Biostratigraphy and Biochronology of the Latest Wasatchian, Bridgerian, and Uintan North American Land Mammal “Ages”

Albright, L. B. III, ed., 2009. Papers on Geology, Vertebrate Paleontology, and Biostratigraphy in Honor of Michael O. Woodburne. Museum of Northern Arizona Bulletin 65, Flagstaff, Arizona. BIOSTRATIGRAPHY AND BIOCHRONOLOGY OF THE LATEST WASATCHIAN, BRIDGERIAN, AND UINTAN NORTH AMERICAN LAND MAMMAL “AGES” GREGG F. GUNNELL1, PAUL C. MURPHEY2, RICHARD K. STUCKY3, K. E. BETH TOWNSEND4, PETER ROBINSON5, JOHN-PAUL ZONNEVELD6, AND WILLIAM S. BARTELS7 1Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109; [email protected] 2Department of Paleontology, San Diego Natural History Museum, 1788 El Prado, Balboa Park San Diego, CA 92101; [email protected] 3Department of Earth Sciences, Denver Museum of Nature and Science, 2001 Colorado Boulevard, Denver, CO 80205; [email protected] 4Department of Anatomy, Arizona College of Osteopathic Medicine, Midwestern University, 19555 N. 59th Avenue, Glendale, AZ 85308; [email protected] 5Section of Paleontology, University of Colorado Museum of Natural History, Boulder, CO 80309; [email protected] 6Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3; [email protected] 7Department of Geology, Albion College, Albion, MI 49224; [email protected] ABSTRACT—Biochronological zones for the late Wasatchian, Bridgerian, and Uintan North American Land Mammal Ages are formally defined in this paper. These zones include Wasatchian biochron Wa7, Bridgerian biochrons Br1a, Br1b, Br2, and Br3, and Uintan biochrons Ui1a, Ui2, and Ui3. We argue for the existence of an additional early Uintan biochron between Ui1a and Ui2 and informally recognize Ui1b, although it is not possible to formally define it at this time. For each biochron we designate a stratotype section as well as referred sections where appropriate. In addition we present complete biostratigraphic range information for all known mammalian taxa from stratotype and referred sections and use these ranges to define biochrons based on index taxa and first and last appearance information. Biochron Wa7, all of the Bridgerian biochrons, and Ui1a have stratotype sections in Wyoming, either in the Wind River Basin (biochrons Wa7 and Br1a) or in the Green River Basin. The stratotype sections for Uintan biochrons Ui2 and Ui3 can be found in the Uinta Basin in Utah. Most referred sections can be found in these areas as well although there are isolated referred sections in Colorado (Sand Wash and Huerfano), Texas (basal Devil’s Graveyard Formation), and the Washakie Basin in Wyoming. INTRODUCTION As now understood, the early and middle Eocene North American Land Mammal “Ages” consist of (from oldest to youngest) the Wasatchian, Bridgerian, Uintan, and Duchesnean as originally codified by Wood et al. (1941). In each case, these NALMAs derived their names from rock units that had been defined earlier (Hayden, 1869, 1878; Comstock, 1875) and each had been subdivided based on either lithostratigraphy (Veatch, 1907; Matthew, 1909; Kay, 1934; Wood, 1934; Van Houten, 1944;) or biostratigraphy (Granger, 1910, 1914; Kay, 1934; Wood, 1934). A detailed history of the development of North American Eocene lithostratigraphy and biostratigraphy can be found in Krishtalka et al. (1987) and Robinson et al. (2004). Most recently, Woodburne (2004) has recognized biochronological zones for all NALMAs. Only the lower and middle biochrons of the early Eocene Wasatchian Land Mammal “Age” have been formally defined with designated stratotype sections and refined biostratigraphic control (see Gingerich, 2001). In the pages that follow we formally define the latest Wasatchian biochron (Wa7), four Bridgerian biochrons (Br1a through Br3) and three of four of the Uintan biochrons (Ui1a, Ui2 and Ui3; for various reasons it is not yet possible to formally define Uintan biochron Ui1b), and offer a discussion of the transition between 279 MUSEUM OF NORTHERN ARIZONA BULLETIN 65 the Bridgerian and Uintan (an interval often referred to as the Shoshonian) which, to date, is not fully understood. It is hoped that a more formal definition of the Duchesnean will soon follow. Procedurally, we follow the general steps laid out in the North American Stratigraphic Code for designation and formalization of chronostratigraphic and geochronologic units (NACSN, 2005). Practically this protocol consists of designating stratotype sections for biostratigraphic units or intervals that are defined by the first and last occurrences of fossil vertebrates contained within the rocks representing those intervals – these units or intervals are essentially equivalent to chronozones as defined in article 75 of the NASC. Units of time, here termed biochrons and designated with a letter and number (e.g., Wa7), are defined based on these