Over the next few months we intend to post a series of short commentaries exposing common myths in paleontological research that have biomechanical implications or relationships. In some cases, these myths extend to work on living animals, as well. Today's little "myth" is one of those that does, in fact, crop up in both studies on fossil animals and living ones.
Here is a cross section at the midshaft of the humerus of the pterosaur Bennettazhia; the trabecular braces have been digitally removed to just show the outer shell of bone (the braces are little struts that run from one side to the other inside the bone):
As you can see, this bone is exceptionally hollow. A similar degree of hollowness is seen in a few living birds, such as pelicans. Many other birds also have hollow limb bones (though usually not quite as hollow as seen above in Bennettazhia) and nearly all birds have some hollow bones in the core of their body (vertebral column, especially).
The typical conclusion from observing these hollow bones is that birds and pterosaurs have (or had) light skeletons. It's simple, clean, and intuitive. Unfortunately, it's also wrong. In reality, we have known since the 1970's that birds do not have lighter skeletons than that of other terrestrial vertebrates (e.g. mammals), and there is no evidence that pterosaurs had light skeletons, either (see work by Prange and others in 1979). This may seem rather odd, but there are two key elements here:
1) A hollow bone is not necessarily lighter than a solid one if they do not have the same diameter. Birds and pterosaurs tend to have wide bones.
2) A hollow bone is not necessarily lighter than a solid one, even given the same diameter, if the density of the material making up the bone is different. Betsy Dumont showed very nicely in 2010 that birds have denser bone material than mammals.
So, as it turns out, while the bones of many birds and pterosaurs are/were extremely hollow, they are also wide, dense, and stiff - and this means that the total mass of bone is about the same as for a "typical" skeleton, like that of a mammal (i.e. thick walls and filled with semi-liquid marrow). This might mean that the mechanical efficiency of pterosaur skeletons was very high - in other words, they may have had very strong skeletons for the same mass of material. My work shows that this is true for part of the pterosaur skeleton (base of the wings, particularly) but no one has done an exhaustive look across the entire pterosaur skeleton to see if this holds true overall.
The issue of hollow bones and the potential for lightening the skeleton (or now) has been well addressed by Matt Wedel with regards to sauropods. Check out his excellent blog post on it here.
But there you have it - birds don't have "light" skeletons, and it is likely that pterosaurs didn't, either. As a parting bit of fun, here are some CT scans through the middle of the upper arm (humerus) and thigh (femur) bones of various birds, showing the extreme variation in the degree of hollowness: