Paleobiology, 34(2), 2008, pp. 282–300

Stable isotopes in early as indicators of forest canopy structure and resource partitioning

Ross Secord, Scott L. Wing, and Amy Chew

Abstract.—The three dimensional structure of vegetation is an important component of ecosystems, yet it is difficult to reconstruct from the fossil record. Forests or woodlands prevailed at mid-lati- tudes in North America during the early Eocene but tree spacing and canopy structure are uncer- tain. Here we use stable carbon isotope values (␦13C ) in early Eocene mammalian faunas to infer canopy structure. We compare ␦13C values in two diverse fossil assemblages from the central Big- horn Basin to values predicted for mammals in a variety of open and closed habitats, based on modern floras and faunas. We conclude that these early Eocene faunas occupied an open canopy forest. We also use carbon and oxygen isotopes to infer diet and microhabitat. Three higher level taxa have significantly different mean ␦13C values, and values are negatively correlated with body mass. The pattern suggests diets high in leaves for larger mammals, and fruit or other non-foliar plant organs for small ones. A preference in the larger mammals for wetter habitats with high water availability to plants may also have contributed to the pattern.

Ross Secord.* Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Post Office Box 37012, NHB MRC 121, Washington, D.C. 20013-7012 Scott L. Wing. Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Post Office Box 37012, NHB MRC 121, Washington, D.C. 20013-7012 Amy Chew. Department of Anatomy, School of Medicine, Stony Brook University, T8-040 HSC, Stony Brook, New York 11794-8081 *Present address: Florida Museum of Natural History, 206 Dickinson Hall, Museum Road and Newell Drive, Gainesville, Florida 32611. E-mail: [email protected]

Accepted: 14 January 2008

Introduction Houten (1945) argued from the prevalence of hoofed mammals that savanna-like habitats The three-dimensional structure of vegeta- prevailed in the early Eocene of the Rocky tion is important for many reasons. It affects Mountain region. Upchurch and Wolfe (1987), the albedo of land surfaces, hydrologic cy- however, inferred from paleobotanical evi- cling, atmospheric circulation near the earth’s dence that midlatitude Eocene forests were surface, and carbon storage, all of which affect similar to modern closed canopy tropical rain climate and biogeochemical cycles on a global scale. Vegetation also forms the habitat in forests. On the basis of mammalian body which terrestrial organisms move, and over mass distributions (cenograms), Gunnell time influences the evolution of their locomo- (1997) also concluded that closed canopy for- tor adaptations. In spite of the climatic, bio- ests were present. Subsequent paleobotanical geochemical, and evolutionary importance of work suggests, however, that early Eocene cli- vegetational structure, it is difficult to recon- mates at midlatitudes were not tropical, but struct. Inferences about past vegetation struc- rather were warm-temperate to subtropical ture generally rely on rare instances of excep- (Wing et al. 1991; Wilf 2000; Wing et al. 2000). tional preservation or ecological analogies be- Moreover, leaf-area analyses suggest mean tween living and ancient organisms. Such infer- annual precipitation of only ϳ120–140 cm ences, however, become increasingly tenuous (Wilf 2000), which is lower than in modern as older biotas are considered. tropical rain forests and may have been in- We consider the structure of early Eocene adequate to support a closed canopy. forests or woodlands in the Bighorn Basin of We use a new approach to infer canopy Wyoming. Occasional fossilized tree stumps structure and develop a simple model that indicate the presence of trees, but tree spacing uses stable carbon isotope values (␦13C) in and canopy structure are less certain. Van mammalian tooth enamel and modern plants.

᭧ 2008 The Paleontological Society. All rights reserved. 0094-8373/08/3402-0000/$1.00 STABLE ISOTOPES IN EARLY EOCENE MAMMALS 283

The model predicts expected ␦13C values for pretations for early Eocene mammals from fossil tooth enamel (hydroxylapatite) from a North American. variety of habitats and microhabitats, ranging Specimen Provenance. Specimens are early from closed canopy forests to savannas. (In Eocene in age ( land- age, this paper ‘‘habitat’’ refers broadly to vegeta- Fig. 1) and are from overbank floodplain de- tion structure, such as open or closed canopy, posits in the in the cen- whereas ‘‘microhabitat’’ refers to areas within tral Bighorn Basin, Wyoming (Wing et al. a habitat, such as the understory in a closed 1991; Bown et al. 1994). Teeth are from two canopy forest). We infer canopy structure by discrete stratigraphic intervals in the Elk comparing predicted to measured ␦13C values Creek composite section. The lower and upper from two early Eocene mammalian assem- assemblages are from the Upper Haplomylus- blages. Ectocion and Heptodon biozones, respectively, These assemblages also provide a glimpse of the Wasatchian land-mammal age (Schank- into the evolution of mammalian herbivory ler 1980). The lower and upper assemblages about 12 Myr after the beginning of the Ce- occur at times of moderately cool and warm nozoic mammalian radiation. Faunas at this climates, respectively, according to mean an- time contained a mixture of ‘‘archaic’’ ungu- nual temperature estimates (MAT) from leaf lates (e.g., condylarths, tillodonts) and the margin analyses and ␦18O values in hematite first representatives of the extant (Wing et al. 2000). Although MAT during the clades Perissodactyla and Artiodactyla. These cool interval was lower than that of bounding faunas pre-date the spread of grasslands (e.g., intervals (Fig. 1), it was still considerably Stro¨mberg 2004) and contain a higher propor- warmer than in this region today. The lower tion of omnivores and browsers than most assemblage was collected from a thicker strati- post-Eocene faunas (Janis 2000). Dental spe- graphic interval (ϳ22 m) over a greater geo- cializations such as hypsodonty and seleno- graphic area than the upper one (ϳ4 m; thick- donty were rare. Thus, herbivores may have nesses assume that most localities sample an partitioned resources less and had more interval of ϳ4 m). However, although total broadly overlapping diets than younger fau- thickness for the lower assemblage is ϳ22 m, nas. We make the first attempt to recognize re- 76% of the specimens were collected from an source partitioning in faunas of this antiquity interval of only ϳ8 m. Sediment accumulation by using stable isotopes. rates for the upper and lower assemblages were ϳ215 and 422 m/Myr, respectively, ac- Materials and Methods cording to interpretations of paleosol maturi- ty (Bown and Kraus 1993). This implies time- Diet and Locomotion. Most of the mammals averaging of ϳ37,000 and 10,000 years for the included in this study are considered herbi- lower and upper assemblages, respectively. vores, but a few may have been omnivores Seventy-nine percent of the specimens in the (e.g., Gunnell et al. 1995). Diets inferred from lower assemblage were collected from a geo- other studies, on the basis of dental morphol- graphic area of ϳ1.5 km2. Another 15% are ogy and body size, are summarized in the Ap- from a ϳ1 km2 area about 5 km farther north pendix (online at http://dx.doi.org/10. (D-1415, D-1417) and 6% are from a small lo- 1666/06049.s1) and discussed for selected cality (D-1299) about 3 km southwest of the taxa in the ‘‘Resource Partitioning’’ section. main area. The upper assemblage is from a se- Although postcrania are poorly known for ries of localities distributed over ϳ1 km2. many species, it is clear that most were Stable Isotope Conventions. Stable isotope ground-dwelling . Exceptions are ratios are expressed using delta notation in Cantius, which is thought to have been arbo- units of parts per thousand (per mil, ‰): ␦13C 18 3 real, and Esthonyx, which had both arboreal or ␦ O ϭ {[Rsample/Rstandard] Ϫ 1)}·10 , where R and terrestrial adaptations. Didelphodus was ϭ 13C/12C for carbon, and the standard is Cre- also probably arboreal. Rose (2001) summa- taceous belemnite shell from the PeeDee For- rized known postcrania and locomotor inter- mation (vPDB); R ϭ 18O/16O for oxygen, and 284 ROSS SECORD ET AL.

