The great spine-chilling Tyrannosaurus rex has a reputation for having killed its prey in cold blood. But was this ancient dinosaur really a cold-blooded ectotherm?

Strong evolutionary links among reptiles (ectotherms), birds (mostly endothermic, or warm-blooded) and dinosaurs make it hard to conclude whether nonavian dinosaurs were also unable to regulate their own internal body temperatures.

A new method of studying the chemical bonds in a mineral found in the teeth and bones of animals might finally offer a way to settle the debate.

Researchers found that heavy isotopes of carbon and oxygen bond differently in the biological version of the mineral apatite (which is a main component of bones and teeth of animals, both living and long extinct). These rare isotopes are more prone to bond in big clumps at low temperatures, "therefore, if you can measure the clumping accurately enough, you can work out the temperature at which a mineral precipitated," explains Robert Eagle, a postdoctoral researcher in geochemistry at the California Institute of Technology's Division of Geological and Planetary Sciences, and lead author of the new study, which was published online May 24 in Proceedings of the National Academy of Sciences. "In the case of teeth and bone, this will be the body temperature of the organism."

From ancient seas
A similar way of examining ancient limestone and shells has helped researchers estimate ambient rock temperatures by looking at the bonding structures in calcium carbonate.

"Most people were focused on using this to look at climate change in the past and various geological problems like the heating and cooling of the Earth's crust," Eagle says. But that approach gave John Eiler, a professor of geology and geochemistry at Caltech and co-author of the study, the idea that the same analysis might be applied to the bio-apatite lingering in fossilized teeth or bones. And if it worked, the method "would then measure body temperatures of extinct vertebrates," Eagle says.

Eagle and his colleagues tested the method on a variety of living and extinct endothermic animals, including a tooth from a modern Indian elephant (Elephas maximus indicus), a tooth from a modern white rhinoceros (Ceratotherium simum), teeth from extinct late Pleistocene woolly mammoths (Mammuthus primigenius), and enamel from extinct Miocene rhinocerotid species. Their analysis proved to be accurate to 1 to 2 degrees Celsius for the modern animals, thereby lending a highly precise approximation for animals that died some 12 million years ago, such as the Miocene rhinocerotid.

Unknown origin
Finding out whether or not T. rex were ectotherms would not only sate curiosity, it would also shed light on the whole emergence of internal temperature regulation in animals, which, as the authors of the new study noted, is "among the most fundamental aspects of their biology." And currently, the emergence of warm-bloodedness "is a major physiological change that occurred at an unknown stage during the evolutionary transition to mammals and birds."

As part of the study, the group also tested teeth from modern Nile crocodiles (Crocodylus niloticus), teeth from modern American alligators (Alligator mississippienis), and enamel from extinct Miocene alligator species. Comparing the extinct Miocene endotherms with ectotherms from the same site in Florida found a difference of 6 degrees C, revealing "a measurable difference between mammals and ectotherms," Eagle notes—a difference that might be used to compare even older species found together.

The team has not yet figured out just how far back they can use bio-apatite assessment to determine body temperature. The new study examines materials as old as 12 million years, but parsing out dinosaur temperature regulation patterns will require going back tens and hundreds of millions of years, "so that will be a new frontier," Eagle says.

Ancient climes uncovered
Like the calcium carbonate environmental temperature testing method that inspired Elier to turn to vertebrate measurements, bio-apatite can also help researchers get a sense of ambient temperature. Ectotherms, because their internal temperatures mirror those around them, can be a living gauge and record of past temperatures.

Other estimates for ancient climates have been based on plant fossils, core samples and complex reconstruction models but, as Eagle notes, "all are to some extent uncertain as they are influenced by factors other than temperature that can be difficult to control for, particularly in the distant past." Their measurement of bio-apatite "seems only to be a function of temperature," he says, which is "a big advantage."

The method, however, is not infallible: In modern ectothermic species, it is known that the body temperature can change 8 to 12 degrees C each day and some 20 degrees C throughout the year. So the measurements derived from these animals "seemed to be recording more or less an average temperature," Eagle says. And he also posits that larger ectotherms, the size of big dinosaurs, might be able to sustain warmer body temperatures simply by retaining heat in their larger bodies, so future research might have to take that variable into account.

In the meantime, the group is hoping to uncover more ground-shaking news about how cold-hearted the dinosaurs really were.