January 15, 2014
[This is part 4 in an ongoing series on our recent PLOS ONE paper on sauropod neck cartilage. See also part 1, part 2, and part 3.]
Here’s a frequently-reproduced quote from Darwin:
About thirty years ago there was much talk that geologists ought only to observe and not theorise; and I well remember some one saying that at this rate a man might as well go into a gravel-pit and count the pebbles and describe the colours. How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!
It’s from a letter to Henry Fawcett, dated September 18, 1861, and you can read the whole thing here.
I’ve known this quote for ages, having been introduced to it at Berkeley–a copy used to be taped to the door of the Padian Lab, and may still be. It’s come back to haunt me recently, though. An even stronger version would run something like, “If you don’t know what you’re looking for, you won’t make the observation in the first place!”
For example: I started CT scanning sauropod vertebrae with Rich Cifelli and Kent Sanders back in January, 1998. Back then, I was interested in pneumaticity, so that’s what I looked for, and that’s what I found–work which culminated in Wedel et al. (2000) and Wedel (2003). It wasn’t until earlier this year that I wondered if it would be possible to determine the spacing of articulated vertebrae from CT scans. So everything I’m going to show you, I technically saw 15 years ago, but only in the sense of “it crossed my visual field.” None of it registered at the time, because I wasn’t looking for it.
A corollary I can’t help noting in passing: one of the under-appreciated benefits of expanding your knowledge base is that it allows you to actually make more observations. Many aspects of nature only appear noteworthy once you have a framework in which to see them.
So anyway, the very first specimen we scanned way back when was the most anterior of the three plaster jackets that contain the four cervical vertebrae that make up OMNH 53062, which was destined to become the holotype of Sauroposeidon. I’ve written about the taphonomy of that specimen here, and you can read more about how it was excavated in Wedel and Cifelli (2005). We scanned that jacket first because, although the partial vertebrae it contains are by far the most incomplete of the four, the jacket is a lot smaller and lighter than the other two (which weigh hundreds of pounds apiece). Right away we saw internal chambers in the vertebrae, and that led to all of the pneumaticity work mentioned above.
Happily for me, that first jacket contains not only the posterior two-thirds of the first vertebra (possibly C5), but also the front end of the second vertebra. Whoever decided to plow through the second vertebra to divide the specimen into manageable chunks in the field made a savvy choice. Way back in 2004 I realized that the cut edge of the second vertebra was not obscured by plaster, and therefore the internal structure could be seen and measured directly, which is a lot cleaner than relying on the artifact-heavy CT scans. (The CT scans are noisy because the hospital machines we had access to start to pant a bit when asked to punch x-rays through specimens this large and dense.) A figure derived from that work made it into a couple of papers and this post, and appears again above.
But that’s pneumaticity, which this post is allegedly not about. The cut through the second vertebra was also smart because it left the intervertebral joint intact.
Here are a photo of the jacket and a lateral scout x-ray. The weird rectangles toward the left and right ends of the x-ray are boards built into the bottom of the jacket to strengthen it.
And here’s a closeup of the C5/C6 joint, with the relevant radiographs and tracing. The exciting thing here is that the condyle is centered almost perfectly in the cotyle, and the zygapophyses are in articulation. Together with the lack of disarticulation in the cervical rib bundle (read more about that here and in Wedel et al. 2000), these things suggest to us that the vertebrae are spaced pretty much as they were in life. If so, then the spacing between the vertebrae now tells us the thickness of the soft tissue that separated the vertebrae in life.
I should point out here that we can’t prove that the spacing between the vertebrae is still the same as it was in life. But if some mysterious force moved them closer together or farther apart, it did so (1) without decentering the condyle of C6 within the cotyle of C5, (2) without moving the one surviving zygapophyseal joint out of contact, and (3) without disarticulating the cervical ribs. The cervical ribs were each over 3 meters long in life and they formed vertically-stacked bundles on either side below the vertebrae; that’s a lot of stuff to move just through any hypothetical contraction or expansion of the intervertebral soft tissues after death. In fact, I would not be surprised if the intervertebral soft tissues did contract or expand after death–but I don’t think they moved the vertebrae, which are comparatively immense. The cartilage probably pulled away from the bone as it rotted, allowing sediment in. Certainly every nook and cranny of the specimen is packed with fine-grained sandstone now.
