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Stable Isotopes in Early Eocene Mammals As Indicators of Forest Canopy Structure and Resource Partitioning

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Stable Isotopes in Early Eocene Mammals As Indicators of Forest Canopy Structure and Resource Partitioning Pakobzo/ogy, 34(2), 2008, pp. 282-300 Stable isotopes in early Eocene mammals 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 (S13C ) in early Eocene mammalian faunas to infer canopy structure. We compare S13C 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 813C 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 ofPaleobiology, 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 climate and biogeochemical cycles on a global similar to modern closed canopy tropical rain 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 ~ 120-140 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 (813C) 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 813C 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 (Wasatchian land-mammal 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 Willwood Formation 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 813C 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 ls lates (e.g., condylarths, tillodonts) and the margin analyses and 8 O values in hematite first representatives of the extant ungulate (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 Stromberg 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 (~22 m) over a greater geo- cializations such as hypsodonty and seleno- graphic area than the upper one (~4 m; thick- donty were rare. Thus, herbivores may have nesses assume that most localities sample an partitioned resources less and had more interval of ~4 m). However, although total broadly overlapping diets than younger fau- thickness for the lower assemblage is ~22 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 ~8 m. Sediment accumulation by using stable isotopes. rates for the upper and lower assemblages were ~215 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 ~37,000 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 ~1.5 km2. Another 15% are ogy and body size, are summarized in the Ap- from a ~1 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.si) 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 ~1 km2. many species, it is clear that most were Stable Isotope Conventions.—Stable isotope ground-dwelling ungulates. Exceptions are ratios are expressed using delta notation in Cantius, which is thought to have been arbo- units of parts per thousand (per mil, %o): 813C real, and Esthonyx, which had both arboreal or 8i»Q = {[R_pw/R,_d,,d] - 1)} -10%, where R and terrestrial adaptations. Didelphodus was = 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 = 180/160 for oxygen, and 284 ROSS SECORD ET AL. a- .> BIOZONE 700 Ash 52.5 Lambdotherium 52.59: (Wa-7) ±0.12 600 53.0 Heptodon (Wa-6) Upper 500 53.5 "Assemblage en (17.1°0 01 < Bunophorus 400 > (Wa-5) Lower • 54.0 Assemblage Upper (11.5°C) Haplomylus- 300 % 54.5 Ectocion (Wa-4) 200 O <u 55.0 Lower Haplomylus- 100 Ectocion 55.5 (Wa-1 toWa-3) 55.8 Copecion (Cf-3) 10 15 20 25 MAT (°C) leaf-margin analysis 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.
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