LOS ANGELES (Reuters) - Scientists in California say they have for the first time devised a way to accurately take the body temperatures of dinosaurs -- by examining the creatures’ teeth.

A Camarasaurus tooth from the Jurassic Morrison Formation of North America, in a photo courtesy of Caltech. REUTERS/Lance Hayashida/Caltech

Chemical analysis of the Jurassic period fossil teeth from two sauropods -- long-tailed, long-necked dinosaurs that rank among the largest land animals ever to roam the Earth -- showed they were about as warm as most modern mammals.

But they also were cooler than some experts had predicted for animals of such gigantic size.

The findings from a team led by researchers at the California Institute of Technology were published Thursday in an online edition of the journal Science.

“This is like being able to stick a thermometer in an animal that has been extinct for 150 million years,” said Robert Eagle, an evolutionary biologist and post-doctoral scholar at Caltech who was lead author of the report.

The study supports a growing body of research suggesting dinosaurs were more active and energetic than scientists originally believed.

But it leaves unanswered the key question of whether dinosaurs were cold-blooded, relying on their environments for heat, or warm-blooded, with self-regulated metabolism like modern mammals and their evolutionary descendants, birds.

Eagle said that determination will have to come with further analysis of a much greater range of dinosaur species.

The two dinosaurs initially selected for study -- Brachiosauraus brancai and camarasaurus -- were close cousins of the massive plant-eating dinosaur known as brontosaurus.

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The temperature of brachiosaurus was measured at 38.2 degrees Celsius, or 100.8 degrees Fahrenheit. Camarasaurus registered a temperature of 35.7 degrees C, or 96.3 degrees F. Researchers say those figures are accurate to within 2 degrees Celsius.

WARMER THAN CROCS, COOLER THAN BIRDS

While equivalent to the temperature of most modern mammals, that range is warmer than modern and extinct crocodiles and alligators but cooler than birds.

Still, because of their sheer enormous size, sauropod dinosaurs would be expected to retain their body heat more efficiently than smaller warm-blooded animals, like humans, even if dinosaurs themselves were cold-blooded, Eagle said.

To explain this, researchers suggested the dinosaurs may have had some physiological or behavioral adaptation that allowed them to avoid getting too hot. One possibility is they dissipated excess heat through their long necks and tails.

In any case, scientists will learn more as they apply their new technique to other species, such as meat-eating predators like Tyrannosaurus rex or velociraptors, which were smaller and probably faster on their feet, Eagle said.

Researchers previously had to gauge dinosaur body temperatures indirectly, inferring energy needs and metabolism from the spacing of fossil footprints that indicated how fast they ran, or the presumed ratio of predators to prey in the fossil record.

Eagle’s more direct technique was adapted from geological research perfected by other Caltech scientists, he said. It examines concentrations of rare carbon and oxygen isotopes in a mineral found in tooth enamel and bone.

Researchers based their study on an examination of 11 fossil teeth unearthed in Tanzania, Wyoming and Oklahoma and donated by museums.

Eagle said his team started with sauropods because their plant-munching teeth were bigger and contained more enamel to work with. Sauropod teeth are also easier to come by in the fossil world, in part because museums and collectors are less willing to part with the teeth of dynamic predator species like T. rex, he said.