This is corn on the cob:

Corn on the cob, in cross section, stolen from http://www.istockphoto.com/file_thumbview_approve/214165/2/istockphoto_214165-co rn-cob-cross-section.jpg

Corn on the cob, in cross section. Stolen from http://www.istockphoto.com/file_thumbview_approve/214165/2/istockphoto_214165-co rn-cob-cross-section.jpg

This is a shish kebab:

Most tetrapods are like shish kebabs: a whole lot of meat stuck on a proportionally tiny skeleton.  If you don’t believe me, you can look at the human and cow neck torso cross-sections in Matt’s last post, or check out this ostrich-neck cross-section from his 2003 Paleobiology paper:

Ostrich neck in cross section, CT scan.  From Wedel (2003a: fig. 2)

Ostrich neck in cross section, CT scan. From Wedel (2003a: fig. 2)

Remember that this is a freakin’ ostrich — of all extant animals, one of the ones with a most extreme long, skinny neck.  And yet, if sauropods were muscled like ostriches, then their necks would have looked like this in cross section:

Putative shish kebab-style sauropod neck in cross section.  Ostrich soft-tissue from Wedel (2003a: fig. 2), Diplodocus vertebra cross-section from Paul (1997: fig. 4) scaled to match size of ostrich vertebra

Putative shish kebab-style sauropod neck in cross section. Ostrich soft-tissue from Wedel (2003a: fig. 2), Diplodocus vertebra cross-section from Paul (1997: fig. 4) scaled to match size of ostrich vertebra

And soft-tissue reconstructions would have to look like this:

Diplodocus with its neck as fat as an ostrich's.  Modified from Paul (1998: fig. 1F)

Diplodocus with its neck as fat as an ostrich's. Modified from Paul (1998: fig. 1F)

Which, happily, no-one is suggesting.  Instead, published reconstructions of sauropod neck soft-tissue are startlingly emaciated.  As exhibit A, I call this pair of Greg Paul cross-sections:

Diplodocus and Brachiosaurus neck cross-sections, showing very light musculature.  From Paul (1997: fig. 4)

Diplodocus and Brachiosaurus neck cross-sections, showing very light musculature. From Paul (1997: fig. 4)

(Yes, the Diplodocus on the left is the one I used in the photoshopped ostrich cross-section above.  It’s instructive to compare Paul’s original with the What If It Was Like A Big Ostrich version.)

Paul’s reconstructions seem to be widely considered too lightly muscled.  But even the very careful and rigorous more recent reconstructions of Daniela Schwarz and her colleague show a neck much, much thinner than that of the ostrich:

Diplodocus neck cross-sections.  From Schwarz et al. (2007: fig. 7a)

Diplodocus neck cross-sections. From Schwarz et al. (2007: fig. 7a)

Although Schwarz has put a lot more soft tissue onto the neck vertebrae than Paul did, it is still a tiny proportion of what we see in extant animals — even the ostrich, remember, which has a super-thin neck compared with pretty much anything else alive today.  If sauropod necks were muscled as heavily as those of, say, cows, then the soft tissue would pretty much reach down to the ground.  But they weren’t: they were more like corn on the cob, with a broad core of skeleton and relatively little in the way of delicious edibles festooned about it.

So why is this?  Why does everyone agree that sauropod necks were much less heavily muscled than those of any extant animal?

It’s a simple matter of scaling.  A really big ostrich might have a neck 1 m long.  (Actually, ostriches don’t get that big, but let’s pretend they do because it makes the maths easier).  If the x meter-long neck of a sauropod was just a scaled-up ostrich neck, then it would be x times longer, x times taller and x times wider, for a total of x^3 times as voluminous and therefore x^3 times as heavy.  But the cross-sectional area of the tension members that support it is only x times taller and x times wider, for a total of x^2 times the strength.  In total, then, the neck’s mass/strength is x^3/x^2 = x times as great as in the ostrich.  (The sauropod neck’s mass also acts further out from the fulcrum by an additional factor of x, but that is cancelled by the fact that the tension in the neck also acts x times higher above the fulcrum.)

