Caudal pneumaticity in saltasaurines. Cerda et al. (2012: fig. 1).

Earlier this month I was amazed to see the new paper by Cerda et al. (2012), “Extreme postcranial pneumaticity in sauropod dinosaurs from South America.” The title is dramatic, but the paper delivers the promised extremeness in spades. Almost every figure in the paper is a gobsmacker, starting with Figure 1, which shows pneumatic foramina and cavities in the middle and even distal caudals of Rocasaurus, Neuquensaurus, and Saltasaurus. This is most welcome. Since the 1990s there have been reports of saltasaurs with “spongy bone” in their tail vertebrae, but it hasn’t been clear until now whether that “spongy bone” meant pneumatic air cells or just normal marrow-filled trabecular bone. The answer is air cells, loads of ‘em, way farther down the tail than I expected.

Caudal pneumaticity in diplodocines. Top, transverse cross-section through an anterior caudal of Tornieria, from Janensch (1947: fig. 9). Bottom, caudals of Diplodocus, from Osborn (1899: fig. 13).

Here’s why this is awesome. Lateral fossae occur in the proximal caudals of lots of neosauropods, maybe most, but only a few taxa go in for really invasive caudal pneumaticity with big internal chambers. In fact, the only other sauropod clade with such extensive pneumaticity so far down the tail are the diplodocines, including Diplodocus, Barosaurus, and Tornieria. But they do things differently, with BIG, “pleurocoel”-type foramina on the lateral surfaces of the centra, leading to BIG–but simple–camerae inside, and vertebral cross-sections that look like I-beams. In contrast, the saltasaurines have numerous small foramina on the centrum and neural arch that lead to complexes of small pneumatic camellae, giving their vertebrae honeycomb cross-sections. So caudal pneumaticity in diplodocines and saltsaurines is convergent in its presence and extent but clade-specific in its development. Pneumaticity doesn’t get much cooler than that.

Pneumatic ilia in saltasaurines. Cerda et al. (2012: fig. 3).

But it does get a little cooler. Because the stuff in the rest of the paper is even more mind-blowing. Cerda et al. (2012) go on to describe and illustrate–compellingly, with photos–pneumatic cavities in the ilia, scapulae, and coracoids of saltasaurines. And, crucially, these cavities are connected to the outside by pneumatic foramina. This is important. Chambers have been reported in the ilia of several sauropods, mostly somphospondyls but also in the diplodocoid Amazonsaurus. But it hasn’t been clear until now whether those chambers connected to the outside. No patent foramen, no pneumaticity. It seemed unlikely that these sauropods had big marrow-filled vacuities in their ilia–as far as I know, all of the non-pneumatic ilia out there in Tetrapoda are filled with trabecular bone, and big open marrow spaces only occur in the long bones of the limbs. And, as I noted in my 2009 paper, the phylogenetic distribution of iliac chambers is consistent with pneumaticity, in that the chambers are only found in those sauropods that already have sacral pneumaticity (showing that pneumatic diverticula were already loose in their rear ends). But it’s nice to have confirmation.

So, the pneumatic ilia in Rocasaurus, Neuquensaurus, and Saltasaurus are cool because they suggest that all the other big chambers in sauropod ilia were pneumatic as well. And for those of you keeping score at home, that’s another parallel acquisition in Diplodocoidea and Somphospondyli (given the apparent absence of iliac chambers in Camarasaurus and the brachiosaurids, although maybe we should bust open a few brachiosaur ilia just to be sure*).

* I kid, I kid.**

** Seriously, though, if you “drop” one and find some chambers, call me!

Pectoral pneumaticity in saltasaurines. Cerda et al. (2012: fig. 2).

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

There’s loads more interesting stuff to talk about, like the fact that the ultra-pneumatic saltasaurines are among the smallest sauropods, or the way that fossae and camerae are evolutionary antecedent to camellae in the vertebrae of sauropods, so maybe we should start looking for fossae and camerae in the girdle bones of other sauropods, or further macroevolutionary parallels in the evolution of pneumaticity in pterosaurs, sauropods, and theropods. Each one of those things could be a blog post or maybe a whole dissertation. But my mind is already thoroughly blown. I’m going to go lie down for a while. Congratulations to Cerda et al. on what is probably the most important paper ever written on sauropod pneumaticity.

