"Dinosaurs were a very long-lived and diverse group of animals. There is as yet no widely accepted definitive answer as to whether or not they were warm-blooded. It is quite possible that dinosaurs had a metabolism that is different from that of living animals. Indeed, the large size attained by many dinosaurs may have led to what has been called 'inertial homeothermy' or 'gigantothermy.' That is, the large bulk of the animals would have allowed them to maintain a fairly constant temperature even without a high metabolism. Certainly, dinosaurs were active and, in some cases, social animals that successfully competed with warm-blooded mammals during their 160-million-year-long reign on the earth."
Gregory S. Paul, a noted dinosaur researcher and illustrator, adds his perspective:
"It is not yet known whether there will ever be 'definitive proof' of the metabolic status of dinosaurs. At this time, researchers must use multiple lines of evidence to infer dinosaur energetics. Also, the physiological alternatives are themselves complex, and cannot be adequately described by simple terms such as warm or cold blooded. Some or all dinosaurs may have had high aerobic exercise capacity like birds and mammals (tachyaerobic), or low aerobic exercise capacity like reptiles (bradyaerobic). Dinosaurs may have had high resting metabolic rates like birds and mammals, or low resting rates like reptiles. How well dinosaurs could thermoregulate and control body temperature is yet another unanswered question.
"Large hips suggest that most dinosaurs had large, aerobically capable leg muscles like those of big-hipped birds and mammals, rather than small leg muscles like those of bradyaerobic reptiles. Many dinosaur trackways show that dinosaurs walked as fast as do birds and mammals, and much more rapidly than can be sustained by reptiles. Predatory dinosaurs appear to have developed birdlike respiratory systems able to take in large amounts of oxygen. The herbivorous ornithopod dinosaurs may have had a mammallike diaphragm for efficient respiration. The earliest and most primitive dinosaurs, however, had smaller hips and less advanced respiratory systems, suggesting their aerobic capacity was lower than in other dinosaurs, birds and mammals.
"From the fossil evidence, at least some dinosaurs appear to have grown more rapidly than any wild reptile, suggesting the juveniles were very aerobically active and able to seek out large amounts of food.
"The great size and especially the towering height of the sauropod dinosaurs may have required extra-high-pressure, oversized hearts that consumed much more oxygen than is observed in reptiles.
"The evidence for resting metabolic rates is more ambiguous. Some continental birds and mammals, for example, have poorly developed respiratory turbinates or nasal passages no larger than those of dinosaurs of similar size. Some birds and mammals have very high aerobic exercise-to-resting ratios, but no tachyaerobic animal has a reptilian resting metabolic rate. If most dinosaurs were tachyaerobic, then they probably had high resting metabolic rates as well.
"It is even less clear whether dinosaurs thermoregulated as effectively as do birds and mammals. Growing dinosaurs laid down rings inside their bones more often than birds and mammals, and this hints they did not regulate body temperature to the same degree.
"In conclusion, the evidence that most dinosaurs had high aerobic exercise capacity like birds and mammals is very good. It is probable that their resting metabolic rates were higher than in reptiles. It is possible that many dinosaurs did not maintain a constant body temperature. The energetics of dinosaurs almost certainly were not reptilian, but just how closely they approached the avian-mammalian condition is not yet certain."
Philip N. Froelich, director of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, offers yet another view: