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 â« This is the dawning of the age of articular cartilage â« 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.
Mallison, H. (2010). CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus Henning 1915 Swiss Journal of Geosciences, 103, 211-233
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.
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I wanna read ur article on rearing dinosaurs already!! :>
What he said.
I really like this "digital revolution" that Mallison appears to be spearheading. His two digital Plateosaurus papers are similarly awesome, and colorful. One wonders the expense required to laser-scan an entire skeleton. But there's a huge potential to solve a lot of nagging problems with dinosaur movement and posture. In particular, this could shed real light on the stance of ceratopsids and the center of balance of many theropods.
Now which of these stegosaurs is the one featured in that carving on that Cambodian temple?
No, please don't thagomise me, I'm just kidding!
hahaha: to thagomize = forcefully whacking or penetrating something with your thagomizer.
I love your blog, even though I do mainly molecular biology. I also liked the nomenclature discussion.
uh-oh! I guess this means I need to hurry my rearing sauropod paper!
that aside, thank you for the wonderful write-up! There is only one point that needs correction, and it is a huge one: the wonderful laser scans of the Kentrosaurus bones were not made by me, but by Dave Mackie (then of Research Casting International - www.rescast.ca), during the exhibition renovation. Dave is a real digitizing wizard, and created wonderful high resolution scans also of the Giraffatitan skull and the Elaphrosaurus mounted material. I owe him, big time (as well as the rest of the RCI team, especially Dave's wife Carla, who repaired many of the bones from the exhibit, and Kevin Krudwig, the man with the welding torch and the delicate touch, who had a huge part in creating the awesome new skeletal mounts).
Thanks all for comments, especially Heinrich - I'll go and correctly attribute the laser scanning to Dave Mackie, thanks for the heads-up.
The Kentrosaurus and Plateosaurus papers are indeed very awesome. I only just recently got some experience in CT work (on therocephalians), and after seeing this, I'd love to do some more. A lot of work could be done on the posture of basal synapsids with a technique like this.
Also, I had never seen Kentrosaurus before (nor have I paid much attention to the other non-Stegosaurus stegosaurs that I've seen) and I find it quite striking how the dorsal plates gradually become spikes. I never once thought that the plates and spikes could be homologous structures modified differentially in different segments. Very interesting...
Anyone present who can make this happen, put a half-decent head on that elaphrosaurus!
This certainly is my favorite study of stegosaur week. These CAD models of dinosaurs are very interesting. I can only imagine what other taxa will be examined with this method...
Wow, I'm flattered! Thanks for all the nice feedback!
@ croconut: patience, please! This study keeps growing and growing.
@ Zach Miller: thanks! The cost of scanning a skeleton complete (mounted) is minuscle once you have a good scanner. Those cost some thousands or tens of thousands of bucks - eek! However, the drop in price from year to year is incredible. I recently read of a scanner that does building to street sized scans at millimeter resolution, measuring 15.000 points per second! Can you spare 250.000 Euros?
There was a project in the German Research Unit 533 "sauropod Biology" of the German Science Foundation run by H.-C. Gunga for scanning entire skeletons. other members were T. Suthau, A. Bellmann, S. Stoinski and some others. They did some wonderful scans, e.g. in the Zhigong Dinosaur Museum. Expect to hear more from them soon (once Springer gets their act together). :)
@ Dallas Krentzel: Way to go! CT is really the quickest way to get high quality 3D data - but it is an expensive one. You need CT access - ok, that's usually doable, somehow. Then you need the software to extract the data, and that sometimes takes quite some time, e.g. when sediment of nearly the same density sticks to the bones. Sometimes, the docs in the hospital are willing to help, sometimes they can't or it just takes too long. In that case you need to buy an expensive program and a computer able to run it. A one-time expense, but in case of the Berlin museum that cost 40.000 Euros.
