[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.]

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Weird stuff on the ground, Big Bend, 2007.

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!”

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Kent Sanders looking at scans of BYU 12613, a posterior cervical of either Kaatedocus or an anomalously small Diplodocus, at the University of Utah in May, 2008.

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.

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BYI 12613 going through a CT scanner at the University of Utah medical center. We were filming for the “Megasaurus” episode of Jurassic CSI. That shoot was crazy fun.

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.

Sauroposeidon C5 cross section Wedel 2007b fig 14

Internal structure of a cervical vertebra of Sauroposeidon, OMNH 53062. A, parts of two vertebrae from the middle of the neck. The field crew that dug up the bones cut though one of them to divide the specimen into manageable pieces. B, cross section of C6 in posterior view at the level of the break, traced from a CT image and photographs of the broken end. The left side of the specimen was facing up in the field and the bone on that side is badly weathered. Over most of the broken surface the internal structure is covered by plaster or too damaged to trace, but it is cleanly exposed on the upper right side (outlined). C, the internal structure of that part of the vertebra, traced from a photograph. The arrows indicate the thickness of the bone at several points, as measured with a pair of digital calipers. The camellae are filled with sandstone. Wedel (2007: fig. 14).

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.

Figure 11. Fifth and partial sixth cervical vertebrae of Sauroposeidon. Photograph and x-ray scout image of C5 and the anterior portion of C6 of Sauroposeidon OMNH 53062 in right lateral view. The anterior third of C5 eroded away before the vertebra was collected. C6 was deliberately cut through in the field to break the multi-meter specimen into manageable pieces for jacketing (see [37] for details). Note that the silhouettes of the cotyle of C5 and the condyle of C6 are visible in the x-ray.

Fifth and partial sixth cervical vertebrae of Sauroposeidon.
Photograph and x-ray scout image of C5 and the anterior portion of C6 of Sauroposeidon OMNH 53062 in right lateral view. The anterior third of C5 eroded away before the vertebra was collected. C6 was deliberately cut through in the field to break the multi-meter specimen into manageable pieces for jacketing (see Wedel and Cifelli 2005 for details). Note that the silhouettes of the cotyle of C5 and the condyle of C6 are visible in the x-ray. Taylor and Wedel (2013: figure 11).

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.

Figure 12. CT slices from fifth cervical vertebrae of Sauroposeidon. X-ray scout image and three posterior-view CT slices through the C5/C6 intervertebral joint in Sauroposeidon OMNH 53062. In the bottom half of figure, structures from C6 are traced in red and those from C5 are traced in blue. Note that the condyle of C6 is centered in the cotyle of C5 and that the right zygapophyses are in articulation.

CT slices from fifth cervical vertebrae of Sauroposeidon.
X-ray scout image and three posterior-view CT slices through the C5/C6 intervertebral joint in Sauroposeidon OMNH 53062. In the bottom half of figure, structures from C6 are traced in red and those from C5 are traced in blue. Note that the condyle of C6 is centered in the cotyle of C5 and that the right zygapophyses are in articulation. Taylor and Wedel (2013: figure 12).

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.

Figure 14. Geometry of opisthocoelous intervertebral joints. Hypothetical models of the geometry of an opisthocoelous intervertebral joint compared with the actual morphology of the C5/C6 joint in Sauroposeidon OMNH 53062. A. Model in which the condyle and cotyle are concentric and the radial thickness of the intervertebral cartilage is constant. B. Model in which the condyle and cotyle have the same geometry, but the condyle is displaced posteriorly so the anteroposterior thickness of the intervertebral cartilage is constant. C. the C5/C6 joint in Sauroposeidon in right lateral view, traced from the x-ray scout image (see Figure 12); dorsal is to the left. Except for one area in the ventral half of the cotyle, the anteroposterior separation between the C5 cotyle and C6 condyle is remarkably uniform. All of the arrows in part C are 52 mm long.

