It is time to come to the heart of the matter about dinosaurs—or at least the earliest forms. The most important questions are whether or not they were warm-blooded and what kind of respiratory system they had in terms of lung morphology, circulatory system morphology (especially the heart), and physiological adaptations for low oxygen. First, let’s look at the environmental context of their first evolution and existence.
The Triassic was a time of low oxygen—which we can find today at high elevations—but of atmospheric gas pressure equal to, or perhaps even exceeding, that of today. This is beyond our experience. It was a time of very high carbon dioxide. Our species has never experienced this (yet we are trying to make it so). It was also a time when the climate of the planet was hot—much hotter than today: no icecaps, no glaciers even in the mountains, and heat from equator to pole. Is it any surprise, then, that the animals of that time may have been unlike anything of the modern-day?
Perhaps the best reference for all things dinosaurian comes from the marvelous, weighty tome The Dinosauria (Second Edition). Edited by David Weischampel, Peter Dodson, and Halszka Omolska, this compendium has data that are currently being mined to show many trends, and some of these data have already been used in the pages above and the pages to come. But in addition to raw data on dinosaur ranges and geographic occurrences, there are summary chapters by leading specialists on all of the issues invoked above, the only difference being that none of the authors takes into account the radically low-oxygen levels during the time of the dinosaurs’ first evolution, conditions that carried on for many tens of millions of years of the Mesozoic. On the twin and central issues of metabolic rate (endothermy versus ectothermy) and respiratory adaptations, there are assured stances that we should look at.
Phylogenetically, dinosaurs are positioned between crocodiles and birds, the former cold-blooded with hepatic piston lungs, and birds with warm blood and the air sac system. It is thus fair to ask where dinosaurs sit in terms of these two end members: Were they closer to