Friday 13 November 2015

The lifestyle of Tanystropheus, part 1: was that neck too heavy for use on land?

Two Tanystropheus longobardicus tussle in Triassic Europe. There's a distinct lack of water supporting their necks in this scene, and some might suggest this makes such behaviour impossible for these animals. But does it? Read on...
One of the most famous non-dinosaurian denizens of the Mesozoic is Tanystropheus, a spectacularly long-necked reptile which lived across Europe and Asia in Middle-Late Triassic times. We've known about this 5-6 m long animal since fragmentary fossils were pulled from Italian Triassic rocks in 1855, and now regard it as a particularly large and anatomically extreme member of the Protorosauria. This is a Permian-Triassic group of archosauromorphs (all reptiles more closely related to crocodylians and birds that lizards) that spawned numerous aberrant taxa, such as drepanosaurs, Sharovipteryx and Dinocephalosaurus. Within Protorosauria, Tanystropheus can be considered a tanystropheid, closely related to similar, but shorter-necked and smaller-bodied species such as Tanytrachelos and Langobardisaurus. Tanystropheus longobardicus is by far the best known Tanystropheus representative, and the one we always think of when discussing this animal, but something like five Tanystropheus species have been named over the years. It is currently uncertain how many of these should be considered valid and, of those, which ones truly represent Tanystropheus and not some other type of protorosaur. There are hints that longobardicus might be the sole representative of this genus, but work on this is ongoing.

We know a lot about Tanystropheus because it's fossils are not uncommon, and many of them are complete or every nearly so. Its remains occur in Alpine Europe, the Middle East and China and we can conclude that, weird as it seems, the Tanystropheus bauplan and life strategy was a successful one. But exactly what that strategy was remains a bone of contention for palaeontologists. Summarised simply, opinion is divided over whether Tanystropheus was confined to aquatic habits, at least above a certain age and body size, or else capable of living terrestrially as a shore-patrolling, 'animated fishing rod'. Unsurprisingly, the principle source of this contention is its neck anatomy: clearly long and relatively stiff in life, was it so heavy that would over-balance the animal if not supported in water? Or was the neck anatomy not as heavy as commonly supposed and really no great hindrance to life on land? Other aspects of Tanystropheus form have also influenced this debate, including limb structure and tail anatomy, but it seems fair to say these discussions persist because experts disagree about the significance of that crazy neck. Renesto (2005) and Nosotti (2007) provide recent overviews and contributions to this long-running controversy.

I've been wanting to cover Tanystropheus lifestyle here for some time now, and I've ended up with sufficient material to spread discussion over two posts. In the next article I'll be discussing nuances of arguments for aquatic and terrestrial habits, but, first, I want to satisfy some personal curiosity over how Tanystropheus was constructed. Specifically, I'm interested in the mass distribution of this animal: is it really _that_ front heavy? There are plenty of terrestrial animals with very long necks - sauropods, giraffes, some pterosaurs - and we don't worry about them toppling over. Moreover, although neck mass is frequently mentioned as critical to understanding Tanystropheus lifestyle, to my knowledge, there isn't any information available on its body volumes or mass (if I'm wrong, please tell me below). I thought I'd see what I could find out about this myself using the GDI (Graphic Double Integration) method of volumetric mass estimation, a quick and easy way to get a sense of mass and body fractions of fossil animals. It basically involves chopping up drawings of animals to determine volumes of body segments, then multiplying these by a suitable density - check out this excellent SV:POW! summary for a full lowdown.

Tanystropheus longobardicus as reconstructed by Rupert Wild in 1973. Image borrowed from Palaeos.
GDI methods require a clear layout of animal form to measure and divide into segments. There is no shortage of life restorations of Tanystropheus out there, and plenty of photographs of near complete specimens, but objective, modern portrayals of its anatomy are hard to come by. Rupert Wild's skeletal reconstruction from the 1970s (above) seems to remain a common frame of reference, and a David Peters reconstruction is sometimes used as an alternative. Neither really seemed suited for my purposes here, the crouched poses obscuring anatomical details, some specifics of vertebral count being inaccurate to modern interpretations, and the latter being produced with techniques of questionable reliability. I decided to try my hand at producing a new skeletal reconstruction based on the large, near complete Tanystropheus skeleton described in detail by Rieppel et al. (2010): GMPKU-P-1527:

