Ancient latitudinal gradients of C3/C4 grasses interpreted from stable isotopes of New World...

Global Ecology and Biogeography (1999) 8, 137–149

RESEARCH LETTER

Ancient latitudinal gradients of C3/C4 grasses interpretedfrom stable isotopes of New World Pleistocene horse(Equus) teethBRUCE J. MACFADDEN, THURE E. CERLING, JOHN M. HARRIS and JOSE PRADOFlorida Museum of Natural History, University of Florida, Gainesville, Florida 32611, U.S.A., email:[email protected], Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112,U.S.A., George C. Page Museum, 5801 Wilshire Blvd., Los Angeles, CA 90036, U.S.A. and INCUAPA,Universidad Nacional del Centro, Del Valle 5737, 7400 Olavarrıa, Argentina

Pleistocene occurred in the mid-latitudes betweenABSTRACTabout 30 to 40°. The oxygen data, which vary

Carbon and oxygen isotopic data are reported from proportionately with temperature, indicate a116 Pleistocene Equus teeth from sixty-six localities latitudinal gradient (d18O range of 20 parts/mil)in the New World ranging from 68°N (Alaska, between high-latitude and equatorial EquusCanada) to 35°S (Argentina). Equus species have samples. The basic pattern of latitudinal gradientsbeen predominantly grazers, and as such, carbon of C3/C4 grass distribution and temperature asisotopic values of their tooth enamel provide interpreted from these Pleistocene data is similar toevidence of the Pleistocene distribution of C3 and the modern-day. The use of stable isotopes of fossilC4 grasses. The carbon data presented here indicate herbivore teeth represents a new means to interpreta gradient (d13C range of 10 parts/mil) in the relative Pleistocene climates and terrestrial ecology.proportion of C3 and C4 grasses between high Key words. C3 and C4 grasses, latitudinal gradients,latitude and equatorial Equus samples. The largest isotopes, carbon, oxygen, temperature, New World,amount of change from C3 to C4 grasses during the Pleistocene, Equus.

INTRODUCTION Cavagnaro, 1988; Cabido et al., 1997) and occasionalC4 grass species in Arctic regions (Schwarz & Redman,

Some of the most fundamental global biogeographic 1988). The underlying reason for the systematicvariation of the ratio of C3/C4 grasses and latitude atpatterns are those structured by latitudinal gradients

that extend from pole to equator. Of relevance to the a given time and atmospheric CO2 level is temperature(Terri & Stowe, 1976; Ehleringer et al., 1997).current study, the proportion of C3 and C4 grasses in

most modern ecosystems varies with latitude. C3 grasses Within the past decade, combined geochemical/palaeontological studies have revealed that stableare predominant (>90%) in high-latitude steppes and

prairies, whereas C4 grasses are predominant isotopes of carbon and oxygen in Cenozoic mammalteeth preserve a proxy record of ancient environments(>70–90%) in most low-elevation and equatorial

grasslands (Fig. 1; Terri & Stowe, 1976). In modern and global change in terrestrial ecosystems. Specifically,the carbon isotopes preserved in tooth enamel recordecosystems the crossover between C3 v. C4 dominance

in grasslands occurs at about 40–45° latitude in the the photosynthetic pathways (C3, C4, CAM) of plantfoodstuffs ingested by mammalian herbivores (e.g.northern hemisphere (Ehleringer et al., 1997; Epstein

et al., 1997; Tieszen et al., 1997). Exceptions to this Quade et al., 1989; Cerling, Wang & Quade, 1993;MacFadden & Cerling, 1996). Oxygen isotopes ingeneral rule include grasses at high-elevation or

climates (e.g. Mediterranean) with cool-growing modern terrestrial ecosystems are dominated by theisotopic composition of local meteoric water which, inseasons in which the grasses are predominantly C3

regardless of latitude (e.g. Tieszen et al., 1979; turn, is related to local temperature. Cold climates

137 1999 Blackwell Science Ltd. http://www.blackwell-science.com/geb

138 Bruce J. MacFadden et al.

Fig. 1. Proportion of C4 grasses distributed within latitudinalbelts in North America. Modified from Terri & Stowe (1976).

Fig. 2. Plot of the modern-day pattern of both d18O andMean Annual Temperature with respect to latitude. Data takenfrom Rozanski et al. (1993, Table 1) for continental reporting

have d18O depleted (more negative) waters compared stations at elevations of less than 2000 m.to warm climates. Accordingly, oxygen isotopes ofeither tooth enamel carbonate (e.g. Koch, Zachos &Gingerich, 1992; Bocherens et al., 1996; Cerling & far as we know, this is the first study to use data for

ancient land mammals from a single wide-rangingSharp, 1996) or phosphate (Bryant, Luz & Froelich,1994, 1996; Bryant & Froelich, 1995) in fossil mammal genus to interpret broad biogeographic patterns within

ancient terrestrial ecosystems.teeth can be used to identify patterns of temperatureand related palaeoclimatic parameters. This study is restricted to the genus Equus because:

(1) it is exceedingly common in Pleistocene terrestrialThe purpose of this paper is to present carbon andoxygen isotopic data from Pleistocene Equus teeth in sediments throughout the Americas; (2) ecological

studies of extinct and modern Equus indicate that thisorder to understand the ancient distribution of C3 andC4 grasses along a northern and southern hemisphere horse characteristically has been a grazer, i.e. it feeds

predominantly or exclusively on grasses (e.g. Walker,latitudinal transect covering a range of 100° withinthe New World, from 65°N (Yukon, Alaska) to 35°S 1975; Berger, 1986; MacFadden & Cerling, 1996;

