An Isotopic Record from Fossil Horses Benjamin H

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

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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.

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 photosynthetic 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). microwear data (MacFadden et al. 1999b). Sample Pretreatment. Two enamel pretreatment The oxygen isotopic composition of mammalian methods were used in this study. In one method, body water determines that of mammalian apatite organic contamination was removed by soaking

(including tooth enamel) and is positively corre- enamel powder in 3% H2O2 for 24 h. Following this, lated with the isotopic composition of meteoric wa- the samples were rinsed several times in distilled Journal of Geology GREAT PLAINS ENVIRONMENTAL CHANGE 125

Figure 1. Distribution of fossil localities sampled in this study. Dashed lines indicate approximate percentage of

C4 grass species relative to all grass species, using data taken from Teeri and Stowe (1976). North American land mammal age abbreviations are as follows:R p Rancholabrean ;I p Irvingtonian ;B p Blancan ;H p Hemphillian ; C p Clarendonian;.Ba p Barstovian water and then labile carbonates were removed by normalized using NBS-19 calcite and in-house soaking in 1 M acetic acid for 24 h. This was fol- tooth enamel standards. lowed by rinsing and drying. In another method, Volcanic ash samples were cleaned using distilled enamel powder was treated in 3% H2O2 for 15 min, water, dilute hydrofluoric acid, and ultrasonic followed by rinsing and then treatment in 0.1 M equipment. Glass separates (199.5% glass) were an- acetic acid for 15 min, followed by rinsing and dry- alyzed with an ARL 8410 x-ray fluorescence spec- ing. An excess of treatment solution was used in trometer at the University of Utah using methods each step of each method (k0.05 mL solution per described in Perkins et al. (1995). mg sample). Table 1 shows that these treatment procedures did not change the isotopic composition Carbon Isotopic Composition of of the enamel in a predictable or significant way. Ancient Atmospheric CO Sample Analysis. Purified enamel samples were 2 Њ reacted with 100% H3PO4 in vacuo at 25 C for 48 Knowledge of the carbon isotopic composition of h or under positive helium pressure at 90ЊCfor7 13 ancient atmospheric CO2 (d CCO2 ) is necessary for min. Resulting CO2 was cryogenically purified and understanding the meaning of fossil tooth enamel isotopically analyzed using Finnigan MAT 252 and carbon isotope ratios. We follow previous workers Delta Plus mass spectrometers. Isotope ratios were (e.g., Koch et al. 1995; Ekart et al. 1999; Pagani et 126 B. H. PASSEY ET AL.

Figure 2. Generalized diagram showing the sequence of Nebraska fossil localities, time durations of tortoises and alligators in Nebraska, and timing of North American land mammal ages. al. 1999a, 1999b) and use d13C of planktonic fora- ferent species living at different localities during 13 13 minifera calcite (d CPF) as a proxy ford CCO2 . The different time periods (table 2); for consistency, we carbon isotopic composition of atmospheric CO2 is restrict our sampling to data from low latitudes related to that of planktonic foraminifera calcite (30ЊSto30ЊN) and to isotopic determinations that tests by the following relation: were made on the large size fractions (1200 mm) of planktonic foraminifera. The compilation pre- 13 13 3 ∗ 33 C p {( C ϩ 10 )/[(␧ Ϫ /10 ) ϩ 1]} Ϫ 10 , d CO22d PF PF CO sented in table 2 is by no means exhaustive, but it appears that the calculated average value ∗ where␧ Ϫ is the “apparent isotope enrichment” ∗ is appropriate in the (1)‰ ע PF CO2 ␧ p PFϪCO2 7.9 1.1 j between atmospheric CO2 and planktonic forami- sense that it covers most of the observed variability. nifera. The corresponding isotopic compositions of We use values for␧∗ that represent av- PLANTϪCO2 C3,C4, and drought-stressed C3 plants, and of tooth erage C3 vegetation, drought-stressed C3 vegeta- enamel of mammals eating those plants, are cal- ∗∗ tion, and average C vegetation (␧␧Ϫ ,Ϫ , culated in a similar way (ﬁg. 3). 4 C32CO sC 32CO ∗ 13 and␧ Ϫ , respectively). With modern C p The primary tasks are to determine representa- C42COd CO2 Ϫ tive␧ values and to assemble an appropriate time 8.0‰, the following values are calculated using 13 ␧∗ bulk d13C data from Kenyan plants collected be- series of d CPF. The valuePFϪCO2 is variable in modern environments and is inﬂuenced by biological tween 1997 and 2000 (Cerling and Harris 1999; T. E. Cerling and J. M. Harris, unpub. data): ␧∗ p disequilibria (often termed “vital effects”; see Spero C32ϪCO ע Ϫ 13 p Ϫ et al. 1991), the depth at which foraminifera tests 19.6‰ (125 C3 dicots;d C 27.4 1.6 ), 13 ␧∗ p Ϫ are precipitated (because the d C of dissolved in- sC32ϪCO 16.7‰(15C3 dicots collected at Mpala organic carbon decreases with depth), concentra- Reserve during the September through De- and ,( 1.1 ע tion of seawater carbonate (Spero et al. 1997), and cember drought of 1998;d13 C p Ϫ24.6 ␧∗ p Ϫ 13 p other factors. Our approach is to bracket this var- C42ϪCO 4.7‰ (182 C4 monocots; d C ע ␧∗ Ϫ iability by estimatingPFϪCO2 values for several dif- 12.7 1.1). We use the value for the offset be- Journal of Geology GREAT PLAINS ENVIRONMENTAL CHANGE 127