Anyway, barring actual preserved cartilage, this is a best-case scenario for trying to infer intervertebral spacing in a fossil. If articulation of the centra, zygs, and cervical ribs doesn’t indicate legitimate geometry, nothing ever will. So if we’re going to use the fossils to help settle this at all, we’re never going to have a better place to start.
So, by now, you know I’m a doofus. I have been thinking about this problem literally for years and the data I needed to address it was sitting on my hard drive the entire time. One of the things I pondered during those lost years is what the best shape for a concave-to-convex intervertebral joint might be. Would the best spacing be radially constant (A in the figure above), or antero-posteriorly constant (B), or some other, more complicated arrangement? The answer in this case surprised me–although the condyle is a lot smaller in diameter than the cotyle, the anteroposterior separation between them in almost constant, as you can see in part C of the above figure.
Don’t get too worked up about that, though, because the next joint is very different! Here’s the C6/C7 joint, again in a lateral scout x-ray, with the ends of the bones highlighted. Here the condyle is almost as big in diameter as the cotyle, but it is weirdly flat. This isn’t a result of overzealous prep–most of the condyle is still covered in matrix, and I only found its actual extent by looking at the x-ray. This is flatter than most anterior dorsal vertebrae of Apatosaurus–I’ve never seen a sauropod cervical with such a flat condyle. Has anyone else?
The condyle of C6 is a bit flatter than expected, too–certainly a lot flatter than the cervical condyles in Giraffatitan and the BYU Brachiosaurus vertebrae. As we said in the paper,
It is tempting to speculate that the flattened condyles and nearly constant thickness of the intervertebral cartilage are adaptations to bearing weight, which must have been an important consideration in a cervical series more than 11 meters long, no matter how lightly built.
Anyway, obviously here the anteroposterior distance between condyle and cotyle could not have been uniform because they are such different shapes. Wacky. The zygs are missing, so they’re no help, and clearly the condyle is not centered in the cotyle. Whether this posture was attainable in life is debatable; I’ve seen some pretty weird stuff. In any case, we didn’t use this joint for estimating cartilage thickness because we had no reason to trust the results.
Kent Sanders and I had also scanned several of the smaller sauropod vertebrae from the Carnegie collection (basically, the ones that would fit in the trunk of my car for the drive back to Oklahoma). Crucially, we’d scanned a couple of sets of articulated vertebrae, CM 3390 and CM 11339, both from juvenile individuals of Apatosaurus. In both cases, the condyles and cotyles are concentric (that’s what the ‘orthogonal gaps’ are all about in the above figure) and the zygs are in articulation, just as in Sauroposeidon. These are dorsals, so we don’t have any cervical ribs here to provide a third line of evidence that the articulation is legit, but all of the evidence that we do have is at least consistent with that interpretation.
So, here’s an interesting thing: in CM 3390, above, the first dorsal is cranked up pretty sharply compared to the next one, but the condyle is still centered in the cotyle and the zygs are in articulation. Now, the vertebrae have obviously been sheared by taphonomic deformation, but that seems to have affected both vertebrae to the same extent, and it’s hard to imagine some kind of taphonomic pressure moving one vertebra around relative to the next. So I think it’s at least plausible that this range of motion was achievable in life. Using various views and landmarks, we estimate the degree of extension here somewhere between 31 and 36 degrees. That’s a lot more than the ~6 degrees estimated by Stevens and Parrish (1999, 2005). And, as we mentioned in the paper, it nicely reinforces the point made by Upchurch (2000), that flexibility in the anterior dorsals should be taken into account in estimating neck posture and ROM.