MacFadden & Shockey, 1997); and (3) previous studies(Buenos Aires province, Argentina). These newanalytical data provide a comparison of ancient have shown that body-size may influence the d18O tooth

enamel composition in different sized mammals (Bryantlatitudinal gradients with those that have beenempirically established for the modern-day (Figs 1 and & Froelich, 1995; Kohn, 1996), so it is preferable to

analyse a single genus consisting of generally similar-2). Although not presented with respect to latitude(MacFadden et al., 1994; MacFadden, Cerling & sized species (rather than, for example, a diversity of

Pleistocene grazers).Prado, 1996; Wang, Cerling & MacFadden, 1994;MacFadden & Cerling, 1996; MacFadden & Shockey, Given the dating and associated temporal constraints

of our varied fossil Equus samples, one limitation of1997), or with as large a combined data set (Cerlinget al., 1997), previous publications on the isotopic our study is that the results presented here only indicate

broad first-order (106 years) patterns of latitudinalcomposition of Pleistocene Equus teeth from the NewWorld showed the potential for the current study. So gradients throughout the Pleistocene, i.e. the isotopic

1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 137–149

Pleistocene latitudinal gradients 139

data are all combined within a window of time spanning1.8 myr. Thus, at the current level of temporalresolution, we are unable to discern geographicallybroad climatic and phytogeographic cycles on shorterduration time-slices. Many other Quaternarypalaeoclimatic studies have shown that 100,000 yearand more frequent cyclical events undoubtedly haveoccurred, but we are unable to discern these from ourcurrent data set. Despite this limitation, however, ourresults show important broad patterns of latitudinalgradients within Pleistocene terrestrial ecosystems.

CARBON AND OXYGEN ISOTOPESOF FOSSIL MAMMALIANHERBIVORE TEETH

Fossil mammal tooth enamel consists of >95%hydroxyapatite, which has many chemical variations,including a relatively common form Ca10(PO4,CO3)6(OH)2 (Hillson, 1986; Newesely, 1989). The stable Fig. 3. Plot of d13C values v. fraction of C4 (%) contained in

varied kinds of plant communities (i.e. closed canopy v. xericisotopes of fossil tooth enamel are primarily analysedhabitats) and the corresponding enriched (more positive) valuesfrom the structurally bound carbonate (CO3

−2) thatof tooth enamel. This plot assumes: (1) a mean d13C value forsubstitutes in the phosphate site and represents aboutC3 plants of −27‰ with a range of ±4‰; (2) a mean d13C

4% of the mineral mass (i.e. 0.7 lmol/mg). Althoughvalue for C4 plants of −13‰ with a range of ±2‰; and (3)

early studies suggested the possibility of post- that relative to plant foodstuffs, fossil tooth enamel d13Cfossilization chemical alteration (diagenesis) of porous (hydroxyapatite phase) is enriched by c. 13‰ (e.g. Farquhar

et al., 1989; Boutton, 1991; Quade et al., 1992).fossil bones (e.g. Schoeninger & DeNiro, 1987), morerecent research has convincingly demonstrated thatcarbonate in the dense mineral lattice of tooth enamelis a closed system not prone to secondary alteration (Fig. 3). Previous studies of Pleistocene and extant

Equus from low latitudes and low elevations(e.g. Koch et al., 1992; Quade et al., 1992; Wang &Cerling, 1994; Cerling & Sharp, 1996). Accordingly, demonstrate characteristic d13C values of −3 to 1‰,

which indicate that these equids have beenthe use of tooth enamel carbonate of extinct mammalspecies has become a growing field of investigation predominantly or exclusively C4 grazers (e.g. Wang et

al., 1994; Bocherens et al., 1996; MacFadden &that has produced many new palaeoecological andpalaeoenvironmental insights. Shockey, 1997).

The isotopic systematics of oxygen have been studiedThe analysis of stable isotopes of fossil teeth hasbeen particularly illuminating in understanding the for several decades (e.g. McCrea, 1950; Epstein &

Mayeda, 1953; Craig, 1957). Oxygen isotope data arediets of extinct mammalian herbivores. C3 plants havemean d13C values of about −27‰, whereas C4 plants produced along with tooth enamel carbonate data and,

as such, they are relevant to the current discussion. Inhave mean d13C values of about−13‰ (Deines, 1980;Farquhar, Ehleringer & Hubick, 1989; Boutton, 1991). biogenic materials the relative proportions of the two

stable isotopes of oxygen (16O and 18O) are primarilyRelative to ingested plant foodstuffs, the d13C ofmammalian tooth enamel carbonate is about 12 to 14‰ dependent upon parameters such as temperature and

the isotopic composition of ingested H2O (althoughmore positive, i.e. a browser feeding upon exclusively C3

plants will have tooth d13C values <−10‰ whereas a other factors, such as body size, are also involved).With regard to temperature, the heavier isotope, 18O, isgrazer feeding exclusively on C4 grasses will have d13C

tooth enamel values of about −1 to 3‰ (e.g. Koch et preferentially incorporated into biogenic tissues duringcolder conditions, whereas 16O is favoured in warmeral., 1992; Cerling et al., 1997; Cerling & Harris, 1998)



conditions. As such, the ratio of 18O/ 16O (expressed as poorly known until the Miocene (Stebbins, 1981).Despite their poor fossil record, some of the bestd18O, see below) acts as a ‘palaeothermometer’ that

has been used to interpret fluctuations in ancient evidence for the presence of grasses comes from thewidespread diversification and rapid coevolution oftemperatures, either at different locations at the same

time, or in a time-series from the same region. This mammalian grazers with high-crowned teeth duringthe Miocene, about 20 million years ago (MacFadden,can be seen in the plot of d18O of meteoric water

analyses from modern-day polar to equatorial regions 1997).Carbon isotopic studies of tooth enamel from thesein the New World, in which the correlation of d18O

to mean annual temperature is very high (r=0.95; early and middle Miocene mammalian grazers indicatea diet of exclusively C3 plants (e.g. Cerling, 1992; QuadeRozanski, Araguas-Araguas & Gonfiantini, 1993) (Fig.