References

  • Cerda, I.A., Salgado, L., and Powell, J.E. 2012. Extreme postcranial pneumaticity in sauropod dinosaurs from South America. Palaeontologische Zeitschrift. DOI 10.1007/s12542-012-0140-6
  • Janensch, W. 1947. Pneumatizitat bei Wirbeln von Sauropoden und anderen Saurischien. Palaeontographica, Supplement 7, 3:1–25.
  • Osborn, H. F. 1899. A skeleton of Diplodocus. Memoirs of the American Museum of Natural History 1:191–214.

Hello again, old friend

December 5, 2011

This week the SV-POW!sketeers are off to Bonn, Germany, for the Second International Workshop on Sauropod Biology and Gigantism. All three of us will be there, plus SV-POW! guest blogger Heinrich Mallison, plus Wedel Lab grad student Vanessa Graff, plus about 50 other awesome scientists from around the world. So we’ll have a ton of fun, but we probably won’t get much posted.

In the meantime, enjoy this cool encounter from the bone cellar at the Humboldt Museum in Berlin, where Mike and I fetched up at the end of the last IWSBG back in 2008. It’s a transversely-sectioned dorsal centrum of Giraffatitan, one that Janensch illustrated in his 1950 monograph on the vertebrae of Giraffatitan. Mike and I were very familiar with the cross-section image from the paper, so it was cool and a bit unreal to find the actual item.

Reference

Janensch, Werner. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica (Suppl. 7) 3:27-93.

Busy days. I just published a popular article on skeletal pneumaticity as a web feature at the Australian science magazine Cosmos. It’s entitled, “We are all air-heads: of sinus headaches and strangled birds”, and it includes a few things I don’t think I’ve discussed here, so hopefully even you regulars will find it a worthwhile read. I’d tell you more about it, but that would defeat the point, wouldn’t it? Go on over and check it out.

While you’re there, look at all the cool articles by award-winning science blogger and Cosmos Editorial Assistant Bec Crew, who served as my editor in this venture. I’m grateful to Bec for her help getting the article bashed into shape, her patience with my own article revision incontinence (don’t laugh, some writer you know might suffer from ARI), and most of all her enthusiasm where gory tales of science are concerned. If you’re not familiar with Bec’s work at Save Your Breath for Running Ponies, you’re in for a treat. Set your drink down first so you don’t spew it on the keyboard laughing.

When you last saw this rhea neck, I was squeezing a thin, unpleasant fluid out of its esophagus. Previous rhea dissection posts are here and here; you may also be interested in my ratite clearing house post.

We did that dissection back in 2006. Since then I finished my dissertation, got a tenure-track job, and moved twice. The rhea neck followed me, living in a succession of freezers until last spring.

Last spring I thawed it out, straightened it (it had been coiled up in a gallon ziploc), refroze it, and had it cut in half sagittally with a bandsaw. I did all of this for a project that is not yet ready to see the light of day, but there’s a ton of cool morphology here that I am at liberty to discuss, so let’s get on with it.

Throughout the post, click on the images for full resolution, unlabeled versions.

In the image above, you’ll notice that the saw cut was just slightly to the left of the midline, so that almost the entire spinal cord was left in the right half of the neck (the one toward the top of the image; the left half, below, is upside down, i.e. ventral is towards the top of the picture). The spinal cord is the prominent yell0w-white stripe running down the middle of the hemisectioned neck. It’s a useful landmark because it stands out so well. Dorsal to it are the neural arches, spines*, and zygapophyses of the vertebrae, and epaxial muscles; ventral to it are the vertebral centra and the hypaxial muscles.

* If you want to call them that–some of them are barely there!

Here’s the large supraspinous ligament (lig. elasticum interspinale), which is similar to the nuchal ligament of mammals but independently derived. Compare to the nuchal ligament of a horse (image borrowed from here):

Note how the actual profile of the neck is vastly different from what you’d suspect based on the skeleton alone. This is one of the reasons that necks lie. For more on the supraspinous ligament in rheas and its implications for sauropods, see Tsuihiji (2004) and Schwarz et al. (2007).

Birds also have very large interspinous ligaments (lig. elasticum interlaminare), each of which connects the neural spines of two adjacent vertebrae. In the above photo, the blunt probe is passing under (= lateral to) the unpaired, midline interspinous ligament. Rheas are unusual among birds in having such a large supraspinous ligament, and you can see that this interspinous ligament is almost as big. If you tear down the neck of a chicken or turkey, you will find huge interspinous ligaments, and the supraspinous ligament will be tiny if you can identify it at all.

Here’s something I don’t think we’ve ever shown before here on SV-POW!: a photograph of an actual pneumatic diverticulum. That’s the dark hole in the middle of the photo. You can see that we’re in the left half of the neck, lateral to the spinal cord, almost to the postzygapophysis, the articular surface of which is more lateral still (“below” or “deep to” the surface you see exposed in this cut). Usually at each intervertebral joint there is a connection between the lateral pneumatic diverticula that run up the side of the cervical column and pass through the cervical rib loops and the supramedullary diverticula that lie dorsal to the spinal cord inside the neural canal. That connecting diverticulum is the one exposed here.

NB: diverticulum is singular, diverticula is plural. There are no diverticulae or, heaven forbid, diverticuli, although these terms sometimes crop up in the technical literature, erroneously. (I hesitate to point this out, not because it’s not important, but because I’ll be lucky if I didn’t screw up a Latin term elsewhere in the post!)

Here are pneumatic diverticula in a transverse CT section of an ostrich neck (Wedel 2007b: fig. 6; compare to Wedel 2003: fig. 2, another slice from the same neck). In this view, bone is white, muscles and other soft tissues are gray, and air spaces are black. A, lateral diverticula running alongside the vertebral centra. B, air spaces inside the bone. C, supramedullary airways above the spinal cord. This section is close to the posterior end of a vertebra; the flat-bottomed wing-like processes sticking out to either side are the anterior portions of the postzygapophyses. If the slice was a few mm more posterior, we would see the prezygapophyses of the preceding vertebra in contact with them. Also, the vertical bars of bone connecting the centrum to the postzygs would pinch out, and we’d see the diverticula connecting the lateral (A) and supramedullary (C) airways–that’s the diverticulum revealed in the photo two images up.

Here’s another cool section showing a diverticulum and some muscles. Note the short interspinous muscles, which connect the neural spines of adjacent vertebrae. The probe indicates another open diverticulum, and the very tip of the probe is under one of the very thin layers of epithelium that line the diverticula. You can see that this diverticulum lies on the dorsal surface of the vertebra, posterior to the prezygapophysis and anterior to the neural spine. This supravertebral diverticulum is near and dear to my heart, because I have published an image of its traces before.

Lots going on in this photo (remember that you can click for an unlabeled version). This is a middle cervical vertebra of an emu, in anterodorsal view, with anterior towards the bottom of the picture. Bonus geek points if you recognized it as the basis for Text-fig. 9 in Wedel (2007a). I published this photo in that paper because it so nicely illustrates how variable the skeletal traces of pneumaticity can be, even from left to right in a single bone. On the right side of the photo (left side of the vertebra), the bone resorption adjacent to the supravertebral diverticulum produced a pneuamtic fossa, but one without distinct bony margins or a pneumatic foramen. On the other side, the fossa contains a pneumatic foramen which communicates with the internal air spaces, but the fossa is otherwise identical. Fossae like the one on the right are a real pain in the fossil record, because they might be pneumatic, but then again they might not be; such shallow, indistinct fossae can house other soft tissues, including cartilage and fat. This is what I was talking about when I wrote (Wedel 2009: p. 624):

If progressively more basal taxa are examined in the quest to find the origin of PSP [postcranial skeletal pneumaticity], the problem is not that evidence of PSP disappears entirely. It is that the shallow, unbounded fossae of basal dinosaurs are no longer diagnostic for pneumaticity.

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

The other labelled bits in the above photo are all muscle attachment points, and you may find Wedel and Sanders (2002), especially Fig. 2, a useful reference for the rest of the post. The dorsal tubercles, or epipophyses, are rugosities dorsal to the postzygapophyses that anchor most of the long, multi-segment epaxial muscles, which in birds are the M. longus colli dorsalis, which originates on the anterior faces of the neural spines, and M. ascendens cervicalis, which originates on the cervical rib loops. The crista transvers0-obliqua is a low, bony crest connecting each dorsal tubercle to the neural spine; it corresponds to the spino-postzygapophyseal lamina (SPOL) of sauropods (see Tutorial 4: Laminae!), and anchors the Mm. intercristales, a group of short muscles that span the cristae of adjacent vertebrae, like the Mm. interspinales only more lateral.

The carotid tubercles serve as points of origin for the M. longus colli ventralis, one of the largest and longest of the multi-segment hypaxial muscles; they have no obvious homolog or analog in sauropods. The lack of this feature might indicate that the hypaxial muscles were less of a big deal in sauropods, for whom lifting the neck was presumably a bigger problem than lowering it. Alternatively, the M. longus colli ventralis of sauropods might have attached to the medial sides of the parapophyses and the capitula of the cervical ribs, which tended to be larger and more ventrally-directed than in basal sauropodomorphs and theropods.

The unlabeled red arrows mark the lateral tubercles and crests of the cervical rib loop, to which we will return momentarily.

Here you can see a big bundle of long epaxial muscles, including both the M. longus colli dorsalis and M. ascendens cervicalis, inserting on the left dorsal tubercle of the vertebra on the right.  Note that the cut here is quite a bit lateral of the midline, and actually goes through the lateral wall of the neural canal in the vertebra on the right (that vert is the fifth back from the front of the section of neck featured in this post, which is incomplete). That is why you see the big, multi-segment muscles here, and not the shorter, single-segment muscles, which lie closer to the midline.

Here are some more muscle attachment points in a bird vertebra (a turkey this time, courtesy of Mike). The lateral crests and tubercles (tubecula ansae and cristae laterales, if you’re keeping track of the Latin) are the same bony features indicated by the red arrows in the photo of the emu vertebra up above. They anchor both the long M. ascendens cervicalis, which inserts on the dorsal tubercles of more anterior vertebrae, and the short Mm. intertransversarii, which span the cervical rib loops of adjacent vertebrae. Sauropods usually have at least small rugosities on their diapophyses and the tubercula of their cervical ribs (which articulate with the diapophyses) that probably anchored homologous muscles.

Here’s a dorsal tubercle above the postzyg on the neural arch of a juvenile Apatosaurus (cervical 6 of CM 555, shown in right lateral view). Notice that the spinopostzygapophyseal lamina (SPOL) and postzygodiapophyseal lamina (PODL) actually converge on the dorsal tubercle rather than on the postzyg. This is pretty common, and makes good mechanical sense.

Dorsal tubercles again, this time on the world’s most wonderful fossil, cervical 8 of the HM SII specimen of Giraffatitan brancai, in the collections of the Humbolt museum in Berlin. While you’re here, check out the pneumato-riffic sculpting on the lateral faces of the neural arch and spine, and the very rugose texture on the tip of the neural spine, SPOLs, and dorsal tubercles. In fact, compare the numerous pocket-like external fossae on this vertebra with the internal air cells exposed in the cross-sectioned rhea neck. I have argued here before that sauropod cervical vertebrae are pretty similar to those of birds; the main differences are that the cervical rib loops are proportionally much smaller in sauropods, and sauropod vertebrae mostly wore their pneumaticity on the outside.

Farther anteriorly in the neck–the three vertebrae pictured here are the third, fourth, and fifth (from right to left) in this partial neck–and somewhat closer to the midline. Now you can see some short epaxial muscles, probably Mm. intercristales and Mm. interspinales (the two groups grade into each other and are often not distinct), spanning adjacent vertebrae. As in several previous photos, the supravertebral diverticulum is visible, as well as the communicating diverticulum that connects the lateral diverticula to the supramedullary airways. I forgot to label them, but ventral to the centra you can see long, light-colored streaks running through the hypaxial muscles. These are the tendons of the M. longus colli ventralis, and in some of the previous photos you can see them running all the way to their origination points on the carotid tubercles. These extend posteriorly from the short cervical ribs of birds, and are homologous with the long cervical ribs of sauropods.

