Dinosaurs were most likely warm-blooded, according to a new re-analysis of a 2014 study in which the authors concluded that dinosaurs were neither endotherms nor ectotherms, and instead belonged to a third, intermediate category known as mesotherms.
The original paper, published last June in the journal Science, concluded that dinosaurs likely had metabolic rates in-between those of warm and cold-blooded creatures, and were closest to extant mesotherms, creatures that could adjust their body to metabolically favorable temperatures based on a balance of internally-produced metabolic heat and external environmental heat.
However, in taking a second look at the research, Dr. Michael D’Emic, a paleontologist from Stony Brook University in New York, came to a vastly different conclusion. Based on what he known about how dinosaurs grew, he explained that dinosaurs were actually closer to mammals than reptiles in their growth and metabolism, and were likely warm-blooded.
Dr. D’Emic specializes in bone microanatomy (the study of bone structure on scales that are less than the width of a human hair). He re-examined the findings of the original paper and concluded that dinosaurs “fit right within our understanding of what it means to be a ‘warm-blooded’ mammal.”
Use of daily growth rates, exclusion of birds questioned
He focused his analysis on two different aspects of the original study. First, the authors scaled down yearly growth rates to daily ones in order to standardize comparisons, which Dr. D’Emic called “problematic” because many types of creatures do not grow at constant rates throughout the year – many slow or pause their growth during colder, drier conditions.
“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 said, adding that the growth rates had been especially estimated for larger animals and those living in stressful environments.
Dr. D’Emic also found that dinosaurs should not be separated from modern birds for purposes of a statistical analysis, since these warm-blooded creatures are descendants of Mesozoic dinosaurs. Separating dinosaurs from birds in such research is “generally inappropriate,” he said, “because birds are dinosaurs—they’re just the dinosaurs that haven’t gone extinct.”
Re-analyzing the data with birds as dinosaurs provides more evidence that dinosaurs were warm-blooded creatures, and not members of a special, intermediate metabolic category, as the authors of the original paper had suggested.