2). et al., 1992; Cerling et al., 1993, 1997). Although a fewgrass fossils with the characteristic C4 Kranz anatomyIn some of the classic studies of oxygen isotopes,

analysis of deep-sea cores allowed a fundamental have been reported from 12 million-year-old sedimentsin California (Tidwell & Nambudiri, 1989), theunderstanding of temperature cycles that occurred

throughout the Pleistocene (e.g. Shackleton & Opdyke, terrestrial flora was fundamentally C3 until the lateMiocene, about 6–8 million years ago. During the late1976) or geographic gradients at a point in time, e.g.

the last glacial maximum at 18,000 yrs (e.g. Miocene there was a major shift in photosyntheticregimes of terrestrial ecosystems leading to the patternsCLIMAP, 1976; COHMAP, 1988). Although much of

the earlier work using d18O palaeothermometry of C3 and C4 grass distribution seen today (Cerling etal., 1993, 1997). Accordingly, the d13C values from theconcentrated on marine environments, significant

strides have been made more recently using oxygen tooth enamel of grazing mammals prior to the lateMiocene global carbon shift are alwaysisotopes in terrestrial ecosystems (e.g. Swart et al.,

1993). Much of the previous work studying d18O in characteristically more negative than−10‰, regardlessof latitude (Fig. 4). Thereafter, from the late Mioceneancient terrestrial ecosystems concentrated on

palaeosols, lacustrine carbonates, or invertebrates (e.g. to the late Pleistocene, fossil tooth enamel carbonatefrom grazing mammals indicates d13C values rangingLecolle, 1985; articles in Swart et al., 1993), with little

work until recently coming from fossil mammal skeletal from <−10‰ at high latitudes (full C3), to about 0‰(predominantly C4) towards the Equator. This rangetissues. The primary reason for the lack of vertebrate

studies has been because of the potential problem of of carbon isotopic values indicates that latitudinalphotosynthetic gradients similar to those observed insecondary alteration of the original d18O signal due to

chemical exchange of groundwaters surrounding, or modern terrestrial ecosystems originated during, andhave existed since, the late Miocene.percolating through, fossil bones. The key to using

fossil mammals has been to concentrate on the compact This empirical observation of a late Miocene globalchange in terrestrial ecosystems raises the question asmineral phase found in tooth enamel (in contrast to

porous fossil bone), which has been shown to be to what is the underlying cause for this phenomenon.The answer to this question seems to be related torelatively unaffected by secondary alteration (e.g. Wang

& Cerling, 1994). In this context d18O can be analysed adaptive photosynthetic strategies and changing levelsof atmospheric CO2. It has been shown that C3from either the phosphate or carbonate phase of tooth

enamel (e.g. Cerling & Sharp, 1996). photosynthesis is favoured during relatively high levelsof atmospheric CO2, whereas C4 photosynthesis isfavoured in relatively low levels of atmospheric CO2Antiquity of grasses and C3 and C4 (Ehleringer et al., 1991, 1997). Models of the ancient

photosynthesisatmosphere indicate that during most of the Tertiary,CO2 levels were high relative to today (Berner, 1994),The fossil record indicates that plants have existed on

land since the Silurian, i.e. for about 400 million years. and above the threshold in which C3 photosynthesis isfavoured as an adaptive strategy. During the lateIn contrast to this antiquity, however, grasses are

relatively recent arrivals on the global ecological Tertiary, atmospheric CO2 concentrations decreasedand eventually, during the late Miocene, crossed thelandscape. The first-known fossil grass, about 55

million years old, comes from the early Tertiary threshold below which C4 photosynthesis has beenfavoured in hot, arid terrestrial ecosystems. This model(Paleocene/Eocene) of Tennessee, U.S.A. (Crepet &

Feldman, 1991). Thereafter, fossil grasses are relatively of changing levels of atmospheric CO2 thus provides



Fig. 4. Plot of d13C values v. latitude before (>8 million yearsago [Ma]) and after (<8 Ma) the late Miocene carbon shift in

Fig. 5. Hemispheric view of the Americas showing latitudinalterrestrial plant communities as represented in South America.distribution of the sixty-six fossil sites from which PleistoceneSamples were chosen from high-crowned herbivore taxa andEquus samples were analysed for stable isotopes of carbonfrom localities of low elevation (i.e. <2000 m). The regions(d13C) and oxygen (d18O). This western hemisphere projectionand/or localities used to assess latitudinal variation are asbase is reproduced with the permission of Ray Sterner of Thefollows: before 8 Ma: Patagonia (c. 47°S), southern BoliviaJohns Hopkins University Applied Physics Laboratory.(22°S), and northern Bolivia (17°S); 8 Ma and after: Buenos

Aires province and environs (c. 35°S), northern Argentina(27°S), and southern Bolivia (21.5°, 21°S). From MacFaddenet al. (1996).

