What can sauropod sacra tell us about neck posture?

April 22, 2020

Daniel Vidal et al.’s new paper in Scientific Reports (Vidal et al. 2020) has been out for a couple of days now. Dealing as it does with sauropod neck posture, it’s obviously of interest to me, and to Matt. (See our earlier relevant papers Taylor et al. 2009, Taylor and Wedel 2013 and Taylor 2014.)

Overview

To brutally over-summarise Vidal et al.’s paper, it comes down to this: they digitized the beautifully preserved and nearly complete skeleton of Spinophorosaurus, and digitally articulated the scans of the bones to make a virtual skeletal mount. In doing this, they were careful to consider the neutral pose of consecutive vertebrae in isolation, looking at only one pair at a time, so as to avoid any unconscious biases as to how the articulated column “should” look.

Then they took the resulting pose, objectively arrived at — shown above in their figure 1 — and looked to see what it told them. And as you can well see, it showed a dramatically different pose from that of the original reconstruction.

Original skeletal reconstruction of Spinophorosaurus nigerensis (Remes et al. 2009:figure 5, reversed for ease of comparison). Dimensions are based on GCP-CV-4229/NMB-1699-R, elements that are not represented are shaded. Scale bar = 1 m.

In particular, they found that as the sacrum is distinctly “wedged” (i.e. its anteroposterior length is greater ventrally than it is dorsally, giving it a functionally trapezoidal shape, shown in their figure 1A), so that the column of the torso is inclined 20 degrees dorsally relative to that of the tail. They also found lesser but still significant wedging in the last two dorsal vertebrae (figure 1B) and apparently some slight wedging in the first dorsal (figure 1C) and last cervical (figure 1D).

The upshot of all this is that their new reconstruction of Spinophorosaurus has a strongly inclined dorsal column, and consequently an strongly inclined cervical column in neutral pose.

Vidal et al. also note that all eusauropods have wedged sacra to a greater or lesser extent, and conclude that to varying degrees all eusauropods had a more inclined torso and neck than we have been used to reconstructing them with.

Response

I have to be careful about this paper, because its results flatter my preconceptions. I have always been a raised-neck advocate, and there is a temptation to leap onto any paper that reaches the same conclusion and see it as corroboration of my position.

The first thing to say is that the core observation is absolutely right, — and it’s one of those things that once it’s pointed out it’s so obvious that you wonder why you never made anything of it yourself. Yes, it’s true that sauropod sacra are wedged. It’s often difficult to see in lateral view because the ilia are usually fused to the sacral ribs, but when you see them in three dimensions it’s obvious. Occasionally you find a sacrum without its ilium, and then the wedging can hardly be missed … yet somehow, we’ve all been missing its implications for a century and a half.

Sacrum of Diplodocus AMNH 516 in left lateral and (for our purposes irrelevant) ventral views. (Osborn 1904 figure 3)

Of course this means that, other thing being equal, the tail and torso will not be parallel with each other, but will project in such a way that the angle between them, measured dorsally, is less than 180 degrees. And to be fair, Greg Paul has long been illustrating diplodocids with an upward kink to the tail, and some other palaeoartists have picked up on this — notably Scott Hartman with his very uncomfortable-looking Mamenchisaurus.

But I do have three important caveats that mean I can’t just take the conclusions of the Vidal et al. paper at face value.

1. Intervertebral cartilage

I know that we have rather banged on about this (Taylor and Wedel 2013, Taylor 2014) but it remains true that bones alone can tell us almost nothing about how vertebrae articulated. Unless we incorporate intervertebral cartilage into our models, they can only mislead us. To their credit, Vidal et al. are aware of this — though you wouldn’t know it from the actual paper, whose single mention of cartilage is in respect of a hypothesised cartilaginous suprascapula. But buried away the supplementary information is this rather despairing paragraph:

Cartilaginous Neutral Pose (CNP): the term was coined by Taylor for “the pose found when intervertebral cartilage [that separates the centra of adjacent vertebrae] is included”. Since the amount of inter-vertebral space cannot be certainly known for most fossil vertebrate taxa, true CNP will likely remain unknown for most taxa or always based on estimates.

