More out than in

I drew a couple of these a while back, and I’m posting them now both to fire discussion and because I’m too lazy to write anything new.

Here’s the neck of Apatosaurus, my own reconstruction based on Gilmore (1936), showing the possible paths and dimensions of continuous airways (diverticula) outside the vertebrae.

Here’s figure 4 from Lovelace et al. (2007), which first got me thinking about pneumatic traces on the ventral surfaces of the centra and what they might imply. You can see pneumatic spaces between the parapophyses in Supersaurus (A) and Apatosaurus (C) but not in Barosaurus (B).

This is another of my moldy oldies, again based on one of Gilmore’s pretty pictures, showing how I think the soft tissues were probably arranged. The muscles are basically the technicolor version of Wedel and Sanders (2002). Two points:

1. How bulky you make the neck depends mainly on how much muscle you think was present (which of course depends on how heavy you think the neck was…). Here I was just trying to get the relationships right without worrying about bulk, but it’s worth considering.
2. The volume of air inside the vertebra was dinky compared to the probable volume of air outside. In Apatosaurus, either of the canals formed by the transverse foramina has almost twice the cross-sectional area of the centrum.

A fair amount of this has been superseded with better data and prettier pictures by Schwarz et al. (2007), so don’t neglect that work in any ensuing discussion (it’s free, fer cryin’ out loud). And have a happy Thanksgiving!

References

• Gilmore, C.W.  1936.  Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11: 175-300.
• Lovelace, D.M., Hartman, S.A., and Wahl, W.R. 2007. Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos do Museu Nacional, Rio de Janeiro 65(4):527-544.
• Schwarz, D., Frey, E., and Meyer, C.A. 2007. Pneumaticity and soft−tissue reconstructions in the neck of diplodocid and dicraeosaurid sauropods. Acta Palaeontologica Polonica 52 (1): 167–188.
• Wedel, M.J., and Sanders, R.K. 2002. Osteological correlates of cervical musculature in Aves and Sauropoda (Dinosauria: Saurischia), with comments on the cervical ribs of Apatosaurus. PaleoBios 22(3):1-6.

Postscript

Mike asked me to add the labeled version of Nima’s brachiosaur parade, so here you go. Click to embiggen.

Range of motion in intervertebral joints: why we don’t trust DinoMorph

Wait, what?  So 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:

Stevens (2002: fig. 6A), illustrating the fully extended, neutral and fully flexed poses attainable by Diplodocus according to the original DinoMorph model

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?

Diplodocus carnegii head, neck and anterior torso, right lateral view, articulated in habitual posture as hypothesised by Taylor et al. (2009). Skull and vertebrae from Hatcher (1901).

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:

Martin (1987:fig. 2) showing claimed limits of extension of and flexion in the neck of the Rutland cetiosaur

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:

Rutland cetiosaur, anterior part of neck in right lateral view, showing extreme disarticulation between the cotyle of C4 and condyle of C5

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 cervcial 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:

Diplodocus carnegii holotype CM 84, partial neck (cervicals 6-9) in right lateral view, composed from elements in Hatcher (1901: plate III)

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.

References

• 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.

end

What heads tell us about necks

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.

References

• 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.

Sauropods held their necks erect … just like rabbits

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.

The classic reconstruction of Brachiosaurus brancai, from Janensch (1950b: plate VIII. (For some reason, WordPress doesn't allow italics in these captions, hence the roman-font taxonomic names.)

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.

The subhorizontally mounted neck of the Rutland Cetiosaurus skeleton at the Leicester City Museum, in right posterolateral view.

The median part of the subhorizontally mounted neck of the Rutland Cetiosaurus skeleton at the Leicester City Museum, in left lateral view. Mike Taylor for scale.

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.)

Diplodocus carnegii, DinoMorph computer model , showing neutral neck posture, and limits of dorsal and ventral flexibility. From Stevens (2002:fig. 6a). (Note that Stevens's more recent models show a slightly higher neck due to its leaving the torso at a less steep angle.)

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.

A still from Walking With Dinosaurs, episode 2, Time of the Titans, showing Diplodocus in a DinoMorph-compliant posture with a low, horizontal neck. Image copyright the BBC.

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:

Physical DinoMorph model at the AMNH, with horizontal-neck advocate Kent Stevens. Photograph by Rick Edwards, AMNH

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:

Unrestrained awake rabbit, left lateral view, in X-ray, showing vertical neck. From Vidal et al. (1986:fig. 4B)

Amazing.

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):

Taylor et al. (2009: fig. 1), reversed for easy comparison with the previous two images: skull and cervical skeleton of the Cape hare (Lepus capensis) in neutral pose and in maximal extension

(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 selected of life 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:

Diplodocus carnegii head, neck and anterior torso, right lateral view, articulated in habitual posture as hypothesised by Taylor et al. (2009). Skull and vertebrae from Hatcher (1901).

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!

Diplodocus herd -- mostly with necks in habitual raised posture, with one individual drinking. By Mark Witton.

Stay tuned for more on neck posture …

Update

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.

References

• 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.

A fused atlas and axis in Apatosaurus

Here are the first two cervical vertebrae of the Carnegie Apatosaurus, from Gilmore’s 1936 monograph. As you can see, they are fused together. It is a bit weird that we haven’t covered the morphology of the atlas-axis complex here before. And sadly we’re not going to cover it now. I needed to get an image of these verts to a group working on…something secret…and this turned out to be the fastest way to get them the information in a format that would be easy to find for future reference. Hope you don’t feel used.

UPDATE: Here’s something weird: the both verts have facets for cervical ribs, but the cervical ribs had not fused to the vertebrae, even though they normally do, and despite the fact that the vertebrae had fused to each other, even though they normally don’t.

The Aerosteon saga, Part 1: Introduction and background

Pneumatic dorsal vertebrae of Aerosteon (Sereno et al. 2008:fig 7)

Big news this week: Sereno et al. (2008) described a new theropod, aptly named Aerosteon (literally, “air bone”), with pneumaticity out the wazoo: all through the vertebral column, even into the distal tail; in the cervical and dorsal ribs; in the gastralia; in the furcula; and in the ilium. This is huge news, and it’s free to the world at PLoS ONE. Pneumatic vertebrae and ribs are the norm in theropods and most sauropods (hence our interest here), but the axial elements of Aerosteon are extremely pneumatic. A pneumatic furcula was reported in the dromaeosaur Buitreraptor (Makovicky et al. 2005), but Aerosteon appears to be a basal tetanuran so it pushes furcular pneumaticity a good distance down the tree. Most exciting are the pneumatic ilium and gastralia. Ilial pneumaticity has been suspected in some sauropods and non-avian theropods but the evidence has been lacking until now; either the ilial chambers could not be traced to pneumatic foramina, or the suspected pneumatic foramina could not be shown to lead to internal cavities. Pneumatic gastralia are really wacky–according to the paper, it is the first discovery of pneumatized postcranial dermal bone, and I certainly don’t know of any other examples.

Why is this important? In extant birds, the furcula is only pneumatized by diverticula of the interclavicular air sac, and the ilia are only pneumatized by the abdominal air sacs, so the presence of big pneumatic foramina leading to big internal chambers in both the furcula and ilia of Aerosteon is evidence not just for bird-like air sacs, but specifically air sacs from both the cranial and caudal groups within the thorax that are responsible for the flow-through lung ventilation of birds. It’s pretty dynamite stuff.

Right?

Right?

We-ell . . . There is no question that the fossil material is pretty stunning and shows all the morphological features that Sereno et al. claim (and even some that they don’t–stay tuned for Part 2). But there are parts of the paper that I disagree with, and to understand why, I have to tell you a little about recent research on pneumaticity in sauropods and theropods. In this post and the next I’ll be discussing papers by Pat O’Connor and Leon Claessens, as well as my own; all of these are freely available at the links just provided. So, please, if you have a beef with anything I say below, go read all the relevant literature for yourself, weigh the evidence, and make up your own mind.

