May 31, 2009
Since we’re spending a few days on neck posture, I thought I’d expand on what Mike said about bunnies in the first post: in most cases, it is awfully hard to tell the angle of the cervical column when looking at a live animal. Because necks lie.
Take this horse (borrowed from here). You can see that the external outline of the neck, which is what you would see in the living animal, is pointed in a different direction than the cervical column.
And here’s why. Many mammals carry their heads and necks so that the cranio-cervical joint is up high and the head is angled down from it. At the base of the neck, tall neural spines on the anterior thoracic vertebrae support the nuchal ligament, which lifts the body profile far above the cervical vertebrae. Basically, the cervicals run from the lower or middle part of the neck at its base to near the top of the neck at the head end.
This mismatch holds no matter how the neck and head are oriented. When the animal lowers its head to graze, the cervical column is still angled up relative to the apparent angle of the neck defined by its dorsal and ventral margins.
But if you think that’s bad, you ain’t seen nothin’ yet.
In most of the smaller birds, like this budgie (from Evans 1969:fig. 5-6) the neck is much longer and more flexible than you would think based on the external profile. And check out the mismatch between the cervical column (in front) and the trachea (behind). That’s not drawn incorrectly; the trachea is outside the bundle of neck muscles that encloses the vertebrae, and it is free to slide around all over the place, and does so in many birds.
Also note that while the neck is extended past vertical, the extension occurs in the middle of the neck, not at the shoulder. The neck actually goes down from the craniocervical joint, not up. My guess is that there is a lot of this in climbing taxa that hold their torsos elevated. Vultures come to mind here, too. A useful reminder that in natural history we are usually dealing with norms, not laws.
In the pigeon, note again the fact that the mid-cervicals are angled up much more sharply than is the external profile of the neck. In fact, the external profile of the neck is angled forward while the mid-cervicals are angled backward. This excellent reconstruction is from this page, which has several others which also show that necks lie.
Lest anyone think that the pigeon was either an outlier or a case of artistic embellishment, here’s yet another rabbit, this time from Vidal et al. (1986: fig. 5a). Again, the mid-cervicals–actually, almost all of the cervicals–are angled backward, but the neck as a whole is pointing slightly forward.
As an aside, I think possibly it has blown some people’s minds that we have used so many rabbits as examples, both in the paper and in our blog coverage. What can we say? Rabbits are awesome.
Of course not all necks lie. With flamingos, what you see is what you get.
Giraffes: 20 feet of reticulated irony
Let’s see here: necks not vertical.
Necks not vertical.
Trying . . . very . . . hard . . . and . . . just . . . getting . . . to . . . vertical!
(I know it looks like the neck is just slightly less than vertical, but remember that necks lie, and the cervical column is steeper. In this animal, you could drop a plumb bob from the ear and it would track the course of the cervical vertebrae just about perfectly.)
Cat, not trying at all: cervical column past vertical (Vidal et al. 1986: fig. 2).
Here’s the irony: for practically as long as sauropod neck posture has been contentious, giraffes have been held up as THE example of the most extreme (dude!) elevated neck postures out there. But in fact giraffes have to really reach to achieve vertical cervical postures that “ordinary” animals like cats, rats, guinea pigs, chickens, and, yes, rabbits, reach or exceed all the time.
Good paleobiology has to start with good biology. It’s high time that the sauropod neck posture debate got a reality infusion. Giraffe necks are extreme in terms of length, but not in terms of posture.
Speaking of sauropods…
All right, you’ve suffered long enough. Here’s your sauropod vert. Care to guess what it is?
- Evans, H.E. 1969. Anatomy of the budgerigar; pp. 45-112 in Petrak, M.L. (ed.), Diseases of Cage and Aviary Birds. Lea and Febiger, Philadelphia.
- Vidal, P.P., Graf, W., and Berthoz, A. 1986. The orientation of the cervical vertebral column in unrestrained awake animals. Experimental Brain Research 61: 549-559.
Let’s assume for a moment that you accept our contention (Taylor et al. 2009) that, since extant terrestrial tetrapods habitually hold their necks in maximal extension, sauropods did the same. That still leaves the question of why we have the neck of our Diplodocus reconstruction at a steep 45-degree angle rather than the very gentle elevation that Stevens and Parrish’s (1999) DinoMorph project permits.
As a reminder, here is fig. 6A of Stevens (2002), a paper on the computer science behind DinoMorph which used exactly the same models as the 1999 study but which conveniently illustrates them in lateral view:
As you’ll see, not only does the neutral pose show the characteristic subhorizontal neck with the drooping end, but even the maximally extended pose barely gets the head above the level of the back. In the most recent version of his Diplodocus model, Kent has slightly changed the angle at which the neck leaves the torso, due to a reconfiguration of the pectoral girdle, but this still leaves the neck very low.
So why did we do this?
Doesn’t the DinoMorph model show that the posterior cervicals just can’t do this?
Well, maybe not.
