MAMMALS OF THE MIDDLE EOCENE UINTA FORMATION D. Tab Rasmussen Department of Anthropology, Washington University, St. Louis, Missouri 63130 Glenn C. Conroy Department of Anthropology, Washington University, St. Louis, Missouri 63130 Anthony R. Friscia Department of Biology, University of California, Los Angeles, California 90024 K. Elizabeth Townsend Department of Anthropology, Washington University, St. Louis, Missouri 63130 Mary D. Kinkel Department of Anatomy, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, 44272 ABSTRACT INTRODUCTION Fossil mammals from the Uinta Formation, Uinta Fossils from the Uinta Formation (UF) of northeast- Basin, northeastern Utah, served as the primary basis for ern Utah have played a prominent role in our understand- establishing the Uintan North American Land Mammal ing of middle Eocene mammalian evolution in North “Age.” These Uintan mammals still remain of central America. O. C. Marsh of the Yale Peabody Museum importance in understanding mammalian evolution and first explored the fossil-bearing terrestrial sediments of paleoecology of the middle Eocene in North America. the Uinta Basin in 1870 (Betts, 1871; Marsh, 1875a, The Uintan fauna represents a transition between the rel- 1875b). He collected fossil mammals from gray bad- atively stable, tropical conditions of the earlier Eocene, lands near the White River that clearly belong to today's and the more arid and cooler conditions of the later UF (figure 1). This same area was collected more exten- Eocene. Among the key mammalian events during the sively by Francis Speir in the 1880s, working for Prince- Uintan are the appearance and radiation of selenodont ton University. Speir's fossils were studied by the great artiodactyls, and the great family-level diversification of paleontologists W. B. Scott and H. F. Osborn, who pro- rodents. Less spectacular radiations involved carnivo- duced the first substantive publications on UF mammals rans and rhinoceratoid perissodactyls. Other mammalian (Scott and Osborn, 1887, 1890). groups were experiencing extinction or reductions in The work of Scott and Osborn showed that the fauna diversity, including uintatheres, hyopsodontid condy- of the UF represented a distinct time period in the evolu- larths, and North American primates. A typical Uintan tion of North American mammals, later formalized as the community was dominated by the selendont artiodactyls Uintan Land Mammal "Age" (LMA). Initially, the Uin- Protoreodon and Leptotragulus; by several different tan LMA was believed to be latest Eocene. Now it is perissodacyl groups, including brontotheres, tapirs, the positioned squarely in the middle Eocene, following the horse Epihippus, and both amynodont and hyracodont Wasatchian and Bridgerian LMAs, and preceding the rhinos; and by a characteristic assemblage of abundant Duchesnean and Chadronian LMAs. Although the col- rodents, most notably the cylindrodontid Pareumys, the lections of fossil mammals from Utah originally stood as sciuravid Sciuravus, and the ischyromyid Leptotomus. the centerpiece of the Uintan LMA, mammals of similar While these taxa are consistently present and common age have been found also in California, Texas, Wyoming, through time and space, other mammals are unevenly and other places (Black and Dawson, 1966; Storer, 1984; distributed through the stratigraphic section or among Walsh, 1996a; Westgate, 1990; Wilson, 1986). localities, indicating taphonomic and collecting biases The Uintan LMA has received significantly less (especially for the smallest taxa) or paleoecological spe- attention than the four other Eocene LMAs. The cializations. Wasatchian, Bridgerian, and Chadronian LMAs are 402 Vertebrate Paleontology in Utah Utah Geological Survey made to Uintan faunas elsewhere in North America. It is not our purpose to provide a for- mal, revised taxonomic list of UF mammals by stratigraphic level, as this has been accom- plished by Walsh (1996a). REGIONAL GEOLOGICAL HISTORY Utah has been near sea level for most of its Phanerozoic geologic history, only rising much above that level over the last hundred million years or so during the Sevier and Laramide oro- genic events. Far earlier than these two events, sometime between about 1,000 and 600 Ma, an unusual east-west basin or trench (aulacogen) developed along the trend of the present Uinta Figure 1. Typical outcrop of the Uinta Formation showing good exposure and low Mountains in which some 25,000 feet of sand- erosional relief of the soft mudstones and siltstones, with steeper, horizontally band- ed exposures rising in the background. Recent expeditions have logged over 150 stone and shale accumulated. During the so- vertebrate localities, including this one a few miles northeast of where O. C. Marsh called Laramide uplift in the early Cenozoic first commented on the abundance of fossil turtles near the junction of the White and ~ Green Rivers in 1870. ( 66 - 37 Ma), the rise of the Uinta Mountains was linked to downwarping of an adjacent asymmetrical synclinal