American localities (Fig. 5). These relative proportionsreflect the general abundance of specimens in themuseum collections used during this study. Specimenswere selected from Irvingtonian (2.0–0.5 myr) andRancholabrean (0.5 myr to 10 kyr) localities in Northa potential causal mechanism for why there was aAmerica and the roughly equivalent Ensenadanchangeover from predominantly C3-based terrestrial(1.5–0.5 myr) and Lujanian (0.5–10 kyr) localities inecosystems prior to the late Miocene to mixed C3 andSouth America (for land-mammal calibrations, seeC4 ecosystems thereafter (Cerling et al., 1993, 1997).Berggren et al., 1995; Flynn & Swisher, 1995).(Although earlier, Pliocene [Blancan, 4.5–2.0 myr]Equus species lived in North America, no specimensMATERIALS AND METHODSfrom this time period were used because the focus of thepresent study was to limit the latitudinal comparisons to

Pleistocene Equus teethPleistocene localities.)

Studies such as Tieszen et al. (1979), CavagnaroA database of 116 individual Pleistocene Equus teethand their corresponding d13C and d18O values from (1988), and Cabido et al. (1997) have shown that in

modern ecosystems: (1) grasses use the C3sixty-six localities was compiled for this study(Appendix 1). Of these specimens, ninety-one and photosynthetic pathway at elevations above 3000 m;

and (2) between 2000 and 3000 m tropical and manytwenty-five are from respectively, North and South



temperate grassland biomes (except those withMediterranean-style climates) have mixed C3/C4 grassspecies. Thus, in order to eliminate the effects ofelevation, during the current study only specimens ofEquus from fossil localities below 2000 m elevationwere analysed.

Bryant et al. (1994; also see Kohn, 1996) suggestedthat there can be as much as 2‰ variation in the d18Oenamel values along the tooth row within a givenindividual, and a smaller, yet potentially significantvariation also may occur in d13C values. The variationappears to be systematic, with the greatest deviationoccurring in the 1st and 2nd molars relative to theother cheek teeth (P2-P4, M3, uppers and lowers).Accordingly, when there were sufficiently numerousEquus teeth available for analysis, 1st and 2nd molarswere not used in this study.

Stable isotopic analyses

Preparation and analytical procedures used in thisstudy are similar to those described elsewhere(MacFadden & Cerling, 1996) and are summarizedas follows. About 100–500 mg of enamel was cut orotherwise removed from each tooth specimen. These

Fig. 6. Bivariate plot of stable carbon isotopes (d13C) v.enamel fragments were pulverized using a mortar andlatitude for the tooth enamel carbonate samples used in this

pestle and treated with H2O2 and then with 1 aceticstudy (Table 1).

acid to remove respectively, organic and mineralsurficial contaminants not bound in the structurallattice. The treated powders were then reacted with

Although there are considerably fewer data points in theH3PO4 at 25°C for between 40 and 48 h. The resultingsouthern hemisphere, the general d13C gradient seems toCO2 generated was purified and extracted usingbe symmetrical on either side of the Equator. At highstandard cryogenic laboratory procedures (McCrea,latitudes, i.e. >45°N, the carbon isotopic values are1950). The CO2 sample gases were then analysed forcharacteristically more negative than−10‰ (Table 1).their isotopic ratios in a Finnigan mass spectrometerGiven a c. 14‰ positive enrichment in herbivore enamelin the Department of Biology at the University ofrelative to ingested plant foodstuffs, these data indicateUtah.predominantly- to exclusively-C3 plant foodstuffs,Isotopic data are reported in the standard delta (d)which are interpreted to indicate C3 grasses because ofnotation, which represents the deviation, in parts perthe dominantly grazing diet of Equus. The isotopicmil (‰), of the isotopic ratios of the Equus toothtransition between a full C4 signal on the one hand andsample from that of the PDB standard (Pee Deefull C3 on the other hand is best observed in the northernBelemnite; Craig, 1957) for carbon (d13C) and oxygenhemisphere, where the data set is larger. Proceeding from(d18O), respectively, where d=(Rsample/the pole to mid-latitudes, this transition occurs in aRstandard)−1)×1000 and R=13C/12C or 18O/16O.relatively narrow belt of latitude starting at about 45°N,where mean d13C values begin to be more positive that

RESULTS AND DISCUSSION −10‰ (Table 1). Samples from about 35–30°N latitudeas represented in California, Arizona, New Mexico,

Carbon isotopes (d13C) and C3/C4 Texas, and Florida have individual isotopic values asphotosynthetic gradients

positive as 1.1‰, indicating a diet almost exclusively ofC4 grasses. At latitudes below 30°N, including localitiesThe distribution of d13C values for Pleistocene Equus

teeth with respect to latitude is plotted in Fig. 6. in Texas, Florida, Mexico, and Honduras, virtually all



Table 1. Mean values and univariate statistics for d13C and d18O values for Pleistocene Equus from the different latitudes analysedin this study. Also see Appendix 1 for individual sample data.