FIGURE 1. Geochronology and stratigraphic positions of biozones, faunal assemblages, and MAT estimates in Elk Creek and Cabin Fork sections, central and southern Bighorn Basin (except Paleocene MAT estimate from northern Bighorn Basin). Meter levels are relative to base of Willwood Formation. Geochronologic ages are based on linear interpolation between ages for CIE (Ogg and Smith 2004) and a volcanic ash (upper left) (Wing et al. 1991; Smith et al. 2004). MAT estimates for assemblages are based on spline interpolation (see Secord et al. 2006) between leaf- margin MAT estimates from Wing et al. (2000, 2005). MAT error bars are 95% confidence. Mammalian biozones are based on Schankler (1980), Gingerich (1983, 2001a), and Secord et al. (2006). Spline curves were generated with PetroPlot software (Su et al. 1999–2002). CIE, carbon isotope excursion; Clark., Clarkforkian; LMA, Land-mammal age; MAT, mean annual temperature; PAL., Paleocene. the standard is mean ocean water (vSMOW). teeth could have pre-weaning values (e.g., Diet-to-enamel 13C-enrichment was calculated Boisseriea et al. 2005). Samples of enamel hy- using an enrichment factor (␧*): ␧*diet-enamel ϭ droxylapatite weighing 2–3 mg were pretreat- 13 13 {[1000 ϩ ␦ CE]/[1000 ϩ ␦ Cdiet] Ϫ 1}. For our ed to remove organic matter and nonstructur- 13 13 data, ␧* usually differs from ␦ CE Ϫ ␦ Cdiet by al carbonate following Koch et al. (1997). Our only a few tenths per mil, but using ␧* has the protocol differed only in that samples were advantage of being independent of scale (Cer- baked at 200ЊC after pretreatment under vac- ling and Harris 1999). uum for one hour to remove volatile contam- Sampling and Pretreatment. Tooth enamel inants and water, rather than being lyophi- was sampled from 11 species in the lower as- lized. Experiments at the University of Mich- semblage and 17 in the upper (Appendix). igan Stable Isotope Laboratory (UMSIL) Nearly all specimens were isolated teeth, and showed a mean decrease in ␦18O of 0.98 Ϯ all but Coryphodon and Hexacodus were ade- 0.35‰ (2 ␴; p Ͻ 0.001) when unbaked or ly- quate for specific identification. At least three ophilized samples were baked, while ␦13C val- individuals were sampled for species that ues were unchanged. Fourier transform infra- were abundantly represented. To ensure that red spectroscopy (FTIR) indicated that H2O isotope values represent an adult diet we and OH were removed from samples when avoided sampling first molars and deciduous baked, suggesting that baking is necessary to whenever possible, because these remove water that may otherwise contribute STABLE ISOTOPES IN EARLY EOCENE MAMMALS 285 to 18O enrichment (L. Wingate and K. C. Loh- results mostly from environmental factors, mann, UMSIL, personal communication such as light, temperature, soil nutrients, and 2005). water availability (e.g., Broadmeadow and Isotope Analysis. Samples were reacted Griffiths 1993; Heaton 1999). Studies of vege- with phosphoric acid at 76Њ Ϯ 2ЊC in a Finni- tation along light and water gradients show gan MAT Kiel automated carbonate reaction that the ␦13C value in leaves decreases with in- device at UMSIL. ␦13C and ␦18O values of the creasing humidity or precipitation, and in- resulting CO2 were measured on a Finnigan creases with increasing irradiance (Ehleringer MAT 251 triple collector isotope-ratio-moni- et al. 1986; Stewart et al. 1995). Both open and toring mass spectrometer. We assume that the closed canopy forests exhibit a stratification of fractionation factor between hydroxylapatite ␦13C leaf values, whereby values decrease carbonate and CO2 is the same as for calcite from the upper canopy to the base of the forest and CO2 (1.008818), following common prac- (van der Merwe and Medina 1991; Cerling et tice. Our lab standard (LOX; from modern el- al. 2004). This phenomenon is sometimes ephant enamel, courtesy of D. L. Fox) yielded called the ‘‘canopy effect’’ and results largely 18 the following values: ␦ O ϭ 31.03 Ϯ 0.18‰ from decreased irradiance below the canopy 13 and ␦ C ϭ Ϫ5.80 Ϯ 0.06‰ (SD; n ϭ 31). Rep- (Ehleringer et al. 1986; Hanba et al. 1997; Hea- licates were not run for the data presented ton 1999), although recycling of 13C-depleted here, but the mean, median, and range of dif- CO2 under the canopy also contributes (Vogel ferences among replicates of Bighorn Basin 1978; van der Merwe and Medina 1989, 1991). pretreated fossil enamel are 0.10, 0.08, and The lowest ␦13C values in non-aquatic plants 13 0.00–0.21‰, respectively for ␦ C, and 0.11, occur in understory leaves in closed canopy 18 0.08, and 0.00–0.41‰, respectively for ␦ O (n forests, and mean values increase as the can- ϭ 35) (unpublished data). Analytical precision opy becomes more open (Ehleringer et al. based on international standards for carbon- 1987). Thus, the presence of a closed canopy ate (NBS-18, NBS-19) is Ͻ Ϯ0.1‰ (SD) for can be inferred by distinctively low ␦13C val- ␦18O and ␦13C values. ues in understory leaves or by mean floral val- Inferring Canopy Structure from Carbon ues, which are recorded in the teeth of herbiv- Isotopes in Herbivorous Mammals orous mammals (Cerling et al. 2004). The carbon isotope composition of mam- Carbon in plants is derived from atmo- malian tooth enamel (␦13C ) is strongly cor- spheric CO fixed during photosynthesis. E 2 related to the mammal’s diet and serves as a Plants discriminate against 13C in CO to vary- 2 proxy for the ␦13C value of vegetation (Lee- ing degrees as a result of using different pho- Thorp and van der Merwe 1987; Cerling and tosynthetic pathways. Modern floras consist Harris 1999; Passey et al. 2005). Carbon iso- of plants that use C3, CAM (crassulacean acid tope values from a diversity of herbivores metabolism), and/or C4, photosynthetic path- 13 feeding within different microhabitats should ways. Resulting ␦ C values are lowest in C3 plants (Ϫ37‰ to Ϫ21‰), intermediate in reflect the mean value of local vegetation. This is exemplified in the Ituri closed canopy forest CAM plants, and highest in C4 plants (Ϫ19‰ to Ϫ9‰) (e.g., Vogel 1993; Cerling and Ehler- in tropical Africa (Cerling et al. 2004). If we inger 2000). Although plant fossils with living assume a diet-enamel enrichment factor of 13.7‰, based on a 65/35% composition of ru- CAM or C4 relatives are known from Paleo- gene floras in the Bighorn Basin (including cy- minants and non-ruminants (see below), Ituri cads, the aquatic lycopod Isoetites, and pollen mammalian enamel accurately reflects mean similar to modern Chenopodiaceae; Wing et values for vegetation in the entire habitat and al. 1995; Wing and Harrington 2001), all pub- in microhabitats, with the exception of under- lished ␦13C values of dispersed organic matter, story browsers (Fig. 2). The mean value pre- fossil plants, and fossil mammals are consis- dicted for mammals from understory vegeta- tent with C3 vegetation. tion is ϳ2‰ higher than the actual value but 13 Natural variation of ␦ C values in C3 plants the latter is based on only three individuals, 286 ROSS SECORD ET AL.

an open C3 habitat with no appreciable un- derstory, such as a savanna or woodland. An intermediate range should be found in a fauna from an open canopy forest where trees are more closely spaced and lower irradiance causes greater 13C-depletion in the understory. Significance of differences in range can be approximated by comparing variance. Some data sets we compare are not normally dis- tributed, such as the Ituri fauna, and standard parametric tests of variance (e.g., F-test, Lev- 13 FIGURE 2. Comparison of mean ␦ CE values for mam- mals from the Ituri closed canopy forest (open circles) ene’s test) are sensitive to small departures with mean values predicted for mammals from Ituri from normality. Thus, we use Conover’s vegetation (solid circles). Prediction is based on diet- (1999) nonparametric squared ranks test of enamel ␧* of 13.7‰ (see text). Canopy, Gaps (local open 13 areas), and Understory are microhabitats within the Itu- variance. We use species ␦ CE averages in or- ri forest. Note close agreement between actual and pre- der to meet the test assumption that data are 13 dicted mean ␦ CE values for all but understory. Primary independent within the samples being com- data are from Cerling et al. (2004). Solid diamonds show individuals. Error bars Ϯ 2 SD. pared. Predicting Carbon Isotope Values for Early one of which (dwarf antelope; Neotragus batesi) Eocene Mammals had unusually low values (Ϫ26.0‰, Ϫ25.2‰). We develop a model to predict expected 13 13 The range of ␦ CE values is also important, ␦ CE values in herbivores from a variety of but harder to predict. In to reflect the early Eocene habitats. Far more ␦13C values full range of ␦13C values in a flora, some in- have been published for extant plants than for dividuals would have to feed exclusively on extant mammals, and no study has reported 13 vegetation at both extremes. Because most ␦ CE values for a diverse fauna from an open herbivores exploit a variety of vegetation, canopy forest. Thus, we use data from extant however, we expect that the range in mam- plants. We normalize values for modern veg- mals will be smaller than that of the flora. This etation to parameters for the early Eocene of was demonstrated in a recent study that the Bighorn Basin. Factors that need to be con- showed decreased variance in ␦13C from pro- sidered are (1) diet-enamel enrichment; (2) the ducer to consumer (Bump et al. 2007). In the effects of latitude and altitude on ␦13C values 13 Ituri Forest, the ranges of ␦ CE values for can- in vegetation; and (3) changes in the compo- 13 opy, gap (open area), or canopy and gap feed- sition of atmospheric CO2 (␦ CA) between the ers are all considerably smaller than predicted early Eocene and present. by the flora (using 4 SD, capturing 95% of the Diet-Enamel Enrichment. The large-bodied variability), but for the fauna as a whole (all component of many modern faunas is domi- data, Fig. 2) the actual range is greater than nated by ruminants, for which ␧*diet-enamel ϭ predicted. This is due partly to the extremely 14.1 Ϯ 0.5‰ (1 SD) for individuals larger than 13 low ␦ CE values in N. batesi, which suggest it 5 kg (Cerling and Harris 1999). Other mam- was eating vegetation ϳ3‰ lower than any mals have a lower ␧*, probably caused by low- sampled. If N. batesi is removed, the range is er methane production (Passey et al. 2005), 13 smaller than predicted. The range of ␦ CE val- which is correlated to body mass (p Ͻ 0.001, r ues in the Ituri fauna (ϳ12‰) probably rep- ϭ 0.73; data from Langer 2002; both variables resents a maximum that will only be found in logged). Our faunas are composed primarily

