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Research Article

Dinosaur Fossils Predict Body Temperatures

  • James F Gillooly mail,

    To whom correspondence should be addressed. E-mail: gillooly@zoo.ufl.edu

    Affiliation: Department of Zoology, University of Florida, Gainesville, Florida, United States of America

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  • Andrew P Allen,

    Affiliation: National Center for Ecological Analysis and Synthesis, Santa Barbara, California, United States of America

    X
  • Eric L Charnov

    Affiliations: Department of Biology, University of New Mexico, Albuquerque, New Mexico, United States of America, Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon, United States of America

    X
  • Published: July 11, 2006
  • DOI: 10.1371/journal.pbio.0040248

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On Dinosaur Thermophysiology: Comment on "Dinosaur Fossils Predict Body Temperature"

Posted by plosbiology on 07 May 2009 at 22:14 GMT

Author: gilles escarguel
Position: Dr.
Institution: UMR CNRS 5125 Universite Claude Bernard Lyon 1
E-mail: Gilles.Escarguel@univ-lyon1.fr
Additional Authors: Christophe Lecuyer, Romain Amiot, Eric Buffetaut
Submitted Date: July 30, 2006
Published Date: August 3, 2006
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

Contrasting with the conclusion reached by Gilooly et al., we recently published strong biogeochemical evidence that many Cretaceous dinosaurs were endothermic, with mean predicted body temperatures of 36 to 37 degrees Celsius, independent of their body size, geographic context and phylogenetic position. Our approach relies on the analysis of the oxygen isotope compositions of phosphate from biogenic apatites of 26 Cretaceous dinosaur species (including ornithopods, marginocephalians, sauropods and theropods) ranging from 10 to 9,000 kg and coming from 16 fossil localities worldwide located between N10 and N60 in paleolatitude. At paleolatitudes lower than N45, all dinosaur oxygen isotope values are significantly greater (indicating lower body temperatures) than values obtained for turtles and crocodiles from the same localities; the converse is observed at high paleolatitudes, making the inferred latitudinal gradient of difference between dinosaurs and ectothermic reptiles oxygen isotope values not significantly different from the one observed between extant mammals and ectothermic reptiles.
Our study is fundamentally supported by the temperature-dependence of the oxygen isotope fractionation between phosphate and water, a thermodynamic property applicable at any time and place on Earth. It is also based on two, highly robust working hypotheses:
- Cretaceous crocodiles and turtles shared the same ectothermic condition as extant ones
- the measured oxygen isotope compositions did not undergo diagenetic alterations.
Our analysis clearly supports the hypothesis that a 36 to 37 degrees Celsius endothermy was a widespread thermophysiologic condition in dinosaurs, invalidating the hypothesis of inertial homeothermy (gigantothermy) advocated by Gilooly et al. Based on the phylogenetic position of the 26 dinosaur species we analyzed, the most parsimonious hypothesis is that most, if not all dinosaurs were endothermic organisms, and were likely to have inherited this metabolic status from a Triassic last common ancestor. Ultimately, this could actually indicate that endothermy is the ancestral condition of archosauromorphs, implying that crocodilians (and possibly turtles) are derived ectotherms. This hypothesis is compatible with isochore structure evidence, physiological and developmental crocodilian traits, and molecular phylogenies; it now remains to be tested by isotopic studies of primitive, Permian and Triassic diapsid reptiles. The discrepancy between the two studies suggests that there is a systematic bias in the dinosaur body size (M) or maximum growth rate (G) estimates. Due to the greater uncertainties in G than M-estimates, and to its higher weighting in the body temperature estimation function, it is likely that the ontogenic growth curves used to estimate dinosaur G-values systematically underestimate this parameter, and thus overestimate the growth duration times (D) for dinosaurs lower than about 6,000 kg, and conversely overestimate G, and thus underestimate D for larger dinosaurs. Incidentally, such bias in the estimate of dinosaur G could well explain the atypically steep allometric relationship between dinosaur G and adult body mass; this strongly suggests that this scaling law might actually be close to the eutherian one, consistent with our biogeochemical estimates of dinosaur body temperatures. It is likely that this bias roots into the hypothesis of annual periodicity of the growth rings counted in bone histological sections.

Reference:

Amiot R, Lecuyer C, Buffetaut E, Escarguel G, Fluteau F, Martineau F. 2006. Oxygen isotopes from biogenic appetites suggest widespread endothermy in Cretaceous dinosaurs. Earth and Planetary Science Letters 246: 41-54.

No competing interests declared.