Geometry of opisthocoelous intervertebral joints.
Hypothetical models of the geometry of an opisthocoelous intervertebral joint compared with the actual morphology of the C5/C6 joint in Sauroposeidon OMNH 53062. A. Model in which the condyle and cotyle are concentric and the radial thickness of the intervertebral cartilage is constant. B. Model in which the condyle and cotyle have the same geometry, but the condyle is displaced posteriorly so the anteroposterior thickness of the intervertebral cartilage is constant. C. the C5/C6 joint in Sauroposeidon in right lateral view, traced from the x-ray scout image (see Figure 12); dorsal is to the left. Except for one area in the ventral half of the cotyle, the anteroposterior separation between the C5 cotyle and C6 condyle is remarkably uniform. All of the arrows in part C are 52 mm long. Taylor and Wedel (2013: figure 14).

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.

Figure 13. Joint between sixth and seventh cervicals vertebrae of Sauroposeidon. X-ray scout image of the C6/C7 intervertebral joint in Sauroposeidon OMNH 53062, in right lateral view. The silhouette of the condyle is traced in blue and the cotyle in red. The scale on the right is marked off in centimeters, although the numbers next to each mark are in millimeters.

Joint between sixth and seventh cervicals vertebrae of Sauroposeidon.
X-ray scout image of the C6/C7 intervertebral joint in Sauroposeidon OMNH 53062, in right lateral view. The silhouette of the condyle is traced in blue and the cotyle in red. The scale on the right is marked off in centimeters, although the numbers next to each mark are in millimeters. Taylor and Wedel (2013: figure 13).

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.

Figure 15. First and second dorsal vertebrae of Apatosaurus CM 3390. Articulated first and second dorsal vertebrae of Apatosaurus CM 3390. A. Digital model showing the two vertebrae in articulation, in left lateral (top) and ventral (bottom) views. B-G. Representative slices illustrating the cross-sectional anatomy of the specimen, all in posterior view. B. Slice 25. C. Slice 31. D. Slice 33. E. Slice 37. F. Slice 46. G. Slice 61. Orthogonal gaps are highlighted where the margins of the condyle and cotyle are parallel to each other and at right angles to the plane of the CT slice. 'Zygs' is short for 'zygapophyses', and NCS denotes the neurocentral synchondroses.

First and second dorsal vertebrae of Apatosaurus CM 3390.
Articulated first and second dorsal vertebrae of Apatosaurus CM 3390. A. Digital model showing the two vertebrae in articulation, in left lateral (top) and ventral (bottom) views. B-G. Representative slices illustrating the cross-sectional anatomy of the specimen, all in posterior view. B. Slice 25. C. Slice 31. D. Slice 33. E. Slice 37. F. Slice 46. G. Slice 61. Orthogonal gaps are highlighted where the margins of the condyle and cotyle are parallel to each other and at right angles to the plane of the CT slice. ‘Zygs’ is short for ‘zygapophyses’, and NCS denotes the neurocentral synchondroses. Taylor and Wedel (2013: figure 15).

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.

Figure 16. Dorsal vertebrae of Apatosaurus CM 11339. Articulated middle or posterior dorsal vertebrae of Apatosaurus CM 11339. A. X-ray scout image showing the two vertebrae in articulation, in left lateral view. B–D. Slices 39, 43 and and 70 in posterior view, showing the most anterior appearance of the condyles and cotyles.

Dorsal vertebrae of Apatosaurus CM 11339.
Articulated middle or posterior dorsal vertebrae of Apatosaurus CM 11339. A. X-ray scout image showing the two vertebrae in articulation, in left lateral view. B–D. Slices 39, 43 and and 70 in posterior view, showing the most anterior appearance of the condyles and cotyles. Taylor and Wedel (2013: figure 16).

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.

References

It shouldn’t come as a huge surprise to regular readers that PeerJ is Matt’s and my favourite journal. Reasons include its super-fast turnaround, beautiful formatting that doesn’t look like a facsimile of 1980s printed journals, and its responsiveness to authors and readers. But the top reason is undoubtedly its openness: not only are the article open access, but the peer-review process is also (optionally) open, and of course PeerJ preprints are inherently open science.

During open access week, PeerJ now publishes this paper (Farke et al. 2013), describing the most open-access dinosaur in the world.

