Were dinosaurs warm-blooded or cold-blooded? That's a debate that's been going on since 1968, when Yale scientists first proposed that dinos could have been active, agile, and fast.
It's also a debate for which there's not yet a good conclusion. Yes, scientific consensus has shifted away from perceiving dinosaurs as being slow, lumbering, and essentially reptile-like. But just because we now know that some dinosaurs are the ancestors of birds doesn't mean we've solved the question of how dinosaurs — in general — maintained their body heat and governed their metabolism. All the evidence is indirect, for obvious reasons, and being related to birds is not the same thing as being birds.
The trouble with dinosaurs is that they were something totally different. Not birds. Not reptiles. Something else. Likewise, a new paper suggests that the question of whether they were warm-blooded or cold-blooded can't really be answered in a binary way.
For this paper, the scientists decided to perform a metabolic census of sorts. They compared the growth rate, adult size and metabolism of 381 vertebrate species, living and extinct, including 21 dinosaur species. The idea was that the higher the growth rate, the higher the animal’s rate of metabolism would probably be. And higher metabolisms would probably tend to be endothermic, warm-blooded systems.
Sure enough, they found that high growth rates seemed to match up with the higher metabolisms of warm-blooded animals, whereas lower growth rates were linked to the lower metabolic rates of cold-blooded animals. The dinosaurs seemed to sit right in the middle, overlapping with some endothermic and some ectothermic animals. The scientists aptly called them “mesotherms” ("meso" basically means "middle" in Greek.)
There are some intermediate, mesothermic species that exist today, the authors point out: Animals such as tuna and the leatherback turtle are able to somewhat regulate their internal temperatures, though not to the extent that warm-blooded animals do.
Finally, if you want more background information on the debate over dinosaur metabolism, the University of California Museum of Paleontology has a really nice backgrounder on their DinoBuzz site. It sums up the evidence for warm-blooded dinosaurs, the evidence for cold-blooded dinosaurs, and explains the range of top hypotheses that exist. Despite the way the LA Times (and many other outlets) have framed this, the dino blood debate isn't as simple as Once We Believed This, Then We Believed That, Now This New Paper Proves Something Else.