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....