of similar features in Miocyon as well as other “miacoids” suggests an ecomorphologically similar phylogenetic group containing Miocyon, Uintacyon Leidy, 1872, and Vassacyon Matthew, 1909. New specimens from the Uinta Formation are also described of Tapocyon Stock, 1934, Procynodictis Wortman and Matthew, 1899, a new unnamed miacid, and an unnamed small viverravid. These highlight the adaptive diversity of carnivoramorphans during the Uintan North American Land Mammal Age.

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Abstract: The Bridger Formation is located in the Green River basin in southwest Wyoming, and the Uinta and Duchesne River formations are located in the Uinta basin in Utah. These three rock units and their diverse fossil assemblages have great scientific importance and are also of historic interest to vertebrate paleontologists. Notably, they are also the stratotypes for the three middle Eocene North American Land Mammal “Ages,” the Bridgerian, Uintan, and Duchesnean, from oldest to youngest. The fossils and sediments of these formations provide a critically important record of biotic, environmental, and climatic history spanning ~10 million years (49–39 Ma). This article features a detailed field excursion through portions of the Green River and Uinta basins that focuses on locations of geologic, paleontologic, and historical interest. In support of the field excursion, we also provide a review of current knowledge of these formations with emphasis on lithostratigraphy, biochronology, depositional and aleoenvironmental history, and the history of scientific exploration.

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Abstract: The Bridger Formation is restricted to the Green River Basin in southwest Wyoming, and the Uinta and Duchesne River Formations are located in the Uinta Basin in Utah. These three rock units and their diverse fossil assemblages are of great scientific importance and historic interest to vertebrate paleontologists. Notably, they are also the stratotypes from oldest to youngest for the three middle Eocene North American Land Mammal Ages—the Bridgerian, Uintan, and Duchesnean. The fossils and sediments of these formations provide a critically important record of biotic, environmental, and climatic history spanning approximately 10 million years (49 to 39 Ma). This article provides a detailed field excursion through portions of the Green River and Uinta Basins that focuses on locations of geologic, paleontologic, and historical interest. In support of the field excursion, we also provide a review of current knowledge of these formations with emphasis on lithostratigraphy, biochronology, depositional, and paleoenvironmental history, and the history of scientific exploration.

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Abstract: Fossil mammals from the Uinta Formation, Uinta Basin, northeastern Utah, served as the primary basis for establishing the Uintan North American Land Mammal “Age.” These Uintan mammals still remain of central importance in understanding mammalian evolution and paleoecology of the middle Eocene in North America. The Uintan fauna represents a transition between the relatively stable, tropical conditions of the earlier Eocene, and the more arid and cooler conditions of the later Eocene. Among the key mammalian events during the Uintan are the appearance and radiation of selenodont artiodactyls, and the great family-level diversification of rodents. Less spectacular radiations involved carnivorans and rhinoceratoid perissodactyls. Other mammalian groups were experiencing extinction or reductions in diversity, including uintatheres, hyopsodontid condylarths, and North American primates. A typical Uintan community was dominated by the selendont artiodactyls Protoreodon and Leptotragulus; by several different perissodacyl groups, including brontotheres, tapirs, the horse Epihippus, and both amynodont and hyracodont rhinos; and by a characteristic assemblage of abundant rodents, most notably the cylindrodontid Pareumys, the sciuravid Sciuravus, and the ischyromyid Leptotomus. While these taxa are consistently present and common through time and space, other mammals are unevenly distributed through the stratigraphic section or among localities, indicating taphonomic and collecting biases (especially for the smallest taxa) or paleoecological specializations.

