When dinosaurs were first discovered, scientists thought they must have been cold-blooded like the lizards they resembled.

Later, they realized that some dinosaurs are related to modern birds—which have the highest metabolic rate in the animal kingdom. Could dinosaurs, too, have been warm-blooded?

New research reveals the answer is: both.

So-called “cold-blooded” creatures, or ectotherms, are confined mostly to mild climates. They’re more sedentary but require less food and oxygen to survive.

Warm-blooded creatures, or endotherms, can thrive in and migrate to diverse climates—but expend much more energy to maintain constant body temperatures.

All creatures take in oxygen, and new research shows that the waste products of processing that oxygen—different in warm- and cold-blooded animals—are durable enough to be found in fossils.

Molecular analysis of dinosaur fossils suggests an evolutionary break, with cold-blooded, slow-moving herbivores, like Stegosaurus, on one branch, and warm-blooded, highly mobile predators, like Velociraptors, on another.

Larger dinosaurs like Brontosaurus and Tyrannosaurus also had high metabolic rates, but not as high as the ravenous Velociraptors.

These findings indicate that dinosaurs evolved different metabolisms and body temperatures for different environmental niches—just as today’s animals have.