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

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 ( δ 18O ec and δ 13C e) 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 ( δ 18O p) 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 δ 18O ec 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). δ 18O ec seasonal patterns are preserved in Edmontosaurus specimens, and are interpreted to correlate with annual δ 18O 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 δ 13C e values are hypothesized to result from (1) enrichment of δ 13C in ingested plant material due to higher atmospheric δ 13C ( δ 13C atm) in the Late Cretaceous; (2) taxon-specific δ 13C effects of ingested plants (primarily gymnosperms); (3) isotopic enrichment of ingested plant material ( δ 13C p) due to osmotic stress from proximity to the Western Cretaceous Interior Seaway, (4) taxon-specific δ 13C diet– δ 13C e 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.