Microscale δ18O and δ13C isotopic analysis of an ontogenetic series of the hadrosaurid dinosaur Edmontosaurus: implications for physiology and ecology

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Abstract

Stable isotope analysis of vertebrate biominerals, primarily in mammals, has been used to address questions of paleodiet, paleoclimate, trophic level, migration, foraging zone, and thermophysiology with varying degrees of success. Isotopes have been used less commonly to study physiology and ecology through ontogeny in dinosaurs, generally due to (1) the lack of modern analogs as a basis for comparison of observed fossil isotope values, and (2) difficulty in sampling very thin tooth enamel (a non-remodeled biomineral). By utilizing a relatively new technique in microsampling, this study addresses the following questions: Do microscale analyses of oxygen and carbon isotopes from mineralized tissues of hadrosaurid dinosaurs record temporal variation? If so, is the cause of the variation physiological or ecological?

Isotope values from the carbonate component of enamel (δ18Oec and δ13Ce) were obtained by microsampling multiple teeth in a temporal series from the dental batteries of a juvenile, sub-adult, and adult Edmontosaurus from the Late Cretaceous Maastrichtian Hell Creek Formation of South Dakota. To establish isotope variability in an extant archosaur, consecutive teeth in a temporal series from an extant Alligator mississippiensis specimen were microsampled for isotopic analysis and compared to those of Edmontosaurus. To test for diagenesis, bulk samples from the phosphate component (δ18Op) of modern and fossil tooth enamel, bone, and dentine from Edmontosaurus, A. mississippiensis, and extant ratites were analyzed and compared. Edmontosaurus bone and dentine indicate a greater degree of alteration than does enamel, and while absolute δ18Oec values may be altered, the pattern of seasonal cycles appears to be preserved and can provide detailed information on hadrosaur physiology (tooth mineralization times, rates, and seasons) and ecology (dietary information).

δ18Oec seasonal patterns are preserved in Edmontosaurus specimens, and are interpreted to correlate with annual δ18O variation of local meteoric waters rather than thermophysiology, changes in drinking water sources, or migration. All teeth were mineralized in <0.65 year with no consistent season of mineralization. Mean tooth mineralization times are shorter in the juvenile and sub-adult than the adult. Enamel mineralization rates are estimated to be ∼38 mm/year in Edmontosaurus and ∼36 mm/year in Alligator (consistent with mineralization rates for modern ungulates), although the length of time for tooth formation is shorter in the archosaurs compared with mammals. Heavier than predicted δ13Ce values are hypothesized to result from (1) enrichment of δ13C in ingested plant material due to higher atmospheric δ13C (δ13Catm) in the Late Cretaceous; (2) taxon-specific δ13C effects of ingested plants (primarily gymnosperms); (3) isotopic enrichment of ingested plant material (δ13Cp) due to osmotic stress from proximity to the Western Cretaceous Interior Seaway, (4) taxon-specific δ13Cdietδ13Ce fractionation factors for Edmontosaurus that vary from those observed in modern mammals, and/or (5) diagenesis. Microsampling provides a detailed perspective on the physiological (tooth mineralization times, rates, and seasons) and ecological (dietary) mechanisms of oxygen and carbon isotope incorporation in dinosaur biominerals that is not obtainable through bulk sampling alone.

Introduction

Over the last 20 years stable isotope analysis of vertebrate biominerals has emerged as a powerful tool for investigating questions regarding environmental and physiological variation in extinct organisms, especially mammals Kolodny and Luz, 1991, Quade et al., 1992, Barrick and Showers, 1994, Bryant et al., 1994, Cerling and Sharp, 1996, Kolodny et al., 1996, Longinelli, 1996, MacFadden and Cerling, 1996, Koch et al., 1998, Sharp and Cerling, 1998, Feranec and MacFadden, 2000, Fricke and Rogers, 2000, Thomas and Carlson, 2001. Oxygen isotopes are used to study paleoenvironments (because they reflect seasonal variation in temperature and humidity, latitude, and precipitation) and physiology (because they undergo a temperature-dependent biological fractionation in animals; Fig. 1A). Carbon isotopes are valuable for determining ecological information such as diet, niche partitioning, and trophic level because they undergo differential fractionation during photosynthesis and therefore reflect plant type preferences in herbivores (reviewed in Koch, 1998, Kohn and Cerling, 2002; Fig. 1B).

Whereas stable isotope geochemistry is widely used to study fossil mammals, the same is not true of fossil, non-avian dinosaurs. Interpretation of geochemical data from dinosaur enamel is complicated by factors such as diagenetic alteration Nelson et al., 1986, Kolodny et al., 1996, Longinelli, 1996, Goodwin and Bench, 2000; difficulty in interpreting isotopic signals in a group of extinct and physiologically perplexing animals; the difficulty of sampling very thin tooth enamel (<200 μm, this study), which is the only permanent, non-remodeled mineralized vertebrate tissue available Noyes et al., 1938, Lowenstam and Weiner, 1989, Carlson, 1990; and the lack of a continuous long-term isotopic record from a single individual, because dinosaurs continually shed teeth throughout life Owen, 1840–45, Edmund, 1960. Consequently, it has been difficult for researchers using isotope techniques to reconstruct the physiology (e.g. changes in growth rate or timing in teeth) and ecology (e.g. diet) of dinosaurs over an extended time span of months to years, throughout ontogeny.

Although dinosaurs are known to have replaced their teeth many times throughout life, hadrosaurid (“duck-billed”) dinosaurs (including Edmontosaurus, the genus used in this study) possess a broad pavement of interlocking teeth called a dental battery (Fig. 2). The dental battery consists of up to five teeth stacked vertically in one tooth position (or column; Ostrom, 1961), with columns interlocking side by side. As teeth were worn, deeper rows erupted continuously to form an occlusal surface for grinding food. Enamel from successive tooth rows in this dental battery therefore represents a long-term non-remodeled surface suitable for isotopic analysis of growth rates.

