There has been much debate whether dinosaurs were cold-blooded ectotherms, like modern reptiles, or warm-blooded endotherms, like most modern mammals. However, they might have been neither, according to the bold suggestion that has been put forward by John Grady of the University of New Mexico and colleagues. By plotting (in a mass-independent manner) growth rate as a function of metabolic rate for nearly 400 living and extinct animals (using known scaling laws and annual bone rings to obtain dinosaur growth rates), the researchers find that dinosaurs lie in-between the exotherms and endotherms. They wpropose that dinosaurs were likely to have been what they call mesotherms - a class of animal that can raise its body temperature, but does not maintain it at a specific level. This would have allowed the dinosaurs to outcompete sluggish exotherms, and crowd out the large-animal niches, preventing endotherms from becoming bigger and ruling the Earth - at least until an asteroid hit. Modern mesotherms are few, but include tuna and the echidna.
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