In Chapter 8 we looked at the questions of dinosaur metabolism and respiration. The proposal there was that the first dinosaurs were of a kind of animal never seen and not alive today. Through upright posture and an evolving air sac system, they developed respiratory efficiency (the amount of oxygen extracted from air per unit time, or per unit energy expended in breathing) superior to any other then-extant animal. But these early forms may have lost (or never gained in the first place) endothermy, replacing it with a more passive homothermy or even ectothermy, which was attained with larger size. That was their trick—using ectothermy to reduce oxygen consumption while at rest and a superior lung system to allow extended movement without going into rapid anaerobic (and thus poisonous) states. But what of the later dinosaurs? We know that birds, a group of dinosaurs first appearing in the Jurassic, eventually had both endothermy and a very different kind of lung than in any extant reptile. It seems probable that the large and small saurischians parted company, with smaller forms evolving endothermy later in the Jurassic as oxygen levels rose rapidly.
John Ruben’s group stakes out a very different and conservative position that the first true birds had both ectothermy and reptilian, not air sac, lungs. Most other dinosaur and bird specialists are not so sure. Some believe that endothermy and air sac lungs of some kind were present in Archaeopteryx, while others indicate that based on bone pneumaticity the air sac system was present in the bipedal carnivores that gave rise to birds.
With perhaps the exception of the always-fascinating tyrannosaurids, no group of dinosaurs has received more attention in recent times than the basal birds. Vigorous debate centers on their body covering and, most importantly, on when flight first evolved and why. The first birds appeared about 150 million years ago, and the famous first bird remains Archaeopteryx. That is just before the start of the Cretaceous. Oxygen had been rising for 50 million years at that time. Gigantism in dinosaurs was common. The immediate ancestors of the birds were fast, ground-running dinosaurs that may have used their forelimbs for a type of predation, a motion that was preadapted for a wing