Remember that the precursor to the DinoMorph project was John Martin’s (1987) paper on the mounting of the Rutland cetiosaur at the Leicester City Museum, in which he calculated neutral pose and the extreme extended and flexed poses by manipulating actual bones without the benefit of a computer. Martin ended up with a similar result to that Stevens and Parrish were later to get:
But when Matt and I looked at the actual mounted skeleton a few years back, what we saw didn’t fit with this at all:
Check out that huge gap between the centra of the fourth and fifth cervicals! There’s no way to avoid this without putting a comically extreme downward kink in the neck at this point. And there are similar gaps at other points along the neck, including some near the neck-base that would require a strong upward kink in order to articulate both the centra and the zygapophyses simultaneously.
Are we saying that in life, this specimen did have strong kinks in the neck? No, we’re not (despite the pleasant coincidence that this would force the neck into an extreme version of the elevated pose we’re advocating). What we’re saying is that sauropod cervicals are rarely — I’d go so far as to say never — preserved undistorted, and so you just can’t rely on how they seem to articulate, at least not for quantitative analyses. This post-mortem distortion should not be too surprising: unlike femora and other such solid bones, remember that the cervicals were highly pneumatic and composed primarily of laminae, which would be subject to all sorts of taphonomic and diagenetic distortion. In the extreme case of Sauroposeidon, the cervicals, which were up to 140 cm in length, “are of extremely light construction, with the outer layer of bone ranging in thickness from less than 1 mm (literally paper-thin) to approximately 3 mm” (Wedel et al. 2000:110-111) — it’s astonishing that they are not much more smushed up than they are.
So Martin’s cetiosaur seems too distorted to give meaningful articulation results, but what about the specimens that Stevens and Parrish used for the DinoMorph paper? Well, the Apatosaurus model is certainly based on questionable material. As pointed out by Upchurch (2000):
A second difficulty with Stevens and Parrish’s analysis is that their data for Apatosaurus was derived from a single specimen in the Carnegie Museum (CM 3018). This generally well preserved specimen has suffered severe damage at the base of the neck, and the three most posterior cervicals are thus represented by plaster models that cannot provide reliable anatomical data (Gilmore 1936, pers. obs.). Although Stevens and Parrish acknowledge that the morphology of the posterior cervicals is particularly influential in determining the nature of the feeding envelope, they do not mention this problem, and it is not clear how this gap in the data was addressed in their analyses. This deficit could have had a profound impact on Stevens and Parrish’s conclusions.
And Gilmore’s observations are really rather damning: as well as the account of the damaged neck-base, he also noted (p. 195) that “the type of A. louisae [i.e. CM 3018] lacks most of the spine tops, only those of cervicals eight, ten and twelve being complete”. (You would never guess this from Gilmore’s Plate XXIV, which shows all of the cervicals but C5 essentially complete.) So all in all, the DinoMorph study’s Apatosaurus is not one I’d want to build an argument on.
What about the Diplodocus carnegii holotype CM 84, which is the Diplodocus used in the DinoMorph papers? That’s just about the best preserved sauropod skeleton in the world, right? Well, yes. But even that is distorted enough that the neck can’t be articulated without some sleight of hand. I don’t have good photos of the mounted neck, unfortunately (and probably won’t have until someone at the NHM gives me a stepladder and access to the holy of holies that surrounds the mount), but I did have the experience of photoshopping the cervical vertebra illustrations from Hatcher (1901: plate III) in an attempt to get them into a good pose, and I found that even these don’t really fit properly:
You’ll see that, while the condyles are sat nicely in the cotyles, the zygapophyses are not at all well articulated: in particular, the C7-C8 and C8-C9 junctions have the prezygs shoved much too far forward, so that a double downward kink would be necessary to accomodate these articulations without pulling the condyles out of the cotyles.
Finally, while Matt and I were in Berlin last November, as part of the excursion associated with the awesome all-sauropod-gigantism-all-the-time workshop, we got to play with the superbly preserved set of anterior brachiosaur cervicals HMN SI, and we tried to articulate the real bones. We had to stop for fear of breaking them, because they simply would not fit nicely together.
In conclusion, the distortion of all sauropod cervicals renders them poor subjects for quantitative analysis such as that of the DinoMorph project. While the approach of Stevens and Parrish is a real and valuable contribution to rigour in the analysis of posture, the output of DinoMorph is a hypothesis to be tested by other lines of evidence rather than a firmly established fact. (That last bit was quoted verbatim from our paper.)
I’ve gone on much longer than I intended to in what was supposed to be a quick-and-easy post, so I’ll leave it here. In order to keep the recent paper short and snappy, we didn’t go into this in much detail, summarising down to a mere 88 words (Taylor et al 2009: 216-217), so maybe this will bear repeating (in more rigorous form) in a future publication.
- Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11: 175-300.
- Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1: 1-63 and plates I-XIII.
- Martin, J. 1987. Mobility and feeding of Cetiosaurus (Saurischia, Sauropoda) why the long neck? In: P.J. Currie and E.H. Koster (eds.), Fourth Symposium on Mesozoic Terrestrial Ecosystems, Short Papers, 154-159. Box-tree Books, Drumheller, Alberta.
