X-Men Origins: Pneumaticity

May 8, 2009

The respiratory system of a pigeon injected with pink latex. Click for the unlabeled gory version.

The respiratory system of a pigeon injected with pink latex. Click for the unlabeled gory version.

In case you’ve missed it, William Miller has been asking some great questions over in the comment thread for “Brachiosaurus: both bigger and smaller than you think“. Here’s his most recent, which is so good that the answer required a post of its own:

…in birds, the air sacs are obviously useful for flight, and they might have been useful for weight lightening in sauropods: but the common ancestor would have been flightless and too small to need the lightening. So what drove their evolution in the first place, I wonder?

To which I say: oh, Alice, the rabbit hole is a lot deeper than that.

Introduction to the Three Mysteries

First, in birds the diverticula that enter the bones are a comparatively small subset of all diverticula. Visceral, intermuscular, and subcutaneous diverticula run between the guts, between muscles, and under the skin, respectively. These are usually more numerous and more extensive than the diverticula that enter the bones, and with rare exceptions, like the subcutaneous “bubble wrap” in pelicans, we have no idea what they do. If, indeed, they do anything. All a character needs to do to be hereditarily propagated is not compromise the survival and reproduction of its bearer. Diverticula could be mostly functionless products of developmental processes that are usually invisible to selection but sometimes produce useful exaptations, like lightening the skeleton, insulating the body, etc. Sort of the evolutionary equivalent of the fire extinguisher in your kitchen: most of the time it does absolutely nothing, but once in a while it is really, really useful.

Second, postcranial skeletal pneumaticity (PSP) starts in the cervical vertebrae in basal theropods and sauropodomorphs, and possibly also in pterosaurs (Butler et al. 2009). The vertebrae adjacent to the lungs and air sacs are not the first to be pneumatized. Rather, the pneumatic diverticula must have gotten out of body cavity and traveled a ways before they started impacting the skeleton. Assuming that one thing had to come before another and it didn’t happen in one saltatory leap, diverticula must have evolved before they started pneumatizing the skeleton.

Third, in the earliest evolutionary stages of pneumatization in saurischians, the amount of bone removed is completely negligible. In Wedel (2007) I calculated that in Pantydraco (Thecodontosaurus caducus at the time) and Coelophysis the pneumatic spaces in the bones accounted for 0.0017% and 0.17%, respectively, of the body volumes. The fossae in the Pantydraco vertebrae are not absolutely diagnostic for PSP, but they’re in the right place and hard to explain otherwise. The holotype individual is a juvenile, and it is possible that PSP might have been more extensive in an adult, but it could increase one hundred-fold and still only be 1/500 of the animal’s volume, as in Coelophysis. Although I haven’t run the numbers, a similar result probably hold for the basalmost sauropods with definitive PSP.

Pneumatic fossae in the cervical vertebrae of Coelophysis

Pneumatic fossae in the cervical vertebrae of Coelophysis

To sum up:

  1. Most diverticula in birds are not involved with pneumatizing the skeleton, so PSP can’t be the reason for their existence.
  2. In basal saurischians, the diverticula that pneumatized the skeleton must have evolved before they could start pneumatizing the skeleton, so PSP can’t be the reason for their existence, either.
  3. In the early stages of the evolution of PSP in saurischians, the amount of mass saved was negligible and could not plausibly have influenced natural selection, so PSP didn’t initially evolve to lighten the skeleton.

Lighten Up, Fatso

There is a complication on that last point, which requires a little digression on fat.

In birds, pneumatic diverticula don’t just replace bone tissue, they also take up space that would be occupied by fat in mammals, for example in the spaces between muscles and around plexuses of nerve and blood vessels. Any of you who have had the misfortune to dissect the brachial plexus of a mammal know whereof I speak–you spend most of your time carefully picking fat out from around the nerves and blood vessels. This isn’t gross subcutaneous fat that means an animal or person is obese, this is adipose tissue doing its other job of being a lightweight packing material. Mammal bodies put fat in those spaces because they need to occupied by something light and squishy and fat is the cheapest thing your body can build.