First, a brief sketch of what we’ve been up to. Except for the occasional weirdo (surveyed on the third page here), the only extant tetrapods with postcranial pneumaticity are birds. In birds, postcranial pneumaticity is the skeletal footprint of the lung/air sac system. So if we find postcranial pneumaticity in dinosaurs–say, sauropods, or Aerosteon–we can use the ‘rules’ from birds to make inferences about the morphology of the respiratory system. We can’t tell which way the air was blowing in the lungs, but we can tell the minimum extent of the pneumatic diverticula, and we can make some inferences about lung structure. All of the logic of this is really nicely and concisely laid out in O’Connor and Claessens (2005), which is only three pages of text, so if you want to know more, just go read it.

The hypothesis that sauropods and theropods had air sacs like that of birds has been opposed in a couple of ways: pneumaticity doesn’t tell us anything, and vertebral pneumaticity only indicates cervical air sacs. Neither of these counterarguments has gotten much traction, probably because they’re so easily falsified. Let’s have a look.

Historical Misconception #1: Pneumaticity Is Completely Uninformative

“Without integrating functional data into the study, the most that can be inferred from post-cranial pneumaticity in extinct animals is that, as pointed out by Owen (1856), the pneumatized bones received parts of the lung in the living animal… Because pneumaticity has no known functional role in ventilation or thermoregulation or metabolic rates, its usefulness as a hard-part correlate for lung structure and metabolism is, unfortunately, questionable.” (Farmer 2006, pp. 91-92)

Farmer does not distinguish here between inferences based on the presence of postcranial pneumaticity and inferences based on the distribution of postcranial pneumaticity. If all we know about a bone is that it is pneumatic, then she is correct in stating that the most we can conclude is that it was connected to the respiratory system in some way. (The thermoregulatory function of pneumaticity discussed by Seeley [1870] has been demonstrated for cranial pneumaticity [Warncke and Stork 1977] but not for postcranial pneumaticity [Witmer 1997, O'Connor 2006]). But the inference of cervical and abdominal air sacs in non-avian dinosaurs does not depend simply on the existence of postcranial pneumaticity. Rather, these inferences are based on patterns of postcranial pneumaticity that are diagnostic for specific air sacs.

Verdict: Fail.

Historical Misconception #2: Vertebral Pneumaticity Only Comes From Cervical Air Sacs

“Pneumatization of the vertebrae and ribs is invariably accomplished by diverticuli [sic] of the cervical air sacs (McLelland 1989a), which are located outside the trunk and contribute little, if anything, to the respiratory air flow (Scheid and Piiper 1989). Presence of pneumatized vertebrae in non-avian dinosaurs therefore only speaks of the possible presence of such nonrespiratory diverticuli [sic], and cannot be regarded as indicative of an extensive, avian-style abdominal air-sac system.” (Ruben et al. 2003, p. 153)

This remarkable statement is repeated pretty much verbatim by Chinsamy and Hillenius (2004) and Hillenius and Ruben (2004). What’s remarkable about it is that is so thoroughly inaccurate. People have known for more than 100 years that the posterior parts of the vertebral column of birds are pneumatized by diverticula of the abdominal air sacs, and said as much in many papers–for example, Muller (1908), Cover (1953), King (1966, 1975), Duncker (1971), Hogg (1984a, b), and Bezuidenhout et al. (1999). Still, if McLelland said that the vertebrae and ribs are “invariably” pneumatized by diverticula of the cervical air sacs, it’s not their bad, right?

Okay, first, McLelland (1989) is a review paper and presents no new data (this will become really important later on, when we get back to Aerosteon). Second, here’s what McLelland actually said:

“What can be stated with certainty is that in birds generally the cervical air sac aerates the cervical and thoracic vertebrae (Fig. 5. 22) and the vertebral ribs; the clavicular air sac aerates the sternum, sternal ribs, pectoral girdle and humerus (Fig. 5. 23); and the abdominal air sac aerates the synsacrum, pelvis and femur.” (pp. 271-272)

By listing the synsacrum and pelvis separately, McLelland clearly meant that the synsacral vertebrae are pneumatized by the abdominal air sac, and this is confirmed by the sources he cited elsewhere: Hogg (1984a, b).