Now this is true, so far as it goes: it’s usually impossible to know how much cartilage there was, and what shape it took, as only very unusual preservational conditions give us this information. But I don’t think that lets us out from the duty of recognising how crucial that cartilage is. It’s not enough just to say “It’s too hard to measure” and assume it didn’t exist. We need to be saying “Here are the results if we assume zero-thickness cartilage, here’s what we get if we assume cartilage thickness equal to 5% centrum length, and here’s what we get if we assume 10%”.

I really don’t think it’s good enough in 2020 to say “We know there was some intervertebral cartilage, but since we don’t know exactly how much we’re going to assume there was none at all”.

The thing about incorporating cartilage into articulating models is that we would, quite possibly, get crazy results. I refer you to the disturbing figure 4 in my 2014 paper:

Figure 4. Effect of adding cartilage to the neutral pose of the neck of Diplodocus carnegii CM 84. Images of vertebra from Hatcher (1901:plate III). At the bottom, the vertebrae are composed in a horizontal posture. Superimposed, the same vertebrae are shown inclined by the additional extension angles indicated in Table 2.

I imagine that taking cartilage into account for the Spinophorosaurus reconstruction might have given rise to equally crazy “neutral” postures. I can see why Vidal et al. might have been reluctant to open that can of worms; but the thing is, it’s a can that really needs opening.

2. Sacrum orientation

As Vidal et al.’s figure 1A clearly shows, the sacrum of Spinophorosaurus is indeed wedge-shaped, with the anterior articular surface of the first sacral forming an angle of 20 degrees relative to the posterior articular surface of the last:

But I don’t see why it follows that “the coalesced sacrum is situated so that the posterior face of the last sacral centrum is sub-vertical. This makes the presacral series to slope dorsally and the tail to be subhorizontal (Figs. 1 and 4S)”. Vidal et al. justify this with the claim by saying:

Since a subhorizontal tail has been known to be present in the majority of known sauropods[27, 28, 29], the [osteologically induced curvature] of the tail of Spinophorosaurus is therefore compatible with this condition.

But those three numbered references are to Gilmore 1932, Coombs 1975 and Bakker 1968 — three venerable papers, all over fifty years old, dating from a period long before the current understanding of sauropod posture. What’s more, each of those three was about disproving the previously widespread assumption of tail-dragging in sauropods, but the wedged sacrum of Spinophorosaurus if anything suggests the opposite posture.

So my question is, given that the dorsal and caudal portions of the vertebral column are at some specific angle to each other, how do we decide which (if either) is horizontal, and which is inclined?

Three interpretations of the wedged sacrum of Spinophorosaurus, in right lateral view. In all three, the green line represents the trajectory of the dorsal column in the torso, and the red line that of the caudal column. At the top, the tail is horizontal (as favoured by Vidal et al. 2020) resulting in an inclined torso; at the bottom, the torso is horizontal, resulting in a dorsally inclined tail; in the middle, an intermediate posture shows both the torso and the tail slightly inclined.

I am not convinced that the evidence presented by Vidal et al. persuasively favours any of these possibilities over the others. (They restore the forequarters of Spinophorosaurus with a very vertical and ventrally positioned scapula in order to enable the forefeet to reach the ground; this may be correct or it may not, but it’s by no means certain — especially as the humeri are cross-scaled from a referred specimen and the radius, ulna and manus completely unknown.)