That may seem backwards; we think of fat as an energy store and therefore energetically expensive. But it’s cheaper to build than muscule or cartilage or skin, and lighter than any other tissue or fluid in the body. It has been observed that even when mammals are starving, they do not use the fat in the yellow marrow that fills the marrow cavities of long bones. This is utterly unsurprising if you think about how bodies work. Nature really does abhor a vacuum, at least biologically (cosmically, it seems to be the biggest thing ever). If a starving body used the fat in the marrow cavity, it would have to replace it with something else, and all of the alternatives are heavier and more expensive to build. If the fat was not replaced, a partial vacuum would develop which would cause serous fluid to weep into the space, and that would also be heavier and more expensive, and a great site for infection to boot (ask someone who has an edema).

Birds cheat the system by replacing the lightest of tissues with something even lighter: air, held in diverticula that are basically super-thin layers of epithelium. Possibly diverticula had been running around replacing fat for a long time before they first entered the skeleton, in which case the earliest stages of pneumatization would have been a continuation of pre-existing function of replacing superfluous connective tissue (fat and bone are both forms of connective tissue, along with cartilage, ligaments, tendons, mesenteries, fascia, and blood; blood is connective tissue the way snakes are tetrapods).

Although that will be difficult or impossible to test, it actually makes quite a bit of sense. Getting fat out of the way ought to be easy; the lipids can be mobilized into the bloodstream and the flattened cells could either be pushed out of the way or resorbed. Getting bone out of the way requires increasing parathyroid hormone, mobilizing blood-born multinucleated osteoclasts, and convincing them to digest bone where it needs to be digested, which I assume is a more complicated process from a regulatory standpoint (physiologists or cell biologists, please correct me if I’m wrong!). So it seems plausible that diverticula might have acquired the ability to replace fat early on, and the ability to replace bone much later, and that by the time they got started on the skeleton diverticula could have been lightening the body for a long time by removing little bits of superfluous fat.

This does not contradict my statement above that by and large we don’t know what diverticula do. Some diverticula run where there is no fat to replace. And healthy birds do carry some fat, like any healthy tetrapod. One would think that this energy-reserve fat would need to be protected from diverticula that would otherwise resorb it, but I don’t know how or if that happens, and I don’t know if anyone else does either. The amount of research on diverticula is basically nil.

I also think that fat-resorbing diverticula don’t solve the third mystery, they just pushes it back a level. The amount of mass saved by replacing the “packing” fat with air is probably negligible in most animals, and it certainly would have been so in the earliest stages of replacement, so the third mystery still holds if it  is restated as:

3b. In the early stages of the evolution of diverticular replacement of connective tissue in saurischians, the amount of mass saved was negligible and could not plausibly have influenced natural selection, so PSP didn’t initially evolve to lighten the body.

A dorsal vertebra of Haplocanthosaurus in anterodorsal oblique view. It's pneumatic, sure, but not THAT pneumatic.

A dorsal vertebra of Haplocanthosaurus in anterodorsal oblique view. It's pneumatic, sure, but not THAT pneumatic.

The Problem is the Solution

So, we seem to be stuck. We don’t know why diverticula evolved in the first place, and we don’t know what most diverticula do, and even the diverticula that lighten the body could not have initially evolved to do so.

One upshot of all this is that we need more research on possible  physiological functions of diverticula in birds. Oy! Ornithologists and avian physiologists! We’ve thrown you a bone, now throw us some data. Please?

Another upshot is that the erratic evolutionary pattern of PSP in Triassic and early Jurassic ornithodirans is maybe not entirely unexpected. Pterosaurs and theropods seem to have had PSP right out of the gate, but at least in theropods it was not enough to have done any good. Basal sauropodomorphs had little or no PSP, and not enough to have done any good below about the level of Eusauropoda. No non-dinosaurian dinosauromorphs have been found with PSP, but then we only have a handful of them and they’re all pretty dinky, so it’s possible it just hasn’t been recognized yet. Silesaurids, at least, had very pronounced, very thin laminae, which in derived saurischians are almost always associated with PSP. And ornithischians never had PSP at all, as far as we know.

My opinion is that an air sac system is probably primitive for Ornithodira, and that most of these lineages had pneumatic diverticula, but the speed with which they “discovered” extensive, skeleton-lightening PSP–ranging from “almost immediately” in pterosaurs to “after a while” in theropods to “after a long while” in sauropodomorphs to  “never” in ornithischians–varied because it was such an evolutionarily haphazard process. Basically, PSP had to evolve as a developmental accident, and in some lineages it got far enough to become visible to selection, and in others it did not, or took a long time to do so. That’s a pretty picture that makes a lot of sense to me. If I ever figure out a way to test it, I’ll let you know.