Regarding the spikes/plates: Do you read German? If so I recommend Janensch's and Hennig's 1925 papers:
- Hennig, E. 1925. Kentrurosaurus aethiopicus. Die Stegosaurier-Funde vom Tendaguru, Deutsch-Ostafrika. (Kentrurosaurus aethiopicus. The Stegosaur finds from Tendaguru, German East-Africa") Palaeontographica Supplement 7:101-254
- Janensch, W. 1925. Ein aufgestelltes Skelett des Stegosauriers Kentrurosaurus aethiopicus HENNIG 1915 aus den Tendaguru-Schichten Deutsch-Ostafrikas. (A mounted skeleton of the Stegosaur Kentrurosaurus aethiopicus HENNIG 1915 from the Tendaguru layers of German East Africa.) Palaeontographica Supplement 7:257-276.
I have long tried to find the time to translate them, or at least summarize in English - maybe I'll get around to that on my next long flight or train ride.
The arrangement of the spikes and plates on the mount in Berlin is conjecture. We know what the tail tip spikes looked like, we know that tail spikes were paired, that they likely were spread at roughly regular intervals, that the plates and spikes come "handed", that the "shoulder" spikes fit very nicely on the ilium(!), where Hennig and Janensch put them - and that's about it! THe plates on the neck are plaster, put there because Stegosaurus has them, and because the anterior plates that probably go on the trunk of K. have a shape that is similar to those in S.
@ Tim Morris: I'll definitely play with the Elaphrosaurus files in the near future, but right now I can't recall if the fake head was scanned. In any case, CAD allows testing a number of different options quickly and easily.
@ Darren: thanks for fixing!
Wait a minute... Heinrich, are you saying that the parascapular spines belong back on the ilium, or merely that they can fit there if you want them to? Thanks to Gigantspinosaurus, we know that at least some stegosaurs had big parascapular spines, and this led to a reinterpretation of all so-called parasacral spines. Are you saying that some stegosaurs had parasacral spines after all? If so, I wonder if those taxa had parascapular spines too?
There's a mounted Stegosaurus skeleton cast in the geosciences building of the University of Vienna. Someone put it together as his M.Sc. thesis. It's not quite as crouched as the crouched Kentrosaurus, but it probably should be: the spaces for the intervertebral discs of the trunk are wedge-shaped, some pretty strongly so. Mounting it as straight as the "normal walking posture" of Kentrosaurus would look seriously uncomfortable. (It doesn't for Kentrosaurus as far as I can see from the small picture. I'm just talking about Stegosaurus stenops here.)
Of course, there was another problem with the crouched Kentrosaurus mount and many other stegosaur mounts: they have the neck coming out between the shoulder blades, at the same level. People! Dinosaurs are not monkeys! The coracoids should touch in the midline!!!
That of Limusaurus?
...and where they'd be pretty useless, as your reconstruction of tail mobility shows. Shoulder spikes would close the gap between the shoulder and the thagomizer.
Great stuff as always Darren. Will update my second year lecture accordingly! Stego-beginners might likw to browse this Quick Guide I wrote for Current Biology (should be open access)...
http://dx.doi.org/10.1016/j.cub.2009.10.005
[from Darren: sorry, delayed by spam filter]
@ David: Toni Fürst, IIRC, did the Vienna mount. He and I had some talks about this, afterwards ;)
About the keystone-shaped vertebrae the only thing I can say is that Plateosaurus has keystoned caudals, and ONP places the tail underground and the tail tip at the animal's nose (exaggeration).
If we assume (and I tried to make a good case for that) that stegosaurs could use a spread-eagled forelimb pose for defense, a pose during which large forces acted on the shoulder area and anterior trunk, then the forces on the dorsal series would have been high, too, and their ONP position (which distributes compressive forces evenly over the anterior/posterior faces of the centra) should correspond to it, too. The tail of Plateosaurus certainly saw strong ventral bending during locomotion, so wedge-shaped intervertebral discs would serve to even the force distribution. Same thing here, only in the trunk..... In the erect walking pose the vertebrae would gape - but why should this be "uncomfortable"? Check out the hind limbs: stegosaurs were slow walkers, the forces were low.
