DENVER MUSEUM OF NATURE & SCIENCE ANNALS NUMBER 1, SEPTEMBER 1, 2009 Land Mammal Faunas of North America Rise and Fall During the Early Eocene Climatic Optimum Michael O. Woodburne, Gregg F. Gunnell, and Richard K. Stucky WWW.DMNS.ORG/MAIN/EN/GENERAL/SCIENCE/PUBLICATIONS The Denver Museum of Nature & Science Annals is an Frank Krell, PhD, Editor-in-Chief open-access, peer-reviewed scientific journal publishing original papers in the fields of anthropology, geology, EDITORIAL BOARD: paleontology, botany, zoology, space and planetary Kenneth Carpenter, PhD (subject editor, Paleontology and sciences, and health sciences. Papers are either authored Geology) by DMNS staff, associates, or volunteers, deal with DMNS Bridget Coughlin, PhD (subject editor, Health Sciences) specimens or holdings, or have a regional focus on the John Demboski, PhD (subject editor, Vertebrate Zoology) Rocky Mountains/Great Plains ecoregions. 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Gunnell2, and North America Rise and Fall Richard K. Stucky3 During the Early Eocene Climatic Optimum ABSTRACT—Climatic warming at the beginning of the Early Eocene Climatic Optimum (EECO) resulted in major increases in plant diversity and habitat complexity reflective of temporally unique, moist, paratropical conditions from about 53–50 Ma in the Western Interior of North America. In the early part of the EECO, mammalian faunal diversity increased at both local and continental scales in conjunction with a major increase in tropicality resulting from mean annual tem- peratures reaching 23˚C and mean annual precipitation approaching 150cm/yr. A strong episode of taxonomic origination (high number of first appearances) in the latest Wasatchian and earliest Bridgerian Land Mammal Ages apparently was in response to these greatly diversified floral and habitat associations along with increasing temperature and precipitation. This is in contrast to a similar increase in first appear- ances at the beginning of the Wasatchian (Paleocene-Eocene Thermal Maximum, or PETM) that can be traced instead to climate-induced transcontinental immigration. In the later part of the EECO, from Br-1b–Br-3, climatic deterioration resulted in a major loss of faunal 1Department of Geology, Museum of Northern diversity at both continental and local levels, apparently mirroring Arizona, Flagstaff, Arizona 86001, U.S.A. climatic deterioration. Relative abundance shifted from diverse, evenly [email protected] distributed communities to much less diverse, skewed distributions dominated by the condylarth Hyopsodus. Evolutionary innovation 2Museum of Paleontology, The University of through the 53–50 Ma interval included a modest overall increase in Michigan, Ann Arbor, Michigan 48109, U.S.A. body size and increased efficiency in carnivory and folivory as reflected [email protected] by within-lineage patterns of evolution. Rather than being “optimum,” the EECO engendered the greatest episode of mammalian faunal turn- 3Denver Museum of Nature & Science, Denver, over of the first 15 million years of the Cenozoic era, with both first Colorado 80205, U.S.A. [email protected] and last appearances at their highest levels. Both the PETM and EECO faunas were climatically shaped. Woodburne et al. This report highlights the role of climate in shaping teeth at the time of the PETM suggests that MAT actually faunal dynamics of the early Paleogene. The Early reached 26oC (Fricke & Wing 2004), but for the purposes Eocene Climatic Optimum (EECO) recorded the highest of this report the plant-based climatic interpretations are mean ocean temperature of the Tertiary period (Wolfe followed. 1978; Wolfe & Poore 1982; Miller et al. 1987; Prentice & Clyde & Gingerich (1998) report on substantial Matthews 1988; Zachos et al. 2001, 2008). This phenom- modification of the land mammal community during enon began about 53 Ma and persisted to about 50 Ma the PETM, the effects of which persisted at least into (e.g., Clyde et al. 2001; Zachos et al. 2001, 2008), and the later part of the Wasatchian North American Land occurred in the context of the overall relatively warm Mammal Age (NALMA). Wing et al. (2005), Wing & Love- conditions that characterized the early Cenozoic from lock (2007), and Smith et al. (2007) record strong floral the Paleocene to about medial Eocene. Koch et al. (2003) change in the