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Abstract: We described newly discovered baenid specimens from the Uintan North American Land Mammal Age (NALMA), in the Uinta Formation, Uinta Basin, Utah. These specimens include a partial skull and several previously undescribed postcranial elements of Baena arenosa, and numerous well-preserved shells of B. arenosa and Chisternon undatum. Baenids from the Uintan NALMA (46.5–40 Ma) are critical in that they provide valuable insight into the morphology and evolution of the diverse and speciose baenid family near the end of its extensive radiation, just prior to the disappearance of this clade from the fossil record. These Uintan specimens greatly increase the known variation in these late-surviving taxa and indicate that several characters thought to define these species should be reassessed. The partial cranium of B. arenosa, including portions of the basicranium, neurocranium, face, and lower jaw, was recently recovered from Uinta B sediments. While its morphology is consistent with known specimens of B. arenosa, we observed several distinct differences: a crescent-shaped condylus occipitalis that is concave dorsally, tuberculum basioccipitale that flare out laterally, and a distinct frontal-nasal suture. The current sample of plastral and carapacial morphology considerably expands the documented variation in the hypodigms of B. arenosa and C. undatum. Novel shell characters observed include sigmoidal extragular-humeral sulci, and small, subtriangular gular scutes. Subadult specimens reveal ontogenetic processes in both taxa, and demonstrate that diagnostic morphological differences between them were present from an early developmental age.

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Abstract: Numerous geoemydid turtle fossils from the extinct genus Echmatemys have been recovered from the middle Eocene Uinta Formation, Uinta Basin, Utah over the past several decades. Here, we tested whether co-occurring Uintan species Echmatemys callopyge and E. uintensis can be reliably differentiated based on epiplastral morphology, and whether their geospatial distributions overlapped significantly. The geographic spatial and stratigraphic distributions of Uinta Basin E. callopyge and E. uintensis specimens were compared using ArcGIS and analysis of variance (ANOVA). The analysis revealed overlapping geographic distributions of these two species, and no significant differences in stratigraphic dispersal. This finding of extensive geospatial overlap between the two Uintan Echmatemys species highlights the need for accurate taxonomic identification, such as the gular scale morphology validated here.

In addition, we sought to address a methodological question regarding the relative efficacy of data complexity in this context. Using epiplastra from three additional Eocene species of Echmatemys, we employed hierarchical analyses of increasing data complexity, from standard linear dimensions to 2D geometric morphometrics to 3D laser scans, to determine the degree to which data complexity contributes to taxonomic assessments within this genus. Uintan species E. callopyge and E. uintensis were found to differ significantly in epiplastral shape as captured by all three categories of data. These findings verify that these two co-occurring species can be differentiated consistently using the shape of the gular scale, and that the use of geometric morphometrics can improve identification of fragmentary specimens. Among the non-Uintan species, dorsal and ventral 2D landmark data reliably differentiated among species, but the linear dimensions were less useful.

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Abtract: The Uinta Formation, Uinta Basin, Utah, is the type formation for the Uintan North American Land Mammal Age (NALMA). Lack of a high-resolution stratigraphic section in the Uinta Formation has led to confusion regarding the nature of and position of the contact between Uinta B and Uinta C rocks. Historically, this lithostratigraphic boundary has been defined in various ways, which has also affected our understanding of the early Uintan to late Uintan faunal transition. We describe a new high-resolution stratigraphic section in the upper intervals of the Uinta Formation, in the eastern Uinta Basin of Uintah County. Since 1994, fossil collection in this region by parties from Washington University has resulted in more than 200 productive fossil vertebrate localities, with most of these localities correlated to the new stratigraphic section. The section was measured from the lowest productive fossil mammal locality to the contact between the Uinta Formation (UF) and the overlying Duchesne River Formation (DRF) in the Devil's Playground region and the section was linked using marker beds to the western regions of the study area where it was measured to the UF-DRF contact at the top of Deadman's Bench. The stratigraphic section was used to investigate the utility of the "Amynodon Sandstone" unit as a lithologic boundary between Uinta B and Uinta C rocks, and to evaluate previous perceptions of the early to late Uintan mammal faunal transition. Uinta B rocks are characterized by gray-green mudstones interrupted by golden sheet sandstones and the transition from this lithology to the fine-grained red and orange claystones distinctive of Uinta C rocks occurs at 137 to 140 m. The Uinta B-C lithological transition is 73 m above the "Amynodon Sandstone" unit as measured in our section. A preliminary biostratigraphic analysis suggests that the early to late faunal transition occurred 10–20 m below the lithologic transition.

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