3. Distortion

Finally, we should mention the problem of distortion. This is not really a criticism of the paper, just a warning that sacra as preserved should not be taken as gospel. I have no statistics or even systematic observations to back up this assertion, but the impression I have, from having looked closely at quite a lot of sauropod vertebra, is the sacra are perhaps more prone to distortion than most vertebrae. So, for example, the very extreme almost 30-degree wedging that Vidal et al. observed in the sacrum of the Brachiosaurus altithorax holotype FMNH PR 25107 should perhaps not be taken at face value.

Now what?

Vidal el al. are obviously onto something. Sauropod sacra are screwy, and I’m glad they have drawn attention in a systematic way to something that had only been alluded to in passing previously, and often in a way that made it seems as though the wedging they describe was unique to a few special specimens. So it’s good that this paper is out there.

But we really do need to see it as only a beginning. Some of the things I want to see:

  • Taking cartilage into account. If this results in silly postures, we need to understand why that is the case, not just pretend the problem doesn’t exist.
  • Comparison of sauropod sacra with those of other animals — most important, extant animals whose actual posture we can observe. This might be able to tell us whether wedging really has the implications for posture that we’re assuming.
  • Better justification of the claim that the torso rather than the tail was inclined.
  • An emerging consensus on sauropod shoulder articulation, since this also bears on torso orientation. (I don’t really have a position on this, but I think Matt does.)
  • The digital Spinophorosaurus model used in this study. (The paper says “The digital fossils used to build the virtual skeleton are deposited and accessioned at the Museo Paleontológico de Elche” but there is no link, I can’t easily find them on the website and they really should be published alongside the paper.)

Anyway, this is a good beginning. Onward and upward!

References

  • Bakker, Robert T. 1968. The Superiority of Dinosaurs. Discovery 3:11–22.
  • Coombs, Walter P. 1975. Sauropod habits and habitats. Palaeogeography, Palaeoclimatology, Palaeoecology 17:1-33.
  • Gilmore, Charles W. 1932. On a newly mounted skeleton of Diplodocus in the United States National Museum. Proceedings of the United States National Museum 81:1-21.
  • Hatcher, John Bell. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
  • Osborn, Henry F. 1904. Manus, sacrum and caudals of Sauropoda. Bulletin of the American Museum of Natural History 20:181-190.
  • Taylor, Michael P. 2014. Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs. PeerJ 2:e712. doi:10.7717/peerj.712
  • 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
  • Taylor, Michael P., Mathew J. Wedel and Darren Naish. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54(2):213-230.
  • Vidal, Daniel, P Mocho, A. Aberasturi, J. L. Sanz and F. Ortega. 2020. High browsing skeletal adaptations in Spinophorosaurus reveal an evolutionary innovation in sauropod dinosaurs. Scientific Reports 10(6638). Indispensible supplementary information at https://static-content.springer.com/esm/art%3A10.1038%2Fs41598-020-63439-0/MediaObjects/41598_2020_63439_MOESM1_ESM.pdf
    doi:10.1038/s41598-020-63439-0

 

28 Responses to “What can sauropod sacra tell us about neck posture?”

  1. dale m Says:

    I ❤️ the new digital mount in this paper. It still seems that a lot is going to be left up to palaeo-Art to give us an overall picture of sauropod posture. But I am going to throw something in here that will seem absurd and unwanted for now at least. Is there a faint possibility that the cartilage embracing the centra might also implicate a more vertical BIPEDAL posture as well ?? (Look ….. I’m just sticking this out there Mike).

  2. Tyler H Says:

    Doing a bit of arm-waving here, but wouldn’t the angle of the tail vs the torso affect muscle attachments? Does inclining the tail impact the caudofemoralis muscles too much? Is it too energy-intensive?

    Scott Hartman has indicated he has been in the process of reposing his hadrosaur and sauropod skeletals to bring the tails into a more horizontal position, which results in upwardly inclined torsos. It sounds like he was planning on explaining the functional reasons behind this on his blog, but hasn’t been able to get around to it yet. The position and orientation of the scapula also impacts how much the torso and the tail are inclined.


