University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Mammalogy Papers: University of Nebraska State Museum, University of Nebraska State Museum 2002 Environmental Change in the Great Plains: An Isotopic Record from Fossil Horses Benjamin H. Passey University of Utah, [email protected] Thure E. Cerling University of Utah Michael E. Perkins University of Utah Michael R. Voorhies University of Nebraska State Museum, [email protected] John M. Harris George C. Page Museum See next page for additional authors Follow this and additional works at: http://digitalcommons.unl.edu/museummammalogy Passey, Benjamin H.; Cerling, Thure E.; Perkins, Michael E.; Voorhies, Michael R.; Harris, John M.; and Tucker, Shane T., "Environmental Change in the Great Plains: An Isotopic Record from Fossil Horses" (2002). Mammalogy Papers: University of Nebraska State Museum. 202. http://digitalcommons.unl.edu/museummammalogy/202 This Article is brought to you for free and open access by the Museum, University of Nebraska State at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Mammalogy Papers: University of Nebraska State Museum by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Benjamin H. Passey, Thure E. Cerling, Michael E. Perkins, Michael R. Voorhies, John M. Harris, and Shane T. Tucker This article is available at DigitalCommons@University of Nebraska - Lincoln: http://digitalcommons.unl.edu/museummammalogy/ 202 ARTICLES Environmental Change in the Great Plains: An Isotopic Record from Fossil Horses Benjamin H. Passey, Thure E. Cerling, Michael E. Perkins, Michael R. Voorhies,1 John M. Harris,2 and Shane T. Tucker1 Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, U.S.A. (e-mail: [email protected]) ABSTRACT Carbon and oxygen isotope ratios of fossil horse tooth enamel from Nebraska and Texas show evidence for late Neogene environmental changes in the Great Plains. The earliest unambiguous C4 dietary signal among Texas equids coincides with the age of the classic late Hemphillian–age Coffee Ranch fauna, which we suggest is ∼6.6 Ma based on volcanic ash correlations. C4 vegetation was present in the diets of a small fraction of late Hemphillian equids in Nebraska and was thereafter ordinary in the diets of both Nebraska and Texas equids. There is no unequivocal evidence for abundant C4 vegetation in the diets of pre–late Hemphillian equids, and we suggest that the ensuing dietary change reflects C4-biomass expansion in the latest Miocene. Carbon isotope ratios of post-Hemphillian horses in Nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co- occur with horse remains, indicating that the two proxies (carbon isotopes and presence/absence of tortoises) record complementary environmental phenomena. The average d18O values of late Hemphillian and younger fossil localities in Nebraska trend toward bimodal distribution, but more data are needed to confirm this pattern. Oxygen isotope ratios of Barstovian and Clarendonian horses are significantly enriched in 18O relative to Hemphillian horses, which in turn are significantly enriched relative to Blancan and Irvingtonian horses. A large portion of this oxygen isotope decrease appears to have taken place during late Hemphillian time. Secular variation in the Nebraska d18O record correlates with changes in ungulate diversity, the disappearance of crocodilians in Nebraska, and global change in the latest Miocene. Introduction The Earth’s surface underwent significant environ- Northern Hemisphere glaciation in the late Pliocene mental change during the Neogene. Key events dur- and early Pleistocene (reviewed in Raymo 1994). In ing this time include intensified Antarctic glaciation North America, a time of relative faunal stability about 15 Ma (Miller et al. 1987; Lear et al. 2000), represented by the Clarendonian chronofauna was progressive closure of the Panamanian straits from followed by a decline of ungulate diversity such that about 13 to 1.9 Ma (Haug and Tiedemann 1998), the number of genera at 2 Ma was about one-third repeated desiccation and infilling of the Mediterra- that at 15 Ma (Janis et al. 2000). Characteristic mam- nean basin between 6 and 5 Ma (Hsu et al. 1973; malian lineages disappeared from the Great Plains, Krijgsman et al. 1999), global expansion of C4 bio- including the oreodonts, chalicotheres, and rhinos, mass beginning ∼7–8 Ma (Cerling et al. 1997), per- and elements such as proboscideans first appeared manently increased Antarctic glaciation about 5 Ma (Janis et al. 1998). Large reptiles retreated to more (Miller et al. 1987; Lear et al. 2000), and onset of southerly latitudes (Voorhies 1969; Markwick 1994), and faunas came to resemble those of today. The Manuscript received July 28, 2000; accepted June 28, 2001. goal of this article is to establish a geochemical rec- 1 University of Nebraska State Museum, Lincoln, Nebraska ord from the Great Plains that will be meaningful 68588, U.S.A. 2 George C. Page Museum, 5801 Wilshire Boulevard, Los An- in terms of global, regional, and local histories of geles, California 90036, U.S.A. climate and ecological change. [The Journal of Geology, 2002, volume 110, p. 123–140] ᭧ 2002 by The University of Chicago. All rights reserved. 