- Stevens, K.A. 2002. DinoMorph: Parametric modeling of skeletal structures. Senckenbergiana lethaea 82(1): 23-34.
- Stevens, K.A. and Parrish, J.M. 1999. Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science 284: 798-800. [Free subscription required]
- Taylor, M.P., Wedel, M.J. and Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54(2): 213-220.
- Wedel, M.J., Cifelli, R.L. and Sanders, R.K. 2000. Sauroposeidon proteles, a new sauropod from the Early Cretaceous of Oklahoma. Journal of Vertebrate Paleontology 20, 109-114.
May 29, 2009
So far in our coverage of the new paper (Taylor et al. 2009) we’ve mostly focused on necks, following the discovery by Graf, Vidal, and others that when they are alert and unrestrained, extant tetrapods hold their necks extended and their heads flexed. (Although they turn up with distressing regularity, “ventroflexed” is redundant and “dorsiflexed” is an oxymoron; Darren lays down the law here.)
There’s more to the paper; about half of our argument is primarily about heads and only secondarily about necks, and has to do with semicircular canals (SCCs). SCCs are sense organs in the inner ear that determine the orientation and acceleration of the head. Hagfish have a single loop on each side, lampreys have two loops per side, and gnathostomes (jawed vertebrates, like us) have three per ear, all set at right angles to each other to capture position and movement information in all directions no matter how the head is oriented. There’s a brief overview of how the system works here, and here’s what SCCs actually look like (in this case in the theropod dinosaur Ceratosaurus, from Sanders and Smith 2005:fig. 5):
SCCs are relevant to posture and locomotion: animals that move rapidly tend to have big canals, especially big anterior canals, and the horizontal semicircular canals (HSCCs) are usually held more or less level as animals go about their business. It’s the “more or less” part that gets sticky, as we’ll see in a minute. SCCs and inner ear anatomy in general are areas of accelerating research in vertebrate paleontology, because the soft tissues that comprise them (the membranous labyrinth) are housed in dense bone (the bony labyrinth) which is often preserved and can be imaged non-invasively using CT. Even braincases that look pretty crappy from the outside can yield beautifully-preserved bony labyrinths, from which the dimensions of the membranous labyrinth can be measured and the acuity of the system can be estimated.
Where SCCs have really attracted attention in paleontology is the “more or less” horizontal orientation of the HSCCs in living animals. Some authors have argued that if you set the HSCCs level or close to level, you can figure out how the head was oriented in life.
Well, maybe. The problem is that there is a LOT of variation around level. In birds surveyed by Duijm (1951), HSCC orientation varied by 50 degrees among taxa, from 20 degrees below horizontal to 30 degrees above. Furthermore, in humans HSCC orientation varies by up to 20 degrees among individuals. Possibly humans are weirdly variable, but it seems at least equally likely that most critters are and we’ve only discovered that variation in humans because of the huge sample size.
However you slice it, those are darn big error bars around any given head posture. That doesn’t mean that HSCC orientations in dinosaurs and other extinct vertebrates are worthless for determining posture (they may also be a source of taxonomic information). Strictly speaking, it means that preserved HSCCs can get us in the 50-degree ballpark but can’t narrow things down any further. This is one of those areas in paleontology where we’re just going to have to live with a certain amount of uncertainty, at least for now.
We’re not done with heads, though. Once the HSCCs get us in that 50-degree range, we still have to figure out how the neck facilitated those postures. One thing that seems to hold across the board in sauropodomorphs is that when the HSCCs were in the -20 to +30 range around horizontal, the occipital condyles were pointed down. And that has major implications for the posture of the neck, as we’ll see in the following example.
Let’s start with this neatly abstract Apatosaurus skeleton, borrowed from Kent Stevens’s site here. Note that this version is from 2005 and Kent has updated his models considerably since then. I’m using this one because its elegant minimalism made it easy for me to play with, but it doesn’t represent Kent’s current thinking.
Here’s the same image with some lines drawn on to indicate the long axis of the skull, the orientation of the occipital condyle, and the angle of the anterior neck. In Apatosaurus and Diplodocus the occipital condyle is at right angles to the long axis of the skull. That means that if the cranio-cervical joint was held in “neutral pose”, the head would be at right angles to the anterior neck. Recall that extant tetrapods hold their heads flexed on their necks. This Apatosaurus has its head extended by 50 degrees. This is major extension–to see what it feels like, lean your head back until you’re looking straight up, and then lower your head until its almost halfway back to normal. Imagine walking around like that. In this pose the HSCCs are angled down, within the 50-degree ballpark but not level.
Just for the sake of argument, let’s set the HSCCs level and force the craniocervical joint into ONP. Now the head and first few cervicals are okay, but clearly this posture won’t work with the neck in the original pose. We’re going to have to move the neck up to meet the steep angle dictated by the HSCCs and the occipital condyle.
One option is to keep as much of the neck in the original pose as possible, and just elevate the vertebrae closest to the head. This is not so far off from how Apatosaurus has been depicted for more than a century. But it doesn’t agree with the data from extant tetrapods, in which the neck is extended at its base.