Here’s our last specimen, CM 11339. No big surprises here, although if you ever had a hard time visualizing how hyposphenes and hypantra fit together, you can see them in articulation in parts C and D (near the top of the specimen). Once again, by paging through slices we were able to estimate the separation between the vertebrae. Incidentally, the condyle IS centered in the cotyle here, it just doesn’t look that way because the CT slice is at an angle to the joint–see the lateral scout in part A of the figure to see what I mean.
So, what did we find? In Sauroposeidon the spacing between C5 and C6 is 52mm. That’s pretty darn thick in absolute terms–a shade over two inches–but really thin in relative terms–only a little over 4% of the length of each vertebra. In both of the juvenile Apatosaurus specimens, the spacing between the vertebrae was about 14mm (give or take a few because of the inherent thickness of the slices; see the paper for details on these uncertainties).
Now, here’s an interesting thing: we can try to estimate the intervertebral spacing in an adult Apatosaurus in two ways–by scaling up from the juvenile apatosaurus, or by scaling sideways from Sauroposeidon (since a big Apatosaurus was in the same ballpark, size-wise)–and we get similar answers either way.
Scaling sideways from Sauroposeidon (I’m too lazy to write anymore so I’m just copying and pasting from the paper):
Centrum shape is conventionally quantified by Elongation Index (EI), which is defined as the total centrum length divided by the dorsoventral height of the posterior articular surface. Sauroposeidon has proportionally very long vertebrae: the EI of C6 is 6.1. If instead it were 3, as in the mid-cervicals of Apatosaurus, the centrum length would be 600 mm. That 600 mm minus 67 mm for the cotyle would give a functional length of 533 mm, not 1153, and 52 mm of cartilage would account for 9.8% of the length of that segment.
Scaling up from the juveniles: juvenile sauropods have proportionally short cervicals (Wedel et al. 2000). The scanned vertebrae are anterior dorsals with an EI of about 1.5. Mid-cervical vertebrae of this specimen would have EIs about 2, so the same thickness of cartilage would give 12mm of cartilage and 80mm of bone per segment, or 15% cartilage per segment. Over ontogeny the mid-cervicals telescoped to achieve EIs of 2.3–3.3. Assuming the cartilage did not also telescope in length (i.e., didn’t get any thicker than it got taller or wider), the ratio of cartilage to bone would be 12:120 (120 from 80*1.5), so the cartilage would account for 10% of the length of the segment–almost exactly what we got from the based-on-Sauroposeidon estimate. So either we got lucky here with our tiny sample size and truckloads of assumptions, or–just maybe–we discovered a Thing. At least we can say that the intervertebral spacing in the Apatosaurus and Sauroposeidon vertebrae is about the same, once the effects of scaling and EI are removed.
Finally, we’re aware that our sample size here is tiny and heavily skewed toward juveniles. That’s because we were just collecting targets of opportunity. Finding sauropod vertebrae that will fit through a medical-grade CT scanner is not easy, and it’s just pure dumb luck that Kent Sanders and I had gotten scans of even this many articulated vertebrae way back when, since at the time we were on the hunt for pneumaticity, not intervertebral joints or their soft tissues. As Mike has said before, we don’t think of this paper as the last word on anything. It is, explicitly, exploratory. Hopefully in a few years we’ll be buried in new data on in-vivo intervertebral spacing in both extant and extinct animals. If and when that avalanche comes, we’ll just be happy to have tossed a snowball.
- 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]
- Stevens, Kent A., and J. Michael Parrish. 2005. Neck posture, dentition, and feeding strategies in Jurassic sauropod dinosaurs. pp. 212-232 in: Virginia Tidwell and Ken Carpenter (eds.), Thunder Lizards: the Sauropodomorph Dinosaurs. Indiana University Press, Bloomington, Indiana. 495 pp.
- Taylor, Michael P., and Mathew J. Wedel. 2013c. The effect of intervertebral cartilage on neutral posture and range of motion in the necks of sauropod dinosaurs. PLOS ONE 8(10): e78214. 17 pages. doi:10.1371/journal.pone.0078214 [PDF]
- Upchurch, P. 2000. Neck posture of sauropod dinosaurs. Science 287: 547b.
- Wedel, M.J. 2003b. The evolution of vertebral pneumaticity in sauropod dinosaurs. Journal of Vertebrate Paleontology 23:344-357.
- Wedel, M.J. 2007. Aligerando a los gigantes (Lightening the giants). ¡Fundamental! 12:1-84. [in Spanish, with English translation]
- Wedel, M.J., and Cifelli, R.L. 2005. Sauroposeidon: Oklahoma’s native giant. Oklahoma Geology Notes 65 (2):40-57.
- Wedel, M.J., R.L. Cifelli and R.K. Sanders. 2000. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45(4): 343-388.
My spouse, Vicki, the other Dr. Wedel, is a physical and forensic anthropologist. And she’s one of a very small number of scientists who have (a) learned something new about the human body, and (b) used it to help identify dead people. And since that process involves the sciences of hard-tissue histology and skeletochronology–not to mention lots of dead folks–I reckon it might be of interest here. Hence this post.
This started about a decade ago, when Vicki was working on her PhD under Alison Galloway at UC Santa Cruz. Vicki worked with Alison on a ton of forensic cases, including some you probably heard of–they analyzed the remains of Laci Peterson and her unborn baby, Connor, for Scott Peterson’s murder trial. I had the unusual privilege of assisting a couple of times, on other cases, once to take some pictures in the lab while Vicki fished the skeleton out of the bag of skin that was all that was left of the body, and once to crawl around on my hands and knees picking human finger bones out of a muddy slough near Santa Cruz. All in all, I’m happy that my usual victims have been dead a lot longer.
Incidentally, the only show with forensic content that Vicki will watch voluntarily is Dexter. She cannot stand CSI, NCIS, or the other “behind the scenes” forensic investigation shows. We’ve tried watching them, but the inaccuracies drive her crazy (paleo people: imagine getting the Clockwork Orange therapy and being forced to watch Clash of the Dinosaurs). Real cases are solved by teams of specialists, not two omnicompetent protagonists; it takes weeks or months, not half an hour; and if the forensics people carry guns, it’s because they know waaaay too much about how some very bad, very organized people dispose of bodies (the short answer is, not thoroughly enough*).
* Once a guy who was threatening to testify against a certain criminal organization was shot in the head, his body burned, and his burnt remains scattered along the side of the road. Vicki and Alison picked the bone shards out of the roadside gravel, identified some of them as bits of skull, and found bevelling diagnostic of ballistics trauma on some of those. The way the bone had shattered showed that the gunshot had been inflicted perimortem–around the time of death–and before the body was burned. Bottom line, whatever plan you have to get rid of the body, it is probably not going to be enough to keep someone like Vicki from figuring out how you did it. That much, the TV shows do get right.
Not only is hard to really, truly get rid of a human body, it’s also hard to tell exactly when a person died, especially if all you have is a body in the woods. Insects are good–there’s a whole field of forensic entomology, whose practitioners age cadavers based on what insects are present and what stages of their life cycles they’re in. But what if all that is left is a pile of bones in the woods (which happens more often that you might think, and sometimes for completely innocuous reasons)? I’m preaching to the choir here, but bones can survive for a long time, so general wear-and-tear doesn’t tell you much. Rapetosaurus looks like it died last year.
There’s another side to this, which is figuring out how old someone was at the time of death based on their skeleton. Tooth eruption is good, and fusion of the epiphyseal growth plates, but both of those processes are basically done by the time people are in their mid-20s (teeth) to mid-30s (epiphyseal fusion). After that, there are methods based on the morphology of the auricular surface of the ilium and the public symphyses, but these only narrow things down to intervals of 5 to 15 years, and that’s a lot of missing persons reports to sift through. And none of the regular skeletal methods work past the age of 55 or 60. After that, no matter how healthy you are, the primary skeletal changes are attritional (i.e., you’re wearing out), and that process varies so much among individuals and populations that there are basically no predictive guidelines.
All of this was on Vicki’s mind when she was a grad student, so she was alert to anything that might help forensic anthropologists narrow down the possibilities for identifying dead folks. She was teaching in an osteology course and one of her students, Josh Peabody, brought up dental cementum increment analysis (DCIA), which is used in zooarcheology to determine the age and season at death of animal remains found at archaeological sites. Josh wanted to know if the method worked on humans.
At the time–2004–DCIA was being tested for age at death in some historical human populations from archaeological sites, but no-one had tried using it for season at death. So Vicki and Josh set out to see if it would work.
Our teeth, like those of other mammals, are held in their sockets by periodontal ligaments. The periodontal ligament of each tooth attaches via Sharpey’s fibers to the dental cementum on the tooth root(s). Cementum is laid down in annual bands, so you can count the number of bands on a tooth, add the normal age at which that tooth erupts, and get a pretty tight estimate of when the animal died. So much for age at death, which was already being done on humans in a limited way in the early 2000s, albeit in archaeological rather than forensic contexts.
But wait, there’s more. Actually two bands of cementum are laid down every year–a dark band in the winter (roughly October to March) and a light band in the summer (roughly April to September). ‘Dark’ and ‘light’ describe the appearance of the bands under polarized light microscopy. In the summer months, the collagen fibrils that make up the cementum are aligned parallel to the tooth root, so more light comes through. In the winter, the collagen is aligned perpendicular to the root, so less light is transmitted, and the winter bands appear darker by comparison. So not only does the number of pairs of light-and-dark bands tell you the number of years since the tooth erupted, the color of the outermost band tells you in which six-month period the individual died, and the thickness of the outermost band might help you narrow that down even further.
At least, that’s how it works in other mammals. Would it hold up in humans? After all, we’re pretty good at adjusting our environments to suit us, rather than vice versa. If the winter-summer banding pattern was present in humans, it would be a huge boon to forensic science. Even people in their 40s and beyond with no very reliable skeletal indicators of age could be aged to within a year or two, and their season at death narrowed down to a 2-3 month window.
To find out, Vicki and Josh had a dentist in Santa Cruz collect 112 teeth pulled from patients over the course of a year (with full IRB approval and informed consent from the dental patients). For their purposes, a tooth pulled from a live person is just as good as one from a cadaver or skeleton–extraction kills the tooth as surely as death of the body. Better, even, in that it was easier to quickly get lots of teeth with very precise extraction data.
Vicki and Josh cut a few teeth together and they found dark and light bands right away. They presented those preliminary results at the American Academy of Forensic Sciences meeting in 2005. After that, Josh got busy with his own research, but Vicki pressed on (while finishing a dissertation on different project, and being a first-time mom).
If this was a movie, this is the part where there would be a montage of inspirational music to get us quickly past a lot of hard, boring work. Each of the 112 teeth had to be embedded in plastic, a section through the root cut out with a saw, that section mounted on a slide and ground down until it was translucent (this process will be familiar to bone histologists of all stripes, paleo or neo). Then Vicki had to go all the way around the perimeter of the each root to find the place where the cementum bands showed the most clearly, and count them. This part is trickier than it sounds, unless you’ve done some histo and know just how butt-ugly some sections can be under the scope.
The results? In the words of the Bloodhound Gang, which Vicki quotes in her DCIA talks, “You and me baby ain’t nothin’ but mammals”. Here’s the payoff graph:
The one out-of-place measurement was probably caused by the dark band not being thick enough to register clearly on the image.
Now that she knew that DCIA could be used to determine season at death in humans, Vicki started applying it in her forensic cases, of which there have been many. The vast majority of the work of forensic anthropologists is invisible to the public: after analyzing a set of remains, a forensic anthropologist writes a case report for whatever law enforcement office (or, much less frequently, law firm or other entity) brought them in, and that’s that. The case reports are almost always confidential, but they have to be written to exacting standards since they may be used as evidence in court. So forensic anthropologists spend a lot of time toiling over papers that hardly anyone gets to read.
However, sometimes a case is written up for journal publication–if it’s sufficiently novel or unusual, and if permission can be secured from all of the relevant parties. In 2008, Vicki was approached by the Merced County sheriff’s office to help try to identify the remains of a young woman who had been murdered in 1971. That’s the 37-year-old cold case mentioned in the title of this post, and rather than tell you about it, I’ll point you to Vicki’s case report (Wedel et al. 2013), published last month in the Journal of Forensic Identification and freely available here.