So Ruben et al. (2003)–and those who recycled that text–were relying not on any of their own research, or any primary research at all, but on a single review paper that actually says exactly the opposite of what they claim it does, based on other primary research papers (those by Hogg) that themselves say the same (opposite) thing.

Verdict: EPIC FAIL.

The Brave New Post-2005 World

He said, she said, yadda yadda. There are lots of inaccuracies in the literature, and it’s not like birds are extrasolar planets. If we want to know what is going on inside them, we can just look. That’s what O’Connor and Claessens (2005) did, by injecting and dissecting 200+ birds representing 19 avian orders. Know what they found? The cervical diverticula do not EVER go farther down the vertebral column than the middle of the thorax. NEVER EVER. So if you find pneumatic vertebrae in the posterior dorsals, sacrum, or tail, it’s pretty likely that they were pneumatized by diverticula of the abdominal air sacs.

I say “pretty likely” because it’s always possible that dinosaurian diverticula worked differently, and that the air that got into the posterior part of the vertebral column actually came from the cervical air sacs, or the lungs directly, or from arse gills, or possibly magic rocks. We can imagine lots of ways for air to get into the back half of the vertebral column, but the only one that we’ve ever seen work in a tetrapod* is diverticula of the abdominal air sacs. Dinosaurs may have worked differently, and had wacky cervical diverticula or arse gills or whatever. But those are not the obvious choices, and we don’t have any evidence for them; all the available evidence points to abdominal air sacs.

*Some osteoglossomorph fishes pneumatize the vertebral column from the swimbladder–strange but true!

So, great. The old confusion has been swept away by a blood-dimmed tide of bird carcasses and good science. Pneumatization of the posterior vertebral column implies abdominal air sacs. The combination of pneumaticity in the neck, trunk, sacrum, and even tail of many theropods and sauropods shows that both cervical and abdominal air sacs were present (as in Apatosaurus, above), which means air sacs both anterior and posterior to the lungs, which means that most (maybe all) saurischians had at least some of the gear they would need for flow-through breathing like that of birds (O’Connor and Claessens 2005, O’Connor 2006, Wedel 2007).

And yea, verily, anatomical accuracy and scientific clarity reigned throughout the land . . .

. . . until now.

TO BE CONTINUED.

References

• Bezuidenhout, A.J., H.B. Groenewald, and J.T. Soley. 1999. An anatomical study of the respiratory air sacs in ostriches. Onderstepoort Journal of Veterinary Research 66:317-325.
• Chinsamy, A., and Hillenius, W.J. 2004. Physiology of nonavian dinosaurs; pp. 643-659 in Weishampel, D.B., Dodson, P., and Osmolska, H. 2004. The Dinosauria, Second Edition. University of California Press, Berkeley.
• Cover, M.S. 1953. Gross and microscopic anatomy of the respiratory system of the turkey. III. The air sacs. American Journal of Veterinary Research 14:239-245.
• Duncker, H.R. 1971. The lung air sac system of birds. Advances in Anatomy, Embryology, and Cell Biology 45(6),1-171.
• Farmer, C.G. 2006. On the origin of avian air sacs. Respiratory Physiology and Neurobiology 154:89-106.
• Hillenius, W.J., and Ruben, J.A. 2004. The evolution of endothermy in terrestrial vertebrates: Who? When? Why? Physiological and Biochemical Zoology 77:1019-1042.
• Hogg, D.A. 1984a. The distribution of pneumatisation in the skeleton of the adult domestic fowl. Journal of Anatomy 138:617-629.
• Hogg, D.A. 1984b. The development of pneumatisation in the postcranial skeleton of the domestic fowl. Journal of Anatomy 139:105-113.
• King, A.S. 1966. Structural and functional aspects of the avian lungs and air sacs. International Review of General and Experimental Zoology 2:171-267.
• King, A.S. 1975. Aves respiratory system; pp. 1883-1918 in Getty, R. (ed.), Sisson and Grossman’s anatomy of the domestic animals, 5th edition, volume 2. W.B. Saunders, Philadelphia.
• Makovicky, P.J., Apesteguía, S., and Agnolín, F.L. 2005. The earliest dromaeosaurid theropod from South America. Nature 437:1007-1011.
• McLelland, J. 1989. Anatomy of the lungs and air sacs; pp. 221-279 in King, A.S., and McLelland, J. (eds.), Form and Function in Birds, Volume 4. Academic Press, London.
• Müller, B. 1908. The air-sacs of the pigeon. Smithsonian Miscellaneous Collections 50:365-420.
• O’Connor, P.M. 2006. Postcranial pneumaticity: an evaluation of soft-tissue influences on the postcranial skeleton and the reconstruction of pulmonary anatomy in archosaurs. Journal of Morphology 267:1199-1226.
• O’Connor, P.M., and Claessens, L.P.A.M. 2005. Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs. Nature 436:253-256.
• Ruben, J. A., Jones, T. D. and Geist, N. R. 2003. Respiratory and reproductive paleophysiology of dinosaurs and early birds. Physiological and Biochemical Zoology 76:141-164.
• Seeley, H.G. 1870. On Ornithopsis, a gigantic animal of the pterodactyle kind from the Wealden. Annals and Magazine of Natural History, Series 4, 5: 279-283.
• Sereno, P.C., Martinez, R.N., Wilson, J.A., Varricchio, D.J., Alcober, O.A., Larsson, H.C.E. 2008. Evidence for avian intrathoracic air sacs in a new predatory dinosaur from Argentina. PLoS ONE 3(9): e3303. doi:10.1371/journal.pone.0003303
• Warncke, G., and Stork, H.G. 1977. Biostatische und thermoregulatorische Funktion der Sandwich-Strukturen in der Schädeldecke der Vögel. Zoologische Anzeiger 199:251-257.
• Wedel, M.J. 2007. What pneumaticity tells us about ‘prosauropods’, and vice versa. Special Papers in Palaeontology 77:207-222.
• Witmer, L.M. 1997. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Society of Vertebrate Paleontology Memoir 3:1-73.

Seriously. Apatosaurus is just nuts.

Pursuant to a comment I just made on the previous post, here is cervical 8 of YPM 1980, the holotype of Brontosaurus excelsus, now of course known as Apatosaurus excelsus, in anterior and left lateral views, scanned from plate 12 of Ostrom and McIntosh 1966. Look on my cervicals, ye mighty, and despair.

You see? I wasn’t kidding. This thing is beyond crazy. The dorsoventral height of its parapophyses alone exceeds that of the centrum and neural spine together. What the heck was it doing with that thing? Seriously.  It makes no mechanical sense whatsoever.  To the best of my knowledge no-one has ever even advanced a hypothesis about those honkin’ great cervical ribs … and I am not about to break that streak.

Just enjoy.

Reference

• Ostrom, John H., and John S. McIntosh.  1966.  Marsh’s Dinosaurs.  Yale University Press, New Haven and London.  388 pages including 65 absurdly beautiful plates.

Your coccyx is contemptible

Those of you who have been paying attention to my recent posts will have pretty much known this was coming. I’d hate to disappoint you, so here it is:

What you’re looking at here is the first caudal vertebra (i.e. the first tail bone) of Apatosaurus ajax, the newish specimen NSMT-PV 20375 described by Upchurch et al. (2005). The drawings are all from the plates at the end of that lavishly illustrated paper: all I’ve done is composite them. The top row, from left to right, shows the vertebra in anterior, left lateral, posterior and right lateral views. Below the left lateral view is a dorsal view, with the front pointing to the left (as in the left lateral view).

Oddly, the size of this vertebra doesn’t seem to be stated in the paper, but two lines of evidence suggest that it’s about 65 cm in total height. First, measuring the caudal on the skeletal reconstruction that is the frontispiece, and comparing with that figure’s 1m scale-bar, yields a height of 64 cm; second, the neural spine’s height (measured from the ventral margin of the poztzygapophyses) is given in Table 9 as 392 mm, and that extrapolates, using the posterior view figure, to a total height of 665 mm. So about 65 cm, then.