C3 habitats with extreme heterogeneity, such of non-artiodactyls, and therefore non-rumi- as closed canopy tropical rain forests. The nants. A controlled diet study found that pigs 13 range of ␦ CE values in the combined canopy and rabbits, which bracket the body sizes of and gap feeders (ϳ4‰) should be similar to, most species in our Eocene assemblages, had or slightly less than, what would be found in ␧* ϭ 13.3 Ϯ 0.3‰ and ␧* ϭ 12.8 Ϯ 0.7‰, re- STABLE ISOTOPES IN EARLY EOCENE MAMMALS 287 spectively (Passey et al. 2005). Thus, we use a temperature gradient in the early Eocene was mean non-ruminant ␧*diet-enamel of 13.1‰. lower than today, the difference in MAT Latitude Correction. A mean increase of would have been greater at higher latitudes. ϳ0.3‰/10Њ latitude in ␦13C values occurs in Taking into account that surface area decreas- leaves receiving the same amount of light and es from the equator to the poles, we calculate water (our calculation; data from Ko¨rner et al. that global MAT was ϳ7ЊC warmer in the ear- 1991). Ko¨rner et al. attributed the effect to air ly to middle Eocene (data from Greenwood temperature. To compensate, we normalized and Wing 1995; Fricke and Wing 2004). Be- modern data to 37ЊN, where present-day cause of higher MAT, fractionation between mean annual temperature (MAT) in the south- dissolved carbon in the ocean and atmospher- ern United States is similar to that estimated ic carbon would have been ϳ0.8‰ lower for the Bighorn Basin at the time of our assem- (based on Mook 1986; Lynch-Stieglitz et al. 13 blages (average MAT ϳ14ЊC; Fig. 1). 1995). This translates to a 0.6‰ greater ␦ CA Altitude Correction. A mean increase in value in the early Eocene, when the 0.2‰ tem- ␦13C values of ϳ1.2 Ϯ 0.90‰/km of elevation poral difference in foraminifers is subtracted. was found in 12 plant species sampled at dif- Of greater impact is a decrease of ϳ1.5‰ in 13 ferent elevations (Ko¨rner et al. 1988: Table 3). the ␦ CA value caused by industrialization However, when mean values for a much wider over the last two centuries (Friedli et al. 1986). diversity of C3 species are considered, the in- Taken together these effects imply that the 13 crease is considerably less (ϳ0.65‰/km [our ␦ CA value at the time of our assemblages was calculation]; data from Ko¨rner et al. 1988). ϳ2.1‰ more positive than today. Fricke and Wing (2004: p. 627) estimated an An additional factor is the influence that the 2Ϫ early Eocene paleoelevation of 0.6–1.3 km carbonate ion concentration ([CO3 ]) of ma- (mean ϭ 0.95 km) for intermontane basins in rine water has on ␦13C values in foraminiferal Wyoming on the basis of modern lapse rates tests. Spero et al. (1997) showed that in two and MAT estimates from leaf-margin analy- species of planktonic foraminifera ␦13C values ses. Accordingly, we normalized modern data in tests were negatively correlated with 2Ϫ 2Ϫ to a mean elevation of 0.95 km using the more [CO3 ]. Recent studies suggest that [CO3 ] conservative rate of 0.65‰/km. was lower in the Eocene (Tyrrell and Zeebe Atmosphere Correction. Changes in atmo- 2004; Locklair and Lerman 2005), which could 13 13 13 spheric ␦ C (␦ CA) can be estimated from the have resulted in higher ␦ C values in tests. 2Ϫ tests of fossil foraminifers because they incor- Lower [CO3 ] was presumably the result of porate dissolved carbon with predictable frac- higher atmospheric CO2 concentrations (Tyr- tionation from vital effects (Koch et al. 1995; rell and Zeebe 2004). Although this suggests 2Ϫ Passey et al. 2002). Carbon is rapidly ex- that a correction for the [CO3 ] effect is need- changed between the atmosphere and surface ed, there is considerable variability in the ocean in near equilibrium. In turn, surface amount of effect in the two modern species and deep waters are mixed over centuries or studied, and the effect has not been docu- millennia (Sundquist 1993). Thus, benthic for- mented in benthic species. Also problematic is aminifers should be in approximate isotopic the observation that the massive influx of CO2 equilibrium with the atmosphere. We used (ϳ4500 Gt) into the atmosphere during the Pa- data compiled by Zachos et al. (2001) from leocene/Eocene boundary thermal maximum two lineages of benthic foraminifers, adjusted (PETM) appears to have had little effect on the 13 for vital effects, to calculate changes in ␦ CA. magnitude of the carbon isotope excursion in

The mean foraminifer value at the time of our foraminifers. An increase in CO2 should cause 2Ϫ assemblages is only 0.2 Ϯ 0.5‰ more negative a decrease in [CO3 ], resulting in an increase than the mean pre-industrial value for the last in ␦13C values in foraminifers and a muting of 1000 years. the carbon isotope excursion (CIE). The mut- 13 Fractionation between ␦ CA values and the ing should be greater in planktonic species, surface ocean is, however, moderately tem- but the magnitude of the CIE is actually lower perature dependent. Because the latitudinal in benthic species (Bowen et al. 2004). For 288 ROSS SECORD ET AL. these reasons we refrain from correcting for riched (Ayliffe and Chivas 1990; Iacumin and 2Ϫ [CO3 ], which would result in lower predict- Longinelli 2002; Balasse et al. 2003; Levin et al. ed ␦13C values for early Eocene mammals. 2006). This is due to enrichment of leaf water The sum of latitudinal, altitudinal, and at- in dry climates where evapotranspiration mospheric differences results in adjustments rates are high and to enrichment of the mam- of ϩ1.5 to ϩ2.9‰ in ␦13C values of modern mal’s body water through physiological pro- 18 vegetation. After applying an enrichment fac- cesses. Levin et al. (2006) showed that ␦ OE 13 tor of 13.1‰, the predicted mean ␦ CE value increased with increasing aridity in evapora- for early Eocene herbivores in a closed canopy tion-sensitive (ES) species, but not in evapo- forest is ՅϪ14‰, and for an open canopy is ration-insensitive (EI) ones. They quantified between Ϫ10‰ and Ϫ13.5‰, with values in the magnitude of enrichment between ES and the upper range (ՆϪ12‰) representing dry, EI species (␧ES-EI) and suggested that it could open forest, woodland, or savanna, and values be used to predict differences in water deficit in the lower range (ՅϪ13‰) representing me- among habitats. sic and/or dense open canopy forest. Values Of the early Eocene mammals studied here, for individual understory browsers in a closed Coryphodon is the best EI candidate because it canopy forest range from Ϫ18‰ to Ϫ23‰, was probably semiaquatic or at least closely and for confident recognition must be associated with water (see below). However, it ϽϪ17‰. is unclear which, if any, of the fossil species were aridity sensitive. Thus, we refrain from Isotopic Differences Among Species using the ␧ES-EI relationship to estimate water Isotopic differences in ␦13C values among deficit. We note, however, that the overall 18 mammalian herbivores can result from feed- range of ␦ OE values in a fauna should in- ing in different microhabitats or on different crease in arid regions because of 18O-enrich- foods. Fruit, seeds, flowers, or bark generally ment in ES taxa, and compare the range of 13 18 have higher mean ␦ C values than leaves from ␦ OE assemblage values with those in the the same habitat. Young, tender leaves also modern faunas considered by Levin et al. have higher ␦13C values than mature leaves (2006). (e.g., Sobrado and Ehleringer 1997). Mean ␦13C values were 1.4–1.7‰ lower for leaves than for Results 13 18 other organs in C3 plants from tropical rain Figure 3 shows mean ␦ CE and ␦ OE values forest and savanna biomes, and canopy fruit for mammal species in the lower and upper exposed to direct sunlight had some of the assemblages. Isotopic values and descriptive highest values (Cerling et al. 2004; Codron et statistics are reported in the Appendix and in 13 al. 2005). Thus, we expect folivores to have Table 1, respectively. The range of ␦ CE values 13 lower mean ␦ CE values than frugivores. Low is slightly smaller in the lower assemblage, but 18 values are also expected for mammals feeding the range of ␦ OE values is nearly identical in 13 18 in microhabitats that are dark or where water both. Variance in neither ␦ CE nor ␦ OE val- is easily available to vegetation, such as ripar- ues, however, is significantly different be- ian and paludal settings. tween the assemblages (p ϭ 0.16, p ϭ 0.20, re- Oxygen isotopes in mammals reflect the spectively). 18 13 18 ␦ O composition of local surface and plant Mean ␦ CE and ␦ OE values increased by water, with varying degrees of 18O-enrichment 0.3‰ and 1.6‰, respectively, from the lower (e.g., Bryant and Froelich 1995; Kohn 1996). to upper assemblage, but only the increase in 18 18 Oxygen isotope values in tooth enamel (␦ OE) ␦ OE values was significant with 95% confi- in carnivorous mammals and species that are dence (t-tests: p ϭ 0.08 and p Ͻ 0.001, respec- 18 associated with water and are obligate drink- tively). The increase in ␦ OE values is consis- ers have been shown to reflect meteoric water tent with the direction expected from ␦18O val- values, whereas values in species that derive a ues in hematite sampled from the same strati- significant portion of water from leaves and/ graphic sections, which should track surface or live in arid regions are sometimes 18O-en- water values (Bao et al. 1999). It is also consis- STABLE ISOTOPES IN EARLY EOCENE MAMMALS 289

13 18 FIGURE 3. Mean ␦ CE and ␦ OE values for mammals from the upper and lower fossil assemblages. Error bars 1 SD. Species without bars are represented by a single sample, except Didelphodus (upper assemblage), which is based on three combined samples. Artiodactyls are shown as circles and perissodactyls as squares (tapiroids black, equids white background). Shaded areas show regions within which mean values for artiodactyl or perissodactyl species 13 occur. Note close grouping and elevated mean ␦ CE values for Artiodactyla in upper assemblage.

tent with the increase in MAT inferred from TABLE 1. Descriptive statistics for ␦13C and ␦18O values leaf-margin analyses (Fig. 1) (Wing et al. from lower (LA) and upper (UA) assemblages. 2000). Oxygen isotopes in modern precipita- 13 18 ␦ C(vPDB) ␦ O(vSMOW) tion are strongly correlated to MAT at mid LA UA LA UA and high latitudes (Dansgaard 1964; Kohn n 33 52 33 52 and Welker 2005), and although the slope of 18 Mean Ϫ13.2 Ϫ13.0 19.7 21.4 the ␦ Ometeoric water/MAT relationship may Minimum Ϫ15.0 Ϫ14.8 16.8 18.6 have been different in the past (Boyle 1997), a Maximum Ϫ12.1 Ϫ11.3 21.7 23.7 Range 3.0 3.5 4.9 5.0 positive correlation is still expected. SD 0.74 0.83 1.28 1.21 Body mass is significantly negatively cor- SE 0.13 0.11 0.22 0.17 13 related with mean ␦ CE values for species in Ϫ Ϫ Ϫ Ϫ Skew 0.55 0.13 0.69 0.33 the upper assemblage, using either parametric 290 ROSS SECORD ET AL.