0022-1376/2002/11002-0001$15.00 123 124 B. H. PASSEY ET AL. Stable carbon and oxygen isotopes in mammalian ter (Longinelli 1984; Luz et al. 1984, 1990; Luz and tooth enamel are resistant to diagenesis (Lee-Thorp Kolodony 1985; Huertas et al. 1995), which in turn and van der Merwe 1987; Quade et al. 1992; Boch- is positively correlated to mean annual tempera- erens et al. 1996) and are therefore useful in pa- ture, especially within continental settings (Dans- leoenvironmental reconstruction. Carbon isotope gaard 1964). Previous studies have shown that the ratios in mammalian tooth enamel largely reflect d18O of apatite can also relate to humidity (Ayliffe the fraction of C4 vegetation in an animal’s diet. and Chivas 1990; Luz et al. 1990). Animal behavior C4 plants are dominantly grasses, and their abun- and physiology further modify the oxygen isotopic dance is positively correlated with temperature, es- signal in apatite (e.g., Bocherens et al. 1996; Kohn pecially with growing season temperature (Teeri 1996). and Stowe 1976; Ehleringer et al. 1997). Physiolog- The outline of this article is as follows: we dis- ical considerations of the C4 photosynthetic path- cuss methods used in this article, and then con- way, along with growth-chamber experiments, struct a simple model of the carbon isotopic com- show that C4 vegetation is better adapted to low position of atmospheric CO2 for the past 20 m.yr. concentrations of atmospheric carbon dioxide than based on the carbon isotopic composition of plank- is C3 vegetation (Tissue et al. 1995; Ehleringer et tonic foraminifera. This model allows us to eval- al. 1997). C4 vegetation appears to have existed only uate the implications of fossil tooth carbon isotopic at background levels before about 8 Ma (Cerling et signatures. Next, we present volcanic ash correla- al. 1997), but today it is the dominant grass in trop- tions that constrain the age of the classic late ical and warm-temperate grasslands. In the modern Hemphillian–Coffee Ranch fauna. We present the Great Plains, the “crossover” latitude where C4 carbon and oxygen isotope data and conclude with grasses are equal in abundance to C3 grasses (in some interpretations of the data. terms of species) is in the vicinity of 40ЊN (Teeri and Stowe 1976). Plants using the C3 photosyn- thetic pathway account for about 75% of the ter- Methods restrial net primary productivity (Ehleringer et al. Sample Selection. Nebraska fossils were sampled 1997) and include most trees and shrubs and the because the region preserves an excellent and abun- cool growing season grasses. Plants using the third dant record of Neogene horses and because it is major pathway, crassulacean acid metabolism presently situated at the transition between south- (CAM), include many of the succulent plants and ern C4-dominated and northern C3-dominated today do not account for a large fraction of terres- grasslands (Teeri and Stowe 1976). If this distribu- trial primary productivity. tion were true of the past, then small temperature In this article, we use equid diet as an indicator or PCO2 fluctuations might be greatly amplified in 13 of the presence of C4 biomass in an ecosystem. d C values of horses as the C3/C4 fringe moved Modern equids are known to graze, and all late north and south of the region. Furthermore, Ne- Hemphillian and younger equid genera that have braska has limited topographic relief and thus been isotopically analyzed (Pseudhipparion, Neo- should yield isotopic results that reflect regional hipparion, Nannippus, Cormohipparion, Astrohip- rather than local climate. The Texas samples are pus, Dinohippus, and Equus) have representatives from the High Plains region, which is significantly with C4 dietary components (Wang et al. 1994; La- warmer than present-day Nebraska. These should torre et al. 1997; MacFadden et al. 1999b). Koch lend climatic perspective and could allow a check (1998) and Koch et al. (1998) have shown that of whether climatic differences between the two within some Pleistocene ecosystems in Texas and regions can be identified. The distribution of sam- Florida, Bison, Mammuthus, or both consumed a ple localities is shown in figure 1, and the strati- larger fraction of C4 vegetation than did presumably graphic relationship of Nebraska localities is shown coexisting Equus, and they suggest that Equus was in figure 2. The relative ages of fossils analyzed in a often a mixed feeder, consuming C3 browse and this study were estimated by their placement in C4 grass. This feeding strategy has also been indi- North American land mammal ages (NALMAs), us- cated for equid genera from the latest Hemphillian ing data from Voorhies (1990a) and Woodburne and of Florida on the basis of carbon isotope and enamel Swisher (1995).