A thermometer for modern and extinct vertebrates

Robert Eagle of Caltech and his collaborators have shown that they can determine the body temperature of living and long-dead vertebrates by measuring the abundance of a molecule made of isotopes—an isotopologue—in bones, scales, and teeth. The isotopologue is a heavy version of the carbonate ion CO₃ ^2-. In a typical piece of bone or other biomineral, all but 1.8% of the CO₃ ^2- ions are made of the lightest carbon and oxygen isotopes, ¹²C and ¹⁶O. At around 45 ppm, ¹³C¹⁸O¹⁶O₂ ^2- is barely present, but its scarcity is made up for by a useful property: The isotopologue’s precise abundance depends on the ambient temperature when the biomineral first crystallized. The temperature dependence arises because lower temperatures boost the propensity of a heavy isotope to form a bond with another heavy isotope rather than with a light isotope. Five years ago, Prosenjit Ghosh, who is now at the Indian Institute of Science, and his colleagues extracted CO₂ gas from carbonate crystals they’d made in the lab. From their measurements they derived a robust formula relating the abundance of ¹³C¹⁸O carbonate to its formation temperature. By applying the formula to tooth samples, Eagle could accurately predict the body temperature of five vertebrates, including the white rhino (37 °C) and the sand tiger shark (23 °C). From fossilized samples he could also predict the body temperature of the woolly mammoth (38 °C). Applying the paleothermometer to samples of other extinct vertebrates could reveal when vertebrates first became warm-blooded. (R. A. Eagle et al., Proc. Natl. Acad. Sci. USA 107 , 10377, 2010 http://dx.doi.org/doi:10.1073/pnas.0911115107 .)