Table 1. Comparison of Different Enamel Pretreatment Procedures d13C d18O Sample ID and description Method (‰PDB) (‰PDB) MG-92-578-TG-RP3-7 (modern horse) B Ϫ12.7 Ϫ10.3 NT Ϫ12.5 Ϫ10.5 K00-TSV-223 (modern African buffalo) A 1.7 2.6 B 1.7 2.7 NT 1.5 2.4 K00-NKU-250 (modern zebra) A Ϫ.1 2.4 B Ϫ.2 2.4 NT Ϫ.3 2.4 K00-NKU-255 (modern African buffalo) A Ϫ.3 2.7 B Ϫ.3 2.9 NT Ϫ.7 2.4 K00-AS-168 (modern hippo) A Ϫ2.4 Ϫ2.4 B Ϫ1.9 Ϫ1.7 NT Ϫ2.1 Ϫ1.9 K00-AB-301 (modern black rhino) A Ϫ12.8 .4 B Ϫ12.9 .4 NT Ϫ12.9 .5 K00-AB-302 (modern black rhino) B Ϫ12.0 Ϫ1.3 NT Ϫ12.0 Ϫ2.4 MG-92-578-TG-RP3-4 (modern horse) B Ϫ12.8 Ϫ10.3 NT Ϫ12.6 Ϫ10.5 LACM HC92411 (fossil bison) A Ϫ2.4 Ϫ2.5 B Ϫ2.3 Ϫ2.6 NT Ϫ1.8 Ϫ2.8 LOTH 87 (fossil hippo) A Ϫ.6 Ϫ3.9 B Ϫ.6 Ϫ3.9 NT Ϫ.8 Ϫ3.6 UNSM 1132-93 (fossil equid) A Ϫ5.9 Ϫ8.0 B Ϫ5.9 Ϫ8.1 NT Ϫ5.9 Ϫ8.0 IMNH 113/5623 (fossil gomphothere) A Ϫ8.9 Ϫ10.7 B Ϫ8.9 Ϫ10.7 NT Ϫ8.7 Ϫ10.7 LOTH 126 (fossil proboscidean) B Ϫ1.2 Ϫ1.2 NT Ϫ1.2 Ϫ1.6 Note. MethodA p 24 h in 3% hydrogen peroxide, followed by rinsing and then 24 h in 1 M acetic acid; methodB p 15 min in 3% hydrogen peroxide, followed by rinsing and then 15 min in 0.1 M acetic acid;NT p no treatment; PDB p peedeebelemnite isotpe standard. tween diet and enamel carbonate reported by Cer- Age of the Coffee Ranch Local Fauna ע ␧∗ p ling and Harris (1999;ENϪDIET 14.1 0.5 ). 13 The time series of d CPF was assembled using The fossil assemblage at Coffee Ranch, Texas, is data from Whitman and Berger (1993), Pagani et al. the reference fauna for the late Hemphillian (Ted- (1999a, 1999b), and Veizer et al. (1999). These re- ford et al. 1987). It is capped by and partly incor- cords were used because they are from low-latitude, porated into a silt-sized gray vitric tuff. Fission low-productivity regions, because measurements track (FT) ages of Obradovitch (reported in Izett were made on 1200-mm-size fractions, and because 1975 and later revised in Naeser et al. 1980) indicate Ma 0.2 ע Ma (FT glass) to 6.8 0.3 ע they are from relatively continuous and well-dated an age of4.9 cores. These data were grouped into 0.5-Ma bins, (FT zircon) for this ash bed. In addition, Boellstorff Ma for 0.4 ע and the resulting time series allowed calculation of (1976) reported a FT glass date of5.5 expected carbon isotope ratios of tooth enamel the Coffee Ranch ash bed, which is in accord with from mammals feeding on C3,C4, or drought- the revised ages for the Coffee Ranch ash bed re- stressed C3 vegetation. The results (ﬁg. 4) show that ported by Naeser et al. (1980). enamel values characteristic of pure C3 and pure We have compared elemental compositions of

C4 feeders are in general more positive than are glass shards in the Coffee Ranch ash bed to those characteristic of the modern environment. Further in an extensive University of Utah database of implications of this model will be discussed below. glass-shard analyses of western United States ash 128 B. H. PASSEY ET AL.

ash. The value D2 is equivalent to the statistic x2 and so has a x2 distribution. This statistic involves

estimation of jk of a population of shards; if the

estimates of jk are similar to the true values, then repeated sampling of a single population will yield values of D2 that fall within a x2 distribution. If the estimate is not characteristic of the population, the values of D2 will fall on the extreme left or right tails of the x2 distribution. The values of D2 reported in table 3 are the statistical distances between each ash and the type Blacktail Creek ash,

using jk values reported in Perkins et al. (1998; table 3). The 95% confidence interval for 12 elements (11 df) is3.8 ≤ D2 ≤ 21.9 ; thus, 95% of multiple 12- element analyses from a single ash bed should yield values of D2 within this range. Table 3 shows that both the Coffee Ranch ash and “distal” Blacktail Creek ash are within the this range, while the oth- Figure 3. Diagram showing the method used to cal- ers are outside of this range; thus, we argue that culate expected tooth enamel d13C values, starting with the Coffee Ranch ash is statistically indistinguish- d13C of planktonic foraminifera. The “isotope enrich- able from the 6.62-Ma type Blacktail Creek ash. ␧ ment”AϪB between two substances A and B is equiva- Magnetopolarities for the Heise volcanic field 13 3 13 lent to the following: ␧ Ϫ p [( C ϩ 10 )/(d C ϩ A BAd Btuffs are also listed in table 3. Note that all but the 33Ϫ # 10 ) 1] 10 (Craig 1954). Asterisks indicate that equi- tuff of Blacktail Creek record reversed polarities. librium fractionation is not assumed. See text for expla- ∗ The tuff of Blacktail Creek and of the Coffee Ranch nation of each␧ quantity. ash bed are normal (Lindsay et al. 1976). This pro- vides additional support for the correlation of the beds (Perkins et al. 1995, 1998). These comparisons Coffee Ranch ash bed with the Blacktail Creek ash indicate that the Coffee Ranch ash bed has a com- bed. The dating of Morgan et al. (1999), along with position typical of ashes erupting along the Yellow- the revised magnetopolarity timescale of Cande stone hotspot track (Perkins et al. 1995, 1998) and, and Kent (1995), suggest that the Coffee Ranch ash in particular, is most similar to ash beds from bed was deposited in chron C3An.2n and not in sources in the Heise volcanic field, located along chron C3An.1n, as estimated by Lindsay et al. the hotspot track in the eastern Snake River Plain (1976), Tedford et al. (1987), and Woodburne and (Pierce and Morgan 1992). Table 3 lists analyses of Swisher (1995). proximal ash beds associated with the four major Late Hemphillian fossils from northern New ash-flow tuffs in the Heise volcanic field along with Mexico have been 39Ar/40Ar dated between 6.95 and analyses of correlative distal ash beds (including the 6.75 Ma (McIntosh and Quade 1995). The youngest Coffee Ranch ash bed). Examination of these anal- dated medial Hemphillian fauna appears to be the yses shows both the distinctiveness of ash beds Rattlesnake fauna of east-central Oregon. This from individual eruptions and the good match be- fauna lies beneath the Rattlesnake Tuff, which re- ,.Ma (i.e 0.01 ע tween the Coffee Ranch ash bed and the type Black- cent 40Ar/39Ar dating places at7.05 39 40 tail Creek ash bed, which has an Ar/ Ar date of older than the previously available K-Ar dates of 6.62 Ma (Morgan et al. 1999). 6.6–6.8 Ma; Streck and Grunder 1995; Tedford et The significance of ash correlations can be eval- al. 1987, respectively). Thus, the transition to late uated using the multivariate statistical distance Hemphillian faunas is reasonably placed in the function (Perkins et al. 1995): 7.05–6.62 Ma interval.

