The debate about whether dinosaurs were cold-blooded or hot-blooded has long intrigued palaeontologists.

Now, an international team led by California Institute of Technology, claims to have unveiled a new method to measure the body temperature in extinct vertebrates by analysing rare isotopes in the animals’ bones, teeth, and eggshells.

“This is not quite like going back in time and sticking a thermometer up a creature’s back end. But it’s close,” Prof John Eiler, who led the team, said.

The only way scientists have had to study temperature regulation in such creatures was to make inferences based on what is known about their anatomy, diet or behaviour till now.

The new technique looks at the concentrations of two rare isotopes —— carbon-13 and oxygen-18.

“These heavy isotopes like to bond, or clump together, and this clumping effect is dependent on temperature. At very hot temperatures, you get a more random distribution of these isotopes, less clumping. At low temperatures, you find more clumping,” said team member Robert Eagle.

In living creatures, this clumping can be seen in the crystalline lattice that makes up bioapatite —— the mineral from which bone, tooth enamel, eggshells, and other hard body parts are formed.

“When the mineral precipitates out of the blood — when you create bone or tooth enamel — the isotopic composition is frozen in place and can be preserved for millions of years.

“In addition, work in lab has defined the relationship between clumping and temperature, allowing measurements of isotopes in the lab to be converted into body temperature. The method is accurate to within one or two degrees of difference.

“A big part of this paper is an exploration of what sorts of materials preserve temperature information, and where,” Mr. Eagle said.

To do this, the team looked at bio-apatite from animals whose form of body-temperature regulation is already known. .

“We know, for instance, that mammals are warm-blooded; all the bio-apatite in their bodies was formed at or near 37 degrees centigrade,” he said.

After showing proof of concept in living animals, the team looked at those no longer roaming the earth. For instance, the team was able to analyse mammoth teeth, finding body temperatures of between 37 and 38 degrees.

Going back even further in time, they looked at 12-million-year-old fossils from a relative of the rhinoceros, as well as from a cold—blooded member of the alligator family tree. “We found we could measure the expected body temperature of the rhino—like mammal...,” Mr. Eagle said.

There are, however, limitations to this sort of temperature sleuthing. “When we look at tooth enamel, for instance, what we get is a record of the head temperature of the animal when the tooth grew. If you want to know what his big—toe temperature was two years later, too bad,” he said.

The findings have been published in the ‘Proceedings of the National Academy of Sciences’ journal.