Supporters of the warm-blooded dinosaur notion cited certain evidence in support of their view, such as the well-defined joints of limb bones of theropods such as Deinonychus, that implied an active life. They also cited just about everything else they could think of – which was not always convincingly relevant.
Critics of this view pointed to dinosaurian features that were like those found in modern lizards and absent in modern birds and mammals – features they took to imply a less metabolically-expensive life. One of those features was LAGs, thought to be usually absent in mammals.
But Köhler and colleagues’ work shows LAGs are present, and in mammals from the tropics to the Arctic. So their occurrence in dinosaurs doesn’t preclude endothermy, and one line of evidence against the warm-blooded dinosaur notion has been removed.
But this doesn’t mean we can assume that all, or any, dinosaurs were warm-blooded. After all, it isn’t even clear that all mammals are warm-blooded in the same sense. Some, such as echnidas, seem to have variable body temperatures, but others, humans for example, usually don’t.
The problem, obviously, is that dinosaurs are extinct. If they weren’t, we could measure their temperatures and metabolic rates.
Metabolic rate, what is really of interest here because it enables an active lifestyle, generates the “warmth” of warm-blooded animals. For extinct creatures metabolic rate has to be inferred from other indicators.
But being warm-blooded is not a simple, single property, but a suite of properties. Just because these properties all occur together in living animals, doesn’t mean that we can assume this association is necessary. Maybe some of these features once conferred benefits unrelated to “warm-bloodedness."
Feathers in birds are now linked with warm-bloodedness, but may have arisen for their value in showy displays, and not for insulation. Albert Einstein reputedly said that science should be made as simple as possible, but no simpler.
Too often both proponents and opponents of warm-blooded dinosaurs have tried to make this notion simpler than possible, as Köhler and her colleagues have shown. And we can expect more such demonstrations to come.