Here’s the partially Vidal-compliant version, with the cranio-cervical joint in ONP and the base of the neck extended. To be fully Vidal-compliant, the head would have to be flexed on the neck. In the diagram, that would have the effect of turning changing the angle between the long axis of the skull and the anterior cervicals from a right angle to an acute one. Since the orientation of the head is “fixed” by the semicircular canals (in this example), that means the neck would have to be even more steeply inclined.
One more for the road. Here the HSCCs are angled up by 20 degrees, which is in the upper part of the range but certainly not an extreme value for either birds or mammals; chances are you and your cat carry your HSCCs at about the same angle (intraspecific variation caveat applies!). Angling the HSCCs up moves the occipital condyle further down, which makes the neck steeper still.
You may look at that last picture and think it’s impossible or crazy, and I don’t blame you if you do. Remember that all I’ve shown you is two possibilities from within the 50-degree ballpark defined by the HSCCs. But even if we put the HSCCs at the very bottom of that range, the occipital condyle still points down at something like 25 degrees below horizontal, which means the anterior neck has to be angled up at 25 degrees just to keep the cranio-cervical joint in ONP; if the head is flexed on the neck, it has to be steeper.
The moral of the story is that, even within the broad range of postures allowed by the HSCCs, head posture still constrains neck posture to be elevated in most if not all sauropods. It will be VERY interesting to see how the skull of Brachytrachelopan is put together, when one comes to light.
- Duijm, M. 1951. On the head posture in birds and its relation to some anatomical features. II. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, Series C 54: 260–271.
- Sanders, R.K. and Smith, D.K. 2005. The endocranium of the theropod dinosaur Ceratosaurus studied with computed tomography. Acta Palaeontologica Polonica 50 (3): 601–616.
- Taylor, M.P., Wedel, M.J., and Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54 (2): 213–220.
Update (later the same day)
We’ve added scans of the print-edition coverage that we got in the UK’s national newspapers (and the London-only freebie Metro). Somehow, seeing it in an actual newspaper still feels more real than the same newspaper’s web-site. Scan are at the bottom of the paper’s home page.
May 28, 2009
[I wrote this in the cafe on the ground floor of the BBC's Millbank studios, where I spent much of yesterday, just before I headed off for Paddington and the train home. I have lightly edited it since the original composition.]
It’s been a day spent doing publicity for the new SV-POW! paper on sauropod neck posture.
Overall, there’s been a little less interest than we were able to rustle up for Xenoposeidon, but we nevertheless got a live TV interview on Channel 4 News, plus radio interviews on BBC Radio 4’s Today programme, BBC Scotland, BBC Radio Solent (twice) and finally BBC Wales (which turned out to be my favourite). In the mean time, Darren was being interviewed on BBC Radio 5 Live. So a very BBC-centric day, with Channel 4 the only independent to take up the story. (That contrasts with Xeno, when I seemed to spend the whole day doing interviews on the mobile phone for various independent radio stations as I was rushing between studios for the big boys.)
We got pretty good coverage in print, too. I bought all the national dailies and went through looking for sauropod-neck news. There was a good third-of-a-page story in Guardian (thanks to their fine science reporter Ian Sample who also did such a good job on Xeno), and smaller spots in the Times and Independent. The Telegraph, oddly, included a nice photo of the NHM Diplodocus with an inset of Mark Witton’s artwork, but accompanied it with no text other than a 38-word caption. Go figure. There were brief mentions in the early editions of the Mirror and Sun, although they dropped out in later editions; I couldn’t find anything in the Mail, the Express or the Star — I think that’s everything. There was a nice bonus in Metro, London’s free daily, which had half a page on the story including a nice big photo of the Berlin brachiosaur, with me by its elbow for scale.
As I write this, I’ve not been able to check on the net and see what the online coverage has been like, beyond a very quick informal scan this morning before I left the house I was staying at for the first radio interview. I did find a story in the Times that was considerably more detailed that what made it into the print edition, so the same may have been true of other papers, too. I’ll see what Google News digs up for me when I get home. [Update: we're tracking Internet coverage on this page.]
A few themes emerged as the sequence of interviews progressed. Most predictably, lots of interviewers wondered whether this meant that the NHM would have to remount its Diplodocus skeleton. Not at all: the pose that it’s in is still a perfectly valid one, which it would have gone through in the transition between drinking and browsing poses; it’s just not what we think would have been the habitual pose. Paul Barrett was quoted for the counter-view in several of the printed reports, and made that point (though usually it was reported in truncated form). The BBC web-site’s coverage was unusually good in carefully reporting what we’d actually told everyone, that the mounted pose is one that would have been adopted from time to time, so hopefully no-one at the NHM will come away from thinking we were getting at them.
Another recurring theme was whether Seymour’s blood-pressure argument was good evidence that our proposed habitual posture is wrong. I didn’t want to say too much about this, because our thoughts on the subject are still in the process of approaching their final form and are not ready to be published, but hopefully I was able to say enough to satisfy the interviewers and listeners without giving it all away.
