The Evolution Of The Air Sac System

The complex air-sac-lung system found in birds had to have evolved from a reptilian, sac-like lung, and all evidence indicates that this happened in the middle Triassic. The most primitive thero-pods from this time (the first dinosaurs) do not show bone pneumatization, but their ribs became 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, Late Triassic Coelophysis, show the evidence of the bone pneumatization, consistent with the proposal that more air sacs had evolved, this time those 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.

By the time that Archaeopteryx had evolved in the middle part of the Jurassic, there may have been a great diversity of respiratory types among the 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 saurischian carnivores (Wedel, 2003)

Perhaps the greatest contribution to the subject of air sacs both modern and ancient was the seminal paper by O'Connor & Claessens (2005). They pointed out that the entire community has misunderstood which air sacs penetrate which bones in birds. By injecting liquid rubber into the air sacs of modern birds, they showed that the sacral, or tail ward parts of the body are most important in producing the characteristic "bird breathing" pattern. They also showed a great variability in modern bird pneumaticity, with diving birds showing different patterns from small flyers, and different again among ground dwellers and birds of large size. With this new and improved understanding of the size, morphology, and positions of pneuma-tized bones and their attendant air sacs, O'Connor & Claessens (2005) then showed evidence of nearly identical holes in homologous bones of found in saurischian dinosaurs.

Why would this the air sac system have evolved? First, metabolism. If we accept that at least some dinosaurs were warm-blooded, then it is immediately apparent that they would have needed of highly efficient oxygenation mechanisms; and this would be even more important in a low oxygen world. All modern (cold blooded) reptiles have to warm up at the start of the day, and thus there is little early morning activity other than behavioral movement to acquire heat from the external environment. If the first bipedal dinosaurs - all predators - did not have to do this, they would have been able to forage freely on the slower ectotherms in the morning or nighttime hours. But what is the price paid for this? At rest, all endotherms use as much as 15 times the amount of oxygen as do ectotherms (there is a 5-15 times range based on experimental observation). In our oxygen-rich world this is not a problem for the warm-blooded animals. There is so much oxygen available that there is no penalty. But in the oxygen-poor Late Triassic and into the Jurassic, such was surely not the case. And the energy and oxygen necessary for endothermy would not have been necessary if the dinosaurs moved toward large size. With larger body size the ratio of surface area (from which heat is lost) to body volume becomes increasingly favorable.

+1 0

Post a comment