There is strong evidence to believe that pterosaurs possessed a respiratory system similar to what can be found in modern birds. We know that pterosaurs were powerful fliers and therefore would have required prodigious amounts of oxygen in order to power their huge wings. But how did they supply oxygen quickly enough and in sufficient amounts throughout their bodies in order to maintain the energy needed for flight? There is much that we know (or at least can guess) from the numerous fossilized skeletons of these flying reptiles. David Unwin, a paleontologist from Leicester, England has written a marvelous book on pterosaurs and in it he describes how the pterosaur respiratory system worked.1
Pterosaur abdomens were tightly packed with various internal
organs, leaving little room for inhalation; however, when the pterosaur flexed
the muscles on the sides of the abdomen, sufficient space was provided for the
lungs to expand.
The pterosaurs relied on air sacs to provide them with needed oxygen. Air sacs are membranes extending from the lungs, usually forming several sets of “air bags” throughout the torso. Air sacs also extend into the interior of the bones, in the form of tiny tubes, pumping additional oxygen to more remote areas of the body. In order to have sufficient space in their bones for these air sacs tubes, the pterosaurs developed hollow bones. Hollow? Yes, their bones were like hollow pipes through which ran air sac tubules bringing fresh oxygen quickly to many parts of the body all at the same time, a very important principle when exerting tremendous amounts of energy while flying. Pterosaurs weren’t the only Mesozoic denizen to have hollow bones and air sacs. Many of the Saurischian dinosaurs possessed air sacs, especially within their bones. And today, modern birds, like their Saurischian ancestors, also have air sacs within their abdominal cavity as well as within their bones. Why are these air sacs so important? The table below provides some answers.
|Lightening the Load||When thin-walled air sacs appear in bones (called pneumatization) they replace part of the interior of the bones resulting in lighter bones, an important asset for flying pterosaurs or a massive Supersaurus.|
|Greater Circulation of Oxygen||The combination of lungs and air sacs provides more fresh oxygenated air to the body than lungs alone. This was important for animals that were large like Brachiosaurus and those that expended tremendous amounts of energy such as pterosaurs when flying. Fresh oxygen could quickly reach not only the lungs, but other areas of the body, powering muscles and fueling the brain.|
|Reduction of Body Temperature||With air sacs placed throughout the body (their location varied depending on the dinosaur or pterosaur) it was easier to cool down a body heated by strenuous activity or external temperatures. And if the body didn’t have a good internal thermometer to start with (perhaps they were exothermic), this ability would become essential.|
Peter Ward in his book Out of Thin Air tells us that modern birds are 33 percent more proficient at obtaining oxygen from the air (at sea level) than mammals. As altitude increases, so does the difference between bird and mammalian abilities at extracting oxygen. At 5,000 feet, a bird may be as much as 300 times more efficient than a mammal.2
Modern birds have a very efficient method of breathing, unlike mammals. When a mammal (such as us) inhales, air is pulled directly into our lungs. Here, in the lungs, a small percentage of the oxygen is extracted from the air and goes into the bloodstream. At the same time, the bloodstream releases carbon dioxide into the lungs to be exhaled. The next breath starts the process anew: inhaling as the diaphragm pulls in air and exhaling as the diaphragm pushes it out. This all sounds pretty efficient, especially since it keeps us alive. But the respiratory system of a bird is even more efficient since it can extract more oxygen from the air than a mammal.
How a bird circulates the air in its body is also a key factor in how they utilize oxygen. As we saw above the air in human lungs goes two directions: in and out. But the air flow in birds passes through the lungs in only one direction. How does this work? Well, when a bird breathes in air, it goes directly into air sac network not the lungs. After it has circulated in the air sac network, the carbon-dioxide-laden air passes into the lungs where the exchange of air occurs and is exhaled. This way the air only flows through the lungs in one direction immediately ridding the carbon dioxide from the body. This entire process enables the efficient absorption of oxygen from the air as well as the dispersal of CO2 from the body. It is this same respiratory process (or at least one similar to it) that paleontologists think was used by both pterosaurs and many Saurischian dinosaurs.3 Only recently has it been discovered (2009) that the crurotarsi used this same means of respiration. For more on this recent discovery see the article on the cru../Dinosaurs/crurotarsi.html#respirationrotarsi.
A pterosaur aided by a lighter skeletal structure, powerful wings, an excellent respiratory and ventilation system, and a big flocculus was able to rule the Mesozoic skies.
1. Unwin, pp. 119-122
2. Ward, Out of Thin Air, p. 174.
3. Bakker, Dinosaur Heresies,pp. 363-364