BioEd Online

The 'clumping' of rare isotopes of carbon and oxygen in the bones and teeth of extinct animals offers a method for determining their body temperatures.

The discovery could provide key evidence in the debate surrounding the body temperature of dinosaurs, and might also allow researchers to glean information about the temperature of ancient oceans, say the authors of a paper published this week in Proceedings of the National Academy of Sciences.

When a mineral known as bioapatite forms in bone, teeth and scales, two heavy isotopes of carbon and oxygen — carbon-13 (₁₃C) and oxygen-18 (₁₈O) — tend to cluster together in the mineral lattice more often than would be expected by chance. But heat disrupts the process, scattering the atoms. "At 5 °C you'll get much more ₁₃C–₁₈O bonding than you would at 100 °C," says lead author Robert Eagle, a geochemist in the Division of Geological and Planetary Sciences at the California Institute of Technology in Pasadena.

To measure the clumping of ₁₃C and ₁₈O in bones and teeth, Eagle and his team used a method² for studying geological deposits pioneered by co-author John Eiler, also a geochemist at California Institute of Technology. This involves dissolving bits of teeth enamel, bone or shell in acid and running the resulting carbon dioxide gas through a mass spectrometer. The level of ₁₃C–₁₈O bonding in this gas reflects the degree to which these isotopes were clumped in the bioapatite.

Reading the bones

Eagle and colleagues first tested their findings on samples of teeth from living species, including an Indian elephant (Elephas maximus indicus) and a Nile crocodile (Crocodylus niloticus). "We discovered that we could reconstruct their body temperatures with about a one or two degree error," he says.

Reassured, they turned to teeth from the woolly mammoth (Mammuthus primigenius) dating back to the Late Pleistocene, finding that the animals had a body temperature of approximately 38 °C, "about what you would expect for a large mammal very similar to an elephant", says Eagle.

The scientists then applied their method to two older fossils: a 12-million-year-old rhinoceros and an equally old alligator. The authors determined body temperatures of 36.6 °C for the rhino and 30.4 °C for the alligator, which is comparable with the temperatures of their modern descendants.

Up to now, this is as far back as the team has looked. But it is only the beginning. "This has the potential to really answer some questions about the evolution of warm-bloodedness in birds and mammals and their ancestors," says Eagle. Not to mention dinosaurs. "That's going to be our next target," says Eagle. "Keep your eyes peeled for that in the next year or so."

Hot or cold?

Previous attempts to measure dinosaur body temperatures have relied solely on the abundance in their bones of ₁₈O. But that method is not as precise as the clumped-isotope method, Eagle says, because the level of ₁₈O in the bones varies with its level in the environment.

"There is no way we would ever know with any certainty what the ₁₈O composition of an extinct organism's blood would be," he says.

Isotope clumping, on the other hand, is independent of such variations.

Other scientists are impressed. The finding, says Michael Bender, a biogeochemist from Princeton University in New Jersey, is a "terrific contribution" that "conclusively demonstrates" that the method can be used to determine body temperatures. The main difficulty, he says, will be that it requires finding bones or teeth that have not been altered by subsequent geological processes that might have also changed their isotope clumping.

Like Eagle, he says that palaeontology is what everyone is initially going to be interested in. "The most obvious major application is to resolve the long-standing controversy over the extent to which different dinosaurs regulated their body temperatures," he says. "But some of the most exciting applications probably remain to be imagined."

Eagle says that the method has uses in other palaeoclimate studies. The body temperatures of fish and sharks are determined by the waters in which they live, and these temperatures will be reflected in their bones. "You can start looking at the geological past and say something about the oceans," says Eagle.