FarkeEtAl2013-parasaurolophus-fig4

It’s a baby Parasaurolophus, but despite being a stinkin’ ornithopod it’s a fascinating specimen for a lot of reasons. For one thing, it’s the most complete known Parasaurolophus. For another, its young age enables new insights into hadrosaur ontogeny. It’s really nicely preserved, with soft-tissue preservation of both the skin and the beak. The most important aspect of the preservation may be that C-scanning shows the cranial airways clearly:

FarkeEtAl2013-parasaurolophus-fig9

This makes it possible for the new specimen to show us the ontogenetic trajectory of Parasaurolophus — specifically to see how its distinctive tubular crest grew.

FarkeEtAl2013-parasaurolophus-fig11

But none of this goodness is the reason that we at SV-POW! Towers are excited about this paper. The special sauce is the ground-breaking degree of openness in how the specimen is presented. Not only is the paper itself open access (and the 28 beautiful illustrations correspondingly open, and available in high-resolution versions). But best of all, CT scan data, surface models and segmentation data are freely available on FigShare. That’s all the 3d data that the team produced: everything they used in writing the paper is free for us all. We can use it to verify or falsify their conclusions; we can use it to make new mechanical models; we can use it to make replicas of the bones on 3d printers. In short: we can do science on this specimen, to a degree that’s never been possible with any previously published dinosaur.

This is great, and it shows a generosity of spirit from Andy Farke and his co-authors.

But more than that: I think it’s a great career move. Not so long ago, I might have answered the question “should we release our data?” with a snarky answer: “it depends on why you have a science career: to advance science, or to advance your career”. I don’t see it that way any more. By giving away their data, Farke’s team are certainly not precluding using it themselves as the basis for more papers — and if others use it in their work, then Farke et al. will get cited more. Everyone wins.

Open it up, folks. Do work worthy of giants, and then let others stand freely on your shoulders. They won’t weigh you down; if anything, they’ll lift you up.

References

Farke, Andrew A., Derek J. Chok, Annisa Herrero, Brandon Scolieri, and Sarah Werning. 2013. Ontogeny in the tube-crested dinosaur Parasaurolophus (Hadrosauridae) and heterochrony in hadrosaurids. PeerJ 1:e182. http://dx.doi.org/10.7717/peerj.182

OMNH baby Apatosaurus

I was at the Oklahoma Museum of Natural History in March to look at their Apatosaurus material, so I got to see the newly-mounted baby apatosaur in the “Clash of the Titans” exhibit (more photos of that exhibit in this post). How much of this is real (i.e., cast from real bones, rather than sculpted)? Most of the vertebral centra, a few of the neural arches, some of the limb girdle bones, and most of the long bones of the limbs. All of the missing elements–skull, neural arches, ribs, appendicular bits–were sculpted by the OMNH head preparator, Kyle Davies. Kyle is one of those frighteningly talented people who, if they don’t have what they need, will just freaking build it from scratch. Over the years he has helped me out a LOT with the OMNH sauropod material–including building a clamshell storage jacket for the referred scapula of Brontomerus so we could photograph it from the lateral side–so it’s about time I gave him some props.

Atlas-axis model with Kyle

Case in point: this sweet atlas-axis complex that Kyle sculpted for the juvenile Apatosaurus mount.

Atlas-axis model by Kyle Davies

Most fish, amphibians, and other non-amniote tetrapods only have a single specialized vertebra for attaching to the skull. But amniotes have two: a ring- or doughnut-shaped first cervical vertebra (the atlas) that articulates with the occipital condyle(s) of the skull, and a second cervical vertebra (the axis) that articulates with the atlas and sometimes with the skull as well. Mammals have paired occipital condyles on the backs or bottoms of our skulls, so our skulls rock up and down on the atlas (nodding “yes” motion), and our skull+atlas rotates around a peg of bone on the axis called the odontoid process or dens epistrophei (shaking head “no” motion). As shown in the photos and diagrams below, the dens of the axis is actually part of the atlas that fuses to the second vertebra instead of the first. Also, reptiles, including dinosaurs and birds, tend to have a single ball-shaped occipital condyle that fits into the round socket formed by the atlas, so their “yes” and “no” motions are less segregated by location.

Anyway, the whole shebang is often referred to as the atlas-axis complex, and that’s the reconstructed setup for a baby Apatosaurus in the photo above.  In addition to making a dull-colored one for the mount, Kyle made this festive version for the vert paleo teaching collection. Why so polychromatic?