The caudal vertebrae of diplodocids such as Apatosaurus, Diplodocus and Barosaurus are unusually complex for sauropods, having been somewhat “dorsalised”, i.e. taking on some of the complex morphology of posterior dorsals rather than being the rather dull round-centrum-with-a-flat-spine-on-top affairs you get hanging off the rear end of brachiosauruids. You’ll notice that the lateral processes, or “caudal ribs”, take the form of tall, broad plates, so that the middle part of the vertebra is trapezoidal in anterior view. This is as different as can be from the boring, stick-like caudal ribs of Brachiosaurus. (What actually are caudal ribs? So far as I can tell, amazingly, no-one really knows. They might be homologous with the diapophysis of dorsal vertebrae, or with the parapophysis, or perhaps both of them fused, or one or both fused with an actual rib.)

Oh, yes: also in the picture is your coccyx, that is, the four or five bones that make up your vestigial tail. It is, needless to say, contemptible. It’s surprisingly hard to find a reference for how big it should be, but by cross-scaling from illustrations of whole human skeleton and sacra, I’ve come up with a figure of about 2.5 cm, and that’s what I’ve used here. If you want to compare your tail with Apatosaurus’s, remember that Apato had about eighty caudals: they diminish in size posteriorly, of course, but they do stay about the same anteroposterior length for much of the tail. In fact, diplodocids have tremendous tails, something like half the entire length of the entire animal. One of my long-standing bugbears is that the biomechanics of sauropod tails gets almost no attention (except for speculations about whip-cracking) compared with the love and care lavished on their necks. One day, one of us might do something about that.

That concludes our short but humiliating series of abuse directed at your frail human body. I’ll have to come up with something else next time it’s my turn. Hope you’ve enjoyed the ride.

Finally, good news for everyone who was intererested in Matt’s Aegyptosaurus post: he’s made a PDF of Stromer 1932 so you can see that mystery vertebra for yourselves.

Bibliography

• Upchurch, Paul, Yukimitsu Tomida, and Paul M. Barrett. 2005. A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. National Science Museum Monographs No.26. Tokyo.

Tutorial 5: Neurocentral Fusion

I was going to write about mystery cervicals of the Cloverly Formation, but that requires knowing something about juvenile vertebrae and Pleurocoelus, so I decided to write about Pleurocoelus, but that still requires knowing something about juvenile vertebrae. So I’m writing this tutorial to lay the groundwork for more goodness to come.

Sutures

Vertebrae do not spring forth fully formed, like Athena from the mind of Zeus. They grow from bits, and the bits come together at different times in development. The bits themselves start out as anlagen–bone precursors–made of cartilage, and these anlagen start to ossify–turn into bone–at one or more ossification centers. From those centers, the bone grows outward and replaces the cartilage like some kind of science fiction blob monster taking over its host. As the bone replaces the cartilage, the contacts between different bony elements are sometimes left behind as sutures, like the sutures in the bones of your skull. Once the replacement of cartilage by bone is complete, most of the new bone growth happens at the suture margins. This is easy to demonstrate experimentally: if you cut out the suture and glue together the bones on either side, the combined element will not grow to the normal length. Premature suture closure can be a big problem, if the bones that are now fused (say, skull bones) can’t grow fast enough to keep up with whatever is inside (say, a brain). And many of the sutures between skull bones stay open even into old age, at least in humans and other mammals (birds are another story), because sutures also serve another purpose, which is to help the skull respond to mechanical stress without cracking like an egg. However, the eventual fate of most sutures is to close as the bones on either side finally fuse. Some old folks do eventually fuse up most or all of the sutures in their skulls.

If you look across the whole skeleton, the closures of different sutures–in the vertebrae, in the long bones, in the limb girdles, in the skull–are spread out through time. This is nice if you want to determine the age at which something–or more often, someone–died (forensic anthropologists get a lot of mileage out of this). But it can also be a pain, because it means that some things are hardly ever found intact. Sauropod skulls are particularly prone to exploding. There are complete, articulated sauropod skeletons for which the skull is either scattered all over the place or simply gone.

