Airheads

December 8, 2008

human-sinuses

A 3D reconstruction of the paranasal sinuses in a human (from Koppe et al. 1999). You also have paratympanic sinuses that pneumatize the mastoid process of the temporal bone (feel for an inferiorly-directed, thumb-size protuberance right behind each ear).

pig-skull-x-ray

An x-ray of a pig skull, from here. Can you see the outline of the brain-shaped endocranial cavity?

rhino-skull-ct

How about in this x-ray of a rhino skull? Image courtesy of Kent Sanders.

cow-sinuses-small

A sectioned cow skull. The bottom half of the endocranial cavity is exposed in the horizontal cut. The vertical cut shows the tiers of sinuses that make up most of the volume of the skull. I think that the middle tier (the large, butterfly-shaped space) is the front part of the endocranial cavity and housed the most rostral bits of the brain; note that it is completely surrounded by sinuses.

bighorn-small

Part of a bighorn sheep skull. The pneumatic horncores of bighorns are a useful antidote to the idea that pneumatic bones must be weak.

elephant-skull-sectioned-480

A cross-section of an elephant skull, courtesy of Project Gutenberg. The cavity at the back marked ‘b’ is the endocranial cavity that holds the brain. The big tube running through the middle is the nasal airway. Everything else is pneumatic. Note that the brain is entirely surrounded by sinuses.

exploded-proboscidean-skull-480

A blown skull of a proboscidean from the bone cellar at the Humbolt Museum. I snapped this on the last day in collections,  on a mad scramble to get whatever non-sauropod pics (gasp!) I might want later. The bumps to the upper right are the occipital condyles; the skull is in left lateral view facing down and to the left.

alligator-paratympanic-sinuses

Paratympanic sinuses (green) surrounding the brain (blue) of an alligator, from David Dufeau’s homepage. Go there for a lot more mind-blowing images of sinuses. The snout of this gator is filled with paranasal sinuses, they’re just not shaded in here.

rhinoceros-hornbill-480

Sectioned skull of a rhinoceros hornbill, which is pretty much completely filled with paranasal and paratympanic sinuses. Even the lower jaw is pneumatized.

Okay, so now you know that mammals, crocs, and birds are all air-heads. What does any of this have to do with sauropods? Well…

  • Archosaurs and mammals evolved cranial pneumaticity independently. Does that mean that cranial pneumaticity is easy to evolve (since it evolved more than once) or hard to evolve (since it only evolved twice)? This is relevant to the question of how many times postcranial pneumaticity evolved.
  • Archosaurs evolved cranial pneumaticity before they evolved postcranial pneumaticity. Does that mean that postcranial pneumaticity is the application of a pre-existing developmental program (bone pneumatization) to a new anatomical region (the postcranial skeleton)? Or did the developmental control of pneumatization have to evolve de novo in the postcranium?
  • The development of cranial pneumatization in mammals and postcranial pneumatization in birds seems to  follow similar rules. Does that mean that we can apply lessons learned from, say, the development of human sinuses to understand the development of sauropod vertebrae?
  • Sauropods and big-headed mammals like elephants have this in common: at the front end they’ve got a big chunk of pneumatic bone. In sauropods, it’s the neck; in elephants, it’s the head. In both cases the big pneumatic organ makes up close to a tenth of the animal’s volume. I don’t know what else to make of that, but maybe you can get mileage out of it at a cocktail party.

I posted these because I was inspired by Darren’s post on dome-headed elephants, because they’re cool, to maybe demystify sauropod pneumaticity a little, or perhaps to re-mystify skeletal pneumatization in general. You have a pneumatic cavity between your brain and your monitor right now. How much time have you spent thinking about that (when you didn’ t have a sinus headache)?

Next time: more Berlin goodness.

original-gangsta

UPDATE: By utter coincidence, Ohio University put out a news story about Larry Witmer’s work on sinuses yesterday. Hat tips to Yasmani Ceballos Izquierdo, who posted the link on the DML, and to Mike for sending it on to me. As long as you’re going over there, remember that Larry is one of the Good Guys and puts his papers up for public consumption; the new dino sinus paper is here. It’s great, but it makes the pictures I used here look pretty pathetic. Go have fun!

