Tutorial 3: Pneumaticity
November 3, 2007
It’s come up here a few times already–it’s hard to talk about sauropod vertebrae without bringing it up–but now it’s time to get it out in the open. In almost all sauropods, and certainly in all the ones you learned about as a kid, at least some of the vertebrae were pneumatic (air-filled). Now, this is a very strange thing. Most bones are filled with marrow, so if we find a bone that is filled with air, somebody’s got some ‘splainin’ to do.
Figure 1. Pneumatic bones of various animals. Compare the air spaces in the skull of a cow (A) and a hornbill (B) with those in the vertebrae of a turkey (C) and Apatosaurus (D). The front of the turkey vertebra was worn off with sandpaper. Erosion did the same thing for Apatosaurus. The vertebra, OMNH 1312, has a preserved height of 53 cm, but the neural spine is missing.
How does the air get into the bone?
You probably know more about pneumatic bones than you think, because you’ve carrying some around your whole life. Some of the bones of your skull are pneumatic, and we call the air-filled spaces sinuses. Your sinuses are connected to your nasal passages or the air-filled spaces in your middle ear—but connected by what? These connections are made and maintained by diverticula, which are pouches of epithelium (tissue that lines your internal surfaces) that grow out into the surrounding bones. For example, when you were a baby, pouches of epithelial tissue in your nose pushed up into the bones of your forehead. The spaces enlarged as you grew up, and today they form your frontal sinuses. But those sinuses are still lined with epithelium that is much like the inner lining of your nose, and the sinuses are still connected to your nasal passages, as you may discover when you have a cold. The air-filled pouches of epithelium that fill your sinuses are called pneumatic diverticula. The growth of the diverticula into the bones produces the pneumatic cavities, or holes in the bone, that house the diverticula.
In mammals, pneumatic bones are normally only found in the skull (there are very rare cases of diverticula getting loose and invading the first cervical vertebra). But in birds almost any bone in the body can be pneumatized, by diverticula of the lungs and air sacs. The lungs of birds are very different from our lungs–in fact, they are unique in the animal kingdom. The lungs themselves are small and not very flexible, but they are attached to a system of large air sacs in the thorax and abdomen. These air sacs are empty—in other words, they contain no tissue except a thin lining of epithelium. Like us, birds breathe by movements of muscles and bones, but instead of expanding and compressing the lungs as we do, the breathing movements of birds expand and compress the air sacs, and the air sacs blow air through the lungs. The air sacs are connected in such a way that birds get fresh air blown through their lungs when they inhale, and then again when they exhale (fresh air is stored in some of the air sacs between inhalation and exhalation). This constant flow of fresh air through the lungs (which are arranged into tubes rather than small sacs, like ours) means that birds have the ability to pull much more oxygen out of the air than mammals can.
In addition to providing large amounts of oxygen, the air sacs give rise to a network of pneumatic diverticula. These diverticula spread throughout the body: in between the internal organs, between the bodies of the muscles, and even under the skin. If one of these diverticula comes into contact with a bone, it may press into the bone in the same way that the diverticula of your nasal cavities pressed into the bones of your forehead when you were young. Because the diverticula go just about everywhere, they can pneumatize almost all of the bones in the body. In some birds, such as pelicans, almost the entire skeleton is pneumatic, but in most birds only the vertebrae, sternum, hip and shoulder bones, and humeri and femora (upper arm and leg bones) are pneumatic.
Figure 2. CT slices through cervical vertebrae of Apatosaurus (left) and a swan (right). Although the two animals are very different in size, the construction of their vertebrae is very similar. The Apatosaurus vertebra, OMNH 1094, is 51 cm long. The swan vert is 2.5 cm long (1/20 as large).
What does this have to do with sauropods?