13 FIGURE 4. Comparison of mean ␦ CE values. A, Predicted mean values for diverse mammal faunas from various habitats (solid circles) and microhabitats (solid squares). B, Data from fossil assemblages (open circles). Note that fossil means plot in the area expected for an open canopy fauna. Predictions are based on ␦13C values from modern floras, normalized to 37ЊN latitude, 950 m elevation, corrected for a 2.1‰ difference in atmospheric ␦13C values, and a diet-enamel ␧* of 13.1‰ (see text). ‘‘Wet’’ and ‘‘dry’’ refer to water availability for vegetation. Error bars show 95% confidence of mean (Ϯ1.96 SE). CC, closed canopy; OC, open canopy. Data sources: a, Cerling et al. (2004); b, Ehleringer et al. (1987); c, Codron et al. (2005); d, Yan et al. (1999); e, Mooney et al. (1989).

(Fisher’s least significant difference [LSD]; p ϭ canopy forest, and 95% confidence intervals 0.006; r ϭ Ϫ0.65) or nonparametric tests do not overlap with closed forest. Figure 5 (Spearman’s rank correlation; p ϭ 0.001, r ϭ shows the predicted range of ␦13C values for Ϫ0.75) (Body size transformed to natural log various habitats and microhabitats. The range 13 in both). The Shapiro-Wilk test suggests nor- of ␦ CE values in both fossil assemblages is mal distribution (p Ն 0.24), but even if possi- narrower than expected for closed canopy for- ble body mass outliers (Coryphodon and Didel- est, but consistent with open forest. No indi- phodus) are removed, correlation is still signif- vidual fossil values are in the area predicted icant at ␣ ϭ 0.05. Body mass is also signifi- for closed canopy understory browsers cantly negatively correlated with ␦13C values (Յ17‰). The distribution of fossil values is in the lower assemblage (Fisher’s LSD; p ϭ nearly even (skew ϭ Ϫ0.55, Ϫ0.13; Table 1); 0.02, r ϭ Ϫ0.66). The correlation is only mar- this contrasts with the Ituri fauna, which has ginally significant using Spearman’s rank cor- a long left tail resulting from understory relation (p ϭ 0.08, r ϭ Ϫ0.55), but the Shapiro- browsers (skew ϭ Ϫ1.65). Wilk test suggests normal distribution (p Ն We perform a pairwise comparison of all 0.29). Body mass is negatively correlated with genera and orders represented by three or 18 ␦ OE values in both assemblages, but neither more isotope values in each assemblage using correlation is significant (all data; lower: p ϭ analysis of variance tests (ANOVA). Fisher’s 0.38, r ϭ Ϫ0.30; upper: p ϭ 0.78, r ϭ Ϫ0.07; LSD test indicates that many taxa have signif- 13 18 without body mass outliers; lower: p ϭ 0.25, r icantly different mean ␦ CE and ␦ OE values ϭ Ϫ0.40; upper: p ϭ 0.37, r ϭ Ϫ0.25; all tests (Table 2, Fig. 6). However, this test does not parametric). adjust for the large number of pairs being con- 13 Figure 4 compares mean ␦ CE values for the sidered (e.g., Sokal and Rohlf 1997). A com- fossil assemblages with those predicted for parison of 11 genera (55 pairs) will result in a early Eocene herbivorous faunas feeding in false indication of significance for 2.8 pairs at 13 various habitats. Mean ␦ CE values for both ␣ ϭ 0.05 using standard ANOVAs. Thus, we fossil assemblages plot in the area for open also apply Tukey’s post hoc test, which adjusts STABLE ISOTOPES IN EARLY EOCENE MAMMALS 291

13 FIGURE 5. Comparison of ␦ CE ranges. A, Predicted ranges for various early Eocene habitats (means: solid circles) and microhabitats (means: solid squares). Predicted range is Ϯ1.6 SD of floral range (see text). B, Actual ranges for fossil assemblages (solid diamonds) and the modern Ituri fauna (solid triangles) (means: open circles). Note how 13 range narrows from closed to open habitats and that no fossil ␦ CE value plots in the range for unequivocal un- derstory browsers (shaded area). See Figure 4 for normalization of mean values for plants and Ituri mammals, and for abbreviations. for number of pairs, and find that several pairs Regarding (1), the tapiroid perissodactyls remain significant. In the upper assemblage, and Coryphodon had moderate to well-devel- 13 ␦ CE values in Artiodactyla are significantly oped shearing lophs, which are used in elevated above those in Perissodactyla and modern mammals to slice mature leaves and Pantodonta (p ϭ 0.004, 0.002, respectively), other tough vegetation with high fiber content and marginally above Primates (p ϭ 0.08). At (e.g., Collinson and Hooker 1991). The equoid the generic or specific level Diacodexis is sig- perissodactyls also had shearing lophs, al- nificantly elevated above Coryphodon (p ϭ though weakly developed. This implies that 0.009), and Bunophorus is marginally higher (p almost half of the individuals sampled were ϭ 0.07). Differences in ␦18O values were mar- capable of masticating at least some mature ginally significant for Artiodactyla-Pantodon- leaves. Moreover, young leaves are softer and ta (p ϭ 0.08). Using Tukey’s test for the lower easier to digest than mature leaves, and would assemblage, we find no pairs with significant- have been available to an even wider range of ly different ␦13C values, but Primates and Can- species. Young leaves have ␦13C values ϳ2‰ 18 tius have significantly higher ␦ OE values higher than mature leaves of the same plant than Perissodactyla and Homogalax, respec- (Sobrado and Ehleringer 1997), but mammals tively (p ϭ 0.06, 0.04). eating young leaves in a closed canopy un- derstory should still record an understory sig- Discussion nal. Interpretation of Forest Structure. Our inter- Regarding (2), we targeted taxa that were pretation relies on several assumptions that most likely to be understory browsers, judg- warrant discussion: (1) the assemblages con- ing from dental morphology and body size. A tained mammals capable of consuming un- total of 85 individuals were sampled, repre- derstory leaves; (2) the numbers of species and senting 11 species in the lower assemblage individuals sampled were great enough to de- and 17 the upper one. Because some species tect understory browsers; and (3) results were may have browsed in both closed and open not biased by diagenetic alteration or time av- habitats, as do some modern species (e.g., Hy- eraging. lochoerus meinertzhageni), the number of indi- 292 ROSS SECORD ET AL.

13 18 TABLE 2. Matrices of pairwise probabilities of mean differences in ␦ CE and ␦ OE values among higher level taxa (A, B, E, F) and genera (C, D, G, H) in upper and lower assemblages. Values shown in bold indicate significance for ␣ Յ 0.05. using Fisher’s least significant difference test. Pairs that were significant using Tukey’s post hoc test are indicated by asterisks (*␣ Յ 0.10, **␣ Յ 0.05).

13 A. ␦ CE 1 2 3 4 5 6 Upper Assemblage 1. Artiodactyla 1.000 2. Condylarthra 0.060 1.000 3. Pantodonta 0.001** 0.014 1.000 4. Perissodactyla 0.001** 0.292 0.040 1.000 5. Primates 0.008* 0.214 0.274 0.516 1.000 6. Tillodontia 0.065 0.658 0.079 0.799 0.495 1.000

18 B. ␦ OE 1 2 3 4 5 6 Upper Assemblage 1. Artiodactyla 1.000 2. Condylarthra 0.104 1.000 3. Pantodonta 0.008* 0.156 1.000 4. Perissodactyla 0.025 0.928 0.128 1.000 5. Primates 0.928 0.232 0.030 0.163 1.000 6. Tillodontia 0.607 0.521 0.084 0.435 0.635 1.000

13 C. ␦ CE 1 2 3 4 5 6 7 8 9 10 11 Upper Assemblage 1. Bunophorus 1.000 2. Cantius 0.050 1.000 3. Coryphodon 0.002* 0.269 1.000 4. Diacodexis 0.454 0.009 0.001** 1.000 5. 0.356 0.331 0.043 0.112 1.000 6. Esthonyx 0.217 0.489 0.078 0.057 0.776 1.000 7. Heptodon 0.236 0.328 0.031 0.047 0.913 0.834 1.000 8. Hyopsodus 0.647 0.132 0.010 0.238 0.627 0.431 0.503 1.000 9. Phenacodus 0.180 0.554 0.095 0.044 0.700 0.920 0.747 0.372 1.000 10. Protorohippus 0.120 0.696 0.139 0.026 0.558 0.762 0.585 0.270 0.840 1.000 11. Systemodon 0.081 0.717 0.127 0.013 0.496 0.705 0.511 0.212 0.786 0.956 1.000

18 D. ␦ OE 1 2 3 4 5 6 7 8 9 10 11 Upper Assemblage 1. Bunophorus 1.000 2. Cantius 0.938 1.000 3. Coryphodon 0.014 0.030 1.000 4. Diacodexis 0.695 0.802 0.021 1.000 5. Eohippus 0.046 0.084 0.630 0.072 1.000 6. Esthonyx 0.541 0.633 0.083 0.752 0.202 1.000 7. Heptodon 0.013 0.032 0.852 0.020 0.742 0.095 1.000 8. Hyopsodus 0.195 0.285 0.190 0.303 0.418 0.571 0.222 1.000 9. Phenacodus 0.215 0.297 0.235 0.324 0.474 0.567 0.277 0.964 1.000 10. Protorohippus 0.104 0.164 0.404 0.161 0.722 0.352 0.479 0.665 0.717 1.000 11. Systemodon 0.988 0.930 0.017 0.703 0.055 0.550 0.017 0.213 0.231 0.117 1.000

13 E. ␦ CE 1 2 3 4 5 6 Lower Assemblage 1. Artiodactyla 1.000 2. Condylarthra 0.226 1.000 3. Pantodont 0.189 0.724 1.000 4. Perissodactyla 0.141 0.795 0.852 1.000 5. Primates 0.462 0.653 0.492 0.484 1.000 6. Tillodontia 0.504 0.665 0.508 0.514 0.983 1.000 STABLE ISOTOPES IN EARLY EOCENE MAMMALS 293