n (x Ϫ x¯ )2 2 p ͸ k1 k D 2 , kp1 2jk Carbon Isotopes wherexk1 is the concentration of the kth element

of the ash in question,x¯ k1 is that of the kth element Texas High Plains. The carbon isotopic record

of the type ash, and jk is the standard deviation of from Texas and Nebraska is presented in ﬁgure 5, the concentration of the kth element in a typical and data are listed in appendix 1, available from Journal of Geology GREAT PLAINS ENVIRONMENTAL CHANGE 129

Table 2. Apparent Isotope Enrichments of Planktonic Foraminifera Calcite Relative to Atmospheric CO2 13 13 ␧∗ Time period/location Species d CPF (‰PDB)d CCO2 (‰PDB) PFϪCO2 0–30 ka: 3ЊS, 83ЊW Neogloboquadrina dutertrei 1.2–1.7A Ϫ6.2 to Ϫ6.9B,C,D ∼8 Preindustrial: 19ЊN, 20ЊW Globigerinoides ruber 1.1E Ϫ6.5B 7.6 19ЊN, 20ЊW Globigerina bulloides Ϫ.9E Ϫ6.5B 5.6 19ЊN, 20ЊW Orbulina universa 1.8E Ϫ6.5B 8.4 19ЊN, 20ЊW Globorotalia inﬂata .3E Ϫ6.5B 6.8 Modern: 19ЊN, 20ЊW G. ruber .5E Ϫ7.8F 8.4 19ЊN, 20ЊW G. bulloides Ϫ1.5E Ϫ7.8F 6.3 19ЊN, 20ЊW O. universa 1.6E Ϫ7.8F 9.4 19ЊN, 20ЊW G. inﬂata .1E Ϫ7.8F 7.9 30ЊS–15ЊN, 70Њ–95ЊW G. ruber 1.0G Ϫ7.5H 8.6 30ЊS–15ЊN, 70Њ–95ЊW Globigerinoides conglobatus 1.1G Ϫ7.5H 8.7 30ЊS–15ЊN, 70Њ–95ЊW Globigerinoides sacculifer 1.4G Ϫ7.5H 9.0 1.1a ע 7.9 Sources. A, Shackelton et al. 1983; B, Friedli et al. 1986; C, Jesse Smith et al. 1999; D, Indermu¨ hle et al. 1999; E, Beveridge and Shackelton 1994; F, Quay et al. 1992; G, Duplessy et al. 1981; H, Keeling et al. 1989. Note.PDB p peedeebelemnite isotope standard. a Value given as 1j.

The Journal of Geology’s Data Depository upon re- are greater than the modeled upper limit for quest. Clarendonian horses from the Texas high drought-stressed C3 vegetation, indicating that C4 plains have d13C values ranging between Ϫ11.5‰ vegetation was a widely utilized food resource fol- Ϫ and 9.1‰, suggesting predominantly C3 diets but lowing late Hemphillian time. None of these in- allowing for a small component of C4, drought- dividuals indicate “pure” C4 or ”pure” C3 diets, in- stressed C3, or CAM vegetation (see “Discussion”). dicating that ecosystems contained a mix of C3 and

Two early Hemphillian equids from Box T Quarry C4 grasses or that horses consumed a significant have values of Ϫ10.7‰ and Ϫ8.3‰, and an indi- amount of browse during those times. vidual from Janes-Prentice Gravel Pit has a value Nebraska. The pre–late Hemphillian carbon iso- Ϫ of 9.3‰, again suggesting a C3-based diet. Carbon tope record from Nebraska (fig. 5) suggests C3-based 13 isotope ratios suggest C4 vegetation in the diets of diets, but as in Texas, many of the d C values are several late Hemphillian individuals. Wang et al. significantly greater than the expected values for

(1994) and Cerling et al. (1998) showed that C4 veg- “average” C3 diets. The exceptions to this pattern etation was consumed by some Coffee Ranch are two Cormohipparion occidentale individuals equids. We have examined material from two ad- from the late Clarendonian Pratt Slide locality, ditional late Hemphillian sites (Bailey Farm and whose values of Ϫ12.4‰ and Ϫ12.6‰ are the most