Tanystropheus cf. longobardicus specimen GMPKU-P-1527, as depicted by Rieppel et al. (2010).
I wanted a large animal because the Tanystropheus neck seems to increase in length disproportionately to body size (Tschanz 1988). I want to give this animal the best chance of falling over, so it makes sense to use the largest neck possible. GMPKU-P-1527 is articulated and includes most of the neck, missing only the relatively short anterior 3.5 vertebrae (of 13), the skull, and the end of the tail. I reconstructed these missing parts using smaller Tanystropheus specimens (from Nosotti 2007) and Wild's widely-used 'adult' skull reconstruction. These came together to form a skeleton measuring 3.5 m as reconstructed, and likely over 4 m if the vertebral column were completely straightened. This is not as large as we think this animal could get, but is c. 70% of maximum size, and the minimal amount of proportional inference and cross scaling means we should be looking at a fairly authentic image of Tanystropheus form. The results are below.

Tanystropheus cf. longobardicus skeletal reconstruction, almost entirely based on GMPKU-P-1527. See text and illustration below for details on which bits are borrowed from other specimens.
The length of the limbs here is quite striking. Note that they aren't depicted in a true sprawling pose, because foreshortening would impact measurements for the mass calculation, but I depicted a crouched pose which hopefully conveys something of a low, sprawling gait. I also followed Nosotti's (2007) suggestion of digitigrady, which boosts the standing height a bit. Despite the low pose, I immediately get a different vibe from this image to that of Wild's classic, sitting reconstruction. Simply putting the animal on its feet gives the impression of the limbs and body being more proportionate to the neck. The arc of the neck follows that preserved in GMPKU-P-1527 quite closely, a pose also occurring in several other articulated Tanystropheus specimens. As depicted, I don't think it conflicts with recent interpretations of Tanystropheus neck arthrology (e.g. Renesto 2005). The body outline should be non-controversial, pretty much following the outline of the skeleton and hitting major muscle landmarks.

Time to chop this guy and up see what it's made of. Ideally, we'd want full orthographic views for a GDI mass estimate, but I've not had time to produce a multi-view skeletal. This means we're going to have to make predictions of body width. For the neck, body and tail, I decided to calculate width as 2/3 of body segment height, this being indicated by the proportions of Tanystropheus neck and tail verts, and the fact the dorsal ribs straighten out as they approach the lateral margins of the body. The 2/3 figure is a little arbitrary and arm-wavy, but seems more precise than assuming a circular cross section across the entire body. Other elements - the head and limbs - were modelled as having circular cross sections, however. You can see how I chopped the reconstruction up below: note that this uses an earlier, differently posed version of the skeletal shown above, and that the limbs are somewhat straighter. The bone sizes are no different, however, so influences on mass estimation should be negliable.

GDI mass estimation on Tanystropheus cf. longobardicus. Grey portions of the skeleton show which parts were modelled on other specimens. Numbers in parentheses give mass fractions for each body component, and the grey shapes indicate the cross-sectional shape used for that part of the body.
The entire animal shakes out to 26.7 L, and using a middle-of-the-road reptile density of 0.85 kg/L, the animal masses 22.9 kg. Of more interest to us is the mass percentages of each component, which are indicated in parentheses in the illustration above. You can see that the neck and head together are a hair away from 20% of the body mass, despite accounting for something like half the length of the animal. Virtually all of this 20% represents neck, of course, the head being less than 1% of the overall mass. As is usual for tetrapods, the trunk volume dominates all, being 50% of overall bulk despite only just exceeding 50 cm long in a 3.5 m long animal.

What do these figures actually mean? 20% doesn't seem like that much in the grand scheme of things, it being balanced by the other 80% of the body. These are certainly not values which make me think this animal perpetually toppled over unless it was in water. But can we be more precise here - how does this neck fraction stand up to other long-necked animals? For brachiosaurid sauropods, Mike Taylor (2009) suggested the neck accounted for 14% of the body mass, while Don Henderson (2010) suggested 8% for the same animals, noting that this was the largest neck mass fraction in his dataset of 10 volumetric sauropod mass predictions. Mitchell et al. (2013) did not report exact head and neck mass fractions for a large set of giraffes, but eyeballing their data suggests male giraffe necks and heads account for around 14% of body mass, with females slightly less than that. These are all significantly lower than the 20% I've estimated for Tanystropheus, implying that my gut feeling might be wrong: maybe it did have quite a heavy neck and, perhaps, was at greater risk of overbalancing.