Atlas-axis model key

Because in fact he built two: the fully assembled one two photos above, and a completely disassembled one, some of which is shown in this photo (I had to move the bigger bits out of the tray so they wouldn’t block the key card at the back). I originally composed this post as a tutorial. But frankly, since Kyle did all of the heavy lifting of (a) making the thing in the first place, (2) making a color-coded key to it, and (d) giving me permission to post these photos, it would be redundant to walk through every element. So think of this as a self-study rather than a tutorial.

Atlas-axis model by Kyle Davies - labeled

Oh, all right, here’s a labeled version. Note that normally in an adult animal the single piece of bone called the atlas would consist of the paired atlas neural arches (na1) and single atlas intercentrum (ic1), and would probably have a pair of fused cervical ribs (r1). Everything else would be fused together to form the axis, including the atlas pleurocentrum (c1), which forms the odontoid process or dens epistrophei (etymologically the “tooth” of the axis).

Romer 1956 fig 119 atlas-axis complex

Here’s the complete Romer (1956) figure from the key card, with a mammalian atlas-axis complex  for comparison. Incidentally, the entire book this is drawn from, Osteology of the Reptiles, is freely available online.

Apatosaurus axis-atlas complex Gilmore 1936 figs 5 and 6

And here’s the complete Gilmore (1936) figure. Sorry for the craptastic scan–amazingly, this one is NOT freely available online as far as I can tell, and Mike and I have been trying to get good scans of the plates for years. Getting back on topic, single-headed atlantal cervical ribs have been found in several sauropods, especially Camarasaurus where several examples are known, so they were probably a regular feature, even though they aren’t always preserved.

Also, as noted in this post, it is odd that in this specimen of Apatosaurus the cervical ribs had not fused to the first two vertebrae, even though they normally do, and despite the fact that the vertebrae had fused to each other, even though they normally don’t. Further demonstration, if any were needed, that sauropod skeletal fusions were wacky.

Varanops atlas-axis complex Campione and Reisz 2011 fig 2C3

For comparison to the above images, here is the atlas-axis complex in the synapsid Varanops, from Campione and Reisz (2011: fig. 2C).

Those proatlas thingies are present in some sauropods, but that’s about all I know about them, so I’ll say no more for now.

There is a good overview of the atlas-axis complex with lots of photos of vertebrae of extant animals on this page.

Previous SV-POW! posts dealing with atlantes and axes (that’s right) include:

References

Fuzzy Apato Juvenile by Niroot

Well, this is rad. And adorable. Brian Switek, whom we adore, commissioned a fuzzy juvenile sauropod from Niroot, whom we adore, for his (Brian’s) upcoming book, My Beloved Brontosaurus, which I am gearing up to adore. And here is the result, which I adore, borrowed with permission from Love in the Time of Chasmosaurs.

There is much to like here. Here’s my rundown:

  • Small forefeet that are the correct shape: good. Maybe too small, given that young animals often have big feet. But better too small than too big, given how often people screw this up.
  • Pronounced forelimb-hindlimb disparity: win.
  • Fat neck: pretty good.

In fact, let me interrupt the flow of praise here to put in Brant Bassam’s dorsal view of his mounted Phil Platt model Apatosaurus skeleton. I’ve been meaning to post about this for a while now and haven’t gotten to it, so now’s a good time: just look at how friggin’ FAT that neck is, and how it blends in with the body, and how the tail gets a lot skinnier a lot quicker (and, yeah, caudofemoralis, but not that much).  Now, go look at a bunch of life restorations of Apatosaurus–drawings, paintings, sculptures, toys, whatever–and see how many people get this wrong, by giving Apatosaurus a too-skinny neck. The answer is, damn near everyone.

Apatosaurus lousiae 1/12 scale skeleton in dorsal view, modelled by Phil Platt, assembled and photographed by Brant Bassam. Image courtesy of BrantWorks.com.

Apatosaurus lousiae 1/12 scale skeleton in dorsal view, modelled by Phil Platt, assembled and photographed by Brant Bassam. Image courtesy of BrantWorks.com.