  3. Some good thoughts here. First off, because I am constitutionally unable to not say it: Yeesh that’s an old version of my Mamenchisaurus youngi! Although to be fair the tail still looks a bit uncomfortable in the newer one: https://www.skeletaldrawing.com/sauropods-and-kin/mamenchisaurus And yes, if I’m going to follow Vidal, et al (2020), and (spoiler alert) I am, the back needs to be tilted upwards even more at some point.

    The reason why I would argue that all tails are horizontal (or sub-horizontal) in life is leverage for the m. caudofemoralis longus. If the tailbase is horizontal, the action of the CFL during leg retraction is close to horizontal (it should actually be sub-horizontal to some degree that depends on the position of the insertion on the femur). In that position mechanical advantage starts reasonable and gets better as the leg retracts. If the tail is above horizontal you are sacrificing mechanical advantage across the entire stride, and never approach maximum leverage.

    Of course all organisms are a series of compromises and tradeoffs, but in the absence of any clear benefit it seems unlikely that tetrapods that give such prominence to tail-based retractors would then sacrifice their effectiveness (and the energetic efficiency of walking). And it’s hard not to notice that no CFL-driven tetrapod alive today has a tail habitually above horizontal. This is why I’ve been slowly-but-surely lowering the tails (and so raising the backs) in my sauropod (and hadrosaur, and stegosaur) skeletals since ~2012.

    I realize that the tail can be horizontal (or even potentially subhorizontal) and the posterior face of the sacral cotyle not be vertical. And for the reasons you’ve mentioned it’s worth investigating that relationship more. But having looked at sauropod specimens I’ve drawn where the sacrum is figured, as well as the ones figured in Vidal, et al’s supplementary material it looks to me like their assumption about sacral positioning correlates well with forelimb lengths (and/or contra-keystoned dorsals), suggesting it’s functionally linked. Given this and the importance of locomotor energetics, as a prima facie assumption it seems better than simply making all tail bases horizontal as I have been doing, and MUCH better than making dinosaurs walk around sticking their buts up in the air.


  4. As an aside, I would not follow Vidal’s position of the pectoral girdle exactly – I think it’s a bit too vertical and low, although it’s more plausible than highly horizontal placements IMO. But that’s not just a feature of the dorsal column orientation, it’s also related to the height of the fore and hind limbs. While Vidal, et al (mercifully) do not imitate the subunguligrade position of Greg Paul’s sauropods, their metatarsals are still arguable too steeply angled compared to rigorous mechanical studies (cf. Bonnan, 2005). Further, their pose makes the knees very nearly fully extended, while the forelimbs are flexed, while elephants generally have their knees more flexed than their elbows, cf. Ren, et al., (2008), or the evidence of your eyes: https://physicsworld.com/wp-content/uploads/2018/05/elephant2.png

    So there’s definitely a possible world where Vidal, et al are 100% right about sacral orientation (and knock-on presacral positioning) and pectoral girdles are not in the position they illustrate in the paper.

  5. Mike Taylor Says:

    Dale, I’m not really clear on how this helps much with bipedality. That is really all about whether the centre of mass can be comfortably shifted to just over the hips. I suppose an elevated (and therefore shorter in distance-along-the-ground) torso helps a little with that.

    Tyler and Scott, good thoughts on the mechanical advantage of the caudofemoralis. But I wouldn’t rule out possibilities like the following ugly one: that sauropod tails projected posterodorsally so that they could naturally droop across the first few vertebrae, leaving the main caudofemoralis attachment points about level with the acetsbulum or trochanter. We need to keep weird ideas on the table, at least until we can show that they’re wrong. It’s too easy to assume we have sauropods worked out.

  6. Mike Taylor Says:

    (Scott, I updated the Mamenchisaurus link to point to your current version. Sorry for landing on the old one initially!)

  7. llewelly Says:

    (maybe this amatuer question reveals how little I know of the literature, but …)

    what about gravity?