(interesting aside: a reviewer, despite having the figures available, could not believe that the femur is longer then the tibia, and even the tibia and the metatarsus [not mounted and figured] combined. He assumed a typographical error!)
I agree on the scapula position; please address complaints to the reviewers. I argued initially only for a pretty steep, low and forward position. You can't really make them very narrow (i.e., touching at midline) with the given ribcage shape - but that one, as I explain in the manuscript at length, is itself questionable). A can of worms I did not want to open. We need more fossils :)
@ David and Darren:
I am saying that the spike bases have shapes congruent to the ilia, not to the shoulder area. I conclude therefore that they are parasacral spikes. I disagree on the supposed "uselessness" of spikes on the ilia! For a large predator the "best" way to kill a Kentrosaurus would be to time a tail swing and dart in before the tail comes back. Not that I show that the tail mobility may have been sufficient to have overlap between the body and the spikes, but that I discount this - very likely there was a "dead space" right next to the hind limb where the tail base moved so slowly that it presented no danger, and where the tail tip spikes could not reach. If you put the questionable spike on the ilium - its tip is smack where the predator's belly would be.......
Please remember also that we do not know if there were any spikes on the shoulder of Kentrosaurus - we simply do not know! There may have been spikes, in shape probably much closer to the (unquestionable) shoulder spikes of Gigantspinosaurus than the spikes of Kentrosaurus. The latter do not fit the scapula well, and would thus tend to droop (and who's gonna attack from below? A mesozoic killer ground hog???), while the photographs someone kindly gave of the Chinese genus show a shape that has them secured against rotation.
This is pretty crazy, since haven't checked out your blog in quite a while, but I have decided to get a Stegosaurus fossil tattoo. Now I've been looking around at pictures, and the ones that came up didn't quite look right to me, (Like this Wikipedia one, even though it is quite beautiful) http://en.wikipedia.org/wiki/File:Stegosaurus_Senckenberg.jpg
Could you possibly refer me to a relatively accurate Stegosaurus skeleton profile?
I am still very much surprised that the latest articles on this blog were exactly what I was looking for! Crazy!
hm, I wrote a lengthy reply to David and Darren hours ago, and it was supposed to be "pending" - Darren, can you please check?
YAGTZP! Thanks Darren, and especially Heinrich and those involved in the stegostudy.
It's nice to see support for the thagomizer being an effective weapon. To paraphrase Calvin's report on the T-rex as scavanger and not predator issue: 'I say the thagomizer was a weapon because it would be so bogus if it was just a decoration. The end.'
I do have a question, when you say that "Mesozoic dinosaurs possessed relatively thick (bird-like) articular cartilage caps", what sort of animal was it that had the, implicitly, thinner articular cartilages that was the model for earlier views?
@ Mike from Ottawa: what animals? Supposedly "all" - somehow many people missed that birds and crocs have thick cartilage. What remains are mammals. Just think of all the studies done on human cartilage - well, there's the animal dino cartilage used to be modelled on.
btw, there will be another stegosaur paper by me soon, on thagomizer use :D
It's nice that we're getting to see reconstructions that look living because they have a little bit of elastic tension in their posture and don't just flump like a wet sock -- the problem with so many "ONP" poses.
@Heinrich: I was fortunate enough to have the CT facility and software at AMNH available to me, and I had to deal with exactly what you described, matrix with a similar density to the bone, along with glaring pyrite inclusions. It took me about 4 weeks of going through each 2D slice individually outlining bone (getting only about 20% through the skull) before I upgraded to a more up to date program and used a 3D extraction tool and got 10 times more work done in a single day. It was grueling, but well worth it in the end.
Thanks for the cites, unfortunately I don't read German, but I'll see what I can glean from them. Is it currently the accepted idea that the spikes and plates are homologues, as I assumed earlier? The alternate vs opposite arrangement in the plates and spikes for most stegosaurs makes me a bit skeptical, although I see Kentrosaurus as been interpreted as having oppositely arranged plates.