Lat. (°) d13C d18O

N x (‰) s (‰) Obs. Range (‰) N x (‰) s (‰) Obs. Range (‰)

68 N 2 −10.0 0.2 −9.8 to −9.5 2 −17.9 0.1 −18.0 to −17.965 N 1 −9.5 — — 1 −16.1 — —64 N 2 −11.5 0.3 −11.7 to −11.3 2 −16.9 1.4 −17.9 to −15.951 N 2 −11.1 0.2 −11.2 to −10.9 2 −10.1 2.0 −11.5 to −8.748 N 1 −10.9 — — 1 −9.0 — —44 N 2 −7.0 0.8 −7.5 to −6.4 2 −6.6 0.8 −7.1 to −6.043 N 9 −8.3 1.6 −9.6 to −5.4 9 −10.2 2.1 −13.0 to −7.340 N 5 −9.3 0.2 −9.5 to −9.1 5 −6.1 1.0 −7.6 to −5.236 N 1 −8.4 — — 1 −8.0 — —35 N 3 −3.0 1.2 −4.3 to −2.1 3 −4.1 1.6 −5.5 to −2.334 N 14 −5.5 1.8 −8.5 to −2.0 14 −4.5 2.1 −8.2 to −0.833 N 13 −5.1 2.0 −9.8 to −2.6 13 −5.5 2.7 −10.1 to −0.132 N 6 0.2 1.7 −3.3 to 1.1 6 −5.0 1.2 −6.2 to −2.930 N 5 −4.7 2.6 −7.8 to −1.9 5 −1.9 2.3 −4.4 to 1.528 N 5 −1.6 1.3 −3.5 to −0.2 5 0.6 1.6 −1.6 to 2.027 N 1 −2.4 — — 1 0.7 — —26 N 4 −0.3 0.4 −0.6 to 0.2 4 1.4 0.7 1 to 2.422 N 4 −0.3 0.7 −1.0 to 0.7 4 −2.4 2.0 −4.6 to −0.221 N 4 −2.3 4.0 −8.3 to −0.1 4 −2.7 1.5 −4.2 to −0.619 N 1 −0.8 — — 1 −6.3 — —15 N 1 −7.1 — — 1 −3.4 — —

3 N 2 −1.5 1.1 −2.2 to −0.7 2 0.2 1.7 −1.0 to 1.42 N 2 3.6 2.5 1.8 to 5.4 2 2.2 0.6 1.7 to 2.6

Equator3 S 2 1.1 2.7 −0.8 to 3.0 2 1.6 0.1 1.5 to 1.65 S 1 2.1 — — 1 −3.8 — —

12 S 3 0.7 1.2 −0.6 to 1.7 3 0.7 2.7 −1.4 to 3.721 S 2 0.1 0.1 0.0 to 0.2 2 −0.5 0.9 −1.1 to 0.222 S 9 −3.3 0.8 −4.8 to −2.3 9 −5.4 1.1 −7.2 to −4.032 S 1 −0.8 — — 1 −0.1 — —35 S 7 −9.8 1.2 −11.0 to −7.8 7 0.0 0.5 −0.6 to 0.8

individual d13C values are more positive than c.−2‰. the Equator to c. 32°S are relatively positive(>c.−4‰), indicating a dietary predominance of C4A few outliers, e.g. values of −8.3 (IGCU 4665) and

−7.1‰ (UF 17730), from Mexico and Honduras, grasses. At latitudes of 35°S, corresponding to theBuenos Aires province and environs, d13C values forrespectively, are not easily explained. These outliers

could represent one of several possibilities, such as all individual Equus samples range between −7.8 and−11.0‰, indicating, respectively, an isotopically mixedindividuals living (but not dying) at higher elevations or

inotherhabitats that lackedC4 grasslands,or individuals (but more towards the C3 end), to full C3 diet. Aswith the counterparts at similar and higher northernliving during a time in which C3 grasses were more

abundant, or unusual dietary preferences not observed latitudes, these data are interpreted to represent grazersfeeding predominantly on C3 grasses.in modern equids. A more definitive explanation for

these outliers must await the analysis of additional Terri & Stowe (1976) presented the pattern ofdistribution of modern C4 grasses in North Americasamples from these regions.

Although the data are more sparse in the southern and their results have direct relevance to the Pleistocenedata presented in the current study (Fig. 1). The generalhemisphere, all individual d13C values at localities from



There is only a c. 8‰ increase in d18O values betweenc. 40°N and the Equator.

Although the data set is smaller, the same patternseems to occur in the southern hemisphere between theEquator and 35°S, at the latitude of the Buenos Airesprovince and environs. Thus, as d18O values of meteoricwater are directly proportional to mean annualtemperature, the pattern of oxygen isotopes in the fossilEquus teeth represent a proxy record of latitudinaltemperature gradients.

As shown above (Fig. 1), Rozanski et al. (1993)found about a 25‰ difference in d18O values rangingfrom present-day polar to equatorial continentallocalities. Our data for Pleistocene Equus presents asimilar range of values. Thus, in terms of terrestrialmean annual temperatures in North America, themodern-day latitudinal gradients closely resembles thatof the entire time-averaged Pleistocene and notsurprisingly, the d18O temperature proxy data haveessentially the same pattern as that seen for the d13C.

CONCLUDING REMARKS:POTENTIAL FOR FUTURERESEARCHFig. 7. Bivariate plot of stable oxygen isotopes (d18O) v.

latitude for the tooth enamel carbonate samples used in thisstudy (Table 1). The current study demonstrates the utility of using

wide-ranging fossil mammals to explore latitudinalgradients and patterns of C3/C4 grass distribution andcontinental palaeotemperature during the Pleistocene.distribution of C4 grasses and the steep gradient of

change from predominantly C3 to predominantly C4 There are several benefits of using fossil mammals,such as Equus, in these studies. Firstly, Pleistocene(‘C4/C3 crossover’, see Cerling et al., 1997; Ehleringer

et al., 1997) in mid-latitudes, described above is herbivore remains are plentiful, either in existingmuseum collections or in the field, and as such theyessentially the same for both the Pleistocene and the

modern day, particularly as evidenced from the are a readily available archive. Secondly, the methodsrequired to prepare and analyse carbonate isotopicnorthern hemisphere.samples are relatively affordable and not particularlydifficult or time-consuming. Given the fact that Equus

Oxygen isotopes (d18O) and temperaturewas also very widespread in the Old World during the

gradientsPleistocene, other similar studies could be done toestablish one or more latitudinal transects in EurasiaThe distribution of d18O values for Pleistocene Equus

teeth with respect to latitude are plotted in Fig. 7. The and Africa and to compare these with the one that isnow available for the Americas.general pattern is similar to that of the d13C values

reported above and, not unexpectedly, symmetrical As mentioned above, the major limitation of thecurrent study is the broad time window from whichabout the equator. There is about a 20‰ variation in

mean d18O values from the highest N latitude (68°; the Equus samples are taken. This is a first attempt toshow overall patterns; the next step will be to select amean=−17.9‰) to the lowest N latitude (2°; mean=

2.2‰; Table 1, Appendix 1). Corresponding to the d13C single terrestrial species for which there is a widelatitudinal gradient and a more precise suite ofvalues reported above, the steepest gradients in d18O

values are seen in the higher N latitudes, where there associated radiometric data to calibrate the isotopicdata. These requirements are not out of the question,is a c. 12‰ increase in d18O values from 68 to c. 40°N.



but will prove to be a challenge for future research. projection (Fig. 5) was kindly prepared specifically forthis study by R. Sterner of the Johns HopkinsFor example, with regard to late Pleistocene Equus in

the Americas, samples from different late Wisconsian University Applied Physics Laboratory. L. Walz of theUniversity of Florida skilfully designed and prepared(latest Rancholabrean and latest Lujanian) full-glacial

(c. 20,000 years ago) and interglacial (c. 10,000 years the colour figures. L.B. Albright III and R.C. HulbertJr read earlier versions of this manuscript and madeago) localities that are calibrated with 14C dates could

provide a narrow time-slice from which more numerous suggestions for its improvement. This isUniversity of Florida Contribution to Paleobiologymeaningful interpretations could be made about

palaeoclimatic, ecological, and biogeographic patterns number 485.across the Pleistocene-Holocene transition. These datamight also provide additional insight about latePleistocene large mammal extinctions like the one that REFERENCESextirpated Equus from the Americas by 10,000 years

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Appendix 1. Pleistocene Equus specimens used during this study for isotopic analyses, arranged from N to S latitude.

Museum spec. number1 Tooth2 Country State/ Locality Age3 Lat. d13C d18OProvince (°) (‰) (‰)

CMN 23034 LM3 Canada Yukon Old Crow River 66 RLB 68 N −9.5 −18.0CMN 14318 RM3 Canada Yukon Old Crow River 14N RLB 68 N −9.8 −17.8F:AM 129401 LM3 USA Alaska Cripple Creek e IRV 65 N −9.5 −16.1CMN 47603 RM3 Canada Yukon Sixtymile 5 RLB 64 N −11.3 −17.9CMN 49850 RM3 Canada Yukon Dawson 57 RLB 64 N −11.7 −15.9RSM P663.73 LP2 Canada Saskat. Fort Qu’Appelle RLB 51 N −11.2 −8.7RSM P663.71 Lp2 Canada Saskat. Fort Qu’Appelle RLB 51 N −10.9 −11.5F:AM 129407 LM3 USA North Dakota Lake Agassiz RLB 48 N −10.9 −9.0UWBM 75621-1 lm frag USA Oregon Fossil Lake RLB 44 N −7.5 −7.1UWBM 75621-2 lm frag USA Oregon Fossil Lake RLB 44 N −6.4 −6.0F:AM 129404 LM3 USA Nebraska Hay Springs l IRV 43 N −9.5 −7.4F:AM 129405 RM3 USA Nebraska Hay Springs l IRV 43 N −9.2 −9.5IMNH No 114 frag USA Oregon Rome IRV 43 N −8.5 −7.3IMNH 58001/24427 M frag USA Idaho Duck Point RLB 43 N −9.6 −10.6IMNH 52002/15110 M frag USA Idaho Dam local fauna RLB 43 N −9.5 −11.7IMNH 65003/23325 M frag USA Idaho American Falls Resev. RLB 43 N −9.3 −12.7LACM 4189/105957 tooth USA Oregon Owyhee River IRV 43 N −5.4 −11.1LACM 47/83846 undiff. USA Oregon Owyhee River IRV 43 N −8.9 −13.0UNSM 1132-93 M frag USA Nebraska Backus Pit e IRV 43 N −6.0 −8.9UNSM 104-#1 m frag USA Nebraska Ahrens site—No.-104 IRV 40 N −9.3 −5.9UNSM 104-#2 m frag USA Nebraska Ahrens site—No.-104 IRV 40 N −9.3 −6.5UNSM 104-#3 m frag USA Nebraska Ahrens site—No.-104 IRV 40 N −9.5 −5.4UNSM 1249-79 m frag USA Nebraska Rw-101 mlRLB 40 N −9.5 −7.6UNSM 1075-68 m frag USA Nebraska Rw-101 mlRLB 40 N −9.1 −5.2UCMP 36665 R UM USA California Doolan Canyon RLB 38 N −12.1 −5.1UCMP 55319 RP2 USA California Alameda County IRV 38 N −11.2 −2.5LACM 1209/3319 undiff. USA California Tecopa IRV 36 N −8.4 −8.0LACM 138/135805 M3 USA California McKittrick RLB 35 N −4.3 −5.5TMM 41112-5 frag USA Texas Rock Creek Equus e IRV 35 N −2.1 −4.5

BedsTMM 934-10 frag USA Oklahoma Holloman Gravel Pit IRV 35 N −2.5 −2.3TMM 789-1 frag USA Texas Plainview East RLB 34 N −2.1 −4.2TMM 40919 frag USA New Mexico Big Bear Site RLB 34 N −8.5 −7.8TMM 937-932 frag USA New Mexico Blackwater Draw RLB 34 N −6.2 −0.8LACM 19196 LM3 USA California Rancho La Brea RLB 34 N −2.0 −4.6LACM 19198 RM3 USA California Rancho La Brea RLB 34 N −5.5 −4.6LACM 19198 RM3 USA California Rancho La Brea RLB 34 N −5.3 −4.5LACM 19124 LM2 USA California Rancho La Brea RLB 34 N −6.8 −8.2




LACM 19124 LM2 USA California Rancho La Brea RLB 34 N −6.2 −7.0LACM 19144 LM2 USA California Rancho La Brea RLB 34 N −4.6 −2.8LACM 19204 LM3 USA California Rancho La Brea RLB 34 N −7.7 −5.0LACM 19210 RM3 USA California Rancho La Brea RLB 34 N −4.6 −2.0LACM 24868 LM2 USA California Rancho La Brea RLB 34 N −5.6 −4.8LACM 19019 RP2 USA California Rancho La Brea RLB 34 N −4.8 −3.1LACM 19048 LP4 USA California Rancho La Brea RLB 34 N −6.8 −3.0TMM 31108-75 LM3 USA Texas Old Glory RLB 33 N −2.6 −0.1TMM 31041-138 RM USA Texas Salt Fork Brazos River RLB 33 N −6.1 −3.4LACM 7062/ undiff. USA California R Rosa Indian Reserv. IRV 33 N −9.8 −2.0LACM 66151/17069 tooth USA California Little Diablo Wash IRV 33 N −3.6 −8.2LACM 66151/17069 tooth USA California Little Diablo Wash IRV 33 N −3.1 −5.9LACM 111/3539 RM3 USA California Vallecito Creek IRV 33 N −6.7 −6.7LACM 111/3539 RM3 USA California Vallecito Creek IRV 33 N −6.5 −6.1AZBO 38/148 undiff. USA California Vallecito Creek IRV 33 N −4.8 −4.8AZBO 510/1751-1 undiff. USA California Vallecito Creek IRV 33 N −6.2 −3.7AZBO 272/1023 undiff. USA California Vallecito Creek IRV 33 N −5.7 −10.1AZBO 112/581 undiff. USA California Vallecito Creek IRV 33 N −4.4 −7.3AZBO 112/1335-2 undiff. USA California Vallecito Creek IRV 33 N −2.9 −7.7AZBO 1088/4442 undiff. USA California Borrego Badlands IRV 33 N −4.1 −5.2UA 7354/6552 undiff. USA Arizona Noye’s Prospect IRV 32 N 1.1 −6.2UA 25-0/718A undiff. USA Arizona Curtis Ranch e IRV 32 N 0.7 −5.1UA 25-0718A undiff. USA Arizona Curtis Ranch IRV 32 N 0.7 −5.1TMM 41228-222 frag USA New Mexico Dark Canyon Cave RLB 32 N −3.3 −4.6LACM 5111 undiff. USA Arizona Double Adobe RLB 32 N 0.7 −2.9UF uncatalog. #54 undiff. USA Florida Boy’s Ranch RLB 30 N −1.9 −1.7UF 275345 rm3 USA Florida Ichetucknee River RLB 30 N −2.2 −1.1UF v4066a5 RM1/2 USA Florida Ichetucknee River RLB 30 N −6.7 1.5TMM 41238-59 I USA Texas Cueva Quebrada RLB 30 N −7.8 −4.4TMM 41238-253 rm USA Texas Cueva Quebrada RLB 30 N −4.9 −3.6TMM 18611-4 l pm USA Texas Equus Beds, San Diego IRV 28 N −2.2 −1.6TMM 30967 RM USA Texas Ingleside RLB 28 N −0.8 −0.7TMM 30967-2186 P frag USA Texas Ingleside RLB 28 N −0.2 1.6UF 80047/27965 M3 USA Florida Leisey 1A e IRV 28 N −1.5 2.0UF uncatalogued5 rp2 USA Florida Leisey 1A e IRV 28 N −3.5 1.6UF 1407765 rm3 USA Florida Richardson Rd. e IRV 27 N −2.4 0.7UF uncatalogued5 RP3/4 USA Florida Cutler L. F. l RLB 26 N −0.4 1.0UF uncatalogued5 rp2 USA Florida Cutler L. F. l RLB 26 N −0.5 1.1UF uncatalogued5 rp/m USA Florida Cutler L. F. l RLB 26 N −0.6 2.4UF uncatalogued5 incisor USA Florida Cutler L. F. l RLB 26 N 0.2 1.1UF 8791 undiff. Mexico Aguascalientes Cedazo L. F. e RLB 22 N −0.4 −4.6MSU 10130 RP/M? Mexico Aguascalientes Cedazo L. F. e RLB 22 N −0.6 −3.5MSU 10128 LP3/4 Mexico Aguascalientes Cedazo L. F. e RLB 22 N −1.0 −0.2MSU 10124 RP3/4 Mexico Aguascalientes Cedazo L. F. e RLB 22 N 0.7 −1.3IGCU 9950 lm frag Mexico Jalisco Tecolotlan PLEIS 21 N −0.5 −3.2IGCU 9951 RM1 Mexico Jalisco Tecolotlan PLEIS 21 N −0.1 −4.2IGCU 4665 rm3 Mexico Guanajuato Las Galvanes IRV 21 N −8.3 −2.6IGCU 7758 incisor Mexico Guanajuato Rancho Viejo ?B/IRV 21 N −0.2 −0.6IGCU s/n rm1/2 Mexico Tlaxcala Atlihuetzia RLB 19 N −0.8 −6.3UF 17730 lp/m Honduras Comayaga Orillas del Humuya RLB 15 N −7.1 −3.4UCMP 38100 LM3 Colombia La Venta ‘La Venta, Camp PLEIS 3 N −0.7 −1.0

Huila’ING 184268 undiff. Colombia La Venta Villavieja PLEIS 3 N −2.2 1.4MPN V-3037 m frag Ecuador Le Carolina PLEIS 2 N 1.8 1.7MPN V-68 m frag Ecuador Le Carolina PLEIS 2 N 5.4 2.6




F:AM 131869 RP2 Ecuador Salinas Oil Field ?LUJ 3 S −0.8 1.6F:AM 131868 RP3/4 Ecuador Salinas Oil Field ?LUJ 3 S 3.0 1.5ROM 3471C RM3 Peru Talara Tar Pit LUJ 5 S 2.1 −3.8CM 11032 RP3/4 Brasil Bahia Waterhole deposit RLB 12 S −0.6 3.7Uncatalogued LP3/4 Brasil Bahia Ourolandia, Toc. Ossos LUJ 12 S 1.1 −1.4Uncatalogued RM3 Brasil Bahia Ourolandia, Toc Ossos LUJ 12 S 1.7 −0.3UF uncatalogued RM3 Bolivia Chuquisaca Nuapua-3 LUJ 21 S 0.2 0.2UF uncatalogued RM2/3 Bolivia Chuquisaca Nuapua-1 LU/HO21 S 0.0 −1.1UF uncatalogued-26 undiff. Bolivia Tarija Tarija l ENS 22 S −4.8 −5.5UF uncatalogued-66 undiff. Bolivia Tarija Tarija l ENS 22 S −2.9 −6.3UF uncatalogued-126 undiff. Bolivia Tarija Tarija l ENS 22 S −2.6 −5.0UF uncatalogued-136 undiff. Bolivia Tarija Tarija l ENS 22 S −2.3 −4.4UF 907507 pm Bolivia Tarija Tarija l ENS 22 S −2.9 −4.6UF 908957 m1/m2 Bolivia Tarija Tarija l ENS 22 S −3.5 −7.2UF 906537 undiff. Bolivia Tarija Tarija l ENS 22 S −4.1 −4.0UF 907647 m1 Bolivia Tarija Tarija l ENS 22 S −4.1 −6.5UF 919727 m3 Bolivia Tarija Tarija l ENS 22 S −2.7 −5.8MLP-52-IX-29-91 UP/M Argentina Santa Fe Esperanza LUJ 32 S −0.8 −0.1AMNH 11154 LM3 Argentina Buenos Aires Buenos Aires PLEIS 35 S −10.7 −0.1AMNH 11154 RM3 Argentina Buenos Aires Buenos Aires PLEIS 35 S −10.3 −0.3MLP 91-VI-5-1 undiff. Argentina Buenos Aires Magdalena PLEIS 35 S −10.1 −0.6MLP 52-X-5-3 undiff. Argentina Buenos Aires R. Queguen Salado PLEIS 35 S −8.5 −0.2MLP91-VI-5-1 pm2 Argentina Buenos Aires Fm. Pascua, PLEIS 35 S −11.0 0.8

MadgelenaMLP52-X-5-3 P2 Argentina Buenos Aires Queguen Salado LUJ 35 S −7.8 0.0MLP 80-VIII-13-93 UP/M Argentina Buenos Aires Miembro Guerro LUJ 35 S −10.2 0.4

1 Museum abbreviations are as follows: AMNH, American Museum of Natural History; CMN, Canadian Museum of Nature;CM, Carnegie Museum of Natural History; F:AM, Frick American Mammals at AMNH; IGCU, Instituto de Geologıa,Universidad Autonoma de Mexico, Cuidad Universitaria; IMNH, Idaho Museum of Natural History; ING, Ingeominas; LACM,Los Angeles County Museum of Natural History; MPN, Museo Politecnico Nacional; MSU, Midwestern State University; MLP,Museo de La Plata; ROM, Royal Ontario Museum; RSM, Royal Sasketchewan Museum; TMM, Texas Memorial Museum; s/n,sin numero (unnumbered); UCMP, University of California Museum of Paleontology; UF, University of Florida; UNSM,University of Nebraska State Museum; UWBM, University of Washington Burke Museum.2 Tooth abbreviations: frag, fragment; L/l, left; M/m, molar; P/p, premolar; PM/pm, premolar or molar; R/r, right; undiff.,undifferentated, Upper case=upper molar, lower case=lower tooth, e.g. RP4 is right upper 4th premolar and Lm3 is left lowerthird molar.3 Land-mammal age: ENS, Ensenadan (1.5–0.5 myr); IRV, Irvingtonian (2.0–0.5 myr); LUJ, Lujanian (0.5 myr to 10 kyr); RLB,Rancholabrean (0.5 myr to 10 kyr); PLEIS: Pleistocene, undifferentiated (=Pampean in Argentina); e, early; m, middle; l, late(also B, Blancan; HO, Holocene).4 Taken from Wang et al. (1994).5 Taken from MacFadden & Cerling (1996).6 Taken from MacFadden et al. (1994).7 Taken from MacFadden & Shockey (1997).


Ancient latitudinal gradients of C3/C4 grasses interpreted from stable isotopes of New World...

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