    If I imagine the D. carnegii neck in figure 4 as a neck whose cartilage, tendons, etc, evolved to hold the neck up, contra to the force of gravity, with minimal effort, then that strange upward curl becomes a side effect of tissues that evolved to counter gravity constantly, and thus curl when no longer under the force of gravity.

    Applying gravity, I think, would bend the curl back downward, and the resulting neck posture is potentially similar to what the Vidal et al 2020 method would arrive at (although the neck would be longer).

    (I’m sure that having to include the unknowable (but estimable, maybe, from living analogues) ability of neck tendons and other tissues to resist gravity as yet another unknown in neck posture reconstruction is not going to be appreciated by everyone. )

  8. Mike Taylor Says:

    For sure, everything about the sauropod body-plan was profoundly influenced by gravity. The forces acting on an absolutely large animal are much greater than those acting on a small animal, and that means optimisations that smaller animals could ignore had to be applied to sauropods.

  9. Khon Says:

    I wonder what the implications of this are for the other use of sauropod tails – defense. I’m only an amateur, but it sure looks to me like Sophie’s sacrum has a similar shape https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0138352#pone-0138352-g031. Could it be that such a body plan makes swinging your tail into a theropod’s rib cage either easier or more powerful? Because if so, the trade off for walking efficiently for animals that evolved long necks specifically to reduce their overall amount of walking could well be worth it.

  10. Mike Taylor Says:

    Interesting question, Khon! I don’t have the background to contribute meaningfully to answering it, though.

  11. Khon Says:

    Was wracking my brain for a modern animal that uses its tail for defense, and remembered the iguana. Couldn’t find a good skeletal reference, but in this video you can see it does have that convex shape over the hips. https://youtu.be/HMIxZYnsNRw

    Admittedly iguanas walk nothing like dinosaurs did and that may well have more impact on the shape of their spine and hips, but if a modern animal that uses their tail the same way sauropods did has similar anatomy, that’s still interesting.


  12. Iguanas don’t have upwardly arching caudal verts. I can’t find my own photos, but here’s a web one:
    Museum of Osteology - Common Iguana Skeleton


  13. […] I was figuring out what I thought about the new paper on sauropod posture (Vidal et al. 2020) I found the paper uncommonly difficult to parse. And I quickly came to realise […]


  14. […] say in closing that I intend absolutely no criticism of Daniel Vidal or his co-authors for placing their Spinophorosaurus posture paper in Scientific Reports. He is playing the ball where it lies. We live, apparently, in a world where […]

  15. Allen Hazen Says:

    Just in terms of first impression of overall appearance– the old reconstruction looks like typical (over the past few decades) reconstructions of Diplodoc-whatever-ending-is-most-general types, and the new, in its angling of the back, more like traditional Brachiosaurus.
    The new one involves a very different placement of the pectoral girdle: in the old, the top of the scapula is at a level well up the neural spine of the closest vertebrae, whereas in the new it looks as if the scapula doesn’t extend much, if any, above the top of the centra. Did the authors themselves find this problematic, and is it why they suggested a “hypothesised cartilaginous suprascapula”?


  16. testing testing….


  17. uhu, weirdly, my comment yesterday evening could not be posted. Seems to work now.

    here’s what DIDN’T strike me when I read the paper, but DID pop into my mind immediately when I read the post:

    how much of the wedging is really to allow lifting the tail higher than habitually (for mating etc.)?

  18. dimetroblog Says:

    Be happy, Heinrich, because Vidal shared and replied to your concerns :

    COULD SAUROPODS PERFORM A ‘CLOACAL KISS’? EVIDENCE OF MATING CAPABILITIES FROM A VIRTUAL SPINOPHOROSAURUS

  19. Jura Says:

    Scott wrote: “Iguanas don’t have upwardly arching caudal verts.”