Okay, back to Niroot’s baby:

  • Proportionally shorter neck and tail because it’s a juvenile: win.
  • Neck wrinkles possibly corresponding to vertebrae: okay, just this once.
  • Greenish fuzz possibly functioning as camouflage: We-ell

Yes, it’s true that all of the known sauropod skin impressions show scales, not fuzz. But. We don’t have anything like full-body coverage. And I suspect that there is a collection bias against fuzzy skin impressions. Scaly skin impressions are probably easier to recognize than 3D feathery skin impressions (as opposed to feathers preserved flat as at Liaoning and Solnhofen) because the latter probably just look like wavy patterns on rock, and who is looking for feather impressions when swinging a pickaxe at a sauropod’s back end? And how many sauropods get buried in circumstances delicate enough to preserve dinofuzz anyway? Also, some kind of fuzz is probably primitive for Ornithodira, and scales do not necessarily indicate that feathers were absent because owl legs. So is this speculative? Yes. Is it out of the question? I think not. In the spirit of Mythbusters, I’m calling it ‘plausible’.

Oh, one more thing: Niroot posted this in honor of Brian Switek’s birthday. Happy birthday, Brian! (You owe me a book!)

You may remember this:

Rapetosaurus mount at Field Museum

…which I used to make this:

Rapetosaurus skeleton silhouette

…and then this:

Rapetosaurus skeleton silhouette - high neck

The middle image is just the skeleton from the top photo cut out from the background and dropped to black using ‘Levels’ in GIMP, with the chevrons scooted up to close the gap imposed by the mounting bar.

The bottom image is the same thing tweaked a bit to repose the skeleton and get rid of some perspective distortion on the limbs. The limb posture is an attempt to reproduce an elephant step cycle from Muybridge.

That neck is wacky. Maybe not as wrong as Omeisaurus, but pretty darned wrong. As I mentioned in the previous Rapetosaurus skeleton post, the cervicals are taller than the dorsals, which is opposite the condition in every other sauropod I’ve seen. All in all, I find the reposed Rapetosaurus disturbingly horse-like. And oddly slender through the torso, dorsoventrally at least. The dorsal ribs look short in these lateral views because they’re mounted at a very odd, laterally-projecting angle that I think is probably not correct. But the ventral body profile still had to meet the distal ends of the pubes and ischia, which really can’t go anywhere without disarticulating the ilia from the sacrum (and cranking the pubes down would only force the distal ends of the ilia up, even closer to the tail–the animal still had to run its digestive and urogenital pipes through there!). So the torso was deeper than these ribs suggest, but it was still not super-deep. Contrast this with Opisthocoelicaudia, where the pubes stick down past the knees–now that was a tubby sauropod. Then again, Alamosaurus has been reconstructed with a similarly compact torso compared to its limbs–see the sketched-in ventral body profile in the skeletal recon from Lehman and Coulson (2002: figure 11).

I intend to post more photos of the mount, including some close-ups and some from different angles, and talk more about how the animal was shaped in life. And hopefully soon, because history has shown that if I don’t strike while the iron is hot, it might be a while before I get back to it. For example, I originally intended this post to follow the last Rapetosaurus skeleton post by  about a week. So much for that!

Like everything else we post, these images are CC BY, so feel free to take them and use them. If you use them for the basis of anything cool, like a muscle reconstruction or life restoration, let us know and we’ll probably blog it.

Thanks to the kind offices of the folks at the Field Museum, especially Fossil Vertebrates collection manager Bill Simpson, on Wednesday I got to hop the fence and spend some quality time with FMNH PR 2209, the mounted holotype specimen of Rapetosaurus krausei. I took a tape measure with me, to get some dimensions from the mounted skeleton. Of course I have the detailed descriptive paper (Curry-Rogers 2009), but mounted skeletons are three-dimensional objects and it is often surprisingly difficult to get a sense of a how a skeleton goes together in three dimensions from pictures and measurements of the individual elements. And if these dimensions are not precisely those of the animal in life, because of assumptions made during mounting–concerning, say, cartilage thickness between bones, or the angles of the ribs–at least they’re a starting point for understanding the whole-body proportions of Rapetosaurus.

