stroke in a flyer, according to Berkeley paleontologist Kevin Padian. The fossil record suggests that the ancestors of the first bird were the bipedal carnivorous saurischians known as troodontids or perhaps the dromaeosaurids, forms that appear to have been already feathered (there is much controversy about this).

Could Archaeopteryx fly? Padian thinks so. But there is debate about when true flight took place. Could the late Jurassic “birds” really fly, at a time when their competition in the air would have been the diverse and successful pterodactyls? The fossil record does show that by the lower Cretaceous there was a bird (Eoaluolavis) that had evolved a “thumb wing,” an adaptation that allows greater maneuverability at slower speeds. Thus, within a few million years after Archaeopteryx, fairly advanced flight was present. New discoveries from China have revealed an unexpected high diversity of birds by the early part of the Cretaceous. Flight was an adaptation that stimulated a rapid evolution of new forms.

What new information can be added? Flight is highly energetic. Birds use a great deal of energy to fly, and that, added to their relatively small size and endothermy, makes them great users of oxygen. So the air sac system serves them well. But what of the late Jurassic, when oxygen may have been somewhat lower than now? Could it be that the even lower oxygen of the early and middle Jurassic delayed flight? What about other known flyers? The pterodactyls had long been in the air by the evolution of the first bird, but pterodactyls may not have been as energetic in their flying or might have had an air sac system, for they also show bone pneumaticity consistent with the presence of an air sac system. Hence, there are questions about lung type not only in the first birds but also in other flyers, their immediate ancestors, and in the bipedal, saurischian dinosaurs like T. rex that came along in the Cretaceous.

DINOSAUR REPRODUCTION AND OXYGEN LEVELS

Alas, the total extinction of dinosaurs 65 million years ago (unless birds are considered dinosaurs, now accepted by many) will forever make it impossible to answer many pressing questions about their biology. So it is natural that we try to answer these questions using their



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