Dinosaurs, long considered cold-blooded creatures in the same way modern reptiles are, may have in fact been nothing of the kind, one expert says.

A study in 2014 had proposed dinosaurs might have been something unique, neither cold-blooded nor warm-blooded but rather something in between known as mesotherms.

However, in a new study re-analyzing the research that created the mesotherm suggestion, Michael D'Emic, a paleontologist at Stony Brook University in New York, proposes that dinosaurs were warm-blooded after all.

He says he bases that conclusion on the fact that metabolisms and growth rates of dinosaurs were strikingly similar to what is seen in modern-day warm-blooded mammals.

"I was surprised to see how well dinosaurs fit within our concept of what it means to be a warm-blooded animal today," D'Emic says. "Dinosaurs are a really diverse group of animals that lived over a very long period of time, so I expect that as we study more dinosaurs' growth, we will find more variability in how fast they grew and what their metabolism was like."

The 2014 study creating the mesotherm hypothesis — which D'Emic was not involved in — compared some 20 dinosaur species with mammals, fish, birds, snakes, crocodile and lizards.

The study authors suggested growth rates displayed in dinosaur bones matched neither those of warm-blooded, or endothermic, animals nor cold-blooded, ectothermic ones, leading to the proposition they were in-between mesotherms.

However, D'Emic says his re-analysis showed the earlier study had underestimated the growth rate for a significant number of the dinosaur species examined.

A dinosaur's growth rate can be estimated from growth lines visible in fossilized bones, showing how fast bone size increased — and therefore how fast the creature grew — in the same way that "tree rings" in a tree trunk can indicate growth.

His analysis provided evidence of a much higher growth rate than was put forward in the 2014 study, D'Emic says.

That, and the knowledge that modern birds — descendants of dinosaurs — are warm blooded, convinced him dinosaurs were closer metabolically to warm-blooded mammals, and thus likely warm-blooded themselves, he says.

"Separating what we commonly think of as 'dinosaurs' from birds in a statistical analysis [as was done in the 2014 study] is generally inappropriate, because birds are dinosaurs — they're just the dinosaurs that haven't gone extinct," he explains.

"Upon re-analysis, it was apparent that dinosaurs weren't just somewhat like living mammals in their physiology — they fit right within our understanding of what it means to be a 'warm-blooded' mammal," he says.