Bighorn Basin during the PETM with a indicate that the Eocene warm temperature interval brief virtual disappearance of conifers, but just how this (EWI = EECO), began at about chron C23n.2n, coeval played into the modification of the mammalian com- with the beginning of the Bridgerian NALMA at 51 Ma. munity remains to be determined. Wing & Harrington Smith et al. (2008) proposed a revision of the basal age (2001) suggested that changes in the mammalian fauna of C23n.2n at about 50.5 Ma but still utilize 53 Ma as the in the early Wasatchian likely stemmed from competi- beginning of the EECO. tion between endemic taxa and new immigrants (the Zachos et al. (2001) documented early Cenozoic (ca. “individualistic” pattern of Wing et al. 2005), and this 66.5–46 Ma) global oceanic temperatures that ranged seems plausible for post-PETM intervals of the early from 8–10oC (Fig. 1). In addition to the very short-lived Wasatchian. PETM (Paleocene-Eocene Thermal Maximum; duration This report focuses on the EECO and the plant and of about 170 ky: Bains et al. 2003; Koch et al. 2003; Wing land mammal record from the latest Wasatchian and et al. 2005; Zachos et al. 2008), the ocean temperatures Bridgerian NALMAs. The earliest Eocene hyperthermal during the EECO surpassed the general range by about event (PETM) is considered to have abetted intra- and 2oC from about 53–50 Ma (increased to about 12oC). intercontinental dispersal of plants and land mammals Fossil land plants provide us with estimates of (Tiffney 1994, 2000; Gingerich 2003). The land mammal mean annual temperature (MAT) and mean annual population of the EECO apparently was influenced by a precipitation (MAP) for the Western Interior of North different set of parameters (climate and floral change America. Fig. 1 indicates that in the late Paleocene, MAT absent immigration) as developed below and as initially rose from about 10oC to 18oC, based on paleofloras from suggested by Stucky (1992, 2007). In developing data for the Green River Basin and vicinity. During the PETM, MAT a report such as this we recognize that sampling is likely increased dramatically to about 20oC and then fell back uneven across the temporal intervals utilized, but at the to about 18oC en route to a low of 15oC in Wasatchian present scale this cannot be avoided. biochron Wa-5, just prior to the beginning of the EECO The report begins with a brief summary of plant (about 53 Ma). Subsequently MAT rose rapidly through and mammalian evolution during the Late Cretaceous 17oC (Latham flora, Wa-6, 53 Ma) to about 22oC (Wa-7, to the early Wasatchian in the Western Interior of North Sourdough flora, somewhat younger than 53 Ma), rose America and then follows that background with an further to about 23oC at 52 Ma (Wa-7, Niland Tongue appraisal of the EECO. flora), declined to about 20oC in the early Bridgerian (50 Ma, Little Mountain flora, late biochron Br-1a), and then declined further to about 15oC at the time of the Green River flora (about 47 Ma, Uintan biochron Ui-1). Oxygen isotope composition of biogenic apatite from mammal 2 DENVER MUSEUM OF Nature & SCIENCE ANNALS | No. 1, September 1, 2009 Fig. 1 Ma Epoch Ocean temperature MAT, based on leaf margin MAP, based on leaf area North American Land Mammal Ages Ui-3 Ui-2 Ui-1 Uintan GR GR Eocene Br-3 cooler, drier, Br-2 more open Br-1b KL KL BP L.M. EECO SY Br-1a WR L.M. Bridgerian (high elev.) seas. dry Nil WR BP increased floral Wa-7 SY Nil EECO diversity Wa-6 Lathwarmer, Sour wet Lath Sour Wa-5 (few taxa) humid Wa-4 M.B. W asatchian CB seas. dry Wa-3 asatch CB Wa-2 W PETM PETM Cf-3 Wa-0 Clark ? Cf-2 B. Mul o PETM Clark Cf-1 Ck'n Bison wet 26 C B. Mul Ti-6 Green River Ti-5 Bison D tropical temperatewet Basin ENVER Ti-4 Ti-3 fanian if Ti-2 T Ti-1 ?TR M USEUM Dwcr Dwcr Paleocene Wb, Dbe Dbc Wb, Dbe Dbws FUI Dbws Dbc HCIII OF HCIIb K-D1 N. Dakota K-D1 Le N Lw ? Le Lw ature Lt.Cret. Lt. Cret. o o o o 50 100 150 250 &S 10 15 20 25 200 CIENCE cm/yr Centigrade Eocene ClimateImpactsMammalFaunas Ocean temperature after Zachos et al. (2001) ANN Green River Basin floras after Wilf (2000): Bison = Bison Basin; Clark = Clarkforkian; B.Mul = Big Multi; M.B = main body, Wasatch Fm; Lath = Latham; Sour = Sourdough; Nil = Niland Tongue; L.M. = Little Mountain. seas. dry = seasonally dry. A Denver Basin floras (Wb; Dbe, Dbc, Dwcr, Dbws) after Johnson et al.