I just got word from the History Channel that their documentary “Evolve: Size” will air Saturday, Nov. 8. Kent Sanders, Brooks Britt, and I filmed a long segment for this back in May, covering pneumaticity in sauropods. Hopefully it didn’t all go to the cutting room floor! With any luck, you’ll see the results of this:

sauropods-the-next-generation-4801

Check local listings for showtimes.

UPDATE: IMMEDIATE REACTION

Hey, not bad. Good stuff:

  • I especially liked that they ascribed the evolution of large size in sauropods to several factors–high plant productivity, efficient food gathering (just biting, no chewing), and, yes, pneumaticity. But pneumaticity was at best an accessory adaptation for large size, and not a prime mover. I was worried that its importance would be overstated–“AIR-FILLED bones made these GIANTS into the HUGEST creatures EVAR!!1!” That’s some impressive restraint for a documentary these days.
  • The bit about pneumatic bones being light but also strong is great. I’m glad they worked in the pneumatic horns of bighorn sheep.
  • I’m really happy that they showed the process of CT scanning the vertebra, partly because It’s never been shown before on TV (to my knowledge), and partly for purely selfish reasons: it’s just cool. Too bad they didn’t have time to show Kent Sanders discussing the results of the scan.

Some clarifications:

  • Brooks Britt is not a grad student now, he’s an Associate Professor of geology at BYU. He pioneered the use of CT to study pneumaticity in dinosaurs when he was a grad student at the University of Calgary (Britt 1993). I am glad that they got the bit in about Brooks first suggesting to me that I should CT scan sauropod vertebrae. He got me into this, and it’s nice to have that recognized.
  • At one point the narrator says, “Wedel suspects that the bones were not only light and easy to lift, they also helped get oxygen directly to the muscles, fed by a system of air sacs throughout the neck, similar to birds today.” Woof–I didn’t say that! They got the ventilatory air sacs in the thorax and abdomen–the ones that blow air through the lungs–confused with the pneumatic diverticula up in the neck. There is no evidence that diverticula play any role in gas exchange for the tissues they are adjacent to, and there is strong contrary evidence. Physiologists have measured how much gas exchange goes on in the avian respiratory system, and where that gas exchange occurs. Ninety-five percent of the gas exchange happens in the lungs, and almost all of the remainder happens in the abdominal air sacs, which are immense and fairly convoluted because they enclose the viscera like a nut-shell (thanks to Wetherbee [1951] for that wonderfully accessible image). It’s a fairly minor thing, I guess, it’s just frustrating to spend so much time working on this and then have an obvious mix-up like that sneak in.
  • In the space of about ten minutes, sauropods are described as “freaks of nature” twice! This is a bit irritating–they are only freaks of nature from our limited, human point of view. Big sauropods had appeared by the late Triassic and huge ones by the Early Jurassic, and they stayed huge and successful through the Jurassic and Cretaceous. For all that they were immense and morphologically derived, sauropods were also just critters. They weren’t mutants, they were functioning and apparently successful members of their ecosystems for a long time, like any other organisms. Possibly, though, long exposure has acclimated me to the just-critters aspect of sauropods more than most folks. :-)

It seems churlish to write so much about a segment that was actually pretty great and right on target except for a few, comparatively minor missteps. Overall I’m thrilled that it turned out so well. See it if you get a chance–your own thoughts are welcome, good, bad, or otherwise.

References

  • Britt, B. B. 1993. Pneumatic postcranial bones in dinosaurs and other archosaurs. Ph.D. dissertation, University of Calgary, Calgary, 383 pp.
  • Wetherbee, D. K. 1951. Air-sacs in the English sparrow. Auk 68:242–244.

You know, we have not done what we intended with this blog. We intended to post pretty pictures of sauropod vertebrae, sketch a few lines of text a la our inspiration, and call it good. But not one of us is capable of shutting up–me least of all–so we sit down to write 6 lines and end up writing 60 or 600.

Well, not this time. Here’s BYU 12866, probably a fifth cervical, almost certainly from Brachiosaurus, plus some CT cross-sections (the cross-sections have been straightened up a little to correct distortion in the specimen; see figure 12 here for the unexpurgated version). If they haven’t been defined before, camerae are big chambers and camellae are small chambers.

Hat tip to Mike from Ottawa for the title.

Kthxbai!