A latest study by researchers at the University of New Mexico has found that dinosaurs were neither cold-blooded reptiles nor hot-blooded mammals or birds. Researchers said the pre-historic creatures that dominated the earth for more than 100 million years, instead, belonged to an intermediary group - neither warm-blooded nor cold-blooded.

"Our results showed that dinosaurs had growth and metabolic rates that were actually not characteristic of warm-blooded or even cold-blooded organisms. They did not act like mammals or birds nor did they act like reptiles or fish," evolutionary biologist and ecologist Brian Enquist told Reuters.

Researchers said that warm-blooded animals tend to be more active and grow faster than their cold-blooded counterparts.

Scientists have long debated whether dinosaurs were cold or warm-blooded. In recent times, they hypothesized that they fell into the warm-blooded category based on physiological factors, AFP reports.

That theory was bolstered by the discovery of fast and powerful dinosaurs like velociraptors. Scientists suggested that they must have been warm-blooded to burn enough energy.

In order to determine the growth and metabolism of the extinct species, the researchers analyzed the layers of bone deposits in fossils. Overall 21 species of dinosaur bones were part of the study including super predators Tyrannosaurus and Allosaurus, long-necked Apatosaurus, duckbilled Tenontosaurus and bird-like Troodon. Mammals, birds, fish, sharks, lizards, snakes and crocodiles were included.

The researchers found that the metabolic rates were somewhere between cold and warm-blooded animals.

As a result, the researchers categorised these creatures in an intermediary category called "mesotherms," opposed to ectotherms (cold-blooded animals) or endotherms (warm-blooded). Their metabolic rates are closest to those of tuna, great white sharks and the leatherback sea turtle.

"These animals at times rely on internally-generated metabolic heat to maintain body temperatures, while being subject to external temperatures in others," the researchers said, abc reports.

The researchers said that the intermediary metabolism (neither too fast nor too slow) allowed dinosaurs to grow larger than any other animal and to dominate the ecosystem.

The study titled "Evidence for mesothermy in dinosaurs" is published in the journal Science.

The finding could help scientists estimate the metabolic rates of other animals during Mesozoic Era, from 248 to 65 million years ago.