If you read the previous article on stegosaurs you’ll know that a collection of papers devoted to examination of this fascinating group appeared last year (2010) in a special issue of Swiss Journal of Geosciences (SJG from hereon). These papers resulted from a meeting held at the Sauriermuseum Aathal, Switzerland, in June 2009 (Billon-Bruyat & Marty 2010). Last time round, we looked at the papers on stegosaur systematics and diversity. Here, we begin a look at the remaining papers: they cover such subjects as athleticism and posture, possible feeding behaviour, soft tissue preservation and the role of the plates [image of mounted Kentrosaurus aethiopicus above by Sebastian Wallroth, from wikipedia]. Thanks to Daniel Marty for his help. Anyway…
By far my favourite paper in the issue is not on systematics, phylogeny or diversity, but rather on ranges of movement in the stegosaur skeleton and on what they might mean for behaviour and biology. I’m referring to Heinrich Mallison’s article ‘CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus‘. ‘CAD’ stands for computer aided design: the elements of the mounted Kentrosaurus skeleton at the Museum für Naturkunde in Berlin were scanned with a high-resolution 3D laser scanner by Dave Mackie (then of Research Casting International), and later digitally reconstructed and manipulated with the use of dedicated software (Mallison 2010a). The skeleton is a composite made from several individuals, but – because these were similar in size – this fact doesn’t seem to have created any major problems.
There’s a lot of interesting stuff in Heinrich’s paper; it has implications for all dinosaurs and all fossil archosaurs, not just stegosaurs. If you’ve been following the literature on the biomechanics and soft tissue anatomy of Mesozoic dinosaurs, you’ll know that there’s been quite a bit of interest in articular cartilage lately (check out ♫ This is the dawning of the age of articular cartilage ♫ at SV-POW!). Recent work has confirmed what many of us have been thinking for a while: that Mesozoic dinosaurs possessed relatively thick (bird-like) articular cartilage caps, and that one cannot appreciate joint function and behaviour without taking this (often unpreserved) cartilage into account (Schwarz et al. 2007, Holliday et al. 2010, Mallison 2010a, b). Further confirmation for these contentions is provided by the fact that Kentrosaurus specimens differ significantly in the shape of the ulna: in some, the bone has an abrupt, squared-off proximal end, while in others the same region has an enormously convex, rugose olecranon process. This surely represents an ossified version of a normally cartilaginous structure (Mallison 2010a).
Perhaps more than any other group of dinosaurs, stegosaurs have suffered from extreme ‘bad posture’ [see cartoon above. Larger version available on the Tet Zoo facebook page]: many (often surprisingly recent) reconstructions have shown them with drooping tails, strongly arched backs, hyper-flexed, sprawling forelimbs, and necks that pitch downward such that the head is held close to the ground [below is the Berlin Kentrosaurus in its pre-2005 pose. Image borrowed from here on Peter Bond’s Blog]. Indeed, even the re-mounted*, tail-off-the-ground Kentrosaurus [shown at the very top of this article] was given strongly abducted, flexed forelimbs. Before talking about limb movements and body posture, I want to discuss stegosaur neck posture. Hopefully it will be increasingly appreciated that the sauropod neck posture work published by Taylor et al. (2009) is not only applicable to sauropods, but applies – so it seems – to virtually all terrestrial tetrapods (or, to amniotes at least).
* Re-mounted in 2005.
As I explained in a 2009 article on SV-POW!, the default head-neck posture for stegosaurs should be the one normal for all terrestrial tetrapods: the cervico-dorsal junction should be extended (that is, the neck should be ‘bent upwards’ relative to the back), and the cranio-cervical junction should be flexed (that is, the head should be held, approximately, at a right angle relative to the neck). So, those mounted skeletons and life restorations that show stegosaurs with horizontal necks, or with their necks sloping down, are not accurate representations of ‘normal’ posture. Heinrich acknowledges this, but reconstructs his stegosaurs with their necks in ONP (= osteological neutral pose) since this represents an approximate mid-point in range of motion (note, however, that ONP could not always be achieved with the digital Kentrosaurus. In part this might be because of distortion, but it could also be due to the composite nature of the specimen). For normal locomotion, Heinrich reconstructs Kentrosaurus with its neck angled upwards and its head held somewhat higher than the level of its back (Mallison 2010a). I say again: this should be the normal ‘just walking around’ posture for stegosaurs [image below, provided courtesy of H. Mallison and used with permission, shows digital Kentrosaurus in ‘crouching’ forelimb posture (above) and normal walking posture (below), with translucent neck showing full extension. Scale bar = 50 cm].
Different people have had very different ideas on how stegosaurs used their tails. While some authors have argued that the stegosaur tail was quite stiff (and too stiff for use as a weapon, or for use as a prop used in bipedal rearing), others have argued both that stegosaurs could ‘throw’ or bend the tail right round in order to whack an aggressor or predator, and that they could lean back on the tail when rearing up. Damaged and/or healed Stegosaurus tail spikes, and a perforated Allosaurus tail vertebra, provide strong evidence that Stegosaurus at least could and did deploy its thagomizer as an anti-allosaur weapon (Carpenter et al. 2005). The big, overlapping plates of Stegosaurus might mean that lateral movement in its tail was somewhat constrained (Carpenter 1998), but this awaits proper testing.