However, it strikes me that giraffes and sauropods are not particularly good analogues for Tanystropheus, because their anatomy is built around a fundamentally different set of demands: processing of plant material. Herbivores need large guts to get the most from their nutrient-poor diet, equating to proportionally larger trunk volumes. Anyone who's played with volumetric mass estimations will know that even slight adjustments to trunk proportions can have a big impact on absolute mass and tissue fractions because they represent the biggest components of most animal bodies. We therefore can't ignore the requirement for herbivore torsos to be large when comparing them to non-herbivores like Tanystropheus. Our problem here is that finding a long-necked terrestrial carnivore to compare with Tanystropheus is challenging. Such body plans have been rare throughout geological time and are entirely unrepresented nowadays. We're not entirely licked, though: following the laws of monster movie science, any challenge involving a poorly understood, freakish creature is best solved with another poorly understood and freakish animal: in this case a long-necked azhdarchid pterosaur. Azhdarchid palaeoecology has a history of contention and controversy, but no-one believes that they were aquatic animals, or herbivorous, or at risk of toppling forward without environmental aid. This is despite azhdarchids bearing neck/trunk proportions similar to those of Tanystropheus, as well as much larger heads. We just assume they could carry their heads and necks one way or another, because all indications are that they were not adapted for an aquatic existence.

Taking a GDI approach to the Zhejiangopterus linhaiensis skeletal I produced earlier this year, I attempted to gather some data on azhdarchid body volumes and masses. As before, I estimated widths rather than producing full orthogonal views. The head and neck were assumed to be half as wide as tall, with the neck widths not permitted to exceed those of the skull. All other elements are treated as having circular cross sections. Azhdarchid torsos, forelimbs and necks are all highly pneumatised, so I gave these low tissue densities of 0.7 kg/L (about the lowest density recorded for modern birds), while the legs were given a more typical density of 0.85 kg/L. The breakdown and results:

Zhejiangopterus linhaiensis gets the GDI treatment. As above, numbers in parentheses indicate mass fractions, and the grey shapes indicate cross sectional area used in the calculations. Skeletal based on data in Cai and Wei (1994).
First things first, I was happy to see the animal come out at 7.9 kg - that's in line with most post-2000 interpretations of pterosaur mass, and seems about right for an animal with a 2.5-3 m wingspan. That makes me think the constituent volumes and masses are probably in a sensible ball park. In terms of body component masses, the torso - famously small in derived pterodactyloids like azhdarchids - provides less than 25%, the neck is just over 25%, and the head and paired forelimbs are 22% each. The legs account for just under 6%, and the tail might as well not exist. This puts - wow - almost 50% of the mass in front of the shoulders in this reconstruction. But even accepting that I've been generous with neck tissue in my reconstruction (following a reptilian, rather than avian pattern of neck musculature), and that some cross sectional shapes used here could be refined, it seems unlikely we could slim the head and neck tissues down to the mass fractions seen in long-necked herbivores. Even if my estimates are out by a factor of 2, the neck and head will still account for more mass than the same components in Tanystropheus. This finding makes the neck tissue fraction of the Tanystropheus model look a lot less aberrant, as well as verifying the suspicion that lifestyles, and not just anatomy, are important factors when comparing animal bauplans.

Let's bring all this together. While the sums outlined here are provisional, back-of-the-envelope-type stuff, I find them sufficient to at least make me sceptical of claims that Tanystropheus has a terrestrially-untenable mass distribution. At least one group of non-aquatic Mesozoic carnivores seem more front heavy, and a basic model of Tanystropheus mass distribution does not raise major alarm bells about relative neck and head weight. I could be convinced otherwise, and obviously there's a lot more than could - and should - be done to investigate this issue, but I currently don't see neck mass as a significant barrier to terrestrial habits. This exercise has also brought home the fact that we might not know much about the adaptive and structural significance of extremely long necks in carnivorous animals, and that we should be careful comparing them to other long-necked creatures. Perhaps our unfamiliarity with this extinct bauplan, along with our generally poor intuitive sense of animal mass and tissue fractions (see this discussion and comment field at SV:POW!), means we should be extra cautious about gut-feeling interpretations of such creatures. I guess the bottom line is that running numbers to test our intuitions is an essential part of understanding unfamiliar animal types, especially if we're suggesting those assumptions are significant for extinct animal behaviour and lifestyle.