TABLE 2. Continued

18 F. ␦ OE 1 2 3 4 5 6 Lower Assemblage 1. Artiodactyla 1.000 2. Condylarthra 0.929 1.000 3. Pantodont 0.948 0.867 1.000 4. Perissodactyla 0.214 0.122 0.186 1.000 5. Primates 0.248 0.136 0.277 0.006* 1.000 6. Tillodontia 0.866 0.772 0.917 0.148 0.328 1.000

13 G. ␦ CE 1 2 3 4 5 6 7 8 9 Lower Assemblage 1. Arenahippus 1.000 2. Cantius 0.481 1.000 3. Cardiolophus 0.901 0.581 1.000 4. Coryphodon 0.976 0.517 0.890 1.000 5. Diacodexis 0.176 0.487 0.233 0.215 1.000 6. Esthonyx 0.504 0.984 0.595 0.532 0.528 1.000 7. Homogalax 0.728 0.773 0.823 0.735 0.360 0.772 1.000 8. Hyopsodus 0.626 0.877 0.721 0.643 0.428 0.870 0.900 1.000 9. Phenacodus 0.885 0.594 0.985 0.876 0.240 0.607 0.837 0.734 1.000

18 H. ␦ OE 1 2 3 4 5 6 7 8 9 Lower Assemblage 1. Arenahippus 1.000 2. Cantius 0.022 1.000 3. Cardiolophus 0.513 0.103 1.000 4. Coryphodon 0.293 0.270 0.660 1.000 5. Diacodexis 0.328 0.242 0.713 0.947 1.000 6. Esthonyx 0.244 0.321 0.582 0.916 0.863 1.000 7. Homogalax 0.176 0.002** 0.067 0.037 0.043 0.030 1.000 8. Hyopsodus 0.270 0.293 0.623 0.961 0.908 0.955 0.033 1.000 9. Phenacodus 0.422 0.133 0.886 0.758 0.813 0.675 0.052 0.719 1.000 viduals sampled may be more important than lineages during the CIE associated with the the number of species. Also important is the Paleocene/Eocene boundary (Koch et al. 1995; number of individuals with lophodont denti- Fricke et al. 1998). ␦13C values in carbonate pa- tions, which we assume were most likely to leosol nodules (Bowen et al. 2001) and organic consume leaves. We sampled a total of 46 in- carbon (Magioncalda et al. 2004) from the dividuals with lophodont or proto-lophodont same stratigraphic interval as the teeth also dentitions, 27 in the lower assemblage and 19 decrease in the CIE. All of these materials ob- in the upper one. These individuals represent tain their primary ␦13C signal from plants, 13 species with body masses Ն5 kg (online ap- leaving little doubt that ␦ CE values preserve pendix) that are thought to have been terres- a primary shift in atmospheric CO2 composi- trial, except for possibly Esthonyx (Rose 2001). tion (Koch et al. 2003). Paleosol carbonate nod- With regard to diversity, the lower and upper ules bracketing our assemblages provide ad- assemblages contain nine and 19 species, re- ditional evidence. Unaltered paleosol carbon- spectively, that exhibit some degree of lopho- ate is enriched in 13C through pedogenic pro- donty. We sampled six (67%) and nine (47%) cesses by ϳ15‰, relative to local vegetation of the most common of these species, respec- (Koch 1998). Thus, paleosol nodules should be 13 tively. This sample size should be more than ϳ2‰ higher than mean ␦ CE values, assum- adequate to detect understory browsers. ing an enrichment factor of 13‰ for mam- Regarding (3), the strongest evidence that mals. These values should converge through 13 13 primary ␦ CE values are preserved in the diagenesis. Mean ␦ C values in paleosol car- Willwood Formation is a consistent decrease bonates stratigraphically bracketing our as- 13 of ϳ3–4‰ in ␦ CE values in three mammal semblages (localities D1200, D1493, D1289 for 294 ROSS SECORD ET AL.

semblages. The range of variability is narrow in both assemblages (Table 1, Fig. 5), however, and the range in the lower assemblage, which is from a thicker stratigraphic interval and was collected over a larger geographic area (see ‘‘Methods’’), is narrower than that in the upper one. This is consistent with the smaller number of species and specimens sampled, but not with the idea that temporal or spatial averaging increased the range of isotopic val- ues. Our results suggest that early Eocene for- ests in the Bighorn Basin had an open canopy. The faunal assemblages plot closest to the val- ues predicted from a subtropical open canopy forest in southern China (Ehleringer et al. 1987) and a riparian microhabitat in a South African savanna (Fig. 4) (Codron et al. 2005). The former receives monsoonal rainfall, with ϳ200 cm mean annual precipitation (MAP). Rainfall in the Bighorn Basin may also have been seasonal (Bown and Kraus 1981; Kraus and Riggins 2007). The southern China open

13 18 forest is a localized patch with a well-exposed FIGURE 6. Values of ␦ CE and ␦ OE for higher-level taxa in the fossil assemblages. Symbols indicate means; error understory, adjacent to closed canopy forest. bars show 95% confidence (Ϯ1.96 SE). Artiodactyla and Although this is essentially a microhabitat, Pantodonta have, respectively, the highest and lowest 13 Ehleringer et al. (1987) sampled a high diver- ␦ CE values in both assemblages. See Table 2 for signif- icance matrix. sity of plants species (n ϭ 23) and forms, and the mean value should be comparable to that of a more regional mesic open forest. The ri- lower; D1162, D1250, D1204 for upper; ␦13C parian microhabitat had the lowest mean ␦13C data from Koch et al. 2003) are 2.8‰ and 2.6‰ value of any in the savanna. The area receives 13 13 higher, respectively, than ␦ CE mean faunal low MAP (30–50 cm). The low ␦ C values values. This agrees well with expectations for were attributed to high water availability due unaltered enamel. to close proximity to a perennial water source Our interpretation of canopy structure does (Codron et al. 2005, p. 1765). Because many of not appear to have been affected by temporal the mammals used in our study occur in or spatial averaging. The home ranges of the floodplain deposits, it is probable that they in- species sampled would probably have been habited riparian areas where water was also less than 1 km2, according to body size, except readily available to plants. Thus, part of the for Coryphodon, which would have been ϳ6 ␦13C signal may be related to riparian micro- km2 (Jetz et al. 2004). Time-averaging, how- habitat. Although riparian areas typically 13 ever, could have substantially increased this have dense vegetation, mean ␦ CE assemblage area. A fauna derived from multiple strati- values would be expected to be even lower if graphic levels might sample different micro- a closed canopy had been present. Moreover, habitats or habitats, effectively increasing the no individual values are low enough to sug- sampling area. Climate variation through the gest feeding in a closed canopy understory sampled interval could also result in mixing (Fig. 5). mammals from different climatic regimes. In An open canopy is also suggested by the 13 any of these cases the effect would be to in- even distribution and narrow range of ␦ CE crease overall isotopic variability in the as- values. Both of these parameters are indepen- STABLE ISOTOPES IN EARLY EOCENE MAMMALS 295 dent of changes in atmospheric ␦13C values. If leaf-area analyses, which suggest moderately a closed canopy had been present, we would high precipitation (ϳ120–140 cm) (Wilf 2000), expect a left tail on the overall distribution be- and a cenogram analysis, which suggests a cause of 13C-depletion in understory browsers humid climate in the early Eocene of the Big- (Fig. 5, Ituri fauna). Instead, the distributions horn Basin (Gunnell 1997). are almost even, especially in the upper as- Resource Partitioning and Microhabitats. The 13 semblage, which contains the greatest num- negative correlation between ␦ CE values and bers of species and individuals. The range of body mass suggests that larger species con- values is lower than expected for a closed can- sumed a greater portion of leaves than smaller opy forest, which should have greater hetero- ones. This is consistent with body size/diet geneity between the understory and other mi- relationships in extant mammals and with di- crohabitats (e.g., Cerling et al. 2004). Variance ets inferred from dental morphology. Small- in both fossil assemblages is significantly low- bodied mammals have higher metabolic rates er than in the Ituri fauna (p Ͻ 0.001). Thus, all and therefore need to consume foods that are parameters in both assemblages are consistent rich in nutrients and digest rapidly, such as with a dense open canopy forest where water fruit and insects. These foods are often not is readily available to vegetation. abundant enough to support large mammals, A mesic habitat with high relative humidity which must adapt to foods that take longer to 18 is also suggested by the narrow range of ␦ OE digest and have lower nutrient content, such values in both assemblages (4.9, 5.0‰, Table as mature leaves. Richard (1985: Fig. 5.13) de- 1). The range is smaller than in any of the scribed a general correspondence between modern faunas with a comparable sample size body size and diet in euprimates. For body reported by Levin et al. (2006). Of these, Nak- mass: insectivores Ͻ frugivores/insectivores 18 uru has the lowest range of ␦ OE values Ͻ frugivores/folivores Ͻ folivores. Because (6.8‰, vSMOW) and a low water deficit (448 fruit is more 13C-enriched than leaves (Codron mm). However, only six species were sampled, et al. 2005), a negative correlation between 13 and additional species may increase the range. body size and ␦ CE values might result. The Ituri (including all 22 species from Cerling et correlation would be enhanced if larger-bod- al. 2004) had the lowest water deficit (Ϫ80 ied species were consuming aquatic vegeta- 18 mm) but a moderately high ␦ OE range (10‰, tion, or feeding on leaves in darker and/or vSMOW). However, the range is more than wetter areas, such as riparian or paludal mi- doubled by two extreme positive outliers of crohabitats. 13 Colobus. The high values in these folivorous Differences in mean ␦ CE values among monkeys suggest dependence on an evaporat- taxa are significant only in the upper assem- ed water source, such as leaves (Cerling et al. blage (Tukey’s posthoc test; Table 2). This may 18 2004). Without Colobus the range of ␦ OE val- be because of the smaller sample size, greater ues is 4.4‰ (vSMOW, n ϭ 35), which is only time-averaging, or greater homogeneity in the slightly below that of the fossil assemblages. lower assemblage. Although time-averaging In contrast, less diverse samples from regions does not appear to have influenced our inter- with greater water deficits show a greater pretation of canopy structure, it could have in- 18 range of ␦ OE values. For example, Mpala (12 creased intraspecific variability (as discussed species, n ϭ 105), Tsavo (12 species, n ϭ 128), above), thereby decreasing the ability to dis- and Turkana (9 species, n ϭ 40) have water def- tinguish among taxa. In spite of the reduced 18 icits of 751, 1059, 1588 mm and ␦ OE ranges ability to distinguish among taxa in the lower of 11.0, 10.5, and 14.3‰, respectively (data assemblage, Artiodactyla and Pantodonta from Levin et al. 2006). Because of outliers, (i.e., Coryphodon) occupy the same relative po- differing sample size, and differing species di- sitions in both assemblages. In both assem- versity, it is difficult to evaluate these differ- blages, Artiodactyla has the highest mean 13 ences statistically. Nevertheless, an interpre- ␦ CE value of any herbivore order, and Pan- 13 tation of a mesic, humid habitat for the assem- todonta has the lowest (Fig. 6). Mean ␦ CE val- blages is consistent with other proxies, such as ues in Artiodactyla are elevated above those in 296 ROSS SECORD ET AL.