What's going on here? Why are ornithischians so lame?

What's going on here? Why are ornithischians so lame?

The Solution is the Problem

The absence of PSP in Ornithischia is still a right sod. Pterosaurs, theropods, and sauropodomorphs all evolved some level of PSP in the Late Triassic, even if it wasn’t enough to significantly lighten their skeletons at first. Why not ornithischians? If air sacs are primitive for Ornithodira, then ornithischians had the gear for 160 million years and never exploited it, when the other three major lineages of ornithodirans discovered PSP pretty fast out of the gate. And if air sacs are not primitive for Ornithodira, three out of four ornithodiran lineages still discovered PSP on their own, so why not Ornithischia? It’s a big mystery, any way you slice it.

What do you think?

References

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32 Responses to “X-Men Origins: Pneumaticity”

  1. William Miller Says:

    Wow, thank you very much.

    …Pigeons really look skinny without their feathers, don’t they?

    That’s very interesting, and mysterious.

    While (as you say) the diverticula don’t necessarily need to cause a benefit to exist, if they didn’t in a particular lineage, wouldn’t they have disappeared from that lineage over 100+ million years of evolution simply because mutations that prevented their developing weren’t selected against? Kind of like eyes in cave fish: usually when there’s no evolutionary pressure, it’s easier to lose a structure than build one – random mutations are more likely to destroy something than create.

    On the other hand, the fat thing might have made that not an issue – but that would only be true if nutritional-expense was the crucial limiting factor, and I kind of doubt it was for Mesozoic herbivores. (Isn’t there a theory that food-supply isn’t usually limiting for herbivores in general, otherwise the Earth would be mostly denuded and the herbivores barely surviving – not mostly green and mostly healthy? My ecology class’s supplemental materials mentioned this, but I don’t know how well accepted it is, and Google didn’t help much because I didn’t know what to search.)

    Hmmm, that offers a possible line of attack on the problem. Sauropods were generally huge – even small ones pushing elephant size, which is really big for an ornithischian. Maybe the sauropods had a bigger need to conserve nutrition than ornithischians. Is there a way to tell from fossil bones how well nourished an individual was? If so, a study could show if sauropods were generally worse fed than ornithischians…

    Alternately, how early in the sauropodomorph lineage does PSP really become significant? Is it before or after they get really big?

  2. Zach Miller Says:

    Well, like the graphic suggests, maybe ornithischians (other than ceratopsians) are just lame. :-)

    But it really is a wierd problem. I was discussing it yesterday with Scott Elyard after reading the new pterosaur article. And I do forget that about non-skeletal diverticulae. Perhaps we’re looking at the problem the wrong way. Instead of asking why ornithischians did NOT develop PSP, maybe we should be asking why sauropods and non-avian theropods DID. It’s pretty obviously advantageous for birds and pterosaurs, but big and small carnivorous mammals (and squamates) do just fine without it.

    I’m with you on this one–pneumatic diverticulae probably is primitive for Ornithodira, but PSP probably evolved independantly in pterosaurs and saurischians. Sauropods went crazy with it (perhaps it allowed them to become enormous?) and theropods experimented with it throughout their evolution.

    I know that’s not much of an answer (or opinion) but it’s the best I can do.

    Also, I love that Sony introduced their handheld during the Mesozoic. I wonder if they supported it with decent software back then, because they’re doing a shitty job these days.

  3. LeeB Says:

    I don’t know if ornithischians are lame but they certainly are different.
    While the Pterosaurs, Theropods and Sauropods were busy removing bone from their skeletons the Ornithischians were equally busy adding it.
    Horns, plates, spines, armour, and crests.
    Titanosaurs also had armour but not to the extreme ankylosaurs did.
    Does this suggest some underlying difference in their physiology?

    LeeB.

  4. Will Baird Says:

    Does this suggest some underlying difference in their physiology?

    Thought: compare the scarring from muscle attachment on saurischians and ornithischians. Aim for specimens of approximately the same size and same locomotive methodology. Hunch says they’ll be different, in a nontrivial way.

  5. Matt Wedel Says:

    …Pigeons really look skinny without their feathers, don’t they?

    That is a nontrivial observation. Except for ratites, cranes, and storks, in most birds the plucked body outline is radically different from the feathered one. In The Inner Bird, Gary Kaiser argues that for most birds, feathers are like a full-on environment suit that is pierced mainly by beak, eyes, and feet.

