It's not quite as easy as putting thermometers under their tongues and waiting 30 seconds, but scientists have discovered a way to measure the average body temperature of animals that lived millions of years ago. The findings will help biologists learn more about extinct animals' physiology and give paleoclimatologists a powerful new tool for gauging ancient environmental temperatures.
Modern mammals are warm-blooded, meaning they maintain a body temperature of about 37˚C, whereas modern cold-blooded animals, such as reptiles, fish, and amphibians, tend to have lower body temperatures that often fluctuate depending on their environments. When this distinction first arose is unclear, though, and researchers have debated whether ancient animals such as dinosaurs and pterosaurs were warm- or cold-blooded.
"It's been a long and kind of fractious debate, really," says Robert Eagle, a geochemist at the California Institute of Technology in Pasadena.
For more than 50 years, scientists have tried to solve the puzzle by measuring the levels of isotopes—variants of elements with different atomic weights—in fossils. For example, the amount of the isotope oxygen-18 in biominerals such as calcite, aragonite, and apatite depends on an animal's body temperature. But the problem with oxygen-18, Eagle says, is that its level in these minerals is also influenced by other factors, such as its abundance in an animal's internal fluids. Because evidence of fluids from long-extinct creatures is only rarely preserved, scientists studying the fossilized minerals could only guess at how body temperature alone affected isotope levels.
Adapting a technique that had been used to determine temperature fluctuations in limestone, Eagle and his colleagues analyzed the isotope composition of the mineral bioapatite, which is found in bones and teeth. In bioapatite, the isotopes carbon-13 and oxygen-18 tend to stick together. But as body temperatures rise, those bonds fall apart. The researchers realized that by measuring the amount of carbon-13–oxygen-18 bonding in bioapatite, they could infer an extinct animal's average body temperature when it died.
Eagle's team members verified their technique by gauging the bonding in tooth enamel from modern elephants, rhinoceroses, crocodiles, alligators, and sharks. They found that their method's predictions matched the animals' actual body temperatures to within 3˚C. Then they tried the technique on enamel from two woolly mammoths, an extinct rhinoceros, and an extinct alligator, all of which date to about 12 million years ago. The vanished animals' predicted body temperatures closely matched those of their living modern relatives, giving the researchers confidence that the technique works. The team's report appears  online today in the Proceedings of the National Academy of Sciences.
"There is the potential then to go back further in time to the ancestors of birds and mammals and start trying to ask questions about at what point in time did warm-bloodedness evolve," Eagle says.
What's more, because cold-blooded animals' body temperatures reflect their environment, researchers looking into ancient climate conditions could use extinct animals as a kind of "paleothermometer," he says. "By making this measurement, you could say how hot was the ocean in the Cretaceous," which would help verify data geologists have collected through other means, Eagle says.
Henry Fricke, a geochemist at Colorado College in Colorado Springs, says the paper is a good proof-of-concept that the method works, at least for more recent fossils. "Having said that, working on more ancient systems, like dinosaurs, is a different animal," due to the fossils' longer exposure to environmental conditions that can muddle isotope readings, he adds.
Fricke will be collaborating with Eagle on an upcoming study to find out whether the model can reliably determine body temperatures for dinosaurs and other animals alive during that time. "The potential is there," he says, "and that's the exciting part."