Heinrich,
Thanks! I have eaten plenty of chicken 'drumsticks' but had never paused to think of how thick the cartilage in a chicken's knee is compared to the size of the femur and tibia and how thin human cartilage is in proportion. And, of course, once you mention it, it is blindingly obvious that crocodilians and birds would be better models than mammalian cartilage.
I wonder if the relative thinness of mammalian cartilage is one of the things that kept them from ever approaching the size of some of the larger dinosaurs.
I'll keep my eyes on TetZoo for more news on the thagomizer.
David - yes, the Limusaurus head would be ideal.
That makes sense, thanks!
I guess you've addressed the fact that the vertebrae wouldn't be jammed together as in older reconstructions and could actually move. Still, it bothers me that the computer animations I've seen of an ancient crocodile swimming show it wagging a stiff tail like a board, while films of living crocodiles show them sinuous in water from the head down. I watched a couple of dinosaur/crocodile nature programs last night and as a nature observer I was a little disappointed in typical animations. They are slowly getting better but in general anything walking is animated to be heaving its centre of mass up and down in the style of video-game monster. Can someone at least watch mammals, lizards, and terrestrial birds in motion? The centre of mass usually doesn't move more than a few inches when the critter is just walking. And when in a hurry, they do more than trot. A baby crocodile running flings both legs forward together to get a long "galloping" stride. Imagine a stegosaur in an emergency doing the same.
Please go over your animations with an eye to how real animals move, especially if the software is a general animation program, because whoever builds the animals probably starts with a skeleton and motion rules designed for video games. You might have to review and adjust assumptions about degrees of freedom or where the flexibility lies. Probably a lot of them have a stiff spine and all the motion at the leg joints, with the bouncing added to make it look like it's doing something.
The "evolutionary" trial-and-error program, using a supercomputer according to the show, eventually settled on very smooth gaits, but again spines seem to be interpreted as simple rods. Basically, I'm pleading for more flexibility between vertebrae and elsewhere.
Sorry about the rant. I just get cranky when I see yet another animated fossil lurching across the screen.
Even animals that look like their legs would only go forward and back can often spread them out surprisingly far without damage, e.g. dogs.
That Kentrosaurus aethiopicus at the top, with spines on its upper arms: could it roll its elbows out a bit to point the spines out towards attackers?
No, those spines are on the shoulder blades. The upper arms are mounted almost horizontal, the left elbow faces the camera.
@ Dallas Krentzel: pyrite! EEK!!!!!
And I feel with you about new software versions; I remember too well the trouble I had with mechanical digitizing - only to find the problem vanish in a puff of logic when a new update came out.
as for the papers: email me, and I'll type up a short summary - do you have PDFs or should I send them?
@ Mike from Ottawa: it is IMO highly likely that the ability to use thick cartilage was a major point in sauropod gigantism. Cartilage distributes stresses across the bone surface, so the risk of surface "spalling" on impact (missteps, e.g.) is much smaller.
Are there any "deep" reasons a mammal bent on extreme giganticism couldn't evolve thick articular cartilages? To my ignorant mind, it seems like an "easy" thing to evolve.
Yes. As Heinrich will tell you, mammal articular cartilage is avascular and has to rely on diffusion for nutriment. Dinosaurs have blood vessels in the cartilage. A few of us got the memo a couple of years ago. See...
Graf, J., Stofft, E., Freese, U. & Niethard, F. U. 1993. The ultrastructure of articular cartilage of the chicken's knee joint. International Orthopaedics 17, 113-119.
@ Andreas: indeed, as Darren wrote, it is a fundamental difference, which is directly linked to bone growth. Archosaur cartilage grows on the end caps, where cartilage is ossified, while mammals have epiphyses. Going from the latter to the former would be difficult. And the blood vessels are the key for both growth and thickness.