Perissodactyla by 1.0‰ in the upper assem- frugivore and is equivalent to those of foli- blage (p ϭ 0.004, Table 2, Figs. 3, 6). Possible vores (Figs. 3, 6). Although this could be ex- reasons for this are a higher diet-enamel ␧* in plained by a preference for fruit with lower Artiodactyla, differences in diet, and/or dif- than average ␦13C values, it is more probably ferences in microhabitat. the result of a lower enrichment factor in Can- Higher ␧* in modern ruminant artiodactyls tius. Cerling et al. (2004) calculated ␧* ϭ 12.8 appears to be caused by greater methanogen- Ϯ 0.6‰ for primates in the Ituri Forest, which esis associated with rumination (Passey et al. is only slightly lower than the 13.1‰ that we 2005). Rumination provides a way of extract- assumed for our assemblages. However, the ing nutrients from vegetation high in cellu- Ituri primates are anthropoids, are larger lose, such as mature leaves and unripe fruit. (mean ϳ16 kg), and are considerably more de- This is advantageous for large-bodied mam- rived than Cantius. Cantius was a basal pro- mals, but mammals the size of Diacodexis and simian, similar to modern lemurs, and Hexacodus (Յ2 kg, Appendix) would probably weighed only ϳ2–4 kg (Appendix). Also, no gain no advantage by ruminating (e.g., Janis study has calculated ␧* for primates on a con- 1976; Demment and Van Soest 1985). Diet is a trolled diet. Thus, the primate ␧* from the Itu- 13 more plausible cause for the higher ␦ CE val- ri Forest may not be valid for Cantius. 18 ues in the artiodactyls. The artiodactyls had The high ␦ OE values in Cantius, especially bunodont dentition, a condition that is not in the lower assemblage, suggest that it ob- adapted to chewing tough vegetation. Thus, tained water from an evaporated source. Ex- they would have selected foods with less fiber tant arboreal primates often avoid drinking and higher nutrient content, such as fallen ground water and instead obtain a large por- fruit or berries, buds, shoots, young leaves, tion of their water from arboreal cisterns and and flowers. These items are generally en- foods with high water content, such as fruit riched in 13C above leaves from the same plant (Jolly 1985). Water in arboreal cisterns is likely 13 18 by ϳ1.4–1.7‰. Hence, higher ␦ CE values are to have elevated ␦ O values caused by evap- expected in artiodactyls if the perissodactyls oration, and fruit water is also enriched in 18O were consuming mature leaves, as suggested relative to ground water (Dunbar and Wilson by their dentition. Microhabitat preference 1983). Thus, avoidance of ground water is a 18 could also be a factor if perissodactyls pre- plausible explanation for the high ␦ OE values ferred more closed, and/or more poorly in Cantius. 13 drained areas than artiodactyls. The postcra- Coryphodon has the lowest mean ␦ CE val- nial morphology of early Eocene perissodac- ues in both assemblages and is by far the larg- tyls is most similar to that of modern est taxon (ϳ600 kg). It is often considered (Janis 1984), which often prefer riparian hab- semiaquatic (e.g., Simons 1960: p. 70). Its low 13 itats, frequently spend time in water or mud, ␦ CE values are consistent with feeding on and may consume semiaquatic plants (e.g., plants that lose little water from evapotrans- Nowak and Paradiso 1983). These habits piration, such as aquatic vegetation or plants 13 should result in low ␦ CE values, and al- around the periphery of a river or pond. More- though isotopes in modern tapirs are poorly over, aquatic mammals are expected to have 18 known, Neogene tapirs have some of the low- lower variability in ␦ OE values than terres- est ␦13C values among contemporaneous spe- trial mammals (Clementz and Koch 2001), and cies. (MacFadden and Cerling 1996; Mac- Coryphodon in the upper assemblage has the Fadden et al. 1996; Koch et al. 1998). Thus, lowest variability of any taxon in this study both diet and microhabitat are plausible con- (Figs. 3, 6). This is strong support for a semi- 13 tributors to the difference in ␦ CE values be- aquatic interpretation. Teeth sampled from the tween Perissodactyla and Artiodactyla. upper assemblage were collected on different The arboreal primate Cantius is widely occasions from a single locality, and they ap- thought to have been a frugivore, on the basis pear to represent at least two individuals. of its dentition (e.g., Covert 1995). However, its Even for a single individual, however, the var- 13 18 mean ␦ CE value is lower than expected for a iability in ␦ OE values would be markedly STABLE ISOTOPES IN EARLY EOCENE MAMMALS 297 low. Variability is considerably higher in Cor- onomic groups is high in both assemblages 13 yphodon in the lower assemblage and at other and differences in ␦ CE values are significant localities (Fricke et al. 1998), probably reflect- only in the better sampled upper assemblage. ing differences among water bodies. Variabil- In the upper assemblage, the negative corre- 13 ity in surface water is caused primarily by sea- lation between body size and ␦ CE values is sonal fluctuations in the ␦18O values of precip- probably the result of larger species consum- itation. In continental habitats seasonal vari- ing greater portions of leaves, and smaller ation is muted most in lakes or large ponds species more fruit and seeds. The significantly 13 and to varying degrees in rivers, depending elevated ␦ CE values in Artiodactyla suggest on input from groundwater (e.g., Dutton et al. that fallen fruit, berries, and/or seeds were an 2005). Thus, the low variability of Coryphodon important component of their diet. Differenc- in the upper assemblage suggests that these es between Artiodactyla and Perissodactyla individuals inhabited large ponds or large may also have been amplified by a preference rivers with low seasonal fluctuation. in the latter for microhabitats with high water Didelphodus plots as an outlier from the availability to plants. The amount of resource main cluster in the upper assemblage by hav- partitioning in the ungulates at this stage of ing the lowest mean ␦18O value, but a high ␦13C evolution appears to have been small, but value (Fig. 3; mean based on combined sam- studies of diverse modern faunas from C3 hab- ples from three individuals). Its tribosphenic itats are needed for comparison. This study molar design and small body size (160 g) in- demonstrates that even in mammalian faunas dicate that Didelphodus had an insectivorous of great antiquity resource partitioning can be diet (e.g., Gunnell et al. 1995). Very little is recognized by using stable isotopes. known about isotopes in insectivores, but one study suggested that insectivores should have Acknowledgments ␦18 lower O values than herbivores (Sponhei- We thank K. D. Rose for providing access to ␦18 mer and Lee-Thorp 2001). The low OE value specimens; B. J. MacFadden, P. L. Koch, and in Didelphodus is consistent with this predic- two anonymous reviewers for helpful com- tion. ments; L. L. Wingate and K. C. Lohmann at Conclusions the University of Michigan Stable Isotope Lab for mass spectrometry analysis; W. Boykins, S. The carbon isotope composition of tooth J. Jabo, and P. Kroehler for help with sampling enamel from both fossil assemblages suggests and preparation equipment; T. M. Bown for that early Eocene forests in the Bighorn Basin Hyopsodus measurements; J. W. M. Thompson, had an open canopy. Although some authors S. P. Zach, R. J. Emry, and R. W. Purdy for help have suggested that Eocene forests had a with specimens; and F. Marsh for help with ␦13 closed canopy, no single CE value is low software. Funding was provided by a Smith- enough to suggest feeding in the understory sonian Institution fellowship to R. Secord and of a closed canopy forest, even though we by the Evolution of Terrestrial Ecosystems sampled a large diversity of potential under- (ETE) group (this is ETE publication number story browsers. The range and distribution of 106). 13 ␦ CE values in the herbivores, both of which are independent of changes in the composi- Literature Cited tion of atmospheric CO2, are also consistent 13 Ayliffe, L. K., and A. R. Chivas. 1990. Oxygen isotope compo- with an open canopy. Mean ␦ CE values fall at sition of the bone phosphate of Australian kangaroos: poten- the low end of the range expected for open tial as a palaeoenvironmental recorder. Geochimica et Cos- canopy forests, suggesting dense vegetation mochimica Acta 54:2603–2609. Balasse, M., A. B. Smith, S. H. Ambrose, and S. R. Leigh. 2003. and/or high water availability to plants. Val- Determining sheep birth seasonality by analysis of tooth ues compare closely to a riparian microhabi- enamel oxygen isotope ratios: the Late Stone Age site of Kas- tat, suggesting that many of these mammals teelberg (South Africa). Journal of Archaeological Science 30: 205–215. consumed vegetation near rivers or ponds. Bao, H. M., P. L. Koch, and D. I. Rumble. 1999. Paleocene-Eocene Overlap of isotopic values among most tax- climatic variation in western North America: evidence from 298 ROSS SECORD ET AL.