How Vertebrae Form

Vertebrae form as several distinct pieces. The centrum starts from paired ossification centers on either side of the cartilaginous notochord (see Tutorials 1 and 2 if you need to brush up on vertebral anatomy). The neural arches also start out as left-and-right paired elements, each of which forms one half of the arch over the spinal cord. The left and right components of the centrum fuse very early in development, and the left and right halves of the neural arch come together later. All of these events can and do fail on occasion. The anlagen may form asymmetrically or not at all, parts may not ossify, and left-right halves can fail to fuse. The best known pathology associated with vertebral development in humans is spina bifida, in which the two halves of the arch and spine fail to unite. The growing spinal cord can stick out through the hole and cause all kinds of problems.

I’ll not dwell long on the embryology of vertebrae; there are whole textbooks full of that stuff if you’re curious and some good websites, too. In sauropods, in all of the embryos that have been discovered to date the vertebrae are not yet ossified, so there’s nothing to talk about.

Usually in juvenile dinosaurs you find the centrum as a single unit and the neural arch and spine as another (the exception is the atlas, the first cervical vertebra right behind the head, which is so weird that it will have to be dealt with in a separate post). The centrum and neural arch complex come together at the neurocentral sutures, a pair of zipper-like tracts of rough bone (and, in life, cartilage) on either side of the neural canal. These sutures stay open for a long time, usually until the dinos are around half-grown.

Here are a couple of cervical centra from a juvenile Apatosaurus (in this and other photos in this post, click on each picture to see the unlabeled version). They are facing left, and the giant depressions in the sides are the pneumatic cavities. The centra (C3 and C4 if you’re curious) are propped up on their oversized parapophyses, which are typical for Apatosaurus.

Here’s C6 from the same specimen, in anterior view. For this shot I put a thin sheet of foam over the centrum so that I could put the neural arch in anatomical position and see how the whole vert (minus cervical ribs, which form separately and fuse even later) would have looked.

Same vert in left lateral view. Compare to the first picture above to see how the rough patches on top of the centra match the shape of the corresponding patches on the neural arch.

Here are C7 and C8 in right lateral view. The neural arches and centra are preserved together, but the sutures are still plainly visible. Had this individual lived longer, the neural arches and centra would have grown together, and the sutures would have been gradually erased by bone remodeling.

Stages of Neurocentral Suture Fusion

Because neurocentral sutures close over time, they can tell us something about how mature an individual dinosaur was when it died. Each vertebra goes through several stages as the sutures close. It’s a continuous process and you could divide it into any number of stages depending on how picky you want to be, but for this case I’m going to use four:

1. completely unfused, with the neural arch and centrum as separate pieces that come apart after death
2. partly fused, in which the neural arch and centrum are starting to grow together but the suture is still clearly visible on the surface
3. mostly fused, with the neural arch and spine co-ossified, but with a suture still visible as a small line or scar on the surface
4. fully fused, with no visible trace of the suture, which has been obliterated by bone remodeling

Forensic anthropologists usually divide vertebrae into three bins based on neurocentral suture closure: unfused (1 above), fusing (2 and 3), and fused and obliterated (4). The problem is that you can’t really tell 2 and 3 apart based on external examination; a vertebra with a visible suture might be pretty well fused or it might be held together by only a handful of tiny bars of bone. X-rays or histological sectioning can solve the problem, but usually it’s not warranted; those tests cost time and money and the human skeleton has many other and better indicators of age.

For paleontologists the problem is even worse, because we can’t tell 1 apart from 2 or 3. A vertebra with a visible suture line might not be fused at all; the centrum and arch might just be preserved in full articulation. The c7 and C8 shown above could be in 1, 2, or 3; without cutting them up it’s probably impossible to say. I doubt that even the current generation of medical CT scanners could resolve the sutures–which are convoluted in all three dimensions and probably packed with dense matrix–well enough. If there is no trace of a suture we say that it is closed or fused, and if the suture is visible (or if the arch and spine are preserved as separate pieces, as in C6 above) we say that it is open or unfused.

