The complex air sac-lung system found in birds had to have evolved from a reptilian, sac-like lung. Here is the pathway envisioned by Paul in his 2002 book.
The first of the so-called archosaurs were the primitive late Permian through early Triassic reptilian group (that we have called diapsids), which would eventually give rise to crocodiles, dinosaurs, and birds. Examples of this group included the quadruped form Proterosuchus (described above as one of the earliest Triassic archo-saurs). They would have had a reptilian septate lung. Inspiration may have been aided by a primitive abdominal pump-diaphragm system (more primitive, perhaps, than the system found today in modern crocodiles). Successively, however, evolution of the air sac system may have progressed fairly rapidly. By the time the first true dinosaur was seen in the middle Triassic part of the air sac system may have been in place.
The most primitive theropods from this time (the first dinosaurs) do not show bone pneumatization, but Paul suggests that the lung itself may have become inflexible and smaller—a bird-like trait based on rib anatomy. The ribs also become double headed, showing that the rib cage itself was capable of a great ventilation capacity. Perhaps as a consequence of going bipedal, these dinosaurs may have switched from the more primitive abdominal pump system to the first air sac system—one with only the abdominal air sac found in modern-day birds. Soon after, descendents of these first dinosaurs, forms such as the well-known, upper Triassic Coelophysis, showed the evidence of bone pneu-matization, consistent with the proposal that more air sacs had evolved, this time in the neck region. With the Jurassic forms such as Allosaurus, the air sac system may have been essentially complete (but still much 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 lungs that were ventilated with the same system now found in crocodiles—a hepatic air pump diaphragm, that operates by muscles attached to the pelvis. While admitting that some dinosaurs had pneumatized bones, Ruben does not think this is evidence of air sacs. But if not air sacs, what system was used by the dinosaurs? According to Ruben and his colleagues, we have only to look at respiration in crocodiles to see how—and with what organs—dinosaurs breathed.
We have already described the crocodilian system, called the hepatic (liver) piston pump. Like mammals, a diaphragm system inflates the lungs, but, unlike our system, the crocodiles move their entire liver region, as their diaphragm muscles attach partly to the pelvis and partly to the soft tissue of the liver itself along a broad band. The entire liver is pulled back toward the pelvis like a piston, and in so doing the lungs inflate with air. But a piston must fill its cylinder, and such a system would by necessity have a partition right across the body cavity—es-sentially a subdivision of the forward or thoracic part of the interior of the body from the posterior or abdominal region. Crocodiles have this, and Ruben et al. think that dinosaurs breathed in the same way. They also cite aspects of dinosaur skeletons that seem to preclude the possibility of abdominal air sacs at least. As we have seen, the ribs of birds are capable of extensive movement because of a hinged contact between the posterior ribs and the sternum (the breastbone).
As a further nod to the crocodiles, the Ruben camp does not support warm-bloodedness in dinosaurs. Thus, their views could hardly be more different from the air sac dinosaur endothermy advocates. Voila—there could hardly have been a better recipe for controversy. Unfortunately, neither camp could land any knockout blow to the other's prime hypothesis based on the evidence at hand. And that is why the discovery of exquisitely preserved feathered dinosaurs from China brought the controversy to a boil.
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