This is valuable because AFAIK this specimen is the only mounted titanosaur in North America, and maybe the only one outside of South America and China. [UPDATE: Alert commenters pointed out that I forgot about the Opisthocoelicaudia in Warsaw, which is almost entirely real, and the Argentinosaurus in Georgia, which is almost entirely fake.] And because Rapetosaurus is far out, man. ALL of the neural arches are unfused, even in the distal caudals–even the Arundel Astrodon (formerly Pleurocoelus) material has fused arches in the distal caudals (Wedel et al. 2000: fig. 15). So it’s a very young juvenile, but the neck is already more than twice the length of the body. I say ‘already’ because there is pretty good evidence that the cervical vertebrae grew proportionally longer over the course of ontogeny in at least some sauropods (Wedel et al. 2000:368-369). The neck is 336 cm long, and the femora are 69 cm long. If we isometrically scaled this animal up to have a 2-meter femur, the neck would be 10 meters long, without any such ontogenetic telescoping of the vertebrae. The implications of this for possible neck lengths in the supergiant titanosaurs are pretty darned interesting. The vertebrae of Rapetosaurus don’t really look anything like those of Argentinosaurus. Nevertheless, a sauropod with an Argentinosaurus-sized femur (2.5 meters for the largest known) and Rapetosaurus proportions would have a 12-meter neck–again, that’s assuming this very young Rapetosaurus already has adult proportions, when in fact it may be ontogenetically short-necked (now there’s a thought). In Apatosaurus and Camarasaurus, the cervicals grew in proportional length (i.e., relative to diameter) by 30-50% over ontogeny, but that’s starting from tiny baby vertebrae. The Rapetosaurus vertebrae are already very long, proportionally, but it is interesting to consider the possibilities that they might have been even longer in adults, and that that scaling might have been shared with other titanosaurs.

The tail in this mount is oddly short. Only about every third vertebra is real, with the rest sculpted, so the tail length inevitably depends on how many intermediary vertebrae were added. But unless there are a LOT of missing vertebrae, it’s probably not far off. I can tell you that when I first saw the mount I looked at the tail and said, “No way”. But up close, seeing the real vertebrae and the interspersed intermediates, it looked pretty reasonable, in part because the individual caudal vertebrae are proportionally short. This is one of those things where we may just have to wait for more and better material–although that might be a long wait, because this skeleton is already freakin’ gorgeous. For someone who is used to dealing with hideously incomplete and groadily distorted fossils, this Rapetosaurus material is just mouth-wateringly beautiful.

There’s loads more weird stuff to talk about, like how the cervical vertebrae are taller than the dorsals, which is opposite the condition in every other sauropod I’ve gotten to look at, and the shape of the ilium, and the conformation of the rib cage, but those will all have to wait for future posts. This one is already much longer than I intended it to be (standard).

For the curious, here are all of my measurements. Neck length, dorsal length, etc. are lengths of those sections of the column as mounted–that is, including both the vertebrae and the spaces between them. I haven’t compared any of these to the published measurements, these are straight from the tape measure to my notebook to you. I’m giving them in mm, because that’s what I naturally think in, but they’re all rounded to the nearest cm because given my methods–hand-holding a physical tape measure up next to a bone while I crouch contorted under a fragile mounted skeleton–giving measurements to the nearest mm would be illusory precision.

  • Skull length: 290
  • Neck length: 3360
  • Dorsal length: 1210
  • Sacrum length: 480
  • Tail length: 1720
  • Total length of skeleton, snout to tip of tail (sum of above): 7060
  • Glenoid height (ground to top of socket): L – 1110 (forefoot off floor by a few cm), R – 1080
  • Acetabular height (ground to top of socket): 1320 on both sides
  • Max height of body (ground to top of 5th sacral spine): 1630
  • Gleno-acetabular distance: L – 1500, R – 1440
  • Width across acetabula: 440 between weight-bearing centers, 470 to outer margins of ilia
  • With across glenoids (at bottom of scap-coracoid joints): 710
  • Femur length: 690 on both sides
  • Tib/fib length: 470 on both sides
  • Vertical height of foot: L – 90, R – 120 (different poses)
  • Humerus length: L – 530, R – 500
  • Radius/ulna length (between articular surfaces, not including olecranons): L – 370, R – 360
  • Metacarpus length (MT3): 190 on both sides

References

Matt and I have been looking in more detail at indications of maturity in sauropod skeletons, as we prepare the submission of the paper arising from our response to Woodruff and Fowler (2012) [part 1, part 2, part 3, part 4, part 5, part 6].  Here is an oddity.