Cleo Hibbard Ranch) and have found C4 signatures depleted numbers in this study. Early and medial at both. In all, seven of 17 late Hemphillian equids Hemphillian equids also had C3-based diets. 13 from the Texas high plains have d C values greater C4 signals appear in two out of 20 late Hemp- than the modeled Ϫ7‰ upper limit for drought- hillian samples analyzed, as opposed to seven out stressed C3 vegetation during the late Hemphillian, of 17 in Texas (when the drought-stressed C3 upper Ϫ suggesting that C4 vegetation was a widely avail- limit of 7‰ is used). Of these, only one specimen able food resource during some parts of late Hemp- (UNSM 122035, from the recently excavated Rick hillian time. As a note of interest, the results from Irwin locality) shows a large C4 component in its Coffee Ranch are consistent with the ﬁndings of diet. Interestingly, this individual is Dinohippus, MacFadden et al. (1999b) that Neohipparion eury- the taxon that MacFadden et al. (1999b) found to style consumed a relatively larger fraction of C4 have a large C3-browse dietary component relative vegetation than did Dinohippus and Astrohippus to other equid genera. As in Texas, C4 dietary com- (although MacFadden et al. [1999b] studied Dino- ponents are greater in Blancan time than in late hippus mexicanus and Astrohippus stockii, where- Hemphillian time, suggesting that the time of as the data from Coffee Ranch are from Dinohippus marked C4 expansion postdated the deposition of interpolatus and Astrohippus ansae). most of the late Hemphillian localities. The two All individuals from the Blancan, Irvingtonian, samples from our latest Hemphillian locality (∼5 13 and Rancholabrean NALMAs have d C values that Ma; Devil’s Nest Airstrip) yielded C4 signals. 130 B. H. PASSEY ET AL.

13 Figure 4. a, Modeled time series for expected ungulate tooth enamel d C values associated with C3, drought-stressed 13 C3, and C4 plant diets over the past 20 m.yr. b, Detail of predicted tooth enamel d C values for ungulates with C3 13 diets. Arrows indicate that ungulates feeding on C3 vegetation in closed canopy environments may have d C values 13 that are signiﬁcantly depleted in C relative to “average” C3 feeders because of the “canopy effect,” which results 13 13 from recycling of C-depleted respired CO2. Also note that the modern tooth enamel d C values associated with average C3 diets are signiﬁcantly more negative than those expected over the past 20 m.yr.

Fossils from Blancan and Irvingtonian localities Barstovian and Blancan time. The former has an 18 record variable amounts of C4 vegetation in diet. average d O value of Ϫ3.2‰, and the latter averages The Hall Gravel Pit, Broadwater, Quinn Gravel Pit, Ϫ6.6‰ (table 4). The d18O differences between the Big Springs, and Angus Quarry localities all record Clarendonian and Hemphillian, and between the substantial C4 vegetation in equid diets, while the Hemphillian and Blancan, are Ϫ1.5‰ and Ϫ1.0‰, South Wind Prospect, Albert Ahrens, and Hay respectively. Both differences are significantly dif- Springs (MacFadden et al. 1999a) localities suggest ferent at the 95% level (t-test; table 5). The majority more C -based diets. 18 3 of the d O change that occurred between the Clar- endonian and Hemphillian is accommodated in the Oxygen Isotope Record late and latest Hemphillian interval, which have an average d18O value of Ϫ6.3‰, as opposed to Oxygen isotope data are listed in appendix 1 and Ϫ4.7‰ for the pooled medial Hemphillian, early are presented in figure 6. There are several fossil Hemphillian, and Clarendonian intervals (signifi- localities in Nebraska from which we analyzed cantly different at the 95% level; table 5). There is multiple individuals, allowing calculation of local- no significant difference between Clarendonian and 18 ity d O averages for that region. We suggest that early through medial Hemphillian horses nor be- NALMA and locality averages are more meaningful tween late through latest Hemphillian and Blancan than individual data for understanding regional pa- horses (table 5). Thus, the late Hemphillian appears leoenvironmental trends because they attenuate 18 18 to be the time of most marked O change. d O variability caused by local, behavioral, and sea- d sonal processes. Sampling in Texas was not ade- Another noteworthy feature of the oxygen data is quate to produce a good time series of locality d18O an apparent bimodal pattern in postmedial Hemp- 18 averages, so we focus our discussion on the Ne- hillian locality d O averages (fig. 7). The statistical braska data. significance of the bimodality is uncertain, and The pattern that is most clear in the record is more sampling is needed to confirm this result. We that d18O values decrease progressively between note that the bimodal pattern remains after rejec- Table 3. Age, Magnetopolarity, and Glass Shard Composition of Ash Beds from Heise Volcanic Field Sources

Fe2O3 CaO Ba Mn Nb Rb Sr Ti Zn Zr Th Ce Ash bed Age (Ma) Polarity chron Sample (wt%) (wt%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) D2 a b Type Kilgore 4.45, 4.4 C3n.1r rir93-02 1.39 .48 790 244 48 156 21 1002 43 273 23 121 330 Distal Kilgore … … oqm92-14 1.43 .45 692 227 49 156 16 1029 41 273 24 147 295 a Connant Creek 5.51 C3n.4r btr92-218 1.08 .46 472 209 56 162 15 689 68 182 22 151 307 Wolverine Creek … … btr92-213 1.20 .45 474 212 57 164 16 694 66 180 24 146 303 Distal Wolverine Creek … … acb88-08 1.19 .44 464 205 57 164 16 690 68 174 24 145 314 a b Walcott 6.27, 6.32 C3An.1r wal93-01 1.05 .38 812 224 43 166 22 1085 41 212 28 118 207 Distal Walcott … … mor94-713 1.06 .41 792 239 42 163 27 1080 48 203 27 139 180 Type Blacktail a b Creek 6.62, 6.6 C3An.2n btr92-221 1.17 .42 470 236 37 187 23 1003 43 200 28 140 0 Distal Blacktail Creek … … hp93-466a 1.12 .43 462 243 37 186 23 1035 33 192 31 113 9.4 Coffee Ranch 6.8,c 5.5d Normal 41261-46 1.16 .41 450 237 36 194 22 1025 38 203 28 125 4.2 Note. Chron assignment based on polarity determinations of Pierce and Morgan (1992) and magnetopolarity timescale of Cande and Kent (1995). Statistical distance relative to type Blacktail Creek (see text). Analyses by x-ray ﬂuorescence spectrometry using methods described in Perkins et al. (1995). a40Ar/39Ar laser fusion dates (Morgan et al. 1999). b Average of K-Ar dates in Pierce and Morgan (1992). c Fission-track zircon dates (Naeser et al. 1980). d Fission-track glass date (Naeser et al. 1980). 132 B. H. PASSEY ET AL.

