Scientists have long wondered if dinosaurs were cold-blooded, like the reptiles with which they are categorized, or warm-blooded, like the birds that are believed to be their closest living relatives.

In the latest findings, researchers have proposed that dinosaurs were actually mesotherms, creatures with blood temperatures that come somewhere in between cold- and warm-blooded, Live Science reported.

Scientists have found that "dinosaurs do not fit comfortably into either the cold-blooded or warm-blooded camp--they genuinely explored a middle way," said lead study author John Grady, a theoretical ecologist at the University of New Mexico, as quoted by Live Science.

While rare, mesotherms exist in such modern examples as the great white shark, tuna and leatherback sea turtle as well as the echidna, an egg-laying mammal from Australia. Mesotherms are similar to mammals, generating enough body heat to stay warm, but they are also like reptiles in that their body temperature does not stay consistent.

"For instance, tuna body temperature declines when they dive into deep, colder waters, but it always stays above the surrounding water," Grady told Live Science. "Mesotherms have adaptations to conserve heat, but they do not burn fat or shiver to warm up. Unlike us, they don't boost their metabolic rate to stay warm."

Grady and his colleagues examined "growth rings" in dinosaur bones, analyzing how the bone was deposited in layers to see their growth rate. While studying how the animals grew from birth to adults, the researchers discovered that their growth rates didn't make them similar to mammals or reptiles, instead coming in somewhere in a middle ground.

Despite the new findings, not every dinosaur should be automatically classed as a mesotherm.

"Dinosaurs were a big and diverse bunch, and some may have been endotherms or ectotherms," said Grady, as quoted by Live Science. "In particular, feathered dinosaurs are a bit of a mystery. What do you call a metabolically intermediate animal covered in feathers? Is it like the mesothermic echidna? Or just a low-power endotherm?"

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