There'll be more on Tanystropheus in the next post, where the plan is to review recent arguments for and against different lifestyles in this animal. In the mean time, I'm very curious to know what others make of the ideas presented here. Would you interpret these results differently? Would you have reconstructed Tanystropheus in a different way? The comment field is open...

Tanystropheus was brought to you by Patreon, a weekend of downtime and the letter 'S'

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  • Cai, Z., and Wei, F. (1994). "On a new pterosaur (Zhejiangopterus linhaiensis gen. et sp. nov.) from Upper Cretaceous in Linhai, Zhejiang, China." Vertebrata Palasiatica, 32: 181-194.
  • Henderson, D. M. (2004). Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits. Proceedings of the Royal Society of London B: Biological Sciences, 271(Suppl 4), S180-S183.
  • Mitchell, G., Roberts, D., Sittert, S., & Skinner, J. D. (2013). Growth patterns and masses of the heads and necks of male and female giraffes. Journal of Zoology, 290(1), 49-57.
  • Nosotti, S. (2007). Tanystropheus Longobardicus (Reptilia, Protorosauria): Re-interpretations of the Anatomy Based on New Specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Società Italiana di Scienze Naturali e Museo Civico di Storia Naturale.
  • Renesto, S. I. L. V. I. O. (2005). A new specimen of Tanystropheus (Reptilia, Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus. Rivista Italiana di Paleontologia e Stratigrafia, 111(3), 377-394.
  • Rieppel, O., Jiang, D. Y., Fraser, N. C., Hao, W. C., Motani, R., Sun, Y. L., & Sun, Z. Y. (2010). Tanystropheus cf. T. longobardicus from the early Late Triassic of Guizhou Province, southwestern China. Journal of Vertebrate Paleontology, 30(4), 1082-1089.
  • Taylor, M. P. (2009). A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of vertebrate Paleontology, 29(3), 787-806.
  • Tschanz, K. A. R. L. (1988). Allometry and heterochrony in the growth of the neck of Triassic prolacertiform reptiles. Palaeontology, 31(4), 997-1011.


  1. Another way to look at Tanystropheus is to compare the skeletal anatomy of its neck to that of another long-necked animal which we are certain was aquatic, namely a Plesiosaur. If you do so, you will notice that the Plesiosaur has a lot more neck vertebrae, and these vertebrae have much, much bigger processes on the sides of them.

    Bony processes on vertebrae are there to for muscle attachment, and you'd expect a Plesiosaur to a fairly muscular neck, because for the neck to work as a hunting tool, it has to be muscular enough for the animal to move it quickly, acting against the resistance from the water. Tanystropheus by contrast has far fewer neck vertebrae, and these have very small bony processes on them.

    Tanystropheus clearly wasn't doing anything with its neck that needed much muscle, and therefore was operating in air rather than in water. It also had quite large neck ribs, which are a stiffening system; it didn't seem to have had a particularly flexible neck at all.

    Putting this together, this animal was not aquatic as such, but likely hunted by using its long neck to "sneak up" on fish and so on.

    1. Pretty much agreed wholeheartedly - this is all stuff I intend to cover in the next post.

  2. Since we're already comparing it to azhdarchids, maybe we have another creature with a stalker-type lifestyle? Though possibly more dependent on aquatic resources than terrestrial?

  3. There's no mystery to Tanystropheus: it's a Babel fish.
    ""I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
    "But," says Man, "the Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED."
    "Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.
    "Oh, that was easy," says Man, and for an encore goes on to prove that black is white, and gets killed on the next zebra crossing.

  4. I'm interested in animal density - how did you decide on 0.85 kg/l as a standardish reptile density? Could you mention a couple of references? Thanks in advance.

