As we saw in Chapter 8 the first dinosaurs were the bipedal saurischians, and they soon spawned another group, the ornithischians. The ornithischians, which began as relatively small, carnivorous bipeds, quickly evolved into herbivores and stayed that way, with both quadrupeds and bipeds. But they remained a relatively small part of the terrestrial fauna during the first half of the Jurassic. The reason for their rarity relative to the number of saurischians is probably related to oxygen levels. Unlike the saurischians, which showed bone pneumatization, ornithischians never evolved this trait. If bone pneumatization is a consequence of the air sac respiratory system, it can thus be inferred that ornithischians never used this kind of lung. Dinosaur expert Robert Bakker disagrees, suggesting that all dinosaurs used the air sac system, but that it was less developed in ornithischians, having only the abdominal sacs and not the sacs that fit into cervical bones in the neck. It seems more likely that the ornithischians had no air sacs at all.
The Jurassic world was not solely a dinosaur habitat, of course. We know that the Jurassic and the succeeding Cretaceous Period were times of innovations on land, in the sea, and also in the air, and it was the expansion into the air by two major groups of vertebrates that might have been the most radical of all Jurassic evolutionary changes. Three distinct kinds of flyers were in the skies: pterodactyls, pterosaurs, and birds, with the last evolving from saurischian bipeds during the latter parts of the Jurassic. While no one disputes that true birds first appeared in the Jurassic, there is still controversy about when these earliest birds took to the air. Most experts believe that the first birds could fly. Others, such as John Ruben, consider that true flying did not occur until the early Cretaceous. Regardless, true birds were on the scene in the Jurassic, and once again it appears that this extreme evolutionary novelty was a consequence, or was stimulated by, lower-than-now oxygen conditions that existed until near the end of the Jurassic itself.
Finally, the Jurassic (continuing into the Cretaceous) was the time when several lineages of true mammals evolved, all at small size, while in the seas the ichthyosaurs competed with both long- and short-necked plesiosaurs.