    Yes, they do, at least proximally. I discussed this on my site a few years back when everyone in the paleo community was flipping out over “droopy tailed” Stegosaurus in Jurassic World.

    https://reptilis.net/2015/06/09/jurassic-world-and-the-case-of-the-droopy-tailed-stegosaurus/#more-1302

    Upward deflected proximal caudals are pretty common among lizards.

  20. Hunter1324 (Miquel Escribano) Says:

    Ah… This brings me back.

    A year ago I attended Daniel Pidal’s talk (actually I sort of attended it twice since he presented the results of his skeletal digitalizations at both EJIP and SEP congress held in 2019), and during the question round I asked him (based mostly on your own previous work, btw) why he assumed that soft tissue wouldn’t affect the articulation of the bones. He said that because its impossible to predict to which extent it would affect his morphology they simply assumed it had no effect.

    Needless to say this tick me off, mostly because during the whole talk he had been laying it on thick about all the mistakes made in previous reconstructions and mounts and an air of un-apologetic confidence on the results they obtained.

    Since I’m not an expert on anatomy let alone Sauropod anatomy I couldn’t exactly retort this points any further, but I imagined that once they got on print, they will be scrutinized and that maybe I will feel vindicated to some extent by it.

    And here we are, don’t get me wrong, I appreciate all the effort and detail that went into their study, but seeing this points being brought up still puts a smile on my face.

  21. David Marjanović Says:

    Two questions about the chondrological neutral posture:

    1) Do pro- and opisthocoelous animals have that much articular cartilage on their centra? We’re not talking about intervertebral disks here, archosaurs don’t have those.

    2) Why would there be that much more articular cartilage on the centra than on the zygapophyses? It makes sense that there would be more – there was a lot more compressive force on the centra –, but that much more?

    Did the authors themselves find this problematic, and is it why they suggested a “hypothesised cartilaginous suprascapula”?

    I have to say that birds and mammals are unusual in ossifying the scapula all the way to the end. In other tetrapods it generally stops somewhere, and the rest, which remains cartilaginous, is called “suprascapula”.

    This is not like Paul’s (1988) completely speculative “cartilaginous episternum”.

  22. Mike Taylor Says:

    1. We don’t know anything — at least I don’t — of how cartilage thicknesses vary in different animals, across different shaped of vertebral articulation, etc. Everything Matt and I were able to discover on the subject, we summarised in Taylor and Wedel (2013c): table 4. I would love someone to do a systematic study, and indeed I have tried on multiple occasions to interest people on working in this area.

    On whether archosaurs have intervertebral discs: things are more complicated than they are often presented. At least some crocs have them in at least some parts of the vertebral column (as we pointed out in Taylor and Wedel 2013c), and I seem to recall Matt seeing one in an ostrich — maybe he can confirm or deny. Conversely, we don’t/ have intervertebral discs in our coccyx. In general, there is not the nice, clean binary distinction between synovial and fibrocartilaginous intervertebral joints that we would like.

    2. I have always assumed, like you, that the much thicker intervertebal cartilage between centra compared with between zygapophyses is because it’s bearing a compressive load, whereas as the zygapophyseal cartilage lining is really just a lubricant. But I don’t know if that’s ever been tested.

    On the cartilaginous suprascapula: if crocs have it and birds do not, then once again the EBP doesn’t help us. Does anyone know if we have direct fossil evidence of suprascapulae in saurupods (or any non-avian dinosaurs)?

  23. David Marjanović Says:

    I see, thanks. I knew intervertebral joints were underresearched (much of the merely descriptive research on the distribution of intervertebral discs is very recent!), but still underestimated how bad it is.

    An ossification front should be readily identifiable as an unfinished bone surface. If an edge looks finished, it was not continued in cartilage.

  24. Mike Taylor Says:

    Don’t zygapophyseal facets look finished to you?

  25. Mickey Mortimer Says:

    Re: suprascapulae, going crownward towards birds, Remes (2008) writes “The rugose dorsal edge of the scapula in both Euparkeria and Lewisuchus probably indicates the presence of such a suprascapular cartilage.”

    He further states “the rugose distal end indicates the presence of a cartilaginous suprascapula” in basal ornithischians, and indeed Galton (1974) states in Hypsilophodon “The dorsal edge is thicker where it cuts across the body of the blade and it is rather bumpy. This dorsal end-surface probably carried a cartilaginous suprascapula as described in Parksosaurus by Parks (1926).” It’s actually ossified in Parkosaurus’ holotype, but I don’t have Parks’ description.

    Remes states “As noted by Langer et al. (2007), a thickened, rugose distal edge of the blade indicates the presence of a cartilaginous suprascapula” in Saturnalia. And in Efraasia “The distal rim of the shaft is slightly thickened medially, indicating the suprascapular attachment.”

    In sauropods, Remes states “The distal expansion forms the base for the suprascapular cartilage, the presence of which is indicated by the wrinkled surface texture of the distal edge.” Schwarz et al. (2007) state in Diplodocus “The expanded dorsal end of the scapular blade and its rugose dorsal margin indicate that a cartilaginous suprascapula was present”, in Camarsaurus “The dorsal part of the scapula is craniocaudally expanded, with a rugose vertebral margin, which indicates the presence of a cartilaginous suprascapula.” Harris (2007) write in Suuwassea “The distal end of the blade thickens and presents a rugose surface caudally for the insertion of suprascapular cartilage.” In Opisthocoelicaudia, Borsuk-Bialynicka (1977) writes the scapula’s “upper border is 5 cm thick and roughened for the attachment of the suprascapular cartilage.”

    It seems among birds, at least Struthio has a suprascapula (Jasinoski et al., 2006: Fig. 5A-B, p. 312).

  26. Mickey Mortimer Says:

    I got Parksosaurus’ description. It states “The supra-scapula is a separate and distinct element lying above the scapula, with which it seem to have been in cartilaginous union. The bone is a sub-triangular plate, gently convex externally and slightly thickened on the edge towards the scapula; it measures 51 mm. by 46 mm.” It’s also illustrated in figure 7.

    I checked Mesozoic theropods, but these near universally have very thin distal edges which are often missing in specimens examined for myology and tissue attachments. Nonetheless, there seems to be an assumption suprascapulae were present, e.g. Osmolska and Roniewicz (1969) for Deinocheirus “The dorsal extremity of the scapula consists of thin bone which probably extended into
    a cartilage peripherally”, and Bakker et al. (1992) for Torvosaurus as Edmarka “The upper end of the scapular blade is continued, in the living animal, as a suprascapular cartilage (Fig. 9).” Given its presence in Struthio, a suprascapula might be one of those features whose presence can be inferred, but whose absence is ambiguous.

  27. Mike Taylor Says:

    Wow, Mickey, that’s some compehensive survey of Mesozoic archosaurian cartilaginous suprascapulae! Many thanks.

  28. Jay Says:

    Interesting point, Mickey, regarding the suprascapula in Parksosaurus. But given what we now know of the thoracic anatomy in related neornithischians (Thescelosaurus, Hypsilophodon, Talenkauen, Nanosaurus), and which Parks no doubt would not have been aware of in his time, wouldn’t the triangular fragment in Parksosaurus be better interpreted as a preserved intercostal plate? This suggested intercostal plate of Parksosaurus would occur more or less in the right area anyway.

    To date, all the evidence for suprascapulae in dinosaurs is based on inference from the margin morphology of scapulae, without any ossification still attached/sutured to the scapula ever being found. This is fine, as it is considered to be a mainly cartilaginous structure anyway. But it is odd if the only ossified suprascapula ever found, in Parksosaurus, was also a separate, un-sutured element presumably connected to distal scapula via more cartilage. That would require the invocation two forms of suprascapulae.


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