    1. Colbert (1962) is your reference here: he predicted densities of 0.81 and 0.89 for a gila monster and alligator, respectively. Free pdf from the AMNH:

  5. Wow! Awesome analysis, I see you put a lot of work into it. Totally made me have to re-think this animal and the way its weird body form functions. I cant wait to see how you get into it's lifestyle in the next post. :)

  6. Great post and I have been looking forward to your treatment of these animals. Does raise some interesting questions about what does it take to be a good long necked lunging stalker - if that is where I suspect you are going with this analysis. I think that this bauplan - echoes of it seen in modern day herons (Ardeidae) - is taken for granted because these birds are so common today. But mammals don't seem to achieve this form. Sprawling limbed reptiles either. Archosaurs/archosauromorphs seem to be the best at it. I wonder if a congruence of neck anatomy, upright stance, and potentially weight saving adaptations (pneumaticism) align to provide the proper exaptations to evolve this particularly specialized mode of life? By the way any evidence of pneumaticism/weight saving adaptations (esp in neck) for these animals?

  7. Mark,

    It appears the Tanystropheus specimen that served as the base for your illustration is the Chinese, GMPKU-P-1527 (Rieppel et al. 2010), not the European PIMUZ T 2189.

    Thanks for bringing this specimen to mind. I had ignored it, but now have blog posted on it at (11/15/15) with some comments there that might be helpful.

    Notably the hands and feet of the GMPKU specimen are quite distinct from the European specimens, so it does not appear to be conspecific. A dorsal view would affect your numbers as would the incorporation of hollow bones and a trachea filling the neck. To your question on balancing, I once built a full-scale solid-wood model of Tanystropheus and had to add lead weights aft of the pelvis to keep it from tipping forwards.

    I would avoid the use of building a chimaera of different specimens if possible. The lack of a skull and tail tip in the GMPKU specimen might have been of equal subtractive value to your purposes.

    And finally, if you’re tracing bones from published photographs, (you did not mention visiting the Chinese specimen first hand), and you reported you employed Wild’s drawing of a skull and Nosotti’s little Tany for the missing parts, aren’t you doing exactly what you acuse me of doing, with now OUR work “being produced with techniques of questionable reliability”? Sounds like you traced, you created a chimaera, and you even performed a little free hand if I’m not mistaken… ; )

    I liked your reconstruction, even if you gave it an inappropriate inturned femoral head to support the erect pose.

    Now, fix those typos and let’s stop sniping at each other. Tracing photos is okay. Free handing and chimaeras are questionable.


    1. Thanks for the catch on the specimen number - pulled the wrong one from the paper, and then copy-pasted myself into an embarrassing perpetual typo. All fixed now.

      "as would the incorporation of hollow bones and a trachea filling the neck."

      Pneumatic features are not present in any neck verts of Tanystropheus. The bone mass is reduced, but they are likely not full of air. We have a lot of 3D neck bones of these guys, and pneumatic features are absent from them - I was careful to check this before running the mass numbers. More on this in the next post.

      "I would avoid the use of building a chimaera of different specimens if possible. The lack of a skull and tail tip in the GMPKU specimen might have been of equal subtractive value to your purposes. "

      How else do you propose estimating mass fractions than without having a complete skeleton? This is the standard means of making a complete skeletal reconstruction. I've been very clear where I've inferred anatomy from other specimens, and think it works fine as a model for the intended work.

      "And finally, if you’re tracing bones from published photographs, (you did not mention visiting the Chinese specimen first hand), and you reported you employed Wild’s drawing of a skull and Nosotti’s little Tany for the missing parts, aren’t you doing exactly what you acuse me of doing, with now OUR work “being produced with techniques of questionable reliability”? Sounds like you traced, you created a chimaera, and you even performed a little free hand if I’m not mistaken… ; )"

      I used standard techniques employed for making skeletal reconstructions from a paper: tracing useful bone outlines (mostly from interpretative drawings, not photos), using data from specimen descriptions, measurement tables and as many photos/drawings as I can get my hands on. I use this data to draw bones in appropriate views/complete missing elements where necessary, and always to proportions indicated by preserved elements. Naturally, cross scaling of other animals is required to complete missing skeletal portions. These are tried and tested, non-controversial techniques of basic skeletal reconstruction, and do not include the DGS 'enhancements' that you perform on photographs of fossils.

