Until comparatively recently it was thought the respiratory system of dinosaurs was similar to that of crocodiles. The evidence for this was based on a study of just two fossils encased in rock, but new information is beginning to show the respiratory system of dinosaurs was probably much more bird like. The fossil record has already provided strong indications that dinosaurs and birds are related, and this new evidence helps to reaffirm this hypothesis.

Because the respiratory system of dinosaurs and birds appear to be so similar, understanding the breathing mechanism of birds will also help to clarify how the respiratory system of dinosaurs is thought to have worked.

The Respiratory System Of Birds

The way birds intake oxygen is very different and much more efficient than our own run of the mill respiratory system, not least because carbon dioxide rich air never enters their lungs. Birds’ lungs are small but they are accompanied by two sets of air sacs, a posterior and anterior set. These sacs are so prevalent in the anatomy of birds that they extend into hollow openings in some long bones called pneumatic foramina, and into the trachea.

Air Sacs And How They Work

The air sacs in birds’ respiratory system allows air to move in one direction so that only fresh oxygen enters the lungs. Instead of inflating the lungs first, as the bird inhales the posterior sacs inflate, and then as it exhales the anterior sacs deflate which in turn fills the lungs with air from the posterior sacs. On the next inhalation the posterior air sacs are again filled whilst the anterior sacs fill with air from the lungs. The following exhalation empties the anterior sacs and fills the lungs from the posterior sacs.

This is an extremely efficient respiratory system because stale air never enters the lungs to be exhaled before the next breath, and it helps keeps birds light whilst giving them that extra oxygen rich boost needed for flight.

Comparing Dinosaur Fossils With Modern Birds

The first scientists to combine anatomical and functional studies of birds as possible examples of the internal structures of dinosaurs were Patrick O’Connor (Ohio University) and Leon Claessens (Harvard University). O’Connor and Claessens studied dinosaur fossils from New York, Berkeley, Chicago, Pittsburgh, Washington DC, Berlin and London as well as a 67 million year old complete dinosaur fossil (Majungatholus atopus) O’Connor had discovered in Madagascar in 1996. These fossils were compared to two hundred specimens of modern birds.

The result of their work was to demonstrate that the same pneumatic foramina (openings in the bones for air sacs), although not identical to that of birds were nevertheless present and clearly recognisable in the dinosaur fossils, providing good evidence that the respiratory system of dinosaurs was very similar to that of birds.

Other Studies

Work at Manchester University in the UK has shown that the respiratory system of theropod dinosaurs such as Velociraptor was similar to modern diving birds. The Manchester team lead by Dr Jonathan Codd looked at the fossils of manirapturan dinosaurs and extinct birds such as Archaeopteryx and compared them to modern birds. As well as air sacs as part of their respiratory system, birds also have small bones called uncinate processes that act as levers to move the sternum and ribs during breathing.

Their findings showed that the uncinate processes were also present in the dinosaur fossils. There are three types of uncinate processes in modern birds. The uncinate processes of running birds tend to be short, flying birds are intermediate and diving birds are the longest, and this is the kind found in the dinosaur fossil evidence.

In 1996 a complete dinosaur fossil named Areosteon Riocoloradensis was found on the banks of the Rio Colorado in Argentina. This dinosaur find drew the attention of palaeontologists because the pneumatic foramina were clearly evident in the vertebrae, clavicles and hip bones. In life these hollow spaces would have been lined with soft tissue and would have formed part of the anterior and posterior sacs of the respiratory system. Areosteon has become an important fossil discovery because it shows the early evolution of the use of air sacs in a species that was otherwise unrelated to bird ancestry.

Possible Explanations for Air Sacs in Dinosaurs

Palaeontologists have concluded that there are three possible reasons why a bird like respiratory system evolved in dinosaurs.

Firstly, the development of a more efficient lung system would enable dinosaurs to have a long distance, high energy hunting strategy. Unlike modern mammalian predators dinosaurs would not have needed to rest over long distances making then very effective hunters.

It is thought that some dinosaurs might have had feathers but no sweat system, so the presence of air sacs might have also provided dinosaurs with a cooling system. Some of the air sacs in Areosteon for instance were located in unusual places such as the lower belly ribs, which has lead to some speculation that it may have had air tubes within the skin as a ventilation system.

The final reason is quite simply that air sacs would have reduced the upper body weight of dinosaurs.

The discovery that the respiratory system of dinosaurs closely resembled that of birds has added to the now almost conclusive evidence that birds and dinosaurs are closely related. It has also dispelled the idea that dinosaurs were slow and sluggish, since this very efficient respiratory system would have made dinosaurs very active and able to chase down prey relatively quickly.