Utah Field House Diplodocus 1

Mounted Diplodocus at the Utah Field House of Natural History State Park Museum in Vernal.

I love Utah. I love how much of the state is given over to exposed Mesozoic rocks. I love driving through Utah, which has a strong baseline of beautiful scenery that is frequently punctuated by the absolutely mind-blowing (Arches, Bryce Canyon, Zion, Monument Valley…). I love doing fieldwork there, and I love the museums, of which there are many. It is not going too far to say that much of what I learned firsthand about sauropod morphology, I learned in Utah (the Carnegie Museum runs a close second on the dragging-Matt-out-of-ignorance scale).

DNM baby Camarasaurus

Cast of the juvenile Camarasaurus CM 11338 at Dinosaur National Monument.

There is no easy way to say this so I’m just going to get it over with: Mike has never been to Utah.

I know, right?

But we’re going to fix that. Mike’s flying into Salt Lake City this Wednesday, May 4, and I’m driving up from SoCal to meet him. After that we’re going to spend the next 10 days driving around Utah and western Colorado hitting museums and dinosaur sites. We’re calling it the Sauropocalypse.

UMNH Barosaurus mount

Mounted Barosaurus at the Natural History Museum of Utah in Salt Lake City.

Why am I telling you this, other than to inspire crippling jealousy?

First, Mike and I are giving a pair of public talks next Friday evening, May 6, at the USU-Eastern Prehistoric Museum in Price. The talks start at 7:00 and will probably run until 8:00 or shortly after, and there will be a reception with snacks afterward. Mike’s talk will be, “Why giraffes have such short necks”, and my own will be, “Why elephants are so small”.

Second, occasionally people leave comments to the effect of, “Hey, if you’re ever passing through X, give me a shout.” I haven’t kept track of all of those, so this is me doing the same thing in reverse. Here’s our itinerary as of right now:

May 4, Weds: MPT flies in. MJW drives up from Cali. Stay in SLC/Provo area.
May 5, Thurs: recon BYU collections in Provo. Stay in SLC/Provo area.
May 6, Fri: drive to Price, visit USU-Eastern Prehistoric Museum, give evening talks. Stay in Price.
May 7, Sat: drive to Vernal, visit DNM. Stay in Vernal.
May 8, Sun: visit Utah Field House, revisit DNM if needed, drive to Fruita.
May 9, Mon: visit Rabbit Valley camarasaur in AM, visit Dinosaur Journey museum in PM. Go on to Moab.
May 10, Tues: drive back to Provo, visit BYU collections.
May 11, Weds: BYU collections.
May 12, Thurs: drive to SLC to visit UMNH collections, stay for Utah Friends of Paleontology meeting that evening.
May 13, Fri: BYU collections.
May 14, Sat: visit North American Museum of Ancient Life. MPT flies home. MJW starts drive home.

We’re planning lots of time at BYU because we’ll need it, the quantity and quality of sauropod material they have there is ridiculous. As for the rest, some of those details may change on the fly but that’s the basic plan. Maybe we’ll see you out there.

Clash of the Titans from above

Here’s the “Clash of the Titans” exhibit at the Sam Noble Oklahoma Museum of Natural History, featuring the reconstructed skeletons of the giant Oklahoma Apatosaurus – which I guess should now be called the giant Oklahoma apatosaurine until someone sorts out its phylogenetic position – and the darn-near-T. rex-sized Saurophaganax maximus, which may be Allosaurus maximus depending on who you’re reading.

Now, I love this exhibit in both concept and execution. But one thing that is more obvious in this view from the upper level balcony is that despite its impressive weaponry, a lone 3-to-5 ton Saurophaganax had an Arctic ice cap’s chance in the Anthropocene of taking down a healthy 30-meter, 40-50 ton apatosaur (which is to say, none). I like to imagine that in the photo above, the apatosaur is laughing at the pathetically tiny theropod and its delusions of grandeur.

Clash of the Titans from behind

In this shot from behind, you get a better look at the baby apatosaur standing under the big one, and it hints at a far more likely target for Saurophaganax and other large Morrison theropods: sauropods that were not fully-grown, which was almost all of them. I am hip to the fact that golden eagles kill deer, and some lions will attack elephants – as Cookie Monster says, “Sometime food, not anytime food” – but not only were smaller sauropods easier prey, they were far more numerous given the inevitable population structure of animals that started reproducing at a young age and made more eggs the bigger they got (as essentially all egg-laying animals do).

In fact, as discussed in our recent paper on dinosaur ontogeny (Hone et al. 2016), there may have been times when the number of fully-grown sauropods in a given population was zero, and the species was maintained by reproducing juveniles. The giant Oklahoma apatosaurine is a unique specimen today – by far the largest apatosaurine we have fossils of – but it may also have been an anomaly in its own time, the rare individual that made it through the survivorship gauntlet to something approaching full size.

Amazingly enough, there is evidence that even it was not fully mature, but that’s a discussion for another day. Parting shot:

Oklahoma Apatosaurus neck and head

Reference

Zuniceratops ontogeny - Hone et al 2016 fig 2

Various methods that may be used to determine the age/ontogenetic status of a given dinosaur specimen. Central image is a reconstruction of the skeleton of an adult ceratopsian Zuniceratops, with surrounding indications of maturity (taken from multiple sources and do not necessarily relate to this taxon). (a) Development of sociosexual signals (adult left, juvenile right—modified from [9]), (b) surface bone texture (traced from [17]), (c) large size, represented here by an ilium of the same taxon that is considerably larger than that of a known adult specimen, (d) reproductive maturity, here based on the presence of medullary bone here shown below the black arrow (traced from [18]), (e) fusion of the neurocentral arch—location of the obliterated synchondrosis indicated by black arrow (traced from [19]), (f) asymptote of growth based on multiple species indicated by black arrow (based on [20]). Central image by Julius Csotonyi, used with permission. Hone et al. (2016: fig. 2).

New paper out in Biology Letters:

Hone, D.W.E., Farke, A.A., and Wedel, M.J. 2016. Ontogeny and the fossil record: what, if anything, is an adult dinosaur? Biology Letters 2016 12 20150947; DOI: 10.1098/rsbl.2015.0947.

The idea that dinosaurs had unusual life histories is not new. The short, short version is that it is usually pretty straightforward to tell which mammals and birds are adults, because the major developmental milestones that mark adulthood – reproductive maturity, cessation of growth, macro-level skeletal fusions, histological markers of maturity – typically occur fairly close together in time. This is radically untrue for most dinosaurs, which started reproducing early, often well before they were fully grown, and for which the other signals of adulthood can be wildly inconsistent.

Puny ‘pod

We don’t talk about this much in the paper, but one aspect of dinosaur life history should be of particular interest to sauropodophiles: most of the mounted sauropod skeletons in the world’s great museums belong to animals that are demonstrably not mature. They’re not the biggest individuals – witness the XV2 specimen of Giraffatitan, the giant Oklahoma Apatosaurus, and Diplodocus hallorum (formerly “Seismosaurus”).* They’re not skeletally mature – see the unfused scapulocoracoids of FMNH P25107, the holotype of Brachiosaurus mounted in Chicago, and MB.R.2181, the lectotype of Giraffatitan mounted in Berlin. And histological sampling suggests that most recovered sauropods were still growing (Klein and Sander 2008).

* The Oklahoma Museum of Natural History does have a mounted (reconstructed) skeleton of the giant Apatosaurus, and the New Mexico Museum of Natural History has a mounted reconstructed skeleton of Diplodocus hallorum. But as nice as those museums are, in historical terms those mounts are brand new, and they have not shaped the public – and professional – conception of Apatosaurus and Diplodocus to anywhere near the same degree as the much smaller specimens mounted at Yale, AMNH, the Field Museum, and so on.

OMNH baby Apatosaurus

Apatosaurs large and small at the Sam Noble Oklahoma Museum of Natural History

Basically, very little of what we think we know about sauropods is based on animals that were fully grown – and the same problem extends to many other groups of dinosaurs.

This is kind of a methodological nightmare – a colleague on Facebook commented that he had pulled his hair out over this problem – and in the paper we suggest some ways to hopefully alleviate it. I mean, the biology is what it is, but we can minimize confusion by being really explicit about which criteria we’re using when we assign a specimen to a bin like “juvenile”, “subadult”, and so on.

Supposed Former Evolution Junkie

Personally, I’m more excited about the possibilities that dinosaur life history weirdness open up for dinosaur population dynamics and ecology.

Confession time: I am a recovering and relatively high-functioning evolutionary theory junkie. In grad school I was on the heavy stuff – I read tons of Gould and Dawkins and admired them both without being smitten by either. I took seminars on Darwin and evolutionary morphology, and lots of courses in ecology – ever mindful of Leigh Van Valen’s definition of evolution as “the control of development by ecology”. I read a fair amount of Van Valen, too, until “Energy and evolution” (Van Valen 1976) burned out most of my higher cognitive centers.

I say “recovering” evolutionary theory junkie because after grad school I mostly went clean. The problem is that dinosaurs are good for a lot of things, but exploring the inner workings of evolution is usually not one of those things. As products of evolution, and demonstrations of what is biomechanically possible, dinosaurs are awesome, and we can look at macroevolutionary patterns in, say, body size evolution or morphospace occupation, but we almost never find dinos in sufficient numbers to be able to test hypotheses about the tempo and mode of their evolution on the fine scale. I suppose I could have switched systems and worked on critters in which the machinations of selection are more visible, but for me even the charms of evolutionary theory pale next to the virulent allure of sauropods and pneumaticity.

Anyway, keeping in mind that Van Valenian dictum that evolution stands with one foot in the organism-internal realm of genes, cells, tissue interactions, and other developmental phenomena, and the other in the organism-external world of competition, predation, resource partitioning, demographics, and other ecological interactions, then it stands to reason that if dinosaurs had weird ontogenies – and they did – then they might have had weird ecologies, and weird evolution full stop. (Where by ‘weird’ I mean ‘not what we’d expect based on modern ecosystems and our own profoundly mammal-centric point of view’.)

LACM Tyrannosaurus trio - Hone et al 2016 fig 1

Three growth stages of Tyrannosaurus on display at the Natural History Museum of Los Angeles County. Hone et al. (2016: fig. 1).

Actually, we can be pretty sure that the weird ontogenies and weird ecologies of dinosaurs were intimately linked (see, for example, Varricchio 2010). Like the tyrannosaurs shown here – they didn’t all fill the same ecological niche. This casts some old arguments in a new light. Was T. rex adapted for fast running? Prrrrobably – just not as a full-size adult. The skeleton of an adult tyrannosaur is that of a 500 kg cursor pressed into service hauling around 10 tons of murder. And all of this has some pretty exciting implications for thinking about dinosaurian ecosystems. Whereas mammals tend to fill up ecospace with species, dinosaurs filled up their world with ecologically distinct growth stages.

Does all of this add up to weird evolutionary dynamics for dinosaurs? Possibly. As we say in the paper,

Correct identification of life stage also is relevant to fundamentals of evolution—if the onset of sexual reproduction substantially preceded cessation of growth in dinosaurs then the ‘adult’ phenotype may not have been the primary target of selection. In fact, once juveniles or subadults are capable of reproducing, it is conceivable a population could exist with potentially no individuals making it through the survivorship gauntlet into ‘adulthood’ and close to maximum body size. The occasional hints from the fossil record of anomalously large sauropods like Bruhathkayosaurus [51], and the Broome trackmaker [52] might be explained if many sauropods were primarily ‘subadult’ reproducers, and thus extremely large adults were actually vanishingly rare.

Did that actually happen? Beats me. But it’s consistent with what we know about sauropod life history, and with the observed scarcity of skeletally mature sauropods. And it might explain some other oddities as well, such as the high diversity of sauropods in seasonally arid environments like the Morrison Formation (see Engelmann et al. 2004), and the fact that sauropods – and large dinosaurs generally – are much larger than predicted based on the land areas available to them (see Burness et al. 2001). Because the age structure of sauropod populations was so skewed toward juveniles, the average body size of most sauropod populations was probably fairly modest, even though the maximum size was immense. So maybe a continuously reproducing population didn’t require as much food or space as we’ve previously assumed.

If we can falsify that, cool, we’ll have learned something. And if we can falsify the alternatives, that will be even cooler.

I’ll stop waving my arms now, lest I achieve powered flight and really inspire controversy. Many thanks to Dave and Andy for bringing me on board for this. It was a fun project, and we hope the paper is useful. You can read Dave’s thoughts on all of this here.

References

Baby box turtles 2015-03-21 3

We adopted a couple of 6-week-old box turtles today.

Baby box turtles 2015-03-21 1

They are Three-Toed Box Turtles, Terrapene carolina triunguis, and they are insanely adorable.

Baby box turtles 2015-03-21 4

This one seemed oddly familiar…had I encountered it before?

Baby box turtles 2015-03-21 4-2

Baby box turtles 2015-03-21 4-3

Baby box turtles 2015-03-21 4-4

 

UPDATE: The last few images here are an homage to Mike’s Gilmore sequence from slide 96 in our 2012 SVPCA talk on Apatosarus minimus (link). I would have linked to it sooner, but I couldn’t find the right blog post. Because there wasn’t one. Memory!

[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.]

Big Bend Vanessa 182 small

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!”

OLYMPUS DIGITAL CAMERA

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

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