Manipulation of the digital Kentrosaurus indicates that vertical tail motion is limited. Having said this, the tail could still be bent dorsally to form a gently curved ‘U’. Lateral movement was far more extensive: in the diagram below [provided courtesy of H. Mallison and used with permission], the most ‘extreme’ tail posture involves 6° of movement at each intervertebral joint, and yet still the animal is able to reach well to the side with its thagomizer. However, in extant long-tailed reptiles, about 11° of movement at each intervertebral joint is typical: there’s no reason why Kentrosaurus couldn’t have been this flexible too, so it seems that the animal could easily bend its tail such that the distal-most spikes overlapped the thorax (a somewhat maladaptive thing to do in life, methinks). Kentrosaurus could almost certainly, it seems, deploy its wickedly spiked tail as a weapon and could strike targets “up to heights of 3 m with ease” (Mallison 2010a, p. 229). The neck is also flexible enough to allow the animal to keep a potential target in sight at most orientations (obviously the stegosaur would have to pivot or turn to keep track of an object directly behind it) [In image below, scale bar = 50 cm].
Remember my article from August 2008 about the flexibility of dromaeosaur tails? Heinrich traced the amount of caudal flexibility seen in Velociraptor (it works out at nearly 10° of flexibility at each intervertebral joint) and implies that it is indeed plausible (Mallison 2010a, p. 226), but I’m confused by his statement that Deinonychus would have been different because its elongate zygapophyses and chevrons were more extensive: I don’t think this is so (if I’ve misunderstood, please help me out).
Bipedal rearing is entirely possible and doesn’t seem to have been difficult [adjacent image, provided courtesy of H. Mallison and used with permission, shows digital Kentrosaurus in bipedal rearing pose. Scale bar = 50 cm]. I’ll discuss bipedality in quadruped dinosaurs at a later time, as I’m waiting for Heinrich to publish his article on rearing in sauropods.
I mentioned earlier that the Berlin Kentrosaurus mount is posed with sprawling forelimbs. Debates over the forelimb posture of quadrupedal ornithischians are far from over, though I agree with those who say that scapulocoracoid position, glenoid position and hand anatomy are suggestive of a more parasagittal forelimb posture. The digital Kentrosaurus shows that both sprawling and erect-limbed postures are achievable – maybe the animal adopted both extremes as well as all the postures in between, depending on what it was doing – but glenoid morphology and ichnological evidence leads Heinrich to conclude that an erect-limbed posture is more likely (Mallison 2010a).
In case it’s not obvious, there’s a lot of stuff in this paper worthy of discussion and I haven’t covered all of it. I can’t wait for more work of this sort to be published: congratulations, Heinrich, on a fascinating paper. Time to move on… More stegosaurs next.
Stegosaurs have been mentioned on a few previous occasions on Tet Zoo. Please see…
- A most atypical stegosaur
- Where the scelidosaurs and iguanodontians roam
- Patagonian Mesozoic Reptiles, a book review
- Stegosaur Wars: the SJG stegosaur special, part I
For articles on other thyreophorans, see…
- Ankylosaur week, day 1: Hungarosaurus
- Ankylosaur week, day 2: Tarchia
- Ankylosaur week, day 3: Aletopelta
- Ankylosaur week, day 4: Panoplosaurus
- Ankylosaur week, day 5: Edmontonia
- Ankylosaur week, day 6: Silvisaurus
- Ankylosaur week, day 7: Animantarx
- A month in dinosaurs (and pterosaurs): 3, Minotaurasaurus and giant chasmosaurines
Refs – –
Billon-Bruyat, J.-P. & Marty, D. 2010. Preface: Symposium on Stegosauria proceedings. An international conference on stegosaur finds of the world organized by the Sauriermuseum Aathal (8th and 9th June 2009, Aathal, Switzerland). Swiss Journal of Geosciences 103, 139-141.
Carpenter, K. 1998. Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park, Colorado. Modern Geology 23, 127-144.
– ., Sanders, F., McWhinney, L. A. & Wood, L. 2005. Evidence for predator-prey relationships: examples for Allosaurus and Stegosaurus. In Carpenter, K. (ed) The Carnivorous Dinosaurs. Indiana University Press, Bloomington and Indianapolis, pp. 325-350.
Holliday, C. M., Ridgely, R. C., Sedlmayr, J. C. & Witmer, L. M. 2010. Cartilaginous epiphyses in extant archosaurs and their implications for reconstructing limb function in dinosaurs. PLoS ONE 5(9): e13120. doi:10.1371/journal.pone.0013120
Mallison, H. (2010). CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus Henning 1915 Swiss Journal of Geosciences, 103, 211-233
– . 2010b. 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.
Schwarz, D., Wings, O. & Meyer, C. A. 2007. Super sizing the giants: first cartilage preservation at a sauropod dinosaur limb joint. Journal of the Geological Society, London 164, 61-65.