Sacra of Haplocanthosaurus. Top, H. utterbacki holotype CM 879 in right lateral view, from Hatcher (1903:fig. 15). Bottom, H. priscus holotype CM 572 in left lateral view (reversed), from Hatcher (1903:pl. IV, part 3). To the same scale.

H. priscus is the type species of Haplocanthosaurus; H. utterbacki is the second species, named by Hatcher in the 1903 monograph that described the original material in detail.  As previously noted, the type species is based on adult material, and the referred specimen on subadult material.  This is shown by their different stages of neurocentral fusion, and corroborated by the size of the specimens as indicated in the composite illustration above.

There is a lot of fusion going on in the sacra of dinosaurs:

  1. sacral neural arches fused to their centra
  2. consecutive sacral centra fused together
  3. consecutive sacral neural spines fused together
  4. sacral lateral processes fused to ilia

As we would expect, the less mature of the two Haplocanthosaurus individuals is less fused in most respects: none of the centra were fused either to each other or their respective neural arches, and the ilium was not fused to any of the lateral processes, whereas in the adult all neural arches are fused to their centra, the five sacral centra are all fused together, and the ilium is fused to the lateral processes.

How strange, then, that the consecutive neural spines are more fused in the juvenile!  Not only are spines 1, 2 and 3 fused along their entire dorsolateral length, as in the adult, but spine 4 is similarly fused.  And more: the neurapophysis of spine 5 is fused to that of 4, even though the spines are not fused more ventrally.

What does this mean?  Hatcher (1903:27-28) took it as indicative of species-level separation.  After briefly noting that the posterior dorsal centra of H. utterbacki are more opisthocoelous than those of H. priscus, and speculating that the adult of the referred species was probably larger than that of the type, he continued:

But the most distinctive character is to be found in the sacrum which, in the present species, has the five neural spines normally coössified.  The first four are cocoössified throughout their entire length, forming a long bony plate.  The union between the fourth and fifth is limited to the extremities while medially [sic, presumably meaning half way up the spines] they are separated by an elongated foramen.  In H. priscus only the spines of the three anterior sacrals are coössified, those of the first and second [sic, presumably intending fourth and fifth] sacrals remaining free.  This difference exists notwithstanding that the type of the present species was scarcely adult, the sacral centra being neither coössified with one another nor with their neural arches. By some this character might be considered as of generic importance although I prefer to consider it as of only specific value since in all other parts of the skeleton preserved, there are no distinguishing characters which could be considered as of generic value.

At present, however, the synonymy of H. utterbacki with the type species, proposed by McIntosh and Williams (1988:22), seems to be universally accepted.  If they truly belong to the same taxon then the only realistic possibility is that we are seeing individual variation in the timing of fusion.  That certainly seems to have been the opinion of McIntosh and Williams (1988:14), writing about the sacrum of their own specimen, the H. delfsi holotype CMNH 10380:

As in CM 572 the short to moderately long spines of sacrals one through three are firmly united throughout, and those of sacrals four and five are firmly united to midheight. In CM 572 spines four and five are free, but this is probably an individual character because in the even younger CM 879 all five spines are united.

All of which means: we need to be really careful when drawing conclusions about taxonomy or ontogeny from individual observations of skeletal fusion.

Bonus Pneumaticity Observation: In the image at top, you’ll see that the centrum of sacral 4 in CM 879 has a couple of pneumatic fossae. For more than you probably wanted to know about those specific holes in that specific bone, see this post and the linked paper.

References

  • 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.
  • McIntosh, J.S., and Williams, M. E. 1988. A new species of sauropod dinosaur, Haplocanthosaurus delfsi sp. nov., form the Upper Jurassic Morrison Fm. of Colorado. Kirtlandia 43:3-26.
  • Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.
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