It seems intuitively obvious (which is is code for “I have no way to prove”) that you can’t reasonably expect the neck muscles of a giant ostrich to work ten times as hard as they do in their lesser cousins, which is what you’d need to do for the 10 m neck of, say, Sauroposeidon.  So simple isometric scaling won’t get the job done, and you need to restructure the neck.

But how?  Surely just reducing all the muscle around the vertebrae can’t help?  No indeed — but that is not really what sauropods were doing.  If you look at the typical sauropod-neck life restoration, you’ll see that the proportional thickness of the neck is actually not too dissimilar to that of an ostrich — rather thicker, in fact.  If you scaled an ostrich neck up to sauropod size and compared it with a real sauropod neck, you would find not that the soft tissue was too fat, but that the vertebrae were too thin.

And so we come to it at last: rather than thinking of sauropods as having reduced the amount of soft-tissue hanging on the cervical vertebrae, we do better to think of them as having kept a roughly similar soft-tissue profile to that of an an ostrich, but enlarging the vertebrae within the soft-tissue envelope.  Of course if you just blindly made the vertebrae taller and wider, they would become heavier in proportion, which would defeat the whole purpose of the exercise — but as everyone who reads this blog surely knows by now, sauropod cervicals were extensively lightened by pneumaticity.  By bringing air into the center of the neck, they were effectively able to displace bone, muscle and ligament away from the centre, so that they acted with greater mechanical advantage: higher epaxial tension members, lower hypaxial compression members, and more laterally positioned paraxials.

It’s a rather brilliant system — using the same volume of bone to achieve greater strength by displacing it outwards and filling the center with air (and, in doing so, also displacing soft tissue outwards).  And it will be hauntingly familiar to anyone who loves birds, because it is of course exactly what birds (and pterosaurus) have done in their long bones: the hollow humeri of flying vertebrates famously allow them to attain greater strength — specifically, resistance to bending — for the same volume and mass of bone.  It’s a neat trick when done with long bones, but it takes a truly awesome taxon to do it with the neck.

So maybe sauropods were not corn on the cob after all.  Maybe they were Hostess Twinkies.

Hostess Twinkie.  Not truly pneumatic, as the internal cavity is filled with adipose tissue rather than air, but do you have any idea how difficult it is to find good images of hollow junk food?

Hostess Twinkie. Not truly pneumatic, as the internal cavity is filled with adipose tissue rather than air, but do you have any idea how difficult it is to find good images of hollow junk food? Stolen from http://dixiedining.files.wordpress.com/2008/07/twinkie_070918_ms1.jpg

And now for something completely different

Now that I’ve finished my Ph.D at the University of Portsmouth, what am I going to do with the rest of my scientific life?  I’ve always said that I have no intention of going into palaeo full time: my knowledge is far too narrow for that, so that even if paid jobs were not in insanely short supply, I wouldn’t stand much chance of getting one.  And in any case, I’d hate to get into the all-too-common situation of being up against a friend for a position we both wanted. Throw in the fact that I really enjoy my computer-programming day-job and it seems pretty clear that what I need is an unpaid affiliation that lets me get on with lovely research.

Well: I am absolutely delighted to announce that, as of last month, I am an Honorary Research Associate in the Department of Earth Sciences at UCL.  It’s not just that UCL is such a well-respected institution — see that Wikipedia article for some details — more importantly, it’s where Paul Upchurch hangs out, as Senior Lecturer in Palaeobiology.  Sauropod fans will be familiar with Paul’s characteristically detailed and careful work, from his pioneering work on sauropod phylogeny (Upchurch 1995, 1998), through his and John Martin’s indispensible Cetiosaurus makeovers (Upchurch and Martin 2002, 2003) to the state-of-the art review that he lead-authored for Dinosauria II (Upchurch et al. 2004) and the Tokyo Apatosaurus monograph (Upchurch et al. 2005).  What many of you won’t know is what an excellent collaborator he is — quick, conscientious, insightful and diplomatic.  We’ve already collaborated on a few short papers (Upchurch et al. 2009 and a couple of Phylocode companion-volume chapters that are in press), and I hope there will be more in the future.

References

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!