That’s all I have for this time. If you’d like to see all of this stuff for yourself, turkey necks are cheap and big enough to be easy to work with. Geese are good, too. You can see all the same bits in a chicken or a duck, it’s just harder because everything is smaller (if you’re a real glutton for punishment, try a Cornish game hen).

When I first started working on sauropods, their cervical vertebrae made no sense to me. They were just piles of seemingly random osteology. The first time I dissected a bird neck was an epiphany; ever since then, it is hard for me to look at sauropod vertebrae and not see them clad in the diverticula and muscles that shaped their morphology. Go have fun.

References

How fat was Camarasaurus?

January 16, 2011

For reasons that will soon become apparent (yes, that’s a teaser), Matt and I wanted to figure out how heavy Camarasaurus was.  This is the story of how I almost completely badgered up part of that problem.  I am publishing it as a cautionary tale because I am very secure and don’t mind everyone knowing that I’m an idiot.

Those who paid close attention to my recent paper on Brachiosaurus and Giraffatitan will remember that when I estimated their mass using Graphic Double Integration (Taylor 2009: 802-804) I listed separately the volumes of the head, neck, forelimbs, hindlimbs, torso and tail of each taxon.  In Giraffatitan, the torso accounted for 71% of the total volume (20588 of 29171 litres), and in Brachiosaurus, 74% (26469 of 35860 litres), so it’s apparent that torso volume hugely dominates that of the whole animal.  In the giant balloon-model Giraffatitan of Gunga et al.’s (1995, 1999) estimates, the torso accounted for 74% of volume (55120 of 74420 litres) so even though their fleshing out of the skeleton was morbidly obese, the relative importance of the torso came out roughly the same.  Finally, Gunga et al’.s (2008) revised, less bloated, model of the same Giraffatitan had the torso contributing 68% of volume (32400 of 47600 litres).  So far as I know, these are all of the published accounts that give the volumes of separate parts of a sauropod body, but if there are any more, please tell me in the comments!   (Odd that they should all be for brachiosaurids.)

3D “slim” version of reconstruction of the “Brachiosaurus” brancai mounted and exhibited at the Museum of Natural History in Berlin (Germany).  A. Side view, upper panel; B. top view, lower panel.  The cross in the figure of upper panel indicates the calculated center of gravity.  (Gunga et al. 2008: figure 2)

So it’s evident that, in brachiosaurs at least, the torso accounts for about 70% total body volume, and therefore for about that much of the total mass.  (The distribution of penumaticity means that it’s denser than the neck and less dense than the limbs, so that its density is probably reasonably close to the average of the whole animal.)

Now here’s the problem.  How fat is the sauropod?  Look at the top-view of Giraffatitan in the Gunga et al. figure above: it’s easy to imagine that the torso could be say 20% narrower from side to side, or 20% broader.  Those changes to breadth would affect volume in direct proportion, which would mean (if the torso is 70% of the whole animal) a change in total body volume of 14% either way.  Significant stuff.

So what do we know about the torso breadth in sauropods?  It obviously dependant primarily on the orientation of the ribs and their articulation to the dorsal vertebrae.  And what do we know about that?

Nothing.

Well, OK, I am over-simplifying a little.  It’s been mentioned in passing in a few papers, but it’s never been discussed in any detail in a published paper that I know of.  (There’s a Masters thesis out there that starts to grapple with the subject, but I don’t know whether I should talk about that while it’s still being prepared for publication, so I won’t say anything more.)  The most important published contribution is more than a century old — Holland’s (1910) smackdown of Tornier’s and Hay’s comical Diplodocus postures, which included the following cross-sections of the torsos of several animals at the seventh dorsal vertebra:

(This figure previously appeared on SV-POW! in Matt’s post, Sauropods were tacos, not corn dogs, which as far as I am aware is the only existing non-technical treatment of sauropod torso-shape.)

Holland unfortunately did not discuss the torso shape that he illustrated, merely asserting it.  Presumably it is based on the mounted skeleton of the Diplodocus carnegii holotype CM 84, which is at the Carnegie Museum in Pittsburgh, where Holland was based.  I have no reason to doubt it; just noting that it wasn’t discussed.