Discussion

Carbon Isotopes: 13C-Enriched Pre–Late Hemphillian Equids. The majority of pre–late Hemphillian d13C values lie to the positive side of the modeled av-

erage C3-diet time series. We suggest that this relates to one (or a combination) of four factors:

1. The modeled C3-diet time series is ﬂawed in some aspect. If we assume for the moment that the pre–late Hemphillian carbon isotope record reﬂects

pure C3 diets, then the modeled C3-diet time series is too negative by about 1.5‰. This could result ␧␧∗∗ from too large of values forPFϪCO232 or C ϪCO or too ␧∗ small a value forENϪDIET . The value we use for ␧ C3 is similar to that which can be derived from published d13C data (e.g., Bender 1971; Deines 1980) 13 and is calculated using d C data from C3 dicots. ␧∗ The possibility exists that typicalC32ϪCO values of the middle and late Miocene differed from modern ␧∗ values or thatC32ϪCO values for C3 monocot eco-

systems differ from those of C3 dicot ecosystems. If the former is true, then we expect that the carbon Figure 5. Carbon isotopic record for Nebraska and isotope values of Miocene browsers will also plot Texas High Plains equids. The “modeled diet” time series above the modeled time series (unless they con- 13 are explained in the text and in figures 2 and 3. Plot sumed C-depleted vegetation resulting from the includes data from MacFadden et al. (1999a). “canopy effect”). Data from Argentina, Florida, Turkey, and Europe (Quade et al. 1995; MacFadden and Cerling 1996; MacFadden et al. 1996; Cerling tion of localities with less than three samples or standard deviations 12‰ (white squares in fig. 7). We have not observed sampling patterns relating to taxonomy, geography, or age that could give rise to bimodal locality d18O averages. Seasonal oscil- lations in meteoric water d18O result in intratooth d18O differences (Fricke et al. 1998a). Generation of a bimodal d18O artifact resulting from such season- ality would require that, at each locality, we for- tuitously sample tooth enamel that formed only during the summer or only during the winter. This possibility is exceedingly unlikely given the large number of localities and specimens that we sampled. Oxygen isotope ratios of Texas teeth are several per mil more positive than those from Nebraska. In particular, late Hemphillian equids from Texas average Ϫ0.2‰, while late Hemphillian equids from Nebraska average Ϫ6.3‰. Sampling density in Texas is not adequate to deduce long-term trends, except that the most negative values are found in Pleistocene time (data from MacFadden et al. 1999a) and that the most positive values are found in late Hemphillian time. A bimodal distribution similar to the Nebraska data cannot be re- Figure 6. Oxygen isotopic record for Nebraska equids. solved at this stage, and further sampling is needed Locality averages are reported for localities with two or before any definitive conclusions are made from the more individuals. Plot includes data from MacFadden et data. al. (1999a). Journal of Geology GREAT PLAINS ENVIRONMENTAL CHANGE 133

Table 4. Summary Statistics for Nebraska Carbon and fers from that of modern time (8.3‰) and that there Oxygen Isotope Data is a significant amount of interspecies variability x¯ j 2 Maximum Minimum within single time periods and localities (table 2). NALMA n (‰) (‰) (‰) (‰) It is therefore possible that the diet time series is d13C: affected by artifacts associated with our 7.9‰ es- Ϫ Ϫ Ϫ ∗ Barstovian 14 8.8 .8 7.7 10.9 timate for␧ Ϫ . Ϫ Ϫ Ϫ PF CO2 Clarendonian 18 9.8 1.4 8.5 12.6 2. Great Plains equids regularly consumed C or Hemphillian 50 Ϫ8.6 3.0 Ϫ1.0 Ϫ10.8 4 Blancan 23 Ϫ5.0 4.7 Ϫ2.0 Ϫ10.1 CAM vegetation before the late Hemphillian. Irvingtonian 11 Ϫ7.1 10.5 Ϫ.7 Ϫ9.9 3. Drought-stressed C3 vegetation was the pre- d18O: dominantly available food resource during the time Barstovian 14 Ϫ3.2 1.9 .0 Ϫ5.1 period in question. Ϫ Ϫ Ϫ Clarendonian 18 4.1 1.8 1.3 6.8 4. Carbon isotope ratios of Great Plains tooth Hemphillian 50 Ϫ5.6 4.3 Ϫ1.9 Ϫ9.4 Blancan 23 Ϫ6.6 3.3 Ϫ2.6 Ϫ10.3 enamel are slightly changed by diagenetic pro- Irvingtonian 11 Ϫ6.3 1.9 Ϫ4.5 Ϫ8.5 cesses. These hypotheses might be tested by measuring isotope ratios of Miocene-presumed browsers from et al. 1997) suggest that this is probably not the the Great Plains, given that a pure browsing niche case because a large amount of these data plot existed in the Miocene Great Plains. The second around our modeled value for average C3 vegeta- hypothesis would be supported if presumed brows- tion. If the latter is true, then expected grazers ers have d13C values that are more negative than should be enriched in 13C relative to browsers, but those of equids, and the fourth hypothesis might 13 in this case, it would be difficult to rule out a C4 be supported if they have similar d C values. It dietary component for the grazers. might be difficult to distinguish between hypoth- ␧∗ Our value forENϪDIET was taken from Cerling and eses 3 and 4 on the basis of carbon isotopes alone. Harris (1999) and was calculated using a database So far, there is no published carbon isotope data for of d13C values for enamel and diet for numerous Miocene Great Plains browsers. ungulate taxa in different parts of the world. The Carbon Isotopes: Late Hemphillian Transition to C4 estimates for equids were based on four Hogle Zoo Feeding. The change in equid diet beginning in ␧∗ p (Salt Lake City) zebra (ENϪDIET 14.4 ‰) and six the late Hemphillian may relate to one or both of ␧∗ p horses from Mongolia (ENϪDIET 13.4 ‰). While the following endmember scenarios: more calibration data are still needed, it seems un- 1. C4 vegetation was present in significant quan- ␧∗ ∼ likely thatENϪDIET values will approach the 16‰ tities before the late Hemphillian but was not con- that is necessary to account for the discrepancy be- sumed by equids because of the ready availability tween the modeled time series and the Great Plains of other food resources, including C3 browse and carbon isotope data. C3 grass. In this scenario, a significant vegetation ␧∗ Our estimate ofPFϪCO2 was calculated using data change must have occurred beginning in the late from different times and different parts of the world Hemphillian such that C3 resources decreased in and thus should represent an average value appli- abundance. If the relative proportion of C4 vege- cable to long time scales. Nevertheless, we observe tation was unchanged, then overall ecosystem pro- that our estimate for preindustrial time (7.1‰) dif- ductivity likely decreased significantly, as most ter-

Table 5. t-Test Results for Signiﬁcant Oxygen Isotope Differences between North American Land Mammal Ages Signiﬁcant difference NALMA vs. Mean difference (‰) df tPat 95%? BAR vs. CLR Ϫ.9 30 1.81 .081 No CLR vs. HPH Ϫ1.5 66 2.91 .005 Yes HPH vs. BLA Ϫ1.0 71 2.04 .045 Yes BLA vs. IRV .4 32 .57 .580 No CLR ϩ eHPH ϩ mHPH vs. lHPH Ϫ1.6 66 3.36 .001 Yes CLR vs. eHPH ϩ mHPH Ϫ1.0 44 1.90 .063 No eHPH ϩ mHPH vs. lHPH Ϫ1.2 48 2.17 .034 Yes lHPH vs. BLA Ϫ.3 43 .57 .571 No Post-Miocene: tortoises vs. no tortoises (carbon isotopes) 3.8 32 5.3 .001 Yes Note. NALMA abbreviations are explained in appendix 1, available from The Journal of Geology upon request. 134 B. H. PASSEY ET AL.

sist partially or wholly on C4 vegetation. It is un- clear whether specialized digestive adaptations are

required to successfully process C4 versus C3 vegetation. If profound digestive adaptations are necessary, it seems unlikely that several equid genera would evolve these all during the same interval (late Hemphillian) had the evolutionary pressure existed millions of years prior. Had their digestive systems evolved in response to the availability of

C4 vegetation, it would be expected that there

would be a gradual increase in C4 feeding through time as digestive physiologies became more adept

at processing C4 vegetation. Finally, the timing of

the onset of C4 feeding in the Great Plains is

roughly coeval with the timing of C4-biomass expansion recorded elsewhere in North America, South America, Africa, and Asia (Cerling et al. 1997), suggesting a global vegetation change rather than a regional dietary preference change.

Carbon Isotopes: Correlation between C4 Feeding and Presence of Tortoise Remains. Figure 8 shows that horses from fossil localities with tortoise remains

consumed relatively more C4 vegetation than did horses from localities without tortoise remains. The t-test results indicate that the carbon isotope Figure 7. Histogram of post-Miocene (post-Hemphil- difference between the two groups is signiﬁcant at lian) fossil locality d18O averages (Nebraska localities only). Shaded boxes indicate “best estimate” localities where three or more individuals were analyzed, and standard deviations are less than 2‰. North America land mammal age abbreviations are as in ﬁgure 1. Data from Rw-101 and Hay Springs, and some data from Ahrens, were taken from MacFadden et al. (1999a).

ϩ restrial productivity can be accounted for by C 3

C4 photosynthesis. Before this vegetation change,

C4 vegetation must have been signiﬁcantly less nutritious than coexisting C3 vegetation in order to be so thoroughly avoided as a food resource by horses.

2. C4 vegetation was rare in ecosystems before late Hemphillian time and, beginning in late Hemphillian time, underwent expansion in terms of biomass. In this scenario, C4 vegetation replaced a fraction of C3 vegetation in terms of food availability, and equids were obliged to take up C4 feeding. Of these two scenarios, we suggest that the second is closer to the real history. It seems unlikely that an abundant food resource would be ignored for several millions of years, unless the food re- Figure 8. Plot showing d13C values of post-Miocene source required highly specialized digestive adap- equids from Nebraska, grouped according to whether tor- tations. While some data suggest that C4 vegetation toise remains co-occur or do not co-occur with the fossil is less nutritious than C3 vegetation (see Hecka- teeth. Some data were taken from MacFadden et al. thorn et al. 1999), numerous modern ungulates sub- (1999a). Journal of Geology GREAT PLAINS ENVIRONMENTAL CHANGE 135

199.9% conﬁdence (meandifference p 3.8 ‰; t p perature (Teeri and Stowe 1976), although growing- 5.3,).P ! 0.0001 season temperature, not cold-month temperature,

