Dinosaurs were warm-blooded animals that had many traits in common with mammals, finds controversial new research.
The study, published in the journal Science, counters other popular theories, which say that dinosaurs were either cold-blooded and reptile-like, or occupied a unique intermediate category of animals that were neither fully cold nor warm-blooded.
"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," author Michael D'Emic, a Stony Brook University paleontologist, said in a press release.
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The "re-analysis" refers to D'Emic's work revisiting yet another Science paper, published last year, which compiled a huge dataset on growth and metabolism of hundreds of living animals. D'Emic admits that this dataset was "remarkable" and "unprecedented."
Yet, that earlier study found that dinosaurs would have fit into the intermediate category between ectothermic (cold-blooded, with body temperature controlled largely by the surrounding environment) and endothermic (warm-blooded, like us).
D'Emic looked at that study again, focusing on two primary aspects.
First, the original study had scaled yearly growth rates to daily ones in order to standardize comparisons.
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"This is problematic," D'Emic said, "because many animals do not grow continuously throughout the year, generally slowing or pausing growth during colder, drier, or otherwise more stressful seasons. Therefore, the previous study underestimated dinosaur growth rates by failing to account for their uneven growth. Like most animals, dinosaurs slowed or paused their growth annually, as shown by rings in their bones analogous to tree rings."
He added that very stressful or seasonal environments can also impact growth, and would have really affected dinosaurs. He believes that these factors were underestimated in the earlier study.
The second aspect of his re-analysis took into account the widely held view that all dinosaurs did not become extinct: some evolved to become birds. Today's birds are warm-blooded, so he argues that dinosaurs must have been this way too.
"Separating what we commonly think of as 'dinosaurs' from birds in a statistical analysis is generally inappropriate, because birds are dinosaurs–they're just the dinosaurs that haven't gone extinct," he explained.
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If D'Emic's theory about warm-blooded dinosaurs holds, then this would lead to still more re-evaluation of other data on things like the big sails and head ornaments on some dinosaurs. Certain past studies have linked these to body temperature regulation. If they didn't serve that function, then perhaps they were used more for flashy mating displays or other types of visual communication.
Holly Woodward, an assistant professor in the Center for Health Sciences at Oklahoma State University, supports re-examining prior studies, even those that appear seamless.
As she said, "D'Emic's study reveals how important access to the data behind published results is for hypothesis testing and advancing our understanding of dinosaur growth dynamics."
For now, though, the debate rages on among paleontologists as to whether or not dinosaurs (before the emergence of birds) were warm blooded or not.
There's a surprising human benefit to such research, D'Emic shared. His research on when why, and how pauses or slowdowns in growth are recorded in bones could have implications for treating human bone diseases, such as osteoporosis.