All right then — what about Camarasaurus?  I think it’s fair to say that it’s generally considered to be fairly rotund among sauropods, as for example this skeletal reconstruction by Greg Paul shows:

Camarasaurus lentus skeletal reconstruction, in dorsal and right lateral views. (Paul 2010:197)

Measuring off the height and width of the torso at the seventh dorsal vertebra, using GIMP, I find that they are 341 and 292 pixels respectively, so that the eccentricity is 341/292 = 1.17.  This compares with 1760/916 = 1.92 for Holland’s Diplodocus above, so if both figures are accurate, then Camarasaurus is much fatter than Diplodocus.

But is Paul’s Camarasaurus ribcage right?  To answer that, I went back to my all-time favourite sauropod paper, Osborn and Mook’s (1921) epic descriptive monograph of Camarasaurus (and Cope’s other sauropods).  I knew that this awesomely comprehensive piece of work would include plates illustrating the ribs; and in fact there are four plates that each illustrate a complete set of dorsal ribs (although the associations are doubtful).  Here they all are:

Left dorsal ribs of Camarasaurus (Osborn and Mook 1921:pl. LXXVIII)

Left dorsal ribs of Camarasaurus (Osborn and Mook 1921:pl. LXXIX)

Left dorsal ribs of Camarasaurus (Osborn and Mook 1921:pl. LXXX)

Left dorsal ribs of Camarasaurus (Osborn and Mook 1921:pl. LXXXI)

But hang on a minute — what do you get if you articulate these ribs with the dorsal vertebrae?  Osborn and Mook also provided four plates of sequences of dorsal vertebrae, and the best D7 of the four they illustrate is probably the one from plate  LXX.  And of the four 7th ribs illustrated above, the best preserved is from plate LXXIX.  So I GIMPed them together, rotated the ribs to fit as best I could and …

What on earth?!

I spent a bit of time last night feeling everything from revulsion to excitement about this bizarre vertebra-and-rib combination.  Until I happened to look again Osborn and Mook — earlier on, in the body of the paper, in the section about the ribs.  And here’s what I saw:

(Note that this is the vertebra and ribs at D4, not D7; but that’s close enough that there’s no way there could be a transition across three vertebrae like the change between this and the horrible sight that I presented above.)

What’s going on here?  In the plates above, the ribs do not curve inwards as in this cross-section: they are mostly straight, and in many case seem to curve negatively — away from the torso.  So why do O&M draw the ribs in this position that looks perfectly reasonable?

And figure 70, a few pages earlier, makes things even weirder: it clearly shows a pair of ribs curving medially, as you’d expect them to:

So why do these ribs look so totally different from those in the plates above?

I’ll give you a moment to think about that before I tell you the answer.

Seriously, think about it for yourself.  While you’re turning it over in your mind, here is a picture of the beautiful Lego kit #10198, the Blockade Runner from the original Star Wars movie.  (I deeply admire the photography here: clear as a bell.)

OK, welcome back.

Got it?  I bet most of you have.

The answer was right there in figure 71:

Osborn and Mook 1921:fig. 71. Left rib of Camarasaurus supremus Cope. Rib 4 (Amer. Mus. Cope Coll. No. 5761/R-A-24). (A) direct external view when placed as in position in the body; (B) direct anterior when placed as in position in the body. Capit. capitulum; Sh. shaft; Tub. tuberculum. Reconstructed view, portion in outline.

Osborn and Mook 1921:fig. 71. Left rib of Camarasaurus supremus Cope. Rib 4 (Amer. Mus. Cope Coll. No. 5761/R-A-24). (A) direct external view when placed as in position in the body; (B) direct anterior when placed as in position in the body. Capit. capitulum; Sh. shaft; Tub. tuberculum. Reconstructed view, portion in outline.