the ␦18O of pedogenic hematite. Geological Society of America Conover, W. J. 1999. Practical nonparametric statistics, 3d ed. Bulletin 111:1405–1415. Wiley, New York. Boisseriea, J.-R., A. Zazzo, G. Merceron, C. Blondel, P. Vignaud, Covert, H. H. 1995. Locomotor adaptations of Eocene primates: A. Likius, H. T. Mackaye, and M. Brunet. 2005. Diets of mod- adaptive diversity among the earliest prosimians. Pp. 495– ern and late Miocene hippopotamids: evidence from carbon 509 in L. Alterman, G. A. Doyle, and M. K. Izard, eds. Crea- isotope composition and micro-wear of tooth enamel. Palaeo- tures of the dark: the nocturnal prosimians. Plenum, New geography, Palaeoclimatology, Palaeoecology 221:153–174. York. Bowen, G. J., P. L. Koch, P. D. Gingerich, R. D. Norris, S. Bains, Dansgaard, W. 1964. Stable isotopes in precipitation. Tellus 16: and R. M. Corfield. 2001. Refined isotope stratigraphy across 436–468. the continental Paleocene-Eocene boundary on Polecat Bench Demment, M. W., and P. J. Van Soest. 1985. A nutritional expla- in the northern Bighorn Basin. Pp. 73–88 in Gingerich 2001b. nation for body-size patterns of ruminant and nonruminant Bowen, G. J., D. J. Beerling, P. L. Koch, J. C. Zachos, and T. Quat- herbivores. American Naturalist 125:641–672. tlebaum. 2004. A humid climate state during the Paleocene/ Dunbar, J., and A. T. Wilson. 1983. Oxygen and hydrogen iso- Eocene thermal maximum. Nature 432:495–499. topes in fruit and vegetable juices. Plant Physiology 72:725– Bown, T. M., and M. J. Kraus. 1981. Lower Eocene alluvial pa- 727. leosols (Willwood Formation, northwest Wyoming, U.S.A.) Dutton, A., B. H. Wilkinson, J. M. Welker, G. J. Bowen, and K. C. and their significance for paleoecology, paleoclimatology, and Lohmann. 2005. Spatial distribution and seasonal variation in basin analysis. Palaeogeography, Palaeoclimatology, Palaeoe- 18O/16O of modern precipitation and river water across the cology 34:1–30. conterminous USA. Hydrological Processes 19:4121–4146. ———. 1993. Time-stratigraphic reconstruction and integration Ehleringer, J. R., C. B. Field, Z.-F. Lin, and C.-Y. Kuo. 1986. Leaf of paleopedologic, sedimentologic, and biotic events (Will- carbon isotope and mineral composition in subtropical plants wood Formation, Lower Eocene, Northwest Wyoming, along an irradiance cline. Oecologia 70:520–526. U.S.A.). Palaios 8:68–80. Ehleringer, J. R., Z. F. Kin, C. B. Field, G. C. Sun, and C. Y. Kuo. Bown, T. M., K. D. Rose, E. L. Simons, and S. L. Wing. 1994. Dis- 1987. Leaf carbon isotope ratios of plants from a subtropical tribution and stratigraphic correlation of upper Paleocene and monsoon forest. Oecologia 72:109–114. lower Eocene fossil mammal and plant localities of the Fort Fricke, H. C., and S. L. Wing. 2004. Oxygen isotope and paleo- Union, Willwood, and Tatman formations, southern Bighorn botanical estimates of temperature and ␦18O-latitude gradi- Basin, Wyoming. U.S. Geological Survey Professional Paper ents over North America during the early Eocene. American 1540:1–103. Journal of Science 304:612–635. Boyle, E. A. 1997. Cool tropical temperatures shift the global Fricke, H. C., W. C. Clyde, J. R. O’Neil, and P. D. Gingerich. 1998. ␦18O-T relationship: an explanation for the ice core ␦18O-bore- Evidence for rapid climate change in North America during hole thermometry conflict? Geophysical Research Letters 24: the latest Paleocene thermal maximum: oxygen isotope com- 273–276. positions of biogenic phosphate from the Bighorn Basin (Wy- Broadmeadow, M. S. J., and H. Griffiths. 1993. Carbon isotope oming). Earth and Planetary Science Letters 160:193–208.

discrimination and the coupling of CO2 fluxes within forest Friedli, H., H. Lo¨ tscher, H. Oeschger, U. Siegenthaler, and B. canopies. Pp. 109–129 in J. R. Ehleringer, A. E. Hall, and G. D. Stauffer. 1986. Ice core record of the 13C/12C ratio of atmo-

Farquhar, eds. Stable isotopes and plant carbon-water rela- spheric CO2 in the past two centuries. Nature 324:237–238. tions. Academic Press, San Diego. Gingerich, P. D. 1983. Paleocene-Eocene faunal zones and a pre- Bryant, J. D., and P. N. Froelich. 1995. A model of oxygen isotope liminary analysis of Laramide structural deformation in the fractionation in body water of large mammals. Geochimica et Clark’s Fork Basin, Wyoming. Wyoming Geological Associa- Cosmochimica Acta 59:4523–4537. tion Guidebook 34:185–195. Bump, J. K., K. Fox-Dobbs, J. L. Bada, P. L. Koch, R. O. Peterson, ———. 2001a. Biostratigraphy of the continental Paleocene-Eo- and J. A. Vucetich1. 2007. Stable isotopes, ecological integra- cene boundary interval on Polecat Bench in the northern Big- tion and environmental change: wolves record atmospheric horn Basin. Pp. 37–72 in Gingerich 2001b. carbon isotope trend better than tree rings. Proceedings of the ———. 2001b. Paleocene-Eocene stratigraphy and biotic change Royal Society of London B 274:2471–2480. in the Bighorn and Clarks Fork basins, Wyoming. University

Cerling, T. E., and J. R. Ehleringer. 2000. Welcome to the C4 of Michigan Papers on Paleontology 33. world. In R. A. Gastaldo and W. A. DiMichele, eds. Phanero- Greenwood, D. R., and S. L. Wing. 1995. Eocene continental cli- zoic terrestrial ecosystems. Paleontological Society Papers 6: mates and latitudinal temperature gradients. Geology 23: 273–286. 1044–1048. Cerling, T. E., and J. M. Harris. 1999. Carbon isotope fraction- Gunnell, G. F. 1997. Wasatchian-Bridgerian (Eocene) paleoecol- ation between diet and bioapatite in ungulate mammals and ogy of the western interior of North America: changing pa- implications for ecological and paleoecological studies. Oec- leoenvironments and taxonomic composition of omomyid ologia 120:347–363. (Tarsiiformes) primates. Journal of Human Evolution 32:105– Cerling, T. E., J. A. Hart, and T. B. Hart. 2004. Stable isotope ecol- 132. ogy in the Ituri Forest. Oecologia 138:5–12. Gunnell, G. F., M. E. Morgan, M. C. Maas, and P. D. Gingerich. Clementz, M. T., and P. L. Koch. 2001. Differentiating aquatic 1995. Comparative paleoecology of Paleogene and Neogene mammal habitat and foraging ecology with stable isotopes in mammalian faunas—trophic structure and composition. Pa- tooth enamel. Oecologia 129:461–472. laeogeography, Palaeoclimatology, Palaeoecology 115:265– Codron, J., D. Codron, J. A. Lee-Thorp, M. Sponheimer, W. J. 286. Bond, D. D. Ruiter, and R. Grant. 2005. Taxonomic, anatomi- Hanba, Y. T., S. Mori, T. T. Lei, T. Koike, and E. Wada. 1997. Var- cal, and spatio-temporal variations in the stable carbon and iations in leaf ␦13C along a vertical profile of irradiance in a nitrogen isotopic compositions of plants from an African sa- temperate Japanese forest. Oecologia 110:253–261. vanna. Journal of Archaeological Science 32:1757–1772. Heaton, T. H. E. 1999. Spatial, species, and temporal variations 13 12 Collinson, M. E., and J. J. Hooker. 1991. Fossil evidence of inter- in the C/ C ratios of C3 plants: implications for paleodiet actions between plants and plant-eating mammals. Philo- studies. Journal of Archaeological Science 26:637–649. sophical Transactions of the Royal Society of London B 333: Iacumin, P., and A. Longinelli. 2002. Relationship between ␦18O 197–208. values for skeletal apatite from reindeer and foxes and yearly STABLE ISOTOPES IN EARLY EOCENE MAMMALS 299