If a bone is pneumatic, the air has to get into the bone through a diverticulum, and the diverticulum has to get into the bone through a hole. So almost all pneumatic bones have one or more large holes on the outside, which are the pneumatic foramina. Human medical histories and experiments on birds have shown that these pneumatic foramina must remain open for a pneumatic bone to develop properly and be maintained. If the foramen is closed—for example, by a disease or injury—the air space inside the bone will eventually be replaced by new bone growth.
In addition, pneumatic bones tend to have relatively large, smooth-walled chambers inside, compared to non-pneumatic bones that are filled with marrow. These chambers have a distinct appearance and they are not easily confused with anything else. So if we find a bone with a good-sized foramen leading to big internal chambers, we can infer that the bone was pneumatized. No other anatomical system makes the same traces on the skeleton.
A lot of sauropod vertebrae are crazy pneumatic. In fact, the only non-pneumatic vertebrae we’ve shown so far are these, and you can see big obvious pneumatic foramina in the vertebrae shown here and here and here and here.
Figure 3. Reconstruction of the respiratory system of a diplodocid sauropod. The left forelimb, shoulder, and ribs have been removed for clarity. The cervical vertebra is AMNH 7535, and the caudal vertebra is OMNH 2055.
Air, air, what is it good faer?
(Sorry, that’s my Scottish brogue coming out.) Pneumatic vertebrae tell us some important things about sauropods as living animals. First, sauropods clearly had some kind of air sac system like that of birds. I’ve spilled a lot of ink on that already, and you can find those papers here. Second, we can use the distribution of pneumatic vertebrae to plot the extent of the pneumatic diverticula. Recall that if a bone is to stay pneumatic it has to remain connected to an outside air source. So if we find pneumatic vertebrae from the front of the neck to the middle of the tail, which is the case in most diplodocids, then we know that pneumatic diverticula spanned that whole distance as well.
Finally, it is surely no coincidence that the largest and longest-necked terrestrial animals had mastered ultralight construction. How light is ultralight? Well, the vertebrae of most sauropods were 60% air by volume, and in brachiosaurids like Sauroposeidon that number could be up to 89%. That’s a handy thing to have if you want to hang a long neck off your front end. Every major sauropod clade–Mamenchisauridae, Diplodocoidea, Brachiosauridae, and Titanosauria–had at least one member with a 9-meter neck, and the first three had members with 12-meter necks.
On a personal level, pneumaticity is also a great intellectual playground. There are so many things we don’t know about how it works, even in living birds. How do these spaces form, and what are the physiological controls? Why all the variation among clades, in birds and non-avian dinosaurs alike? Why are some bones 50% air and others 75% and still others 90%? Why do birds pneumatize the bones of their skeletons in the same order as their dinosaurian ancestors? How do body size and pneumaticity influence each other, in development and in evolution?
Morpheus told Neo that the Matrix is a system, with rules, and some rules can be bent and others broken. I want to be Pneo. I want to understand the rules. I want to see the code.
I want to play, too!
Groovy! Pneumaticity is wicked cool, and you don’t have to have a zillion dollars and an ion reflux pronabulator to get a good look at it. The holidays are coming up, and bringing with them the annual spike in the availability of turkey bones (you can get ‘em from the neighbors if you aren’t having turkey yourself). Boil the vertebrae or humeri (the “drumsticks” of the wings) for half and hour or so to get all the soft tissue off, then soak them overnight in ordinary drugstore hydrogen peroxide to degrease them. Then you can use a small hacksaw or a Dremel to cut them open and see the air spaces inside, or you sand off the ends of the bones with sandpaper. The spaces you’ll see are identical to the air spaces in sauropod vertebrae, just a little smaller.
If you’re really ambitious, figure out what the bone-to-air ratio is in turkey vertebrae, and compare that to what you find for the humeri. There are a handful of papers on bone-to-air ratios in bird limb bones, but there is almost zero published data on the same ratios in vertebrae, or on how the bone-to-air ratio compares in vertebrae and limb bones of the same animal. There is a nice opportunity here for someone with little or no formal training to make a real (and publishable) contribution. Gimme a holler if you’d like to know more.