biostratigraphically constrained intervals and are essentially equivalent to geochronological units as defined in article 80 of the NACSN. All of the formally recognized biochrons that we discuss are based on biostratigraphic stratotype sections that occur in Wyoming or Utah (Fig. 1), although two referred sections are in northwestern (Sand Wash, see Stucky et al., 1996) and southern Colorado (Huerfano Park, see Robinson, 1966). The informally recognized biochron (Ui1b) is based on faunal assemblages from the Friars and Santiago formations in California, the Washakie Formation in Colorado, and the Adobe Town Member of the Washakie Formation in Wyoming. The stratotype sections that are the basis for Wasatchian biochron Wa7 and Bridgerian biochron Br1a are found near Shoshoni, Wyoming in the Wind River Basin while the stratotype sections for the remaining Bridgerian biochrons (Br1b through Br3) are found in southwestern Wyoming in the southern Green River Basin. These sections are located just north of the Uinta Mountains along the border between Wyoming and Utah. The transitional Bridgerian-Uintan Bridger E section Figure 1. Map of southern and south-central Wyoming, northeastern Utah, and northwestern Colorado showing the general areas where late Wasatchian, Bridgerian, and Uintan stratotype and referred sections are located. 280 GUNNELL ET AL.—BIOCHRONOLOGICAL ZONATION OF THE BRIDGERIAN AND UINTAN NALMAS (biochron Ui1a) is also found in the southern Green River Basin (West and Hutchison, 1981; Murphey and Evanoff, 2007; Murphey and Walsh, 2007). The stratotype sections of Uintan biochrons Ui2 and Ui3 are found in the Uinta Basin in northeastern Utah, south of the city of Vernal. In addition to stratotype sections, in most cases several referred sections can be designated that represent other areas where biostratigraphically controlled faunal samples have been recovered. MATERIALS AND METHODS A table of the stratigraphic ranges of all taxa from all sections is included as Appendix I. The summary tables found in each subsection below are based on the ranges listed in Appendix I and in most cases include data not only from the stratotype section but also from referred sections as well. In some cases (in particular those from Uintan biochrons Ui2 and Ui3), lists of faunal assemblages were restricted to the stratotype and referred section in the immediate regional area in the Uinta Basin. In the case of the earliest Uintan (Ui1a and Ui1b), it was necessary to include assemblages from other regions because of the compounding factors of provincial endemism and apparent temporal range differences. Additional and detailed analysis beyond the scope of this paper is required to grasp the intricacies of these continental scale distributions. The sections below were jointly written by all authors, but individual sections were supervised as follows: Wa7 (RKS), Br1a (RKS, GFG), Br1b (GFG, JPZ, WSB), Br2 (GFG, PR), Br3 (GFG, PR, PCM), Ui1a-b (PCM), Ui2-3 (KEBT, PCM). WASATCHIAN BIOCHRONOLOGICAL ZONE WA7 The latest Wasatchian biochron, Wa7, has long been recognized as the Lostcabinian Land Mammal “Subage” (see Stucky, 1984c), which was defined as the fossiliferous beds in Western North America that include the paleothere, Lambdotherium popoagicum (Sinclair and Granger, 1911). The type area for Wa7 is the same as the type area for the Lost Cabin Member of the Wind River Formation at Buck Spring in the central part of the Wind River Basin (Stucky 1984a; 1984b). Although Osborn (1909) had earlier suggested that two land mammal faunas could be recognized in the upper part of the Wind River Formation, he later suggested that only one zone was present (Osborn, 1929). In 1984, Stucky recognized through stratigraphic studies that Lambdotherium popoagicum came from the lower beds within the Lost Cabin Member and that Eotitanops borealis (=Palaeosyops borealis) came from higher beds within the member and neither taxon overlapped in stratigraphic distribution in measured stratigraphic sequences. Osborn’s (1929) report that they did was in error. Furthermore, it became apparent that other taxa, including Hyrachyus and Trogosus, occurred with Eotitanops and were not found with the late Wasatchian Lambdotherium. As such, the fauna from the Lambdotherium interval zone does not contain any mammals of Bridgerian aspect at the generic level. There are no sites or stratigraphically documented intervals known in North America where Lambdotherium popoagicum and Eotitanops borealis overlap in distribution. Recently reported co- occurrences of these two taxa from the same horizon have now been recognized to be across a stratigraphic interval with occurrences separated by a fault (Gunnell and Yarborough, 2000; Gunnell, personal observation);

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