Sauropod Vertebra Picture of the Week

I had a new paper come out today. Unofficial supplementary info here, PDF here. I would have had all this ready to go sooner, but the paper came out sooner than I expected. In fact, I didn’t even know that it had been published until Andy Farke (aka the Open Source Paleontologist) wrote me for a PDF. Turnabout’s fair play, I suppose, because last year I congratulated Stuart Sumida on his Gerobatrachus paper before he knew it was out. I guess letting the authors find out through the grapevine that their stuff has been published is part of the “value added” that commercial journals provide. ;-)

Anyway, I’m happy the paper is out, finally. It’s the third chapter of my dissertation, but with teaching and traveling to Spain and such I didn’t get it submitted until last January. I had to forcibly bite my tongue during the Aerosteon saga last fall, when such a big deal was made about the absence of pneumatic hiatuses in non-avian dinosaurs. This despite the facts that there are several good reasons to expect pneumatic hiatuses to be rare, and that pneumatic hiatuses are not the Rosetta Stone or magic bullet for air sacs in saurischian dinosaurs. They’re more like the cinderblock that broke the camel’s back, given all the other evidence for air sacs.

In fact, the structure of the new paper is built around the idea that there are several tiers of evidence for bird-like air sacs in saurischians. Those tiers are:

1. The presence of postcranial pneumaticity at all. Some of the first authors to get interested in the implications of pneumaticity for dinosaurs argued that pneumaticity probably implies an air sac system, and left it at that. Later workers have tended to denigrate this argument as overly simplistic–just because some of the postcranial skeleton is pneumatic does not mean that the animal’s air sac system was necessarily like that of birds–but it’s not actually a bad argument. We can imagine lots of ways to get air into the postcranial skeleton, but for tetrapods the only system that we have any evidence for is diverticula of a lung/air sac system like we see in birds.
2. The distribution of pneumaticity in the skeletons of most saurischians and pterosaurs is  diagnostic for specific air sacs, namely the cervical, clavicular, and abdominal air sacs that we see in birds. This is what Pat O’Connor and Leon Claessens established so firmly with their work on mapping parts of the respiratory system to skeletal domains in birds.
3. The evolutionary patterns of pneumatization in sauropods and theropods parallel the development of pneumatization during ontogeny in birds. Or, more economically, ontogeny recapitulates phylogeny in this system. This is more evidence that the observed patterns of pneumaticity in the skeletons of birds and non-avian saurischians are produced by the same underlying process of diverticula developing from different air sacs in a highly conserved order–even if we don’t know why things evolved, and continue to develop, in the order that they do. And it’s better evidence, because it accounts for more observations (points 1 and 2 can be established from single specimens) and ties postcranial pneumaticity in all saurischians, living and extinct, into a more coherent picture.
4. Pneumatic hiatuses are more evidence that the postcranial skeleton is pneumatized by diverticula from more than one part of the respiratory system. Not the only evidence–we already suspect this quite strongly based on points 2 and 3–but more evidence. It is possible that the diverticula of extinct animals behaved differently than those of all extant birds, and diverticula from a single source could conceivably pneumatize the whole vertebral column. Possible. Conceivably. How likely? Dunno–our n on this is either 1 (if you count all living birds as a batch) or several hundred (if you count each of the species that Pat O’Connor has dissected and injected). Pneumatic hiatuses offer another level of evidence, because they can potentially show that the patterns of pneumaticity in fossil taxa are inconsistent with pneumatization from a single point. That’s how they work in chickens, and that’s how they may work in non-avian dinosaurs, as long as diverticula don’t leapfrog over  some bones without leaving any traces, or at least don’t do that very often.

For the record, I don’t think that pneumatic hiatuses are stronger evidence than point 3; if I was ranking the tiers based on importance I would put 3 at the top. Pneumatic hiatuses ended up being last in the paper because 1-3 were basically review material, and it made sense to group them together before the big  bolus of description.

[UPDATE the next day: also, I just realized that those 4 are not the same as I used in the paper! In the paper I left out 1, advanced 2 and 3, and added a different number 3, which is pneumatization of the pelvic girdle and hindlimb. I tend to forget about that one because the evidence in sauropods is underwhelming so far. And arguably this is just another aspect of 2 (above), or if you like you can think of 5 tiers. They say consistency is the hobgoblin of small minds!]

The importance of pneumatic hiatuses remains to be seen; there might not be enough of them to tell us very much, or we might find that leapfrogging diverticula exist and are common (we’d then need a way to sort hiatuses caused by multiple sources of diverticula from those caused by leapfrogging diverticula). But they’re important to me, for a couple of reasons.

First, they’re probably one of the two or three best ideas that I’ve had in my life. When I realized that pneumatic hiatuses could potentially indicate pneumatization from multiple sources it really was like a light going on in my head. I walked around seeing stars all week.

I got the idea from this figure, from King (1957):

On the left King has drawn the vertebral columns of several chickens, and shaded in the pneumatic regions. Blocks of pneumatic verts separated by apneumatic gaps represent pneumatization from different sets of diverticula. I remember very vividly sitting in the Padian lab reading this paper and thinking, “if we found one of those in a dinosaur it would be the money.” Then I suddenly sat up straight, then stood up, then paced around the room a few times to burn off the discovery energy. I had a very profound need to tell someone. I don’t remember who I told, but it was probably Mike.

The other reason that pneumatic hiatuses are important to me: they are now one of those cool little cases that show that paleontology can be a predictive science. If you want to test a hypothesis in the experimental sciences you manipulate the conditions and see what happens. Historical sciences don’t usually give you that option. But you can play What If? As in, “If hypothesis A is true then we ought to see such-and-such evidence.” In 2003, I predicted that if sauropods had abdominal air sacs we ought to see pneumatic hiatuses once in a while. Finding the evidence that validated the prediction was almost as much of a rush as having the idea in the first place.

The owner of Sauropod Pneumatic Hiatus #1 is Haplocanthosaurus CM 879, which is a cool animal but fairly pathetic as sauropods go. In my dissertation/job talks I would show the above picture and joke that I could probably beat up that animal on a good day. I found out about the pneumatic hiatus by accident, when I was poring over Hatcher (1903). In one of the figures near the end of the paper, Hatcher shows the centra of the fourth and fifth sacral vertebrae. I noticed that sacral 4 had a pneumatic chamber of some sort but sacral 5 did not. Then a few minutes later I had gotten to the plates at the back of the paper, and saw what looked like a pneumatic chamber on the first caudal. Somewhere in the dank, beer-flooded grottoes of my skull, the neuron fired.

This is the figure I put together, using images from Hatcher (1903), for a Jurassic Foundation grant to go see the material in the Carnegie Museum in 2005. It worked; they came through with $1500 for that trip and a week at BYU the same fall (to my immense shame, although the Jurassic Foundation is credited for funding on the first page, I see that I forgot to thank them in the acknowledgments. Belatedly: thanks, you guys rock, I suck). The pneumatic cavities are labeled as foramina because that’s what they look like in the drawing, and not having seen them I didn’t know any better. In fact they are fossae, but they are deep, invasive fossae and their morphology is not consistent with anything other than pneumatic invasion. (Pneumatic invasion!? Flee for your lives!!) See the paper for all the excruciating details. Note that the sacrals have unfused neurocentral sutures, so the animal was not fully mature when it died (there is probably a whole post ahead just on the neurocentral weirdness in this animal).

So that’s the story, for now at least. There are more pneumatic hiatuses coming, but those papers are still in the pipe so I can say no more for now. I’m sure when they come out some alert blogger will notice and e-mail me for a PDF, and then you’ll get the news here.

The moral of the story is that you can make real progress by reading lots of old, obscure stuff. Support–and abuse–your local academic library!

References

• Hatcher JB. 1903. Osteology of Haplocanthosaurus, with a description of a new species, and remarks on the probable habits of the Sauropoda, and the age and origin of Atlantosaurus beds. Memoirs of the Carnegie Museum 2:1–72.
• King AS. 1957. The aerated bones of Gallus domesticus. Acta Anatomica 31:220–230.
• Wedel, M.J. 2009. Evidence for bird-like air sacs in saurischian  dinosaurs. Journal of Experimental Zoology 311A.