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Were Dinosaurs Warm-Blooded?

Some chemical reactions in the body, such as the burning of sugar to produce energy while releasing carbon dioxide and water, are referred to as metabolism. These reactions run most efficiently at a particular optimum temperature. In mammals and birds, the optimum temperature is higher than the normal outside temperature and is regulated internally. So these animals maintain generally higher body temperatures and are called "warm-blooded" or endothermic, for their internal temperature control. Other animals, such as turtles, crocodiles, and lizards, have more variable body temperatures based on their level of activity and regulate body temperature through external sources, such as warming up in the sun or cooling off in the shade. Thus, they are often called "cold-blooded," or more appropriately, ectothermic, for their external temperature control. Were dinosaurs ectothermic or endothermic? Since birds are a group of living dinosaurs, we know that at least some dinosaurs are endothermic. But the question of whether all dinosaurs were endothermic is more difficult to answer. One approach to investigate this question is to look at the microscopic structure of bone. In many ectothermic animals, bone grows in dense, concentric rings, somewhat like tree rings. In endothermic animals, a complex system of closely spaced cavities, called the Haversian system, permeates the bone. By cutting bones of extinct, non-avian dinosaurs into thin slices and examining them under the microscope, we can look for these characteristics. However, many complications exist, such as the need to compare animals of similar size, and no living dinosaurs as large as Tyrannosaurus and Apatosaurus exist today for us to observe. In most non-avian dinosaurs, the microstructure of the bone looks more like that of ectotherms, but the evidence is not conclusive. Because birds are endothermic, it's clear that "warm-bloodedness" evolved somewhere along the evolutionary tree of dinosaurs. But it is not yet possible to identify with certainty which groups of non-avian dinosaurs were "warm-blooded."
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