basin, the Uinta Basin. known by more extensive collections, and have been tar- The Uinta Mountain-Basin couplet is the largest gets of much more research than the Uintan. Fossils of Laramide structure in Utah. Laramide uplift of the Uinta the Duchesnean LMA are less common than those of the Mountains ended after deposition of the Uinta and Duch- Uintan but, in part because of this scarcity, the Duches- esne River Formations (Hintze, 1988). nean's status as an LMA has been controversial, and it has therefore received more than its share of attention (Lucas, 1992; Rasmussen and others, 1999). The UF has not been systematically collected for many decades. Since the Carnegie Museum field trips through the first half of the century (Douglass, 1910; Peterson, 1919; Kay, 1957; Dawson, 1966), only brief visits have been made to the UF, usually by museums interested in obtaining a sample of the typical fauna for their collections. Because most of the work on the UF occurred during an era that emphasized the discovery of large mammals (for example, the brontotheres), the smaller mammals from the formation remained under- represented in collections. Partly for this reason, a series of annual expeditions to the UF was initiated in 1993 by Washington University (WU), St. Louis, which focused on finding smaller taxa, such as primates, insectivores, proteutheres, and rodents (figure 2). The purposes of this review are to introduce the reader to the mammalian faunas of the UF and to provide an update on current field research on this formation. The first section provides an overview of the geological framework and reviews what is known of climate and paleoenvironments. Subsequent sections summarize the Figure 2. Mandible with m2-3 (CM 69899) of a new species of Trogolemur, a diminutive anaptomorphine primate. Recent WU ex- mammals in a paleobiological framework rather than a peditions have concentrated on the recovery of small mammals which formal systematic one. Finally, brief comparisons are were relatively neglected in the early fieldwork in the basin. Mammals of the Middle Eocene Unita Formation - Rasmussen, Conroy, Friscia, Townsend, Kinkel Miscellaneous Publication 99-1 Vertebrate Paleontology in Utah 403 The UF consists of sandstone paleochan- nels and overbank siltstone and mudstone deposits that accumulated on the distributary delta of a lake occupying the basin (figure 3)(Kay, 1934; Grande, 1984; Hamblin, 1987). Traditionally, the formation is divided into three sequential lithologic units, Uinta A, B and C (Osborn, 1895, 1929). Uinta A has rarely yielded fossil mammals and its relationship to the Bridgerian-Uintan boundary remains uncer- tain (Dane, 1954; Walsh, 1996b). Our under- standing of the UF fauna is based almost entirely on fossils from Uinta B and C. The Uinta B fauna is best known from material col- lected at White River Pocket, a loosely circum- scribed region just southeast of the junction of the White River and the Green River, while the Uinta C fauna is known best by collections Figure 3. Sandstone channels (eroding as blocks in the upper right) and mudstones/silt- made at the large sprawling region called stones (eroding as guttered slopes) of the Uinta Formation. Many parts of the for- mation show paleosol development and some freshwater limestone units do occur. Myton Pocket, west of the Green River (Ham- The deposits in this photograph of Uinta C are predominantly bright orange and blin, 1987). Our work has been restricted to a red, which mainly occur high in the section; most of the formation consists of re- duced, drabber gray, greens, and browns. A collector in the lower left provides scale. region well east of the Green River, on land administered by the U.S. Bureau of Land Man- agement (figure 4). The repository for the newly collected material is the Carnegie Muse- um of Natural History (CM) of Pittsburgh (chosen because of its historically important collections from the basin). Within the Uinta Basin the Bridgerian/Uin- tan boundary is currently dated to about 46 Ma (Chron C20r; Prothero, 1996; Prothero and Swisher, 1992). Poorly fossiliferous Uinta A is in sediments of normal polarity and is presently dated to about 46.3 to 47.8 Ma (Chron C21n). Most of Uinta B is reversed polarity and dates to about 43.8 to 46.3 Ma (Chron C20r) although a short normal zone spanning upper Uinta B and lower Uinta C probably correlates with Chron C20n (42.5 - 43.8 Ma). The upper part of Uinta C and lowermost Duchesne River Figure 4. View to the northwest across Devil's Playground, an area of well-devel- Formation (Anderson and Picard, 1972) are oped badlands in the drainage of Kennedy Wash. This and our other collecting reversed polarity and date to about 41.4 to 42.5 areas are on land administered by the BLM. Ma (C19r). The Uintan/Duchesnean boundary occurs within a poorly fossiliferous span in the Uinta C). This simple dichotomy has been usefully middle part of the Duchesne River Formation and lies applied to the UF because most mammalian fossils have within Chron C18n.