Another point that I tried to make when given the opportunity is that we don’t see this paper as closing the debate and settling the issue of posture once and for all — as if that could ever happen for any palaeobiological controversy. What we hope we’ve done is at least to reopen the debate and the end the unchallenged reign of the DinoMorph-compliant hangdog pose. Needless to say, plenty of work remains to be done on the issue of neck posture, and there are now at least two published arguments in favour of each candidate posture. The time may be ripe for a review article. For now, though, we confidently expect a published response from Kent “DinoMorph” Stevens, who we’ve discussed our work with at some length, and who has had a preprint for a few weeks now so that he could get working on it! Ah, the cut and thrust of debate — bring it on!
Update (later the same evening)
I have finally managed to make an MP3 of the last interview — the second one with BBC Radio Solent, with Sasha Twining who was standing in on the Steve Harris Show.
And a plea for help: although the Channel 4 News interview is still available on Channel 4’s own site, I know it won’t last for long — probably no more than a week — so if anyone is able to make an MPEG, AVI, FLV or similar of these, please please do, and send it my way. Thanks!
May 27, 2009
Welcome, one and all, to Taylor, Wedel and Naish (2009), Head and neck posture in sauropod dinosaurs inferred from extant animals. It’s the first published paper by the SV-POW! team working as a team, published in Acta Palaeontologica Polonica, and freely available for download here.
Far, far back in the uncharted depths of history, silly people like Osborn and Mook (1921:pl. 84), Janensch (1950b: pl. 8) and Paul (1988:fig. 1), who didn’t know any better, used to depict sauropods with their necks held strongly elevated.
All that began to change with Martin’s (1987) short paper in the Mesozoic Terrestrial Ecosystems volume, and was then turned upside-down by Stevens and Parrish’s (1999) seminal paper in Science: two and a half pages that transformed the way the world looked at sauropods.
John Martin looked at the cervical vertebrae of the Rutland specimen of Cetiosaurus oxoniensis, and concluded that the joints between them couldn’t be as flexible as people thought. He reconstructed that animal’s neck in a low, near-horizontal pose, and with a very narrow range of movement that didn’t allow it to raise its head far above shoulder level. Stevens and Parrish brought more rigour to this approach by modelling the cervical articulations of two sauropods (Diplodocus carnegii and Apatosaurus lousiae) using a computer program of their own devising, DinoMorph. And as most SV-POW! regulars will probably know, they got results similar to Martin’s, showing neutral positions for both animals that were well below horizontal, and finding restricted ranges of motion. (“neutral pose” here means that the vertebra are aligned such that the zygapophyses overlap as much as possible.)The DinoMorph posture was quickly adopted as orthodox, and got a lot of exposure in the BBC’s classic CGIumentary, Walking With Dinosaurs: episode 2, Time of the Titans, was primarily about Diplodocus, and under Stevens’s consultancy showed them as having obligate low posture throughout the show.
The new horizontal-neck orthodoxy was also reinforced by an exhibition at the American Museum of Natural History featuring a physical metal sculpture of a DinoMorph model:
This brings us pretty much up to date: there’s been very little in the way of published dissent between 1999 and now, and a couple more Stevens and Parrish papers have reinforced their contention. Upchurch (2000) published a half-page response to the DinoMorph paper, and Andreas Christian has put out a sequence of papers arguing for an erect neck posture in Brachiosaurus brancai on the basis that this best equalises stress along the intervertebral joints (e.g. Christian and Dzemski 2007), but otherwise all dissent from the DinoMorph posture has been limited to unpublished venues: for example, Greg Paul has posted several messages on the Dinosaur Mailing List disputing the low-necked posture, but has yet to put any of his arguments in print.
But enough of this dinosaury stuff. Let’s look at a nice, cuddly bunny:
Now here’s the thing: you wouldn’t guess by looking at it, but that rabbit has a vertical neck. In fact, it’s more than vertical: it’s so upright that it bends back on itself. Don’t believe me? Then take a look at this X-ray of an unrestrained awake rabbit:
Can it be that rabbits have unusual cervical vertebrae, such that when you articulate them in neutral pose they curve strongly upwards? No: and to prove it, here is (ahem) Taylor, Wedel and Naish (2009: fig. 1):
(Yes, this is a hare rather than a rabbit, but it’s close enough for government work.) What we found was that it was only possible to get the cervical skeleton anywhere near the habitual life posture by cranking all the proximal cervical joints up as far as they could physically go. In fact, it seems that some of the joints in the live animal flex more than the dry bones can — presumably due to intervertebral cartilage moving the centra further apart.
And this is fully in accord with the findings of Vidal et al. (1986), who X-rayed a selection of live animals (human, monkey, cat, rabbit, rat, guinea pig, chicken, monitor lizard, frog) and found that the neck is inclined in all but the frog. Furthermore, in all the mammals and reptiles, they found that:
- the cervical column is elevated nearly to the vertical during normal functioning;
- the middle part of the neck is habitually held relatively rigid;
- the neck is maximally extended at the cervico-dorsal junction and maximally flexed at the cranio-cervical junction; and
- it is the cranio-cervical and cervico-dorsal junctions that are primarily involved in raising and lowering the head and neck.
(In life, these facts are obscured from view by soft tissue.)
We also looked at unpublished live-alligator X-rays (thanks to Leon Claessens for access to these) and found that even in these ectothermic sprawlers, the neck is habitually elevated above neutral pose. Published X-rays of turtles and even (slightly) salamanders also showed the same tendency.
So what does this mean for sauropods? Simply, unless they were different from all extant terrestrial amniotes, they did not habitually hold their necks in neutral position, but raised well above horizontal. And if they resembled their closest relatives, the birds — and the only other homeothermic and erect-legged group, the mammals — then their necks were strongly inclined. As in, all the proximal cervicals were habitually cranked into the most erect positions they could attain. Kind of like this:
Which is a looong way form the DinoMorph posture that we were all getting used to but couldn’t learn to love. What do you know? Turns out that Osborn and Mook, and Janensch, were right after all.
So that, in a nutshell, is the contention of the first SV-POW! paper: that sauropods held their heads up high. That’s not to say that they couldn’t bring them lower when they wanted to — of course they could, otherwise they’d have been unable to drink — but we believe the evidence from extant animals says that they spent the bulk of their time with their heads held high.
I leave you with this rather beautiful piece that noted pterosaurophile Mark Witton drew to illustrate our favoured posture. Enjoy!
Stay tuned for more on neck posture …
For more cool stuff about the paper, including blog and media coverage and the chance to hear Mike on BBC Radio(!), see our page about the paper on the sidebar.
- Christian, A. and Dzemski, G. 2007. Reconstruction of the cervical skeleton posture of Brachiosaurus brancai Janensch, 1914 by an analysis of the intervertebral stress along the neck and a comparison with the results of different approaches. Fossil Record 10: 38-49.
- Janensch, W. 1950b. Die Skelettrekonstruktion von Brachiosaurus brancai. Palaeontographica (Supplement 7): 97-103.
- Martin, J. 1987. Mobility and feeding of Cetiosaurus (Saurischia, Sauropoda) why the long neck? In: P.J. Currie and E.H. Koster (eds.), Fourth Sympo- sium on Mesozoic Terrestrial Ecosystems, Short Papers, 154-159. Box- tree Books, Drumheller, Alberta.
- Osborn, H.F. and Mook, C.C. 1921. Camarasaurus, Amphicoelias, and other sauropods of Cope. Memoirs of the American Museum of Natural History, new series 3: 246-387.
- Paul, G.S. 1988. The brachiosaur giants of the Morrison and Tendaguru with a description of a new subgenus, Giraffatitan, and a comparison of the world’s largest dinosaurs. Hunteria 2 (3): 1-14.
- Stevens, K.A. and Parrish, J.M. 1999. Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science 284: 798-800. [Free subscription required]
- Taylor, M.P., Wedel, M.J. and Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54(2): 213-220.
- Upchurch, P. 2000. Neck posture of sauropod dinosaurs. Science 287: 547b.
- Vidal, P.P., Graf, W., and Berthoz, A. 1986. The orientation of the cervical vertebral column in unrestrained awake animals. Experimental Brain Research 61: 549-559.
May 20, 2009
Welcome to another episode of the ground-breaking and wonderful Sauropods of 2008 series. Yay! As I’m fond of pointing out, new dinosaurs do not only come from China, or South America: Europe continues to yield surprises. Tastavinsaurus sanzi Canudo et al., 2008 is from the Lower Cretaceous (Aptian) Xert Formation of Spain, and the holotype specimen is pretty good, including dorsal, sacral and caudal vertebrae, ribs, chevrons, and material from the pelvis and hindlimbs (we’ve previously mentioned it here, and figured some of it here). Evidently, only the hindquarters of the animal were preserved. But they’re in good shape, and preserve numerous unique characters: in fact 19 autapomorphies are identified, which is a pretty impressive number and indicates either that Tastavinsaurus was a highly disparate sauropod, or that the morphology of its close friends and relatives is but scrappily known (I have to say that the former possibility looks more likely).
Some of these autapomorphies are in the vertebrae. On the posterior surfaces of their neural spines, the antero-posteriorly short, opisthocoelous dorsal vertebrae sport two small accessory laminae that emerge from the base of a very wide, chunky looking postspinal process. Of more general interest (perhaps) is that the dorsal centra contain ‘big prismatic tubes linked together by slender walls [that exhibit] a honeycomb pattern in cross-section’ (Canudo et al. 2008, p. 713). The ‘honeycomb pattern’ sounds something like somphospondylous texture, but the authors note that the condition present in Tastavinsaurus is distinct, and perhaps represents a new type of pneumatic pattern. Frustratingly, they don’t illustrate the internal texture, so we’re left guessing.
The caudal vertebrae of Tastavinsaurus are not all that different from those of macronarians like Camarasaurus and Brachiosaurus: in the proximal caudals, the centra are wider than they are long, the proximal vertebrae have slightly procoelous centra, and the neural spines are ‘club-shaped’ [proximal caudal above from Canudo et al. (2008), fig. 7]. The more distal vertebrae – those from the 15th position onwards – are slightly amphicoelous. One weird little feature on the distal caudals is a small, centrally placed convexity on both the anterior and posterior articular faces of the centra (see pics below). As Canudo et al. (2008) note, Cedarosaurus and Pleurocoelus nanus both have this as well (Tidwell et al. 1999) [distal caudal below from Canudo et al. (2008), fig. 8].