Actually there is a step 3.5 in the list above, a partly obliterated suture, in which the suture line is visible along part of its length but obliterated elsewhere. These don’t turn up very often–that is, every vertebra goes through a stage like that, but it is evidently pretty brief compared to the other stages because you don’t often come across vertebrae in this condition. Brochu (1996) showed a couple in croc vertebrae, but I’ve never seen one in a sauropod.

There is a final complication, which is that fusion of the neurocentral sutures usually proceeds along the vertebral column like a wave. In some tetrapods in starts in the neck and goes to the tail; in some it goes from tail to neck; in some it starts in the middle and goes in both directions; and in some fusion starts in more than one place. A little work has been done on this in sauropods, but I’ll save that for another post. If you’d like to read up on it in the meantime, see Brochu (1996) and Irmis (2007).

The Point (at last!)

The upshot of all of this is that if you find a sauropod centrum with no arch, or vice versa, you can be sure that the animal was not fully mature. Centra are pretty close to being cylindrical, which is a good shape for surviving the ravages of taphonomy. Neural spines are not, and they fragment pretty easily. There are a lot more juvenile sauropod centra with no arches, both in the ground and in museums, than there are arches with no centra, although I have seen a couple of the latter so they do exist.

Whew! If you made it this far, thanks for sticking around for the long anatomy slog. It’s all groundwork for talking about baby sauropod bits, so your diligence will be rewarded. Stay tuned, true believers.

———

All the vertebrae shown in this post are cervicals of CM 555, from the Carnegie Museum in Pittsburgh.

References

• Brochu, C.A. 1996. Closure of neurocentral sutures during crocodilian ontogeny: implications for maturity assessment in fossil archosaurs. Journal of Vertebrate Paleontology 16:49-62.
• Irmis, R.B. 2007. Axial skeleton ontogeny in the Parasuchia (Archosauria: Pseudosuchia) and its implications for ontogenetic determination in archosaurs. Journal of Vertebrate Paleontology 27:350-361.

Apatosaurus, Russian doll edition

My favorite room in the world is the big bone room at BYU’s Earth Science Museum. It is the only place on the planet that has good material of all six of the best-known Morrison sauropods: Apatosaurus, Barosaurus, Brachiosaurus, Camarasaurus, Diplodocus, and Haplocanthosaurus. So if you are looking at, say, a middle cervical of Apatosaurus and you think, “Hmm, I wonder how this looks in X,” where X is one of the other five genera listed above, you can just go look. It’s phenomenal.

The big vert here is a posterior cervical of Apatosaurus. Those big loops on the side are formed by the diapophyses and parapophyses (sticking out from the vertebra) and the capitula and tubercula of the cervical ribs. Capitula are rib heads, and they articulate with the parapophyses, which are the lower of the two sets of rib articulations on the vertebral centrum. Tubercula are rib tubercles, and they articulate with the diapophyses, which are at the ends of the massive transverse processes sticking out sideways from the neural arch. Here’s a labeled version, just in case the verbal description made no sense. I know Mike covered this stuff back in Tutorial 2, but Apatosaurus is frankly pretty freaky in the cervical rib department.

You’ll notice that the neural spine is split down the middle, which is the case in many diplodocoids but not all of them. Bifurcated neural spines are also found in Camarasaurus, some titanosaurs, and to a lesser extent in some mamenchisaurs, so the character definitely evolved more than once. More about bifid neural spines another day…

At last, to the point. In front of the big vert you can see a smaller one, about the size of a fist. That’s a vertebral centrum from the same part of the neck from a much smaller individual of Apatosaurus, probably somewhere between horse- and elephant-size. And that’s not all–in front of the scale bar, wrapped up in plastic, is a centrum from a wee little baby Apatosaurus about the size of poodle. Why is the vert in plastic? Because it was going off to the micro-CT scanner at the University of Utah, which is in a cleanroom, so all specimens have to be hermetically sealed. I didn’t think to shoot this little growth series lineup until the vert was already bagged, and I haven’t been back since to set it up again.

Alas, the baby Apatosaurus vert and the CTs of its internal structure will also have to wait for another day. We’re such teases…