mean ␦18O values of environmental water. Earth and Plane- Lynch-Stieglitz, J., T. F. Stocker, W. S. Broecker, and R. D. Fair- tary Science Letters 201:213–219. banks. 1995. The influence of air-sea exchange on the isotopic Janis, C. M. 1976. The evolutionary strategy of the and composition of oceanic carbon: observations and modeling. the origins of rumen and cecal digestion. Evolution 30:757– Global Biogeochemical Cycles 9:653–665. 774. MacFadden, B. J., and T. E. Cerling. 1996. Mammalian herbivore ———. 1984. Tapirs as living fossils. Pp. 80–86 in N. Eldredge communities, ancient feeding ecology, and carbon isotopes: a and S. Stanley, eds. Living fossils. Springer, New York. 10 million-year sequence from the Neogene of Florida. Journal ———. 2000. Patterns in the evolution of herbivory of large ter- of Vertebrate Paleontology 16:103–115. restrial mammals: the Paleogene of North America. Pp. 168– MacFadden, B. J., T. E. Cerling, and J. Prado. 1996. Cenozoic ter- 222 in H.-D. Sues, ed. Evolution of herbivory in terrestrial ver- restrial ecosystem evolution in Argentina: evidence from car- tebrates. Cambridge University Press, Cambridge. bon isotopes of fossil mammal teeth. Palaios 11:319–327. Jetz, W., C. Carbone, J. Fulford, and J. H. Brown. 2004. The scal- Magioncalda, R., C. Dupuis, T. Smith, E. Steurbaut, and P. D. ing of space use. Science 306:266–268. Gingerich. 2004. Paleocene-Eocene carbon isotope excursion Jolly, A. 1985. The evolution of primate behaviour, 2d ed. Mac- in organic carbon and pedogenic carbonate: direct compari- millan, New York. son in a continental stratigraphic section. Geology 32:553– Koch, P. L. 1998. Isotopic reconstruction of past continental en- 556. 13 vironments. Annual Review of Earth and Planetary Sciences Mook, W. G. 1986. C in atmospheric CO2. Netherlands Journal 26:573–613. of Sea Research 20:212–223. Koch, P. L., J. C. Zachos, and D. L. Dettman. 1995. Stable isotope Mooney, H. A., S. H. Bullock, and J. R. Ehleringer. 1989. Carbon stratigraphy and paleoclimatology of the Paleogene Bighorn isotope ratios of plants of a tropical dry forest in Mexico. Basin (Wyoming, USA). Palaeogeography, Palaeoclimatology, Functional Ecology 3:137–142. Palaeoecology 115:61–89. Nowak, R. M., and J. L. Paradiso. 1983. Walker’s mammals of the Koch, P. L., N. Tuross, and M. L. Fogel. 1997. The effects of sam- world. Johns Hopkins University Press, Baltimore. ple treatment and diagenesis on the isotopic integrity of car- Ogg, J. G., and A. G. Smith. 2004. The geomagnetic polarity time bonate in biogenic hydroxylapatite. Journal of Archaeological scale. Pp. 63–86 in F. M. Gradstein, J. G. Ogg, and A. G. Smith, Science 24:417–429. eds. A geologic time scale 2004. Cambridge University Press, Koch, P. L., K. A. Hoppe, and D. S. Webb. 1998. The isotopic ecol- Cambridge. ogy of late Pleistocene mammals in North America, Part 1. Passey, B. H., T. E. Cerling, M. E. Perkins, M. R. Voorhies, J. M. Florida. Chemical Geology 152:119–138. Harris, and S. T. Tucker. 2002. Environmental change in the Koch, P. L., W. C. Clyde, R. P. Hepple, M. L. Fogel, S. L. Wing, Great Plains: an isotopic record from fossil horses. Journal of and J. C. Zachos. 2003. Carbon and oxygen isotope records Geology 110:123–140. from paleosols spanning the Paleocene-Eocene boundary, Passey, B. H., T. F. Robinson, L. K. Ayliffe, T. E. Cerling, M. Spon- Bighorn Basin, Wyoming. In S. L. Wing, P. D. Gingerich, B. heimer, M. D. Dearing, B. L. Roeder, and J. R. Ehleringer. 2005.

Schmitz, and E. Thomas, eds. Causes and consequences of Carbon isotope fractionation between diet, breath CO2, and globally warm climates in the early Paleogene. Geological So- bioapatite in different mammals. Journal of Archaeological ciety of America Special Paper 369:1–9. Science 32:1459–1470. Kohn, M. J. 1996. Predicting animal ␦18O: accounting for diet Richard, A. F. 1985. Primates in nature. W. H. Freeman, New and physiological adaptation. Geochimica et Cosmochimica York. Acta 60:4811–4829. Rose, K. D. 2001. Compendium of Wasatchian mammal po- Kohn, M. J., and J. M. Welker. 2005. On the temperature corre- stcrania from the Willwood Formation. Pp. 157–183 in Gin- lation of ␦18O in modern precipitation. Earth and Planetary gerich 2001b. Science Letters 231:87–96. Schankler, D. 1980. Faunal zonation of the Willwood Formation Ko¨ rner, C., G. D. Farquhar, and Z. Roksandic. 1988. A global in the central Bighorn Basin, Wyoming. Pp. 99–114 in Gin- survey of carbon isotope discrimination in plants from high gerich 2001b. altitudes. Oecologia 74:623–632. Secord, R., P. D. Gingerich, M. E. Smith, W. C. Clyde, P. Wilf, and Ko¨ rner, C., G. D. Farquhar, and S. C. Wong. 1991. Carbon iso- B. S. Singer. 2006. Geochronology and mammalian biostratig- tope discrimination by plants follows latitudinal and altitu- raphy of middle and upper Paleocene continental strata, Big- dinal trends. Oecologia 88:30–40. horn Basin, Wyoming. American Journal of Science 306:211– Kraus, M. J., and S. Riggins. 2007. Transient drying during the 245. Paleocene–Eocene Thermal Maximum (PETM): analysis of Simons, E. L. 1960. The Paleocene Pantodonta. Transactions of paleosols in the Bighorn Basin, Wyoming. Palaeogeography, the American Philosophical Society 50:1–99. Palaeoclimatology, Palaeoecology 245:444–461. Smith, M. E., B. Singer, and A. Carroll. 2004. Reply: 40Ar/39Ar Langer, P. 2002. The digestive tract and life history of small geochronology of the Eocene Green River Formation, Wyo- mammals. Mammal Review 32:107–131. ming. Geological Society of America Bulletin 116:253–256. Lee-Thorp, J. A., and N. J. van der Merwe. 1987. Carbon isotope Sobrado, M. A., and J. R. Ehleringer. 1997. Leaf carbon isotope analysis of fossil bone apatite. South African Journal of Sci- ratios from a tropical dry forest in Venezuela. Flora 192:121– ence 83:712–715. 124. Legendre, S. 1986. Analysis of mammalian communities from Sokal, R. R., and J. F. Rohlf. 1997. Biometry: the principles and the late Eocene and Oligocene of southern France. Palaeo- practice of statistics in biological research, 3d ed. W. H. Free- vertebrata 16:191–212. man, New York. Levin, N. E., T. E. Cerling, B. H. Passey, J. M. Harris, and J. R. Spero, H. J., J. Bijma, D. W. Lea, and B. E. Bemis. 1997. Effect of Ehleringer. 2006. A stable isotope aridity index for terrestrial seawater carbonate concentration on foraminiferal carbon environments. Proceedings of the National Academy of Sci- and oxygen isotopes. Nature 390:497–500. ences USA 103:11201–11205. Sponheimer, M., and J. A. Lee-Thorp. 2001. The oxygen isotope Locklair, R. E., and A. Lerman. 2005. A model of Phanerozoic composition of mammalian enamel carbonate from Morea Es- cycles of carbon and calcium in the global ocean: evaluation tate, South Africa. Oecologia 126:153–157. and constraints on ocean chemistry and input fluxes. Chem- Stewart, G. R., M. H. Turnbull, S. Schmidt, and P. D. Erskine. ical Geology 217:113–126. 1995. 13C natural abundances in plant communities along a 300 ROSS SECORD ET AL.

rainfall gradient: a biological integrator of water availability. ———. 1993. Variability of carbon isotope fractionation during Australian Journal of Plant Physiology 22:51–55. photosynthesis. Pp. 29–46 in J. R. Ehleringer, A. E. Hall, and Stro¨ mberg, C. A. E. 2004. Using phytolith assemblages to re- G. D. Farquhar, eds. Stable isotopes and plant carbon-water construct the origin and spread of grass-dominated habitats relations. Academic Press, San Diego. in the great plains of North America during the late Eocene Wilf, P. 2000. Late Paleocene-early Eocene climate changes in to early Miocene. Palaeogeography, Palaeoclimatology, Pa- southwestern Wyoming: paleobotanical analysis. Geological laeoecology 207:239–275. Society of America Bulletin 112:292–307. Su, Y., C. H. Langmuir, and P. D. Asimow. 1999–2002. PetroPlot, Wing, S. L., and G. J. Harrington. 2001. Floral response to rapid a plotting and data management tool set for Microsoft Excel. warming in the earliest Eocene and implications for concur- Distributed by the authors through Lamont-Doherty Earth rent faunal change. Paleobiology 27:539–563. Observatory, Columbia University. http://www.petdb.org/ Wing, S. L., T. M. Bown, and J. D. Obradovich. 1991. Early Eo- petdbWeb/search/PetroPlot/index.html cene biotic and climatic change in interior western North Sundquist, E. T. 1993. The global carbon dioxide budget. Science America. Geology 19:1189–1192. 259:934–941. Wing, S. L., J. Alroy, and L. J. Hickey. 1995. Plant and mammal Tyrrell, T., and R. E. Zeebe. 2004. History of carbonate ion con- diversity in the Paleocene to early Eocene of the Bighorn Ba- centration over the last 100 million years. Geochimica et Cos- sin. Palaeogeography, Palaeoclimatology, Palaeoecology 115: mochimica Acta 68:3521–3530. 117–155. Upchurch, G. R., Jr., and J. A. Wolfe. 1987. Mid- to Wing, S. L., H. M. Bao, and P. L. Koch. 2000. An early Eocene early Eocene vegetation and climate: evidence from fossil cool period? Evidence for continental cooling during the leaves and woods. Pp. 75–105 in E. M. Friis, W. G. Chaloner, warmest part of the Cenozoic. Pp. 197–237 in B. T. Huber, K. and P. R. Crane, eds. The origins of angiosperms and their bi- MacLeod, and S. L. Wing, eds. Warm climates in Earth his- ological consequences. Cambridge University Press, Cam- tory. Cambridge University Press, Cambridge. bridge. Wing, S. L., G. J. Harrington, F. A. Smith, J. I. Bloch, D. M. Boyer, van der Merwe, N. J., and E. Medina. 1989. Photosynthesis and and K. H. Freeman. 2005. Transient floral change and rapid 13C/12C ratios in Amazonian rain forests. Geochimica et Cos- global warming at the Paleocene-Eocene boundary. Science mochimica Acta 53. 310:993–996. ———. 1991. The canopy effect, carbon isotope ratios and food- Yan, C.-R., X.-G. Han, L.-Z. Chen, J.-H. Huang, and B. Su. 1999. webs in Amazonia. Journal of Archaeological Science 18:249– Foliar ␦13C within temperate deciduous forest: its spatial 259. change and interspecies variation. Acta Botanica Sinica 40: Van Houten, F. B. 1945. Early Cenozoic facies in the Rocky 853–859. Mountain Region. Science 101:430–431. Zachos, J., M. Pagani, L. Sloan, E. Thomas, and K. Billups. 2001.

Vogel, J. C. 1978. Recycling of CO2 in a forest environment. Oec- Trends, rhythms, and aberrations in global climate 65 Ma to ologia Plantarum 13:89–94. present. Science 292:686–693.