    Is there a way to tell from fossil bones how well nourished an individual was? If so, a study could show if sauropods were generally worse fed than ornithischians…

    The problem here is that an animal usually has to be chronically malnourished before you see any evidence in the skeleton. And badly malnourished animals are likely to be struck down quickly by diseases or predators. This is one of the classic gotchas in paleopathology: stuff that kills animals quick is impossible to detect, because it’s not around long enough to impact the skeleton.

    While the Pterosaurs, Theropods and Sauropods were busy removing bone from their skeletons the Ornithischians were equally busy adding it.
    Horns, plates, spines, armour, and crests.

    Don’t forget that many pterosaurs and theropods also had flamboyant crests, plus you’ve got long neural spines in spinosaurs, Acrocanthosaurus, Whateverspinax, and Amargasaurus.

    Thought: compare the scarring from muscle attachment on saurischians and ornithischians. Aim for specimens of approximately the same size and same locomotive methodology. Hunch says they’ll be different, in a nontrivial way.

    It’s a good thought, but there are two big problems. First, the range of muscle scarring with a species is usually immense. When the Skulls exhibit was at the Cal Academy of Sciences a few years ago, they had one wall that was covered in 900+ sea lion skulls, arranged from the hugest, gnarliest males to the smallest, most gracile females. It was a sobering thing to see; if we were stuck with bones rather than teeth–as we are, mostly, in sauropods–that range of variation could plausibly be binned into three or four fossil “species”. So I think intraspecific noise would completely overwhelm any phylogenetic signal.

    The second problem is comparability. A good proving run for that kind of analysis would be to try to deduce the relative sizes and strengths of the hip muscles in wolves and deer by looking solely at their pelves. As cursorial mammals they are probably closer in locomotive methodology than any saurischian and ornithischian you could name, but I think in such a study one would have a huge problem determining if differences between the two taxa were caused by physiology, biomechanics (angles of action and relative strengths of muscles), or other factors, even if the intraspecific variation problem could be solved.

    But it IS a good thought, because we can tell something about differences in bone biology among lineages, by looking at bone histology. Specifically, how bones grow in the first place, how (or if) they’re remodeled, and how they heal. I’m not aware of any systemic differences between saurischians and ornithischians yet, but this is a very young science and still very much in an exploratory mode.

  6. William Miller Says:

    The problem here is that an animal usually has to be chronically malnourished before you see any evidence in the skeleton. And badly malnourished animals are likely to be struck down quickly by diseases or predators.

    Hmmm, one idea goes out the window.

    …Well, is there any other way to test the hypothesis that sauropods were living ‘closer to the edge’ than ornithischians?

    I’m not sure it would apply to all ornithischians – hadrosaurs in the Late Cretaceous were pretty ecologically dominant – but they might have locked out those evolutionary options by that point. Also, hadrosaurs seem to have been really social, and that might make a difference… A better comparison might be with contemporary herbivorous ornithischians like stegosaurs, hypsilophodonts, and Camptosaurus.

    Hmmm, maybe growth rates could help? How well are those known? If ornithischians were much faster growing (adjusted for their smaller adult size, of course!) that might be meaningful.

    Isn’t there some way to tell a fossil animal’s diet from isotope ratios? Could that be helpful?

    …Seriously, there ought to be *some* way to test this idea!

    (I wonder if sauropods were nomadic. The effects of such a huge animal, especially in a herd, would be nontrivial. Elephants change the landscape of Africa; I’d think sauropods would do the same, but more. Even their footprints might churn up soft ground, and I wonder if their droppings were a major component of the Late Jurassic Morrison ecosystem?)

  7. William Miller Says:

    Oh! Sorry to comment again, but I can’t edit my earlier comment.

    One other way to test the idea would require a LOT of specimens of a single species. A species which generally didn’t reach full potential growth would show a bigger gap between the average specimen and the largest specimen, right? And it might show more pathologies…

    Are there any herbivorous dinosaurs for which 100+ specimens are known? What herbivorous dinosaur species has the most known specimens?

  8. Mike Taylor Says:

    Lots to comment on here …

    First, a question. Matt wrote: “Getting bone out of the way requires increasing parathyroid hormone, mobilizing blood-born multinucleated osteoclasts, and convincing them to digest bone where it needs to be digested, which I assume is a more complicated process from a regulatory standpoint.” I thought even the most amazingly primitive tetrapods (i.e. us) were constantly resorbing our bone, no? Why would it be such a big deal to step that process up?