In this study, we use stable isotopes to explore several questions related to dinosaur ontogeny. First, does hadrosaur tooth enamel retain a primary environmental seasonal oxygen isotopic composition—and if so, does this signal vary ontogenetically—within a single tooth, among teeth in one individual, and among individuals of different ages? Second, do oxygen and carbon isotopes from successive microscale samples of mineralized tissues of the Late Cretaceous hadrosaurid dinosaur Edmontosaurus vary seasonally? If so, is the cause of the variation physiological, ecological (seasonal), or diagenetic? Third, can oxygen isotopes be used to estimate tooth mineralization rates and tooth formation times in this genus, and is the season of mineralization consistent? Finally, does hadrosaur tooth enamel retain a primary carbon isotope signal, and if so, what dietary information can be obtained from δ13Ce? We analyze modern Alligator enamel in order to establish a baseline for comparison to the extinct dinosaurs. We also perform an analysis of diagenetic alteration by isotopic comparison of Edmontosaurus bioapatite to bioapatites from some of its closest living relatives, Struthio (Ostrich), Rhea, and Alligator.

Previous analyses of environmental (seasonal) patterns from microsampled enamel have focused on mammals to: (1) track environmental responses in modern enamel isotopes to changes in temperature, humidity, and precipitation Stuart-Williams and Schwarz, 1997, Fricke et al., 1998a, Lindars et al., 2001, and (2) apply knowledge of modern enamel isotope patterns to the fossil record in paleoclimate studies Koch et al., 1989, Koch et al., 1998, Fricke and O'Neil, 1996, Fricke et al., 1998b, Sharp and Cerling, 1998, Feranec and MacFadden, 2000. Previous isotope studies of dinosaur bone and enamel have focused on inter- and intra-bone and enamel isotopic variability to determine whether dinosaurs are endothermic or ectothermic Barrick and Showers, 1994, Barrick and Showers, 1995, Barrick and Showers, 1999, Barrick et al., 1996, Barrick et al., 1998, Fricke and Rogers, 2000.

This study represents one of the first attempts to conduct a micro-scale study of carbon and oxygen isotopes from a monogeneric dinosaurian ontogenetic series using a relatively new technique for incremental microsampling of very thin tooth enamel (see also Straight et al., this volume). As such, it provides a detailed ontogenetic perspective on cyclical oxygen and carbon isotope variations and their relationship to dinosaur physiology (tooth mineralization rates, tooth formation times, and season of tooth mineralization) and dietary preferences.

Section snippets

Bioapatite structure and diagenesis

Vertebrate bioapatites (bone, dentine, and enamel) are composed of Ca5(PO4)3OH (carbonate hydroxylapatite, or dahllite) mineralized on an organic framework (Lowenstam and Weiner, 1989). In vivo ionic substitutions are common in the bioapatite crystal lattice, including the substitution of carbonate (4–6% by weight) for phosphate LeGeros et al., 1967, McConnell, 1973, LeGeros, 1981, Carlson, 1990. Ionic substitutions can also occur postmortem in the form of diagenetic alteration by one of two

Specimens

Three maxillae with intact dental batteries were obtained from the Concordia Edmontosaurus bonebed in the Hell Creek Formation, located south of Morristown near the Grand River, in Corson County, South Dakota. The Hell Creek Formation represents Late Cretaceous (Maastrichtian) fluvial nonmarine sediments that were laid down on the western margin of the Western Cretaceous Interior Seaway. Sediments include sandstones, siltstones and mudstones representing channel and floodplain deposits

Oxygen isotopes from microsampled teeth

As mentioned, sampling increments in individual teeth may be used as a proxy for temporal variation (vertebrate teeth are mineralized from crown to root, so that crown enamel is oldest and enamel near the root is youngest; Fig. 2). Thus, increases and/or decreases in δ18Oec within a single tooth represent changes in the δ18O values of the inputs (ingested food or water) during the time of tooth mineralization (Fig. 1A). All teeth with >1 enamel sample from all three Edmontosaurus jaws show a

Evaluating the possibility of diagenetic alteration

Evaluation of diagenesis in fossil bioapatites is best accomplished by utilizing several types of analyses, each of which will independently provide evidence for or against alteration of original isotope values Rink and Schwarz, 1995, Iacumin et al., 1996, Kolodny et al., 1996, Barrick, 1998, Kohn et al., 1999, Sharp et al., 2000. No single test will conclusively indicate pristine preservation (Nelson et al., 1986). Support for at least partial diagenetic alteration of some of the bioapatites

Diagenetic alteration in Edmontosaurus

Analyses of diagenesis in fossil dinosaur enamel, bone (cortical and cancellous), and dentine show a pattern of differential preservation in these tissues. Enamel is likely less altered than are bone and dentine based on (1) preservation of micro-scale crystalline structure in enamel, (2) heavier values in Alligator δ18Oec enamel versus Edmontosaurus, which is predicted if fossil enamel is unaltered, and (3) heavier δ18O from bone and dentine than enamel, which is predicted if bone and dentine

Acknowledgements

We thank: R. Nellermoe for so graciously allowing us access to Edmontosaurus specimens; H. Spero, P. Koch and G. Erickson for helpful discussions and advice with analyses and interpretations; D. Weishampel and P. Higgins for their insightful reviews; M. Goodwin for access to the UCMP collections and abundant help with specimen preparation; N. Kinler for supplying the Alligator specimen; P. Fitzgerald, G. Herbert, G. Jaecks, and I. Montanez for fruitful dialogue; D. Winter and S. Silva for

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