Paleontologists have long debated whether the dinosaurs were cold-blooded, soaking up heat from their surroundings like today’s reptiles, or warm-blooded, able to generate their own heat the way modern birds and mammals do. Some scientists pointed to dark bands called lines of arrested growth in dinosaur bones as evidence that the animals were cold-blooded. These lines, much like tree rings, show times when growth slowed down due to limited resources—perhaps, researchers suggested, during off seasons when the heat-absorbing animal wasn’t able to get enough energy from its environment.
A new study in Nature, however, examined the bones of 40 species of present-day ruminants—many-stomached, very much warm-blooded mammals—from around the globe, and found these animals’ bones show these same lines of arrested growth. Even mammals, it seems, slow down enough in tough times to leave these telltale marks in their bones. (Archaeologists have found similar bands, called Harris lines, in human bones from periods of malnutrition.) While this doesn’t prove dinos were warm-blooded, it knocks down an oft-cited piece of evidence that they weren’t. It’s possible that the dinosaurs evolved high body temperature, and passed the adaptation onto their descendants, today’s birds.
What’s the News: Dinosaur metabolism is one of the biggest mysteries in paleontology. Ever since the giant creatures were first unearthed, scientists have been wondering whether dinosaurs drew their heat from the environment, like the cold-blooded modern reptiles they resemble, or whether they generated heat themselves, like warm-blooded mammals.
Using a geoscience technique to see at what temperature dinosaur tooth enamel formed, scientists have found that at least two large dinosaurs, Brachiosaurus and Camarasaurus, had body temperatures similar to our own. While this study on its own doesn’t explain where the heat came from, it does add to paleontologists’ toolboxes a new, reliable way to probe temperature, which will lead to better inputs into the computational models that may eventually answer the question of whether dinosaurs were warm- or cold-blooded.