Researchers in Spain and Norway reported in the periodical Nature they had found tree-like growth rings on the bones of mammals, a characteristic that until now was thought to be limited to cold-blooded creatures and dinosaurs.
They also found proof that dinosaurs probably had a high metabolic rate to allow fast growth another pointer of warm-bloodedness.
"Our results strongly propose that dinosaurs were hot-blooded," lead author Meike Koehler of Spain's Institut Catala de Paleontologia told AFP.
If so, the findings should punctual a rethink about reptiles, she said.
Modern-day reptiles are cold-blooded, meaning they cannot control their body temperatures through their own metabolic system relying instead on outside means such as basking in the sun.
While the dinosaurs may have been hot-blooded, their other characteristics kept them directly in the reptile camp, said Koehler.
Paleontologists have long noted the ring-like markings on the bones of cold-blooded creatures and dinosaurs, and taken them to designate pauses in growth, perhaps due to cold periods or lack of food.
The bones of hot-blooded animals such as birds and mammals had never been correctly assessed to see if they, too, display the lines.
Koehler and her team found the rings in all 41 hot-blooded animal species they studied, counting antelopes, deer and giraffes.
The finding "eliminates the strongest quarrel that does survive for cold-bloodedness" in dinosaurs, she said.
The team's analysis of fillet tissue also showed that the fast enlargement rate of mammals is related to a high metabolism, which in turn is characteristic of hot-bloodedness.
"If you compare this hankie with dinosaur tissue you will see that they are equal," said Koehler.
"So this means that dinosaurs not only grew very fast but this increase was sustained by a very high metabolic rate, representative hot-bloodedness."
A comment by University of California palaeontologist Kevin Padian that was available with the paper said the study was the latest to chip away at the long-held theory that dinosaurs were cold-blooded.
"It seems that these were anything but characteristic reptiles, and Koehler and colleagues' findings remove another false association from this picture."