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Matt Wedel: Hunting the inflatable dinosaur
You may know that birds have hollow bones (you may even have observed them in your Thanksgiving turkey see photo below) but you may not have considered how those airspaces work. First off, a bird's respiratory system is a bit different from ours: their lungs are ventilated by a set of forward air sacs and another set of rear air sacs. With this arrangement, birds can get oxygen out of air while both inhaling and exhaling an efficient system, which allows birds to perform feats that mammals could never hope to accomplish, like flying over the Himalayas. As a baby bird grows, its air sacs develop a system of outgrowths and tubes that invade and pneumatize the bird's bones, forming hollows. The forward air sacs are connected to the hollow bones at the front of the bird's body, and the rear air sacs are connected to the hollow bones at the back of the bird's body. This system of pneumatized bones provides the bird with a skeleton that is both light and strong.
Matt wondered if the same sort of dual air sac system might be responsible for the airspaces in birds' dinosaur ancestors. But of course, air sacs are built from thin, soft tissue and so, are not preserved in any known dinosaur fossils. How could fossilized pneumatic bones alone reveal which air sacs dinosaurs had? After studying up on chicken development, Matt came up with a way to test the hypothesis that dinosaurs had a fore/rear air sac system. Birds' pneumatic bones always develop in the same way: soon after the bird hatches, the forward air sacs begin to invade the vertebrae in the neck, and later, the rear air sacs begin to invade the vertebrae towards the pelvis. Then the airspaces spread through the vertebrae until the hollows meet in the middle and the whole vertebral column is pneumatized...Well, at least usually that's how it works but Matt knew that occasionally, due to a small developmental glitch, the airspaces in the necks and the pelvises of some birds never quite meet up, leaving them with a few non-pneumatized vertebrae in the middle of their spine. Matt reasoned that if the same dual air sac system were present in dinosaurs, then some dinosaur skeletons should have the same "gap" in the pneumatization of their spines. But he had little hope of actually finding that key piece of evidence: "I didn't really expect to find such things in dinosaurs not because they're not there, but because they don't seem to be common and our sample size is so small."
Matt decided to focus on other research instead of chasing an elusive piece of evidence evidence that turned out to be not so elusive after all. While rereading some classic scientific papers from the early 1900s on sauropod dinosaurs, he noticed that the authors described pneumatized vertebrae in the fore and rear of the animals but solid vertebrae in the middle! The key evidence had been available for a century but no one had even noticed the pattern until Matt came along with a hypothesis to explain it. And emailing a colleague about his "re"-discovery turned up yet another case of the telltale gap in a tyrannosaur.
For Matt, these examples sealed the deal: dinosaurs did have a fore/rear air sac system. As he puts it, "There was already plenty of evidence that saurischians [the dinosaur clade containing theropods and sauropods] had air sacs like birds but this, to me, is the last stake in the coffin...There's just no other way you could get this pattern."
Establishing that dinosaurs had a bird-like air sac system leads to yet another question: did these pneumatized dinosaurs breathe like birds did they use their air sacs to pump air through their lungs in a continuous flow? If saurischian dinosaurs did have this sort of "hot-rod" respiratory system, it could overturn current views of how active dinosaurs were. But answering that question will require more work, suggests Matt. "Solving the problem of how dinosaurs actually breathed is going to take the intersection of several different lines of research. I can bring the air sac piece of the pie, and say 'Well at least they had the soft tissue gear to make this happen.' Somebody else is going to have to look at birds and alligators and as many intermediates as they can, and look at muscle attachment scars to figure out how the trunk muscles evolved...And somebody else is going to have to come with the rib movement and say 'If these muscles contract in such and such a way...it's going to drive air through the lungs.' You've really got to have all three pieces: somebody who knows the bones, somebody who knows the muscles, and somebody who knows the air sacs."
While we wait in the wings to find out how dinosaurs breathed, Matt is delving into new questions: how might developmental constraints have shaped the evolution of pneumaticity, how do pneumatic bones affect dinosaur body mass, and how has that mass change impacted dinosaur evolution? Whatever the answers to these questions, Matt intends to enjoy the journey to the answers: "The most exciting part of science for me is having a problem that you apparently can't solve, figuring out the crucial test for it, and then going and finding the evidence that tells you one way or the other like the problem of whether or not dinosaurs had air sacs like birds. Figuring out the test was a rush because it was taking information from one area that had been known a long time (which is birds) and applying it to this problem in dinosaurs. So it was a fun intellectual trip to figure that out and then it was really exciting to actually start finding examples. And that, to me, is about as good as it gets to be able to go out and satisfy your own curiosity about something by designing a test...and at the end of it, you've learned something about the living world but you've also learned something about how science works and about how much fun it is."
To learn more about Matt's research on this topic, see these related papers:
Photos provided by Matt Wedel.
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