And, my word, isn’t it embarrassingly obvious once you see it?  I’d been blithely assuming that the ribs in O&M’s plates were illustrated in anterior view, with the capitula (which articulate with the parapophyses) located more medially, as well as more ventrally, than the tubercula (which articulate with the diapophyses).  But no: as in fact the captions of the plates state perfectly clearly — if I’d only had the wits to read them — the ribs are shown in “external” (i.e. lateral) view.  Although it’s true that the capitula in life would indeed have been more medially positioned than the tubercula, it’s also true that they were more anteriorly positioned, and that’s what the plates show at the rib heads.  And the curvature that I’d been stupidly interpreting as outward, away from the midline, is in fact posteriorly directed: the ribs are “swept back”.  The ventral portions of the ribs also curve medially, away from the viewer and into the page … but of course you can’t see that in the plates.

The important truth — and if you take away nothing else from this post, take this — is that I am dumb bones are complex three-dimensional objects, and it’s impossible to fully understand their shape from single-view illustrations.  It’s for this reason that I make an effort, when I can, to illustrate complex bones from all cardinal directions — in particular, with the Archbishop bones, as for example “Cervical S” in the Brachiosaurus coracoid post.

Because ribs, in particular, are such complex shapes — because their curvature is so unpredictable, and because their articulation with the dorsal vertebrae is via two points which are located differently on successive vertebrae, and because this articulation still allows a degree of freedom of movement — orthogonal views, even from all cardinal directions, are of limited value.  Compositing figures will give misleading results … as demonstrated above.  PhotoShop is no more use here.  Fly, you fools!

Paradoxically, our best source of information on the shapes of saurpod torsos is: mounted skeletons.  I say “paradoxically” because we’ve all grown used to the idea that mounts are not much use to us as scientists, and are really there only as objects of awe.  As Brian Curtice once said, “A mounted skeleton is not science.  It’s art.  Its purpose is to entertain the public, not to be a scientifically accurate specimen”.  In many respects, that’s true — especially in skeletons like that of the “Brontosaurus” holotype, YPM 1980, where the bones are restored with, and in some cases encased in, plaster so you can’t tell what’s what.  But until digital scanning and modelling make some big steps forward, actual mounted skeletons are the best reference we have for the complex articulations of ribs.

Giraffatitan brancai paralectotype HMN SII, composite mounted skeleton, torso in left posteroventrolateral view (photograph by Mike Taylor)

And I finish this very long (sorry!) post with yet another note of caution.  Ribs are long and thin and very prone to damage and distortion.  It’s rare to find complete sauropod ribs (look closely at the O&M plates above for evidence), but even when we do, we shouldn’t be quick to assume that the shape in which they are preserved is necessarily the same as the shape they had in life.  (If you doubt this, take another look at rib #6 in the third of the four O&M plates above.)  And as if that weren’t enough to discourage us, we should also remember that the vertebra-rib joints would have involved a lot of cartilage, and we don’t know its extent or shape.

So bearing in mind the complicated 3D shape of ribs and of dorsal vertebrae, the tendency for both to distort during and after fossilisation, and the complex and imperfectly known nature of the joints between them, I think that maybe I wasn’t too far wrong earlier when I said that what we know about sauropod torso shape is: nothing.

It’s a sobering thought.

References

Here’s one of those text-light photo posts that we always aspire to but almost never achieve. In the spring of 2008 I flew to Utah to do some filming for the History Channel series “Evolve”, in particular the episode on size, which aired later that year. I always intended to post some pix from that trip once the show was done and out, and I’m just now getting around to it…a bit belatedly.

Utah 2008 01 mountains from museum door

Here’s the view out the back door of the BYU Earth Sciences Museum in Provo, Utah. Not bad–the mountains actually made me drag my eyes away from sauropod vertebrae for a few seconds here and there.

Utah 2008 02 Brooks driving forklift

Here’s the view in other direction, with Brooks  Britt using a forklift to retrieve the big Supersaurus cervical.

Utah 2008 03 Supes and giraffe

And here is said cervical, with a mid-cervical of a giraffe for scale. You may remember the big cervical from this post (and if you click that link, notice how much nicer the new collections area is than the off-site barn where I first encountered the Cervical of Doom). Sauropods FTW!

Utah 2008 04 taping down Diplo vert

While the film crew were shooting Brooks and picking up some establishing shots, I was ransacking the collections for pretty vertebrae. We took our treasures up to the University of Utah med center in Salt Lake for CT scanning. Here Kent Sanders is helping me tape down a Diplodocus cervical.

Utah 2008 05 Kent in reading room

And here’s Kent in the CT reading room playing with the data. Like old times–I spent most of my Saturdays in 1998 and 1999 scanning verts with Kent when he was at the University of Oklahoma Health Sciences Center.

Utah 2008 06 NAMAL main drag

The next morning we went to the North American Museum of Ancient Life in Lehi. Here’s a view down the main drag, with the mounted Supersaurus on the left, mounted Brachiosaurus in the center, and original Supersaurus sacrum (on loan from BYU) in the case on the right.

Utah 2008 07 Matt in lift

The highlight of my day trip year.

I was back at BYU just a few months ago shooting another documentary, but that story will have to wait for the dramatically appropriate moment. Stay tuned!

A section of the cotyle of a presacral vertebra of Alamosaurus (Woodward and Lehman 2009:fig. 6A).

The last time we talked about Alamosaurus, I promised to explain what the arrow in the above image is all about. The image above is a section through the cotyle (the bony socket of a ball-and-socket joint) at the end of one of the presacral vertebra. The external bone surface would have been over on the left; it was either very thin (which happens) or a bit eroded, or both. The arrow is pointing at something weird–a plate of bone inside the vertebra that forms a sort of shadow cotyle deep to the articular surface.

This is weird for a couple of reasons. First, once camellate (small-chambered) vertebrae get above a certain level of complexity, it’s hard to make any sense of the orientation of individual bony struts. Possibly I haven’t seen enough vertebrae, or played with enough 3D models, to figure it out. You would certainly expect that the struts would be oriented to resist biomechanical loads, just like the struts in the long bones of your limbs; the fact that sauropod verts were filled with air whereas your long bones are filled with marrow shouldn’t make any difference. Back in the day, Kent Sanders–who is second author on that super-important paper on unidirectional air flow in croc lungs that you’ve probably heard about (Farmer and Sanders 2010)–speculated to me that the complex of laminae we see in the vertebrae of most sauropods are still there in the inflated-looking vertebrae of titanosaurs and birds, they’re just incarnated in internal struts rather than external laminae. Cool hypothesis for somebody to test.

The other reason that this is weird is that the plate of bone is parallel to the articular surface. One place where I have seen some regularity in terms of strut orientation is in zygapophyses, where in both camerae and camellate vertebrae the internal struts are oriented at right angles to the articular surfaces of the zygs, like beams propping up a wall. In this Alamosaurus section, there are indeed smaller struts that run at right angles to both the cotyle and the internal plate, but I have no idea why they’re so wimpy and the plate is so thick; a priori I would have expected the reverse.

It turns out that this isn’t even the first time that an internal “shadow” of the cotyle has been figured–check out this figure that I redrew from Powell’s (1992:fig. 16) Saltasaurus osteology. But don’t credit me with the discovery. I’d looked at this section a hundred times and even drawn it and never noticed the shadow cotyle, until it was pointed out by Woodward and Lehman (2009)–another reason to read that paper if you haven’t yet. Kudos to Holly Woodward for spotting this and making the connection.

Now that I’ve drawn attention to the weirdness and given credit where it’s due, this is one of those times I’m going to throw up my hands in confusion and open the floor for comments.

References

  • Farmer, C.G., and Sanders, K. 2010. Unidirectional airflow in the lungs of alligators. Science 327:338-340.
  • Powell, J.E. 1992. Osteologia de Saltasaurus loricatus (Sauropoda – Titanosauridae) del Cretacico Superior del noroeste Argentino; pp. 165-230 in J.L. Sanz and A.D. Buscalioni (editors), Los Dinosaurios y Su Entorno Biotico: Actas del Segundo Curso de Paleontologia in Cuenca. Institutio Juan de Valdes.
  • Woodward, H.N.,  and Lehman, T.M. 2009. Bone histology and microanatomy of Alamosaurus sanjuanensis (Sauropoda: Titanosauria) from the Maastrichtian of Big Bend National Park, Texas. Journal of Vertebrate Paleontology 29(3):807-821.
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