I wasn’t sure whether to post about this or not–as cool as they are, murder cases are not our normal stock in trade on this blog. What decided me was talking with Andy Farke. He read Vicki’s paper as soon as it came out, and he said that he really enjoyed getting to see how forensic anthropologists work in the real world. I sometimes take for granted that, since I am married to a forensic anthropologist, I get to see how this works all the time. But that’s a pretty rare experience–if paleontology is a small field, forensic anthropology is positively tiny. So if you want to see an example of the real science that CSI and the like are based on, here’s your window.
What’s next? Vicki has several validation studies on DCIA in progress, for which she and her collaborators have collected a much larger sample size–over 1000 teeth–to try to answer questions like: what tooth is best to use for DCIA? Should the histological sections be made longitudinally or transversely through the tooth root? Does cementum banding vary with latitude? And since banding patterns are reversed in the Southern Hemisphere, following the flip-flopped season, what happens at the equator? Watch this space, and keep an eye out for Vicki’s future publications–including a book due out next year–at her website, Bodies, Bugs, and Bones.
- Wedel, V.L. 2007b. Determination of season at death using dental cementum increment analysis. Journal of Forensic Sciences 52(6): 1334-1337.
- Wedel, V.L., G. Found, and G.L. Nusse. 2013. A 37 year-old cold case identification using novel and collaborative methods. Journal of Forensic Identification 63(1): 5-21.
February 6, 2013
Continuing the recent theme. We’re not giving this a “Things to Make and Do” header because the spirit of that category is to showcase anatomical preparations that average people could do in the comfort of their own homes and gardens (provided they can get hold of dead wallabies, bear skulls, etc.), and freezing and band-sawing a horse is probably outside that envelope for almost everyone (I hadn’t though of that when I posted the gator!).
In the spirit of MYDHHH:
This ain’t mine, it’s a teaching specimen from our vet school, which has a no-kill policy. All of the animal cadavers used in the anatomy labs are donated by the owners at the ends of the animals’ natural lives. So no animals were harmed in the making of this science.
But I wish it was mine. And as long as I’m dreaming, I’d like a pony. Anyone want to go halvesies?
January 24, 2013
Okay, before some wag makes this point, the gator is missing a good chunk of its tail, so this is more like the left half of the anterior two-thirds of a gator. But that would make a lousy title.
We might have more to say about this in the future, but for now, I’m going to let this 1000-word-equivalent speak for itself.
Many thanks to Elizabeth Rega for the use of the gator.
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.
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.
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!
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.
- 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.
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.
Janensch, Werner. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica (Suppl. 7) 3:27-93.
March 14, 2011
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.
February 12, 2011
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.
- Schwarz, D., Frey, E., and Meyer, C.A. 2007. Pneumaticity and soft−tissue reconstructions in the neck of diplodocid anddicraeosaurid sauropods. Acta Palaeontologica Polonica 52(1):167–188.
- Tsuihiji, T. 2004. The ligament system in the neck of Rhea americana and its implications for the bifurcated neural spines of sauropod dinosaurs. Journal of Vertebrate Paleontology 24: 165–172.
- Wedel, M.J. 2003a. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology 29:243-255.
- Wedel, M.J. 2007a. What pneumaticity tells us about ‘prosauropods’, and vice versa. Special Papers in Palaeontology 77:207-222.
- Wedel, M.J. 2007b. Aligerando a los gigantes (Lightening the giants). ¡Fundamental! 12:1-84. [in Spanish, with English translation]
- Wedel, M.J. 2009. Evidence for bird-like air sacs in saurischian dinosaurs. Journal of Experimental Zoology 311A(8):611-628.
- 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.
February 5, 2010
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.
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.
Here’s the view in other direction, with Brooks Britt using a forklift to retrieve the big Supersaurus cervical.
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!
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.
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.
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.
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!