    Second, an observation: Matt’s revised question 3b is: “In the early stages of the evolution of diverticular replacement of connective tissue in saurischians, the amount of mass saved was negligible and could not plausibly have influenced natural selection, so PSP didn’t initially evolve to lighten the body.” But isn’t it the case (absent saltation) that every evolutionary development is “negligible” in its initial expression? Seems that selection can get its teeth into smaller variations than we’d intuitively think.

    Third, a suggestion: you might want to change your pseudosuchian silhouette in future versions: I have only now realised that this is a croc with its head down at bottom left; at first I saw it as a Diplocaulus with its head at top right!

    Finally, a clarification: William Miller wondered whether sauropods were nomadic. Remember that Sauropoda was a huge, and hugely disparate, group, spanning 150 million years, all seven continents and at least a couple of hundred genera — probably many, many more. It would be astonishing if, from a group this size, there weren’t some nomadic species, and some non-nomadic. Bear in mind how much the lifestyles of even closely related extant species vary, and we’ll see how careful we need to be in generalising across groups like Sauropoda.


  9. “Sauropods were generally huge – even small ones pushing elephant size, which is really big for an ornithischian. Maybe the sauropods had a bigger need to conserve nutrition than ornithischians.”

    Carpenter (2006) did suggest that the giant size attained by many sauropods was itself a method for conserving nutrition, and getting as much nutrition as possible out of available, relatively nutrition-poor food. Apparently, studies of elephants and rhinos support the idea that larger size leads to greater nutritional efficiency.

  10. Mike Taylor Says:

    “Apparently, studies of elephants and rhinos support the idea that larger size leads to greater nutritional efficiency.”

    If you have references, I’d like to see these studies. Anecdotally, elephants are surprisingly poor digesters, and their poop tends to be crammed full of tasty, nutritious as-yet-undigested food.

    Which I agree is surprising.

  11. LeeB Says:

    In the book “The evolution of Artiodactyls” by Prothero and Foss there is a chapter by C.M. Janis.
    On page 297 he discusses the lack of Artiodactyl megaherbivores.
    He points out that larger animals have longer passage times for the digested material, this enables them to break down coarser material.
    After 60-70 hours the digestion of any plant material is more or less complete; however if the passage time is too slow methaneogenic bacteria begin to grow and convert acetic acid to methane and carbon dioxide; this gives a huge energy loss.
    This become an acute problem with passage times of more than 4 days.
    To avoid this elephants have shorter and broader guts to speed up the passage rate of digesta.
    The passage time of material through their digestive tracts is less than that of rhinos, which would explain why some material is incompletely digested.

    So to completely break down plant material any large herbivore would want a passage time between 70 and 96 hours or thereabouts.

    Incidentally ruminants have a slow passage time of material through their digestive system and cannot speed it up which is why there are no ruminants of more than 2000kg.

    LeeB.

  12. Darren Naish Says:

    Err, the good Dr Janis is not a ‘he’. But anyway…

  13. Zach Miller Says:

    Hey, BTW, X-Men Origins: Wolverine was a complete waste of time and money. Aside from the coma-inducing plot and really awful treatment of Wolverine’s backstory (in general), the movie made a mockery out of Deadpool (he deserves much better) and some of the effects were Sci-Fi Channel quality.

    However, the new Star Trek just plain rules.

  14. Matt Wedel Says:

    Yes and yes. Loved the Onion story: Trekkies Bash New Star Trek Film as ‘Fun, Watchable.’. As usual, the headline is the best bit.

  15. Graydon Says:

    If the earliest structural function of diverticula was to replace fat, I’d suggest that the exposure to selection was to increase the proportion of body fat available as a metabolic reserve.

    Migrating passerine songbirds have proportionately huge and relatively rapid body mass swings; they’ll gain half or more of their breeding body weight as fat so they can migrate continuously over water for a day or more, for instance, landing having lost all or most of the weight gain. That’s a very highly derived case, but if the root of archosauria is in some Permian desert, it’s easy to see how an increased utilization of fat accumulation would be visible to selection.

    In the case of ornithsichians, none of them seem to have had head mass issues; theropods, sauropods (only more neck mass) and pterosaurs all did. It may be that the only path to PSP started with cervical vertebrae, and a lineage without head-mass issues didn’t ever expose skeletal diverticula to selection.