Tortoise fossils are large, robust, and readily iden- correlates best with percentage C4 species within tifiable. In Nebraska, tortoise remains are present grasslands. We suggest that the coupled changes in throughout the Ogallala group and are present at tortoise and C4 distributions in the post-Miocene all localities 5 Ma or older. Tortoises are first absent Great Plains have recorded periodic climatic change in the early Blancan Lisco fauna (∼4 Ma), but both in terms of temperature or other parameters. large (∼30–50 cm) and giant (∼50–150 cm) species Oxygen Isotopes. The oxygen isotope record for are again present in the medial Blancan (including Nebraska equids shows significant secular varia- Sand Draw and Broadwater) and early late Blancan tion, with the most 18O-enriched values occurring (including Big Springs) faunas (Holman 1972; Rog- in Barstovian and Clarendonian faunas, and the ers 1984). The youngest Blancan fauna in our sam- least-enriched values occurring in Hemphillian (es- ple, South Wind Prospect, has no tortoise remains. pecially late Hemphillian), Blancan, and Irvington- Tortoises are also lacking in the Pleistocene faunas, ian faunas. The 3.4‰ decrease between Barstovian with the exception of Angus Quarry, which con- and Blancan time represents a significant isotopic tains remains of large but not giant tortoises. Ap- difference, and it should be noted that the oceans pendix 1 indicates whether tortoise remains are became enriched in 18O by as much as 1‰ during present or absent at post-Hemphillian localities. this interval due to ice sheet expansions (Lear et This periodic absence of tortoises following the al. 2000), and this would have the effect of reducing Miocene is probably not an artifact of sampling or the isotopic change recorded in Nebraska. Such an changes in depositional environment. The number effect may partially explain why Irvingtonian of identifiable specimens (NISP) of large (11 kg) ver- equids are not significantly depleted in 18O relative tebrates per site is comparable in Miocene and post- to Blancan equids, as might be expected given onset Miocene localities. These sites range from small of Northern Hemisphere glaciation following Blan- (100–200 NISP) at the Miocene Mailbox site and can time. the Pliocene South Wind site to 110,000 NISP at Caution must be exercised when interpreting the Cambridge, North Shore, Norden Bridge, Ha- mammalian oxygen isotope records because nu- zard Homestead, and Annie’s Geese Cross sites merous, sometimes independent, factors contrib- (Miocene) and the Hay Springs, Broadwater, and ute to the overall d18O value of a given individual. Lisco sites (post-Miocene). In the following section, we again make use of an Depositional environments are similar for Mio- endmember approach for data interpretation. cene and post-Miocene localities. All localities Endmember 1. The secular variation in the sampled in this study are of fluvial or fluvio-lacus- d18O record reflects physiological or behavioral trine origin, and all but four (Lisco, Uptegrove, Osh- changes and does not reflect environmental change. kosh, and Ashfall) yield fish bones (i.e., they ac- The Kohn (1996) model estimates that several per cumulated in or near oxygenated water). Although mil differences in d18O can result from differences by no means uniform in details of taphonomic or- in physiological or behavioral parameters. Panting igin (e.g., Norden Bridge is a high-diversity attri- versus sweating, stem feeding versus leaf feeding, tional assemblage deposited by a large river [Voor- dry-food feeding versus succulent-food feeding, and hies 1990b]; North Shore is an attritional site numerous other factors influence the isotopic com- accumulated by large carnivores [Voorhies 1987]), position of mammalian body water. Thus, secular overall, the sites represent a limited range of flood- changes in the behavior or physiology of equids plain settings in areas of low relief. No cave, fissure, could account for the observed 3.4‰ decrease be- eolian, or marine-influenced sites are included in tween Barstovian and Blancan horses, especially if this study. some changes acted in concert. Tortoises have been suggested as a qualitative Differences in d18O between different equid gen- paleoclimate proxy because they require certain era living at the same localities might shed light temperature parameters to form a viable popula- on the degree of oxygen isotope variability that tion. Hutchinson (1982; fig. 2) plots maximum shell can be attributed to physiological or behavioral length of recent tortoises against mean cold month phenomena within equids. At Uptegrove (Ne- (MCM) temperature, showing that tortoises be- braska: late Hemphillian), Dinohippus averages 1j;n p 3 ), and Nannippus averages) 0.7 ע tween 30 and 50 cm long require MCM tempera- Ϫ8.5 n p 3 ), suggesting different water use) 1.1 ע tures of ∼13ЊC, while tortoises 1100 cm long do not Ϫ6.7 live where MCMs drop below 22ЊC. Distributions strategies for the two genera. The same pattern of C4 vegetation are likewise correlated with tem- holds up at Coffee Ranch, but there is significant 136 B. H. PASSEY ET AL.

Not including questionable occurrences .(1998) ע isotopic overlap; Dinohippus averages Ϫ0.5‰ i.e., those followed by a question mark in figs. 22.2-) 1.8 ע n p 40.6 ), and Nannippus averages) 1.0‰ (n p 3 ). Also at Coffee Ranch, Neohipparion av- 8 and 35.2-5 in Janis et al. 1998), there are 47 genera n p 5 ), and Astrohippus aver- recorded from the late late Barstovian (medial and) 0.8 ע eragesϪ1.0 -n p 3 ). At Cambridge (Ne- late Barstovian and early Clarendonian in this ar) 1.4‰ ע ages 0.3‰ braska: medial Hemphillian), there is isotopic ticle), 43 genera from the early Clarendonian (me- n p 6 ), and dial Clarendonian in this article), and 35 genera) 1.3 ע overlap; Calippus averagesϪ3.9 -n p 5 ). Thus, from the late Clarendonian. This number drasti) 0.9 ע Neohipparion averagesϪ4.5 there is some evidence of water use differences be- cally decreases to 22 genera in the early Hemp- tween different equid genera, and although the hillian but then rises to 30 genera in the medial magnitude of the resulting oxygen isotope differ- Hemphillian. The number of genera drops to 21 in ences appear to be small, they might account for the late Hemphillian, and only 15 genera are ac- some of the observed secular variation in the Ne- counted for in the latest Hemphillian. braska record. The oxygen isotope record from Nebraska fol- Endmember 2. The secular variations in d18O lows similar patterns. Values are greatest in the me- are artifacts of diagenesis. While this possibility dial Barstovian (Ϫ3.9‰), late Barstovian (Ϫ2.5‰), cannot be ruled out, it has been shown elsewhere and medial Clarendonian (Ϫ3.7‰). We report no that tooth enamel is resistant to oxygen isotope data from the early Clarendonian, but phosphate diagenesis. For example, Bocherens et al. (1996), oxygen isotope data reported by Bryant et al. (1994) Zazzo et al. (2000), and Cerling et al. (2001) show suggest values similar to those from preceding that fossil hippopotamus enamel is depleted in 18O NALMAs. Oxygen isotope ratios are slightly de- relative to other coexisting fossil taxa, a pattern creased in the late Clarendonian (Ϫ4.2‰), and the that is observed in modern ecosystems and relates early Hemphillian is marked by the most depleted to the aquatic habits of hippos. Fricke et al. (1998b) values up to that point (Ϫ6.4‰). Values rebound in show that the Late Paleocene thermal maximum the medial Hemphillian (Ϫ4.7‰), and the transi- is recorded in the carbonate component of biogenic tion to late Hemphillian (Ϫ6.2‰) marks possibly phosphate. Preservation of seasonal d18O signals the most significant isotopic decrease in the record, within single fossil teeth has been demonstrated by with an average value greater than that for the early Fricke et al. (1998b). Hemphillian but with locality averages that are al- Endmember 3. The secular variations in d18O most 2‰ more negative than any locality in the reflect environmental change. In the modern en- preceding 10 m.yr. The single latest Hemp- vironment, geographic variations in climate give hillian locality also records a depleted value rise to the majority of variation observed in mam- (Ϫ7.2‰). This preliminary correlation between un- malian oxygen isotope data on a global scale. In gulate diversity and oxygen isotopes warrants fur- particular, increased temperature and aridity are as- ther investigation, as it has significant implications sociated with 18O-enriched values and decreased not only for Great Plains paleoenvironments but temperature and greater humidity with 18O-depleted also for ecological hypotheses that seek to explain values. patterns of biological evolution and diversity Several regional, continental, and global changes change in time and space (see Barnosky 2001). correlate with the general features of the Nebraska Remains of crocodilians have not been recovered oxygen isotope record. The “Clarendonian Chrono- from post–late Clarendonian localities in Ne- fauna” represents the last significant chronofauna braska. Like tortoises, crocodilians cannot tolerate in North America and comprises a rich array of extended periods of freezing; they prefer to stay ungulate diversity that was thereafter absent from within an “activity range” of 25Њ–35ЊC, and in ad- North America. Webb et al. (1995, p. 91) summa- dition, they require permanent surface water (see rizes the pattern of diversity change in the Clar- Markwick 1994). Although more oxygen data from endonian Chronofauna as follows: “Large-herbi- the early Hemphillian are desirable, the existing vore diversity reached its apogee in the late data are in accord with environmental changes that Barstovian, about 15 million years ago, stayed high would force crocodilians to retreat to warmer lat- through the Clarendonian, and then entered a series itudes following the late Clarendonian. of devastating extinctions during the Middle and Oxygen isotopes from many of the late Hemp- Late Hemphillian” (plate and latest Hemphillian hillian localities in Nebraska are depleted in 18O in this article). This pattern is illustrated by the relative to those of preceding faunas. The timing faunal data for Great Plains ungulates (artiodactyls, of the late Hemphillian is broadly correlative with perissodactyls, and proboscideans) in Janis et al. global environmental changes marked by the Mes- Journal of Geology GREAT PLAINS ENVIRONMENTAL CHANGE 137 sinian stage in Europe (Hsu et al. 1973; Krijgsman late Hemphillian equids in Texas and Nebraska et al. 1999); increased ice growth in Antarctica (Mil- consumed C4 vegetation, and a larger fraction of ler et al. 1987); C4-biomass expansion in Asia, Af- post-Hemphillian equids from both regions con- rica, South America, and North America (Cerling sumed C4 vegetation. Pre–late Hemphillian equids et al. 1997); a “first-order” mammalian immigra- consumed significantly less, if any, C4 vegetation. tion episode in North America (Webb and Opdyke 2. Post-Miocene localities in Nebraska can be di- 1995); and the first appearance in millions of years vided into those that have or those that do not have of a North American mammalian taxa in South tortoise remains. Equids from localities where tor- America (Butler et al. 1984; Woodburne and Swisher toises remains are present have a significantly

1995). higher C4 dietary fraction than those from localities In contrast to the correlations outlined above, the where tortoise remains are not found. post-Miocene oxygen isotope record shows little 3. The oxygen isotope record for Nebraska equids temporal correlation with other environmental in- shows signiﬁcant patterns of secular variation. d18O dicators, including the presence or absence of tor- values are greatest during Barstovian and Claren- toise remains at fossil localities, the abundance of donian time, decrease between late Clarendonian

C4 vegetation in equid diets, the Great American and late Hemphillian time, and then stabilize but Interchange immigration event starting ∼2.7 Ma trend toward a bimodal distribution of locality d18O (Woodburne and Swisher 1995), and the onset of averages during and following late Hemphillian northern hemisphere glaciation in the latest Plio- time. cene or early Pleistocene. The record displays an 4. The oxygen isotope record shows temporal cor- interesting trend toward bimodal locality d18O av- relations with patterns of diversity changes of erages, especially within the late Hemphillian and Great Plains ungulates, disappearance of crocodil- Blancan faunas. More sampling is necessary to con- ians from Nebraska, and late Miocene global firm this pattern and to determine the extent that change. the pattern records climatic, local environmental, and behavioral factors. ACKNOWLEDGMENTS In summary, there is evidence that the oxygen isotope record reflects components of the first and We thank A. Rigby for help with sample prepara- third endmember scenarios presented above. There tions, C. Cook and M. Lott for help with mass spec- is occasional suggestion of oxygen isotope parti- trometry, and J. Ehleringer for use of the Stable Iso- tioning between different coexisting genera, and tope Ratio Facility for Environmental Research there are compelling correlations between the ox- laboratory. F. Brown provided invaluable assistance ygen isotope record and independent records of en- with early versions of the manuscript. We are in- vironmental change. debted to G. Corner and E. Lundelius, Jr., for sample material and information regarding fossil localities. B. MacFadden and two anonymous reviewers provided comments and constructive criticism that Conclusions greatly improved this manuscript. This work was 1. The Coffee Ranch local fauna was deposited supported by the National Science Foundation and before 6.6 Ma. A fraction of Coffee Ranch and other by the Packard Foundation.

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