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Thursday, 16 May 2013

Another new Plateosaurus

A modern reconstruction of Plateosaurus engelhardti as a relatively bird-like, bipedal animal rather than a tubby, sprawling  quadruped
It turns out that you can teach an old dinosaur some new tricks. Remains of Plateosaurus engelhardti have been known since at least 1837 but, thanks to a flurry of recent research into its anatomy, posture and gait, we've learnt many surprising facts about this animal in the last few years. The restoration offered here is an attempt to portray this animal in its most recent, modern light and, in many details, it contrasts markedly with reconstructions even only a few years ago.

'New looks' for Plateosaurus are not rare, however. Our perceptions of this dinosaur have been rather changeable since its discovery, and few dinosaurs have been through as many iterations of posture, gait and general appearance. Since its discovery in the early 1800s, we've seen Plateosaurus reconstructed in numerous ways. Perhaps the oldest known reconstruction, by O. Jaekel, dates to 1913-1914 and depicts a very different animal to the one we know today. This frequently proportionally inaccurate image shows a very odd-looking bipedal dinosaur which almost has a therizinosaur-like appearance, but perhaps is better compared to Igor from an early Universal horror film. Later depictions of Plateosaurus were of a sprawling quadruped with a barrel-like chest; an erect biped with a dragging tail; a lithe quadruped capable of galloping; a very sauropod-like animal with straight, columnar limbs and a long, low biped with a heavy balancing tail. These interpretations, and others, have been compiled and thoroughly discussed by Heinrich Mallison, a man who clearly loves Plateosaurus as much as a man can love a fossil species (below, from Mallison 2010a).

The many faces of Plateosaurus, compiled by Mallison (2010a). Check out Heinrich's open-access publication for sources of the reconstructions.
This wealth of conflicting interpretations is odd when we consider that, among non-sauropodan sauropodomorphs and perhaps dinosaurs generally, the osteology Plateosaurus is very well known. Numerous articulated skeletons, some essentially preserved standing in miring muds, inform us about details of limb carriage and girdle configuration (see image, below), but it seems that much of this data was overlooked by scientists and artists restoring Plateosaurus for decades. Once the fine anatomy and taphonomic data of these animals was assessed in detail however, it became very apparent that most of our interpretations of Plateosaurus were simply plain wrong. The palms of the Plateosaurus hand were forever facing medially, like those of theropod dinosaurs and unlike many of the reconstructions shown above (Bonnan and Senter 2007). It's forelimbs had an extremely limited range of motion, which more-or-less only permitted movement beneath the body (Mallison 2010a). When the entire skeleton was reconstructed without disarticulated bones, the narrowness of the chest and shoulders became apparent, contrasting with the enormous bellies of many mounted skeletons and sculptures. The rigidity of its avian-like torso skeleton and disproportionate limbs were also brought to light (Mallison 2010a, b, and references therein).

SMNS F33, one of the most famous articulated fossils of Plateosaurus engelhardti. Note the narrowness of the chest and shoulders. Photograph by Heinrich Mallison, uploaded to Wikipedia Commons by FunkMonk.
In essence, it seemed that virtually all reconstructions of Plateosaurus prior to 2010 were committing some sort of major anatomical sin, requiring disarticulation of bones or over-flexed joints in order to achieve their portrayals. Perhaps most obviously, it seems that quadrupedal locomotion, be it a slight sprawl or a horse-like gallop, was not possible for Plateosaurus (and perhaps most other non-sauropodan sauropodomorphs). Rather, the reach, attitude and length of their forelimbs indicate that they were habitual bipeds, and their centre of gravity dictated that they held their backs subhorizontally. Gone too were the round, barrel-like chests: despite being a  6-10 m long herbivore, Plateosaurus was a surprisingly gracile animal that was perhaps even capable of moving relatively quickly on its powerful back legs. The contrast of this new appearance with some of the older, sprawling reconstructions of this animal is rather pronounced.

(Interesting bonus knowledge to stem from recent Plateosaurus studies include new insights into the suppleness of the Plateosaurus neck, which permitted 360° to their owners. Presumably, this reflects a need to keep an eye out for predatory archosaurs which, in the Late Triassic landscapes occupied by Plateosaurus, would not be predatory theropods but large rausuchians like Teratosaurus. [How come there's not many illustrations of predatory interactions between these species? Would probably make for a cool image]. Perhaps even more interestingly, the pedal claws of Plateosaurus show evidence of being used in scratch digging, which may reveal how these animals dug their nests. Neat stuff indeed.)

Junk about the trunk
But it's not just Plateosaurus specific topics that have fuelled the latest changes in its appearance. It's very likely that the tail of Plateosaurus was more massive and heavy than portrayed in most reconstructions because, along with many other dinosaur species, it bore substantial hindlimb musculature along the base of its tail. We can see how large the hindlimb retractor muscles of dinosaurs should have been by looking at homologous anatomies in lizard and crocodile tails (warning: that link is not for the squeamish), and the take-home message is that most dinosaurs had extremely beefy proximal tail regions (e.g. Persons and Currie 2011). As Heinrich covers here, we can see osteological evidence for expansive, tail-anchored hindlimb muscles in Plateosaurus as clearly as we can in nearly all other Mesozoic dinosaur species. The Plateosaurus anterior tail regions was effectively a big tube of meat bulging from a thread of vertebrae, which were themselves enlarged for the attachment of powerful muscles. In life, the hindlimb protractors would probably run continuously from the tail to the hindlimb, with no obvious 'join' between these two body sections. These bulging tails are a world away from the slender and compressed tails seen in many pieces of palaeoart. Furthermore, all this proximal tail muscle probably reduced the flexibility of the tail somewhat (Persons and Currie 2011) suggesting that another palaoart trope - dinosaurs with laterally wavy, sinuous tails - were probably impossible. The extensive muscles at the base of the tail can only be compressed so far by a wiggling tail. Thus, although the tail bones of Plateosaurus seem to offer a fair amount of lateral motion (below; also see Mallison 2010a), there probably wasn't anything close to that level of flexibility available in life. In the picture above, the tail is deliberately a little lifeless, acting purely as a site for hindlimb retractor muscle anchorage and a counterbalance to the heavy, gut-filled torso.
Digital model of Plateosaurus caudal vertebrae in dorsal view, showing 10° flexion between each joint. From Mallison (2010a).
Feathers? Really...?
So, new posture, new build, new tail, and now new integument? We're very used to seeing Plateosaurus and with virtually all other sauropodomorphs with scales, but this may not have been the case  (hat tip here to recent blog posts on this topic by the Internet's favourite sauropod workers, Matt Wedel and Mike Taylor). Sauropodomorphs may not have provided any fuzzy revelatory fossils over the last few years but, because lots of other dinosaur groups have, the landscape of dinosaurian integument has shifted considerably around their feet. Nowadays, it's a commonly known possibility that dinosaurs, and perhaps all ornithodirans, were ancestrally fuzzy. Although we have a few skin impressions from sauropods (Czerkas 1992; Coria and Czerkas 2007), they remain pretty rare across the group as a whole and, to my knowledge, we don't have any from non-sauropodan sauropodomorphs. Our few scraps of sauropod skin suggest they bore typically archosaurian, non-overlapping scales but, even if that's standard for the group (and, given what we know about modern animal integument, it may well not be), it isn't clear when this trait became a typical part of the sauropod bauplan. Thus, it is not inconceivable that some early sauropodomorphs were feathered and, until we have some direct evidence either way, it's probably just as parsimonious (and, arguably, speculative) to draw them with fuzzy hides as it is with scales. In the case of the picture here, I figured throwing an ostrich-like distribution of feathers over a modern Plateosaurus reconstruction added a suitably symbolic purpose to show how far interpretations of this animal have come since its first discovery.

And that may be enough novelties for one day: time to crack on with other things. Here's hoping that this reconstruction of Plateosaurus will stand the test of time, though I'm sure I'm merely tempting fate with those words. Next week, hopefully, we'll see something to do with animals bearing membranes, because this thing isn't very far from being released on the world any more.

References

  • Bonnan, M. F., and P. Senter. 2007. Were the basal sauropodomorph dinosaurs Plateosaurus and Massospondylus habitual quadrupeds? In: Barrett, P.M. and Batten, D.J. (eds.), Evolution and palaeobiology of early sauropodomorph dinosaurs. Special Papers in Palaeontology, 77, 139-155.
  • Coria, R. A. and Chiappe, L. M. 2007. Embryonic skin from Late Cretaceous sauropods (Dinosauria) of Auca Mahuevo, Patagonia, Argentina.  Journal of Paleontology, 81, 1528-1532.
  • Czerkas, S.  A. 1992. Discovery of dermal spines reveals a new look for sauropod dinosaurs. Geology 20, 1068-1070.
  • Mallison, H. 2010a. The digital Plateosaurus II: an assessment of the range of motion of the limbs and vertebral column and of previous reconstructions using a digital skeletal mount. Acta Palaeontologica Polonica, 55,433-458.
  • Mallison, H. 2010b. The digital Plateosaurus I: body mass, mass distribution and posture assessed using CAD and CAE on a digitally mounted complete skeleton. Palaeontologia Electronica 13.2.8A
  • Persons IV, W. Scott, and Philip J. Currie. 2011. The tail of Tyrannosaurus: reassessing the size and locomotive importance of the M. caudofemoralis in non‐avian theropods. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 294.1, 119-131.

13 comments:

  1. The sketch is a bit provocative but I like it.
    Feathers in sauropodomorpha is all about gigantothermism.Their high metabolism is enough to capture heat, and not loose it too quickly.
    A feathery ''coat'' is handicapping in this case ?

    Oliver

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    1. Birds conserve feathers much better than mammals conserve fur. With a complex air sac system as sauropods had, retaining feathers would not be an issue, though it presumably eventually got unnecessary based on skin impressions we have.

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    2. Thanks for the comments, chaps.

      Oliver: I know what you're saying, and was wondering when painting this if Plateosaurus would already be beyond the point where insulation was needed, especially since the Late Triassic was pretty warm. I guess days on Pangaea may have had pretty extreme temperatures though, if a lot of it was very arid and desert-like. Days may have been pretty warm, but temperatures at night may have dipped quite a bit. I do like the idea that feathers may have been partially retained by members of Sauropodomorpha. No reason to think that feathers couldn't be localised to the tips of their tails or around their heads or whatever, just for showing off.

      Gwawinapterus: I'm not clear why the avian-like pulmonary system in sauropods is linked to retaining feathers. Can you elaborate on that a little?

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    3. Largely because a more elaborate control of the animal's body temperature means things like overheating are less of an issue. Ostriches, for example, are less bothered by the heat than antelopes of the same size, and thus retain much more dense feather coats.

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    4. An ostrich's feathers are concentrated on the 'sunny side', and do not lay against the skin as much as they are held up like tents. Don't they play a large role in regulation by casting shade / ie. cooling instead of insulating?

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    5. Well, yes, they do play a large role in cooling the animal down, which if anything is an argument for the preservation of complex feathers in maniraptors of all sides. However, they're still a coat thick enough to preserve temperature even in cold climates, so they're certainly not particularly adapted to loose any heat-preserving function.

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    6. Ostriches naked legs are primarily naked to prevent overheating when sprinting at 70 kmph. This is the reason why "slower' sprinters like Emus ( 50 kmph) have more feathers on their legs than Ostriches, even though Emu's live in a far hotter environment.

      It's not the habitat that made the Ostriches loose their leg feathering, it's their running lifestyle ;)

      Iben

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  2. I wouldn't rule out feathers on sauropodomorphs, just take a look at today Africa's mega-fauna, like elephants or rhinoceros.

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  3. what a cool reconstruction! Excellent work, Mark!

    Incidentally, the SMNS F33 pic is by me, just uploaded to wikipedia by FunkMonk ;)

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  4. on the tail - compared to an alligator it should still look compressed; the haemal arches are very long but the transverse processes not that wide. Thus, high-oval in cross section - but not Sahel-zone like narrow as many artists depicted it.

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    1. Thanks for the comments, Heinrich! I wondered if you may show up. Glad you like the image. Have credited you appropriately for the SMNS F33 image.

      I'm with you on the tail cross sections. I'm thinking of writing a piece entitled '10 common mistakes and tropes' in palaeoart, and will be sure to include those sentiments if and when it appears.

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    2. Mark, that is an excellent idea! Write that piece, and I will happily proofread it and give you a biomech researcher's perspective :)

      Currently, I have an email from a very esteemed colleague waiting to be replied to (and I don't get around to it), one that argues for slim tails! EEK!

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