@ Darren: I was deliberately holding back, wondering how long it takes for that reference to pop up :)
As I pointed out elsewhere (http://www.palaeo-electronica.org/blog/?p=61) medical research can be very helpful for palaeontology.
OK, this makes the difference between mammalian and archosaurian cartilage the coolest thing I've learned so far this year! And I expect it will hold up for at least January.
Is there anything known of which, if either, condition is ancestral?
Thanks again, Heinrich and Darren.
@ Mike from Ottawa: it is indeed cool :) ancestral is the vascular cartilage cap as growth endplate. Us mammals be weird spin-offs ;)
anybody needing a PDF of the Graf et al. cartilage or Hennig and Janensch Kentro papers please email: firstname.lastname_AT_gmail.com
You know, sometimes I really hate the size squeeze in our ancestry. Oh, sure, it allowed us to have parasagittal forelimbs with pronated forearms, but it turned our sacrum into a joke ("sacroiliac joint"... "joint", FFS, in normal amniotes that's a suture!), apparently made us lose the ability to grow blood vessels into cartilage except to ossify it, cut our tooth replacement* short...
* The claim that the requirement for precise occlusion prevents regular tooth replacement is hogwash. When one tooth is replaced at a time, as we humans more or less do it the one time most of us do it, that's not a problem. Camarasaurus had precise occlusion and no sign of mammal-style tooth replacement, right?
David Marjanovic wrote:
There's an hypothesis about that....
Incidentally, keystoned vertebral centra in some tetrapods are usually accompanied by keystoned intercentra. In caudal vertebrae, this is also usually interspersed by the proximal ends of haemal arches, such that there is a wedge-shaped gap between vertebrae in lateral view which would indicate an irregular (wedged!) disc. Such irregular disc structures would be associated with amphiplatyan or biconcave or proplatyan vertebral motion, further permitted by a non-discoid, [but rather] globular intervertebral disc, allowing a broad range of motion and taking on the role of pivot between vertebrae.
Humans have defective spines as a result of our rather bizarre vertical orientation, so that the normally parallel lumbar and thoracic vertebral centra faces are inclined to one another, causing distortion in the discs between them. Poor posture forces the discs out of equilibrium (anything, really, that causes the lordosis to become exaggerated or straightened) as caused by normative, reciprocal bending of the spine in a "ventrodorsal"/anteroposterior axis of motion. This would be solved, generally, by suspending the spine, and taking the load off of the centra, but hey, we're hardly perfection.
ONP using strict philosophy of the alignment of all centra faces and zygapophyses in this sense (as Heinrich alludes) is thus problematic, as it says nothing about the posture itself. Unfortunately, we also lack direct evidence for the shape of intervertebral discs in some fairly odd animals (e.g., extreme pro-/opisthocoely as in ball/socket articulations) due tot he dearth of living organisms with similar vertebral articulations [caveat: and a similar mode of life! Modern crocs have ball/socket vertebrae, but are aquatic and thus suspend their weight in water -- terrestrial crocs typically do not, and lack the extreme socketing of the centra].
Heinrich, fun stuff, and I hope to see more!
There are also a bunch of crocs with differentiated mammalian dentition that, I assume, shed their teeth more often than we do. They seem to have gotten along just fine.
re tooth replacement: the inability to produce exact occlusion in molars(!) is often seen as a disadvantage, because cutting/mashing/grinding is not possible - thus, less good processing of food. However, sauropods show how much of an advantage the absence of oral processing can be!
Sander et al. 2010
http://rsbl.royalsocietypublishing.org/content/early/2010/05/29/rsbl.20…
If Hennig and Janensch put the spikes on the hips, and they fit there, does anyone know why the consensus swung in favour of placement on the shoulders? Was they just deemed to be more useful there, or was it the discovery of Gigantspinosaurus that tipped the scales?
I have to commission an illustration of a Kentrosaurus for a children's magazine, and I'm in a quandary. I'm tempted just to go for broke, and give it parasacral AND parascapular spikes... Kids love spiky things!
See comment 11.