The rest of Tastavinsaurus suggests that it would perhaps have superficially resembled a cross between Camarasaurus and Brachiosaurus. Its ilium looks like a dorsally stretched version of the ilium of Brachiosaurus and, indeed, in general character the specimen would appear to be a brachiosaur-grade titanosauriform. With pneumatic ribs, a lateral bulge on the femur, and caudal vertebrae that have anteriorly positioned neural arches, the rest of Tastavinsaurus agrees with this classification, and in their phylogenetic analysis, Canudo et al. (2008) found Tastavinsaurus to fall within Somphospondyli within Titanosauriformes, and within this clade to be the sister-taxon of Venenosaurus from Utah. If this is correct it weakens the proposal that six sacral vertebrae are a synapomorphy of Somphospondyli (Wilson & Sereno 1998), for Tastavinsaurus only has five.
Well, yet again I’ve done my best to concentrate on CAUDAL vertebrae, given that we have an obvious (and understandable) bias towards cervicals and dorsals. Someone has to speak up for tails. For previous instalments in the Sauropods of 2008 series please see the articles on Eomamenchisaurus, Dongyangosaurus, and Malarguesaurus.
Canudo, J. I., Royo-Torres, R. & Cuenca-Bescós, G. 2008. A new sauropod: Tastavinsaurus sanzi gen. et sp. nov. from the Early Cretaceous (Aptian) of Spain. Journal of Vertebrate Paleontology 28, 712-731.
Tidwell, V., Carpenter, K. & Brooks, W. 1999. New sauropod from the Lower Cretaceous of Utah, USA. Oryctos 2, 21-37.
Wilson, J. A. & Sereno, P. C. 1998. Early evolution and higher-level phylogeny of sauropod dinosaurs. Society of Vertebrate Paleontology Memoir 5, 68 pp.
May 14, 2009
Do you want to know how stupid my co-blogger Matt Wedel is? Having already discussed the ostrich Struthio camelus in Wedel et al. (2000b), that total idiot went on to misspell the trivial name as “camellus” in Wedel and Cifelli (2005:52). What a doofus.
And do you want to know how dumb my other co-blogger Darren Naish is? Throughout Naish and Dyke (2005), he consistently misspelled the species name of Elopteryx nopcsai as “nopscai“, despite extensively discussing Nopcsa, who the species was named after. What a moron.
It’s a good thing I would never do anything so stupid.
Er. Read on …
So I have this paper in press about the two “Brachiosaurus” species and how they are not really congeneric — I think we’ve mentioned it a few times. It’s now very nearly a year since I submitted it, under the title: A re-evaluation of Brachiosaurus altithorax Riggs 1904 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai Janensch 1914. And now — now, a year on, after having re-read this manuscript some insane number of times — I finally notice my own grotesque error: Riggs of course named B. altithorax in 1903. Argh! So in the last few days, I’ve spent some crazy amount of time going through and changing this title in my dissertation (where it pops up as Chapter 2), in my CV, in my on-line publications list … and of course, making a GIGANTIC sign in flashing red neon, to be suspended before my eyeballs at all times, reminding me to fix this in the page-proof when that turns up.
(Actually, I think this error is the most astounding of all: not only did I miss it myself, but so did my Ph.D supervisor, the handling editor at SVP, both peer-reviewers, the self-invited third “reviewer” who sent his unsolicited comments, both of my examiners and the two or three people that I’ve sent preprints to. Incredible that ten or more people could all miss such a horribly obvious mistake right there in the title.)
So. You’d think that just about exhausted Matt’s, Darren’s and my doofosity, right? Oh ho ho. Not so, because we have a paper in press that we wrote together. We submitted it, revised it according to the reviews, commented on the page-proofs and told the journal it was all ready to go. And then — THEN — we noticed a horrible, stupid mistake right in the middle of the abstract. The paper is about osteological neutral pose, but we’d written “osteological neural pose”. And all three of us missed it. (Happy ending: we told the journal what we’d done, and it wasn’t too late to fix.)
So the moral of the story is: we are idiots.
Just thought you ought to know.
- Naish, Darren and Gareth J. Dyke. 2004. Heptasteornis was no ornithomimid, troodontid, dromaeosaurid or owl: the first alvarezsaurid (Dinosauria: Theropoda) from Europe. Neues Jahrbuch für Geologie und Paläontologie Mh. 2004(7): 385-401.
- Taylor, Michael P. In press. A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of Vertebrae Palaeontology.
- Taylor, Michael P., Mathew J. Wedel and Darren Naish. In press (2009). Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54(2).
- Wedel, Mathew J., and Richard L. Cifelli. 2005. Sauroposeidon: Oklahoma’s native giant. Oklahoma Geology Notes 65(2):40-57.