  16. Mike Taylor Says:

    Why would ceratopsians not have “head-mass issues”?

  17. Graydon Says:

    Early ceratopsians have relatively small heads and short necks; later derived ceratopsians have great big heads, but:
    – they’re robust quadrupeds with powerful, relatively short necks
    – they got the great big heads as a side effect of combined display and feeding adaptations, a combination which can plausibly swamp the selective signal from anything else
    – both display and feeding adaptations bias for strength, rather than reach or lightness/endurance, including strength under axial compressive loads (lock horns and push/twist or attempt to gore predator, either way it’s going to put axial compressive load on the neck; it might also produce nasty torsional stress. There might be a way to test for which one of those dominated if there are some really good ceratopsian necks available.) So every selective pressure driving the large head is driving a big, strong, heavy head.

    Theropods, being bipeds, had a combination of mass issues (head and teeth are heavy and way out on one end of the axial skeleton balance beam) and reach issues (that last 5cm making the difference between prey capture and no prey capture); there’s a selective signal for a heavy head (at least one of strong bite or rapid carcass consumption) and a light head (faster head motion, can afford more neck and thus longer reach), so head mass issues. Anything that gets more equivalently-performing neck for the same mass is likely to produce a positive selective signal.

    Sauropods had a selective signal driving longer necks; presumably the ability to reach food. Anything that gave them more equivalently performing neck for the same or less mass is going to have a positive selective signal, too.

    Pterosaurs were volant and had proportionately very large heads and (at least in some cases) an aerodynamic need for a stiff neck. The same “equivalently performing head and neck for less mass” signal will apply.

    Because the big derived ceratopsians have head and neck performance that’s positively correlated with mass, or at least compressive strength, they don’t get that signal. Ornithischians generally have proportionately small heads and strong necks.

  18. Mel. White Says:

    I know this is a very obvious question and that I’m stepping into a realm where I know very little, but could the proportions of the species speak more to why they developed air sacs? I know that in humans, giant humans do very poorly (we stand far off the ground, our weight is distributed on two legs which leads to lots of bone crushing and anatomy problems). Although no one has tried to make them gallop around on all fours as a lifestyle, I suspect they might do better.

    Hmm. I guess what I’m wondering is how far back each lineage is traced before pneumaticity becomes apparent (assuming it may have been there before) and wondering if at that point there’s either some sort of length-to-width ratio that is common or some other proportion that seems to divide the penumatocized from the non-pneumatocized.

    Forgive the novice questions, please. I’m only a volunteer preparator trying to understand the material better.

  19. Matt Wedel Says:

    I know that in humans, giant humans do very poorly (we stand far off the ground, our weight is distributed on two legs which leads to lots of bone crushing and anatomy problems).

    Actually human giants fare poorly because the pathological growth of their bodies outstrips the abilities of their hearts. NBA players show that humans several standard deviations away from the mean–at least!–can function just fine. I can’t think of any a priori reasons why evolution could not produce 10- or 12-foot tall humans, given the right selective pressures. The fact that hormonally abnormal individuals approach those heights and then die of heart attacks is not terribly informative; those unfortunate people are the victims of pathological growth patterns, and their abnormal size comes without the internal coordination among body systems that keeps the rest of us functioning. If they’d evolved to be that size, it would probably be a different story. And I think T. rex and friends show that there is nothing inherently bad about toting your mass around on two legs, at least in the under-20-ton category (which I am in, BTW, but not as comfortably as I’d like).

    I guess what I’m wondering is how far back each lineage is traced before pneumaticity becomes apparent (assuming it may have been there before)-

    See papers from 2006 onward here for some answers. Sorry to be brief and perhaps annoyingly self-referential, but I’m on vacation. :-)

    -and wondering if at that point there’s either some sort of length-to-width ratio that is common or some other proportion that seems to divide the pneumaticized from the non-pneumaticized.

    I don’t know of any, but I haven’t looked.

    Forgive the novice questions, please.

    I refuse! There is nothing to forgive.

    I’m only a volunteer preparator trying to understand the material better.

    Hey, I’m only an ex-grad student trying to understand the material better. Questions are always welcome. If I had all the answers, I’d have to find something else less interesting to work on. And without volunteer preparators, most of us would be out of a job. So a tip o’ the field hat to you, sir!

