different from the bird system, modified as it has been for flying, for even the modern-day flightless birds came from flyers in the deep past), with large thoracic and abdominal air sacs. Yet holes in bones do not an air sac system make, if I may paraphrase Yoda.
By the time Archaeopteryx had evolved in the middle part of the Jurassic, there may have been a great diversity of respiratory types among dinosaurs, some with pneumatized bones, some without. There also may have been a great deal of convergent evolution going on. For instance, the extensive pneumatization in the large sauropods studied with such care by Wedel may have arisen somewhat independently from the system found in the bipedal Saurischians.
Paul considers the evidence at hand as proof of a progressively more complex air sac system appearing in middle Triassic to Jurassic dinosaur lineages. He summarized this view in his 2002 book:
One could hardly ask for a better pattern of incremental evolution progressing to the avian skeletal features needed to operate respiratory air sacs. This fact reinforces the case for pre-avian pulmonary air sac ventilation in predatory dinosaurs. No evidence for progressive evolution of a pelvis based diaphragmatic muscle pump (the system found in modern crocodiles) has been presented.
Yet for all these arguments what the paleontologists had were a series of holes in bones, for in no case was a fossil air sac to be found (nor was one expected to be found, of course). And as might be expected, it was not long before a spirited opposition sprang into action. The leader of that opposition was the already introduced John Ruben. During the 1990s he began an extended debate with the advocates of a pre-avian air sac system in general and Greg Paul in particular.
John Ruben and various coauthors came to an opposite conclusion about almost every aspect of what might be called the “air sac in dinosaurs” hypothesis. And they went well beyond even that. In a summary paper published in mid-2005, Ruben and three coauthors proposed that dinosaurs were ectothermic, as were the earliest birds. According to this idea, birds gained warm-bloodedness and the air sac system only with the evolution of flight, and thus warm-blooded, bird-lunged (air sac) birds may not have evolved until the Cretaceous Period, many millions of years after evolution of the first birds. Ruben and his colleagues proposed that dinosaurs possessed simple, septate