- Mathew J. Wedel, Richard L. Cifelli, and R. Kent Sanders. 2000b. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45(4):343-388.
May 8, 2009
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.
To sum up:
- Most diverticula in birds are not involved with pneumatizing the skeleton, so PSP can’t be the reason for their existence.
- 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.
- 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.
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.
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?
- Butler, R.J., Barrett, P.M., and Gower, D.J. 2009. Postcranial skeletal pneumaticity and air sacs in the earliest pterosaurs. Biology Letters. doi:10.1098/rsbl.2009.0139
- Wedel, M.J. 2007.What pneumaticity tells us about ‘prosauropods’,
and vice versa. Special Papers in Palaeontology 77:207–222.
May 5, 2009
A while back, Matt speculated on the size of the allegedly giant mamenchisaurid Hudiesaurus. At the time, all he had to go on was Glut’s (2000) reproduction of half of Dong (1997:fig. 3), and a scalebar whose length was given incorrectly. The comments on that article gave some more measurements, but we never got around to showing you the figures of the vertebra in question, so here it is:
The measurements of this vertebra given in the paper are:
- Height of vertebra 76
- Length of centrum 55
- Width of centrum anteriorly 42
- Width of centrum posteriorly 39
- Height of neural spine 41
- Width of top of neural spine 27
Dong said (p. 109) that “The mounted type skeleton of Mamenchisaurus hochuanensis, which was the largest sauropod known in China at the time it was described, measures 22 m in length (Young and Chao, 1972). The centrum of the first dorsal in the type of Hudiesaurus sinojapanorum is 1.5 times longer than the type of M. hochuanensis, leading to an estimated skeleton length of 30 m.” This is odd because 1.5 times 22 m is 33 m, not 30 m. But it’s also odd because Young and Zhao (1972:table 1) actually gave the centrum length of D1 as 250 mm — so the alleged 55 cm long centrum of the Hudiesaurus D1 is actually 2.2 times as long, which would yield a length estimate of *gulp* 2.2 x 22 m = 48.4 m. Which would be right up in Amphicoelias fragillimus territory. And that is clearly nonsense for a dorsal vertebra that’s only 76 cm high.
The height:length ratio of the Hudiesaurus vertebra is 76/55 = 1.38. That of the first dorsal of M. hochuanensis is (using Young and Zhao’s measurements) 640/250 = 2.56 — nearly twice as tall. In M. hoch., C19 (the last cervical) has a height ratio of 660/325 = 2.03 — better, but still not very close. C18 has 660/400 = 1.65, C17 has 630/550 = 1.15. So based on proportions, it looks like the Hudiesaurus “dorsal” is not a dorsal but a cervical — furthermore, not even the last dorsal, but the penultimate or antepenultimate cervical. If it is homologous with C17 of M. hoch., then its 55 cm length is exactly the same as that M. hoch.’s C17, which would suggest that Hudiesaurus was the same length (22 m); if it’s homologous with C18. then its 55 cm length is 1.38 times greater than that of M. hoch., suggesting a total length of 30 m. It’s interesting that this last figure is the very one proposed by Dong — could it be that after writing the paper, he reconsidered the serial position of the vertebra and recalculated the total length on the basis of an assignment as the penultimate cervical, while neglecting to update the text? It seems a bit far-fetched, but maybe it’s a possibility.
Finally: is the vertebral morphology consistent with an identity as a cervical? I think so. The strong opisthocoely is a point in favour, as are the lateral fossae. (The dorsals of Mamenchisaurus lack fossae and foramina.) On the other hand, Dong claims that the vertebra has a hyposphene, which is unknown in cervicals — but I see no hyposphene in the photo above. Dong doesn’t really say why he thinks the vertebra is a dorsal, but my guess is that it may be due to lack of a fused cervical rib, and the assumption that there was a free dorsal rib associated with it. But Young and Zhao (1972) say of M. hoch. that “The last cervical and first dorsal vertebrae are generally distinguished by rib morphology. However, a difficulty is posed here by the last cervical’s absence of articulated ribs.” And my own observation of casts of this specimen show that actually the last few cervical ribs are not fused.
In conclusion, it seems to me that Hudiesaurus is probably based on a posterior cervical rather than an anterior dorsal. BUT let me clear that I have never seen the material, and everything I’ve written here is based only on what’s in the literature. I may well have made a dumb mistake, and no-one should take my thoughts on this too seriously. If I was certain, I’d put it in a paper instead of on a blog.
- Dong, Z. 1997. A gigantic sauropod (Hudiesaurus sinojapanorum gen. et sp. nov.) from the Turpan Basin, China; pp. 102-110 in Dong, Z. (ed.), Sino-Japanese Silk Road Dinosaur Expedition. China Ocean Press, Beijing.
- Glut, D.F. 2000. Dinosaurs: The Encyclopedia, Supplement 1. McFarland & Company, Inc. Jefferson, North Carolina. 442 pp.
- Young, C.C., and Zhao, X.-J. 1972. Mamenchisaurus hochuanensis sp. nov. Institute of Vertebrate Paleontology and Paleoanthropology Monographs, A, 8:1-30.