  20. Zach Miller Says:

    @ Graydon: I think ceratopsids did contend with pretty significant “giant head” issues. All those display structures were heavy. Those postorbital horns aren’t hollow structures! Look at Pachyrhinosaurus, too: it’s got parietal fenestrae, but that weight loss is balanced by large parietal spikes and a menagerie of epoccipital growths. And its nose horn is a giant bony boss!

    In fact, the head weight seems to have affected ceratopsid body form: while basal coronosaurs had erect forelimbs, derived members had semi-sprawling forelimbs, probably to counter the weight of the giant head and keep from tipping forward!

    Remember, these weren’t long-bodied animals. To the contrary, in fact: ceratopsid bodies are surprisingly compact, with short tails to boot.

  21. Mel. White Says:

    Thanks for the links, Matt! I don’t mind self-referential papers (err… okay… sometimes I do, but it depends on the quality of the research. If you’re going to start citing papers in “the Jounal for New Age Germantria and Ornithiscians”, I’m gonna smile and ignore that.)

    As a junior preparator, I was NOT prepared for the differences in sauropod vertebra (I thought the left side should look like the right side. Silly me.)

  22. Graydon Says:

    Zach –

    I can’t see how sprawling forelimb posture would help with load-bearing; load bearing tends to produce a graviportal stance.

    I can easily see how a sprawling forelimb posture could help with generating lateral forces for horn-wrestling win one’s fellow highly derived coronosaurs, or for defensive uses, though.

    The point I was trying to make is that theropods and sauropods and pterosaurs all have selection signals that want lighter heads AND selection signals that want heavy (proportionately, anyway) head or neck structures. Derived coronosaurs don’t show much in the way of selection signal for lighter, and lots — feeding *and* display — for heavier.

    I think it’s just as easy to postulate the short body, semi-sprawled forelimbs, and generally robust skeleton as adaptations, like the large horned head, to absorb compressive stress in intra-specific conflicts than it is to postulate mass load. (Especially since if head mass became a disadvantage, not getting larger, or getting larger on a different schedule, are obviously both possible.)

    Also note that (so far as my sadly limited awareness goes) the ball joint attachment of the head never changed; that’s either an indication that head mobility was very important or that head mass never got to the point where restricted joint forms with better passive load bearing capacity were required.

  23. Zach Miller Says:

    Given Farke’s recent suggestion that long-horned ceratopsian routinely jousted with each other, I’d say that the mobile ball joint indicates head mobility as being very important. Good points, though. Still don’t really know what’s up with the semi-sprawling forelimbs, though. Earlier coronosaurs have fully erect forelimbs–even Protoceratops (which has an enormous head). You could very well be on to something in suggesting that the semi-sprawling posture correlates with an increased importance in intraspecific combat.

    Definately something to think about, anyway.

  24. Mel. White Says:

    I don’t think that sprawling limbs (I’m naively assuming you mean a “crocodillian” stance?) would be much of an advantage in butting or pushing fights. But I’m going on modern crocs and other similar creatures. They’re fairly slow runners, though good swimmers (mainly for the tail.)

    The graviportal stance gives more thrust in a forward direction; the motion is straight through the angle of the legs and toward the back. Because hips and shoulder joints are integrated with the body mass, the whole force acts as a full body thrust forward.

    In wider stances, the angle deflects from the amount of force possible, and with a crocidilian stance, the end of the movement actually has the legs in a “v” shape and not straight behind the body. I hope this makes sense. This is a “no coffee” day.

    And… I’m ashamed to admit it, but all I could think of was a couple of triceratops’ beginning a fight by grabbing each other’s beaks and doing the croc “death roll.”

  25. Nathan Myers Says:

    A sprawling stance would have made it harder to tip them over. That cows don’t sprawl is essential to their entertainment value. Imagine frat boys sneaking out to engage in croc tipping.

    This is an argument that ceratopsian competitions were more similar to beetles’ than to goats’.

  26. Nima Says:

    The Ceratopsians’ frills really did nothing for “balance”. The fact that some of them were so light with huge fenestrae (i.e. ALL the earlier Chasmosaurines and Centrosaurines) shows these were not balance structures. The neck muscles were sufficient, as in rhinos.

    As for sprawling stances in Ceratopsians – they’re bunk. There’s no proof from articulated skeletons, and it’s RIDICULOUSLY EASY to take a Ceratopsian with erect (though slightly crouched) arms, swivel them outward so the elbow points sideways instead of back, and CLAIM that Ceratopsians sprawled.

    The one BIG PROBLEM with this is you’d dislocate the elbow and totally screw up the articulation in the shoulder joint and the wrist (assuming you actually wanted the creature’s hands to still face forward like any other land vertebrate).

    Ceratopsian arms, when properly articulated, have the elbows sticking out A LITTLE BIT, at about a 150-degree elbow angle. This is good for shock-absorbing (makes sense considering the pitted joints appear to have contained a lot of cartilage), but still NOT EVEN CLOSE to a sprawling stance – the hand is directly below the shoulder.

  27. Graham King Says:

    Mike Taylor Says:

    May 11, 2009 at 11:06 pm

    “Apparently, studies of elephants and rhinos support the idea that larger size leads to greater nutritional efficiency.”

    If you have references, I’d like to see these studies. Anecdotally, elephants are surprisingly poor digesters, and their poop tends to be crammed full of tasty, nutritious as-yet-undigested food.

    Which I agree is surprising.

    There may be different kinds of efficiency.

    One approach may be ‘get as much nutritional value as you can, out of whatever you eat’.
    Another approach may be ‘get out only as much of the nutritional value as is relatively easy to extract.. out of what you eat. Void the residue, move on, eat more!’

    The scarcity or abundance of food (and other considerations) may determine which approach is taken by any given species/individual/situation.

    A superficially-wasteful-seeming discarding of still-nutritious partly-digested residue may (as well as its ecological benefits) have individual pay-offs: e.g. not having to grow so elaborate a digestive system, nor maintain a more sophisticated gut flora; not having to carry about so much food-mass in process of (prolonged, thorough) digestion; reduced vulnerability to gut infections/toxins/flatulence; whatever..

    (I anticipate some follow-up comment analysing possible adaptive costs/benefits of flatulence..)

  28. Graham King Says:

    Graydon Says:

    May 15, 2009 at 12:35 pm

    If the earliest structural function of diverticula was to replace fat, I’d suggest that the exposure to selection was to increase the proportion of body fat available as a metabolic reserve..

    ..it’s easy to see how an increased utilization of fat accumulation would be visible to selection.

    But.. how would that help? I don’t see it..

    I guess here we are assuming that ‘packing-fat’ (fat with a significant space-filling function) is not available for energy-use (being mechanically essential, or a necessary alternative to pesky vacuum, or somesuch)..

    Ok, so replace packing-fat with air-filled diverticula, wherever such packing is needed. Sure, you can now use all the fat that remains for energy, when needed. But.. that is the same physical amount of fat as was available before!

    You weigh less, is all. Sure, the proportion of fat that is available, has gone up, but that’s just a number game.. You don’t actually get any more ‘available’ fat, or energy, by reducing the total fat content through cutting down ‘non-available’ fat!

    Or have I missed your point?


  29. [...] is relevant when we think about the function of pneumaticity. When I write that pneumaticity lightened vertebrae, I usually mean relative to that same vertebra if it wasn’t pneumatized. But we could also [...]


  30. [...] For more on that problem, see Wedel (2007a) and the post, “X-Men Origins: Pneumaticity”. [...]


  31. [...] I had heard, anecdotally, of networks of diverticula described as looking like bubble wrap. I can now confirm that is true, for at least some networks. What was especially cool about these is that they were occupying space that would be filled with adipose or other loose connective tissue in a mammal, which illustrates the point that pneumatic epithelium seems to replace many kinds of connective tissue, not just bone–something Pat O’Connor has talked about, and which I also briefly discussed in this post. [...]


  32. [...] But that’s not all. The possibility of pneumatic ilia has been floating around for a while now, and most of us who were aware of the iliac chambers in sauropods probably assumed that eventually someone would find the specimens that would show that they were pneumatic. At least, that was my assumption, and as far as I know no-one ever floated an alternative hypothesis to explain the chambers. But I certainly did not expect pneumaticity in the shoulder girdle. And yet there they are: chambers with associated foramina in the scap and coracoid of Saltasaurus and in the coracoid of Neuquensaurus. Wacky. And extremely important, because this is the first evidence that sauropods had clavicular air sacs like those of theropods and pterosaurs. So either all three clades evolved a shedload of air sacs independently, or the basic layout of the avian respiratory system was already present in the ancestral ornithodiran. I know where I’d put my money. [...]


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