      "I liked your reconstruction, even if you gave it an inappropriate inturned femoral head to support the erect pose."

      As mentioned in the text, the limbs are oriented in such a way to optimise measurements for mass estimation, not to reflect life position.

      "let’s stop sniping at each other."

      Please - you're saying I 'snipe' at you? You barely get mentioned in my work at all, and when I must it's only brief, civil comments explaining why I do not want to consider your work. This contrasts with the multiple blog articles and online commentary you've devoted to undermining my illustrations, books, research papers and even professionalism - the article you linked to above contains many examples of this sort of treatment. There's sniping going on here all right, but it's not from me.

  8. Interesting stuff, Mark. However, the proportion of the animal's total mass represented by its head and neck is only a contributing factor of whether or not it will tip forwards. Of much greater significance is whether the animal's centre of mass would be above the quadrilateral defined by it's feet. If it is then it is stable on land, if not it will tip over in the direction that the C of M "overhangs" the quadrilateral.

    Of course, the position that the neck is held in (as well as any acceleration it is undergoing) can greatly affect the animal's C of M. It might be very stable if it could hold it's neck similarly to a giraffe or Brachiosaurus but fall flat on its face if it held its neck straight out in front.

    1. Thanks for the comment, Mark, and you're of course right about centre of mass. As indicated at the end of my post, I appreciate this is a long way from a full argument, and the next obvious step would be the calculations you mention here.

      On your point about bringing the neck up, I do wonder if the neck might be quite flexible at the base. As I recall, existing studies have ruled out neck flexion by looking at the mid-region of the neck, but this is typically one of the less flexible neck regions in most animals (some birds being obvious exceptions). The base of the neck is far more important to neck elevation, and we see it as often strongly extended in articulated Tanystropheus specimens. Moreover, the verts there are short, robust, with large sites for muscle attachment and deep zygapophyses (this doesn't necessarily betray the nature of the articular face of course, but deep zygs generally suggest steep articular surfaces, and thus permission of greater flexion/extension). Would be interesting to see a more detailed study on that.

    2. Thanks Mark. Agree that it doesn't look like there would be a lot of flexibility in the mid-neck but the last two cervicals might provide some ability to raise the neck up somewhat like the boom of a crane (not the bird, obviously).

      Would've made it look more impressive/threatening during sexy time or avoiding someone else's lunchtime.

  9. Excellent post on a wonderful animal!

    I must be woefully behind the times when it comes to Tanystropheus, though; what are those big bones ahead of the chevrons at the base of the tail?

    1. Good question! Some label these as heterotopic ossifications - basically mysterious 'bone where there shouldn't be bone', others as cloacal bones akin to those in some lizards (best known for geckos). Whatever they are, they occur in several specimens and are thus genuine features of this animal - they aren't weird pathologies or the bones from another animal. They've been suggested to be ossified cartilage, but that ideas has gone contested. Their significance is generally ascribed as being something to do with reproductive anatomy, akin to the analogous bones in lizards.

  10. Thinking outside the terrestrial-aquatic issue for a moment, I wonder if Tanystropheus specialized in burrowing animals or buried food, reaching into holes where the rest of its body couldn't go and plucking out anything it found down there. The obvious downside is that any burrow not dead-straight would flummox it, but it seems to me a neck so stiff and horizontal would be most useful if there were something it wanted in a place where a shorter-necked animal couldn't get it, and long, narrow holes are a good example. Since it doesn't seem particularly bendable, the neck doesn't seem to me to be built for speed or agility, but it might be ideal for dragging slow/immobile and stubborn prey without wasting much energy, and an obvious example of what to drag would be a clunky digger fastened to the inside of its hidey-hole and refusing to come out. This might also explain the heavily muscled hind legs, as its back half could focus on pulling while the stiff neck anchors the skull and the jaws hold on tight.

  11. I measured your skeletal in GIMP, axial length is 3.75m, I was surprised, I too expected that following the vertebral column would increase it to over 4m when compared to the 3.5m in an straightline in the pose you used.

  12. Interesting recent update on this which seems to confirm your suspicion: