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Re: DinoMorph Strikes Back!... or does it?



Reposted for Kent Stevens.
___________

On January 3, 2006 4:05:16 PM PST GSP1954@aol.com wrote:

(GSP) < arm posture used by giraffes and okapis). Now, if some sauropods had to partly disarticulate the cervicals to get the head down far enough to drink then it looks like they did so. As long as a joint is not bearing a major load of the body, i. e. a leg joint when on the ground while walking or running, it is possible for some joints to regularly partly disarticulate in order to achieve a normal function (the premeire example being the wrist of the horse). >

Ah, the horse wrist. Some of us remember your admonishments in an earlier DML posting regarding said horse wrist.

But do you know of an intervertebral joint that can be disarticulated (more than once)?

As it seemed to Jeff Wilson and Paul Sereno at an SVP talk, it might appear that camels can dorsiflect their necks so far they seemingly must disarticulate. But as Mike Parrish and I have subsequently shown in our chapter in the Currie Rogers/Wilson volume, it's simply a camel thing. When flexing their necks back sufficiently to rub their occipital glands on their shoulders (a specifically male camel thing apparently) all cervical vertebrae remain comfortably in articulation. They not only preserve a safe overlap at the zygapophyses, but as we also show, there are osteological stops that prevent disarticulation when the postzygapophyses travel posteriorly to the max. Hence camels can achieve the same degree of dorsal flexion as occurs in the death pose. Not so with poor sauropods, by the way. Their death pose exceeds articulation, and thus probably exceeds what they could comfortably achieve in life. Sorry, but the juvenile _Camarasaurus_ CM 11338 is probably preserved in a death pose, unless you believe that they could go around with their necks all out of articulation.

Going back a decade, at the New York SVP, in the question period after a talk I delivered with Mike Parrish, you suggested that I verify the DinoMorph method using a modern analog, such as the giraffe. Well, this last year, as part of the "Dinosaurs: Ancient Fossils, New Discoveries" exhibit, the AMNH kindly CT-scanned an entire giraffe neck and I put that digitized morphology data into DinoMorph, and yes, the virtual giraffe closely replicates the range of motion observed in living, behaving, giraffes (look at the movie on my website under AMNH, or better, go to the exhibit and run the software interactive for yourself) Moreover, I've found the giraffe neck is osteologically braced when laterally flexed to its limit, much as the design in _Diplodocus_ and some other sauropods (for mediolateral flexion, not dorsiflexion, mind).

Thankfully, one simply does not need to postulate sauropods undergoing disarticulation. The "Eeyore model" of sauropod neck posture, depicted in an early drawing of a rather dejected _Diplodocus_ in Hatcher, later in an Osborn and Mook silhouette illustration of _Camarasaurus_ (see the Bozeman SVP presentation PDF at my site), is pretty much what the bones dictate. Sorry.

(GSP) < Kent said that the dorso-cervicals of AMNH 5761 are fused in a straight line. Actually they are fused in a modest upwards kink, >

Sigh... please look again... it should be plain to see that the articulation at the two consecutive centra is indeed straight. This, at least, should not require an infinite loop of argument. For driving instructions, either go my DinoMorph website:

http://www.cs.uoregon.edu/~kent/DinoMorph.html

and in the "research projects" pulldown, select Camarasaurus, then either 1) optionally read a bit of the text there, including instructions on three ways to get a Camarasaurus erect, or 2) just click on the link to the pictures, or 3) just click below and forget reading all the text:

[note that some pc mail programs will not interpret the following URLs correctly if they wrap the URL onto two lines. In that case, you may have to reconstruct the url in your browser by hand.]

http://www.cs.uoregon.edu/~kent/DinoMorph/Camarasaurus/images/AMNH/5761a-
02.jpg
http://www.cs.uoregon.edu/~kent/DinoMorph/Camarasaurus/images/AMNH/5761a-
03.jpg

Warm thanks go to Carl Mehling for the hefting and Rick Edwards for the clicking.

(GSP) <At the opposite end of things, a number of Camarasaurus and some other sauropods of course have the necks in a vertical position, with all cervicals still in full articulation even though they are at their maximum dorsal retraction. >

I'd like to see such specimens. Please provide us all with specific specimen numbers and institutions. For those who have actually studied CM 11338 (either by getting up on a ladder or visiting either Dinosaur National Monument or the Los Angeles Museum of Natural History casts thereof, which are at eye level thankfully), it is patently obvious that the poor juvenile in its death pose is preserved dramatically out of articulation at the zygapophyses! There's a lot more going on there, including a torsional disarticulation at the base of the neck which much have lead astray at least one paleoartist. The same is true of a lovely specimen at the Sauriermuseum Aathal, which sure looks like it's trying to be a "Juraffe", but for the fact that the neck is twisted axially 90 degrees, which is easily misinterpreted visually as suggesting a 90 degree vertical bend. It becomes obvious only when you note that cranially the vertebrae are viewed in lateral aspect, while caudally the neck vertebrae are seen in dorsal aspect; something not obvious to a casual observer.

(GSP) < People forget that because sauopods had so many cervicals that just modest rotation between any two of them adds up to a lot of total flexion.>

We're not forgetting. We also don't forget that there is less angular flexion per joint permitted by the proportions of the zygapophyseal facets relative to their centers of rotation in sauropod design, unless of course one hypothesizes that the vertebrae were routinely capable of disarticulating during everyday neck flexion.

From giraffes and camels to turkeys (and presumably sauropods) the same biomechanical principles hold, and seemingly (but the following is not sufficiently established yet) perhaps the same safety factors apply (e.g. in terms of amount of overlap). We've simply applied the same safety factors to the sauropod geometry. Don't blame us that sauropod osteology does not permit swan-like flexibility.

(GSP) <there is no good reason to conclude that such sauropods (and not all sauropods are like this) when alive could not raise the neck vertically to high browse or get the maximum view of the landscape.>

Yes there is: the cervical vertebral osteology does not permit such extreme dorsiflexion without disarticulation. Is there an echo in here?

(GSP) <What I am getting at is that an odd view has developed, one that seems to think that the only way to legitimitely illustrate or mount dinosaur skeletons is with the cervicals in perfect neutral osteological articulation. There is no such law. ... >

But what if such illustrations nonetheless depict the vertebrae as undeflected (based on close examination of the articular facets along the axial skeleton)?

For instance, examine the _Apatosaurus ajax_ mount at the Yale Peabody Museum. The neck is mounted in a lovely, gentle sigmoid curve. Looks great. Now, look carefully at the zygapophyses (take binoculars, or borrow a ladder as Mike Parrish and I did) and you'll see that the zygapophyses appear to be in neutral position. But before you conclude that this specimen has an intrinsic swan-like curve to it, just tap on the vertebrae and you'll find a whole lot of plaster artfully stained to resemble bone. That is to say, those vertebrae at the base of the neck where the upturn presents itself were reconstructed artfully (I use the term "artfully" again, Scott, in the meaning I intend, not merely as suggesting a subjective, aesthetic sense of art). How did that happen? It seems the available fossil material was first attached to a smoothly curved armature, then the missing pieces were filled in with plaster, and it just so happens that the way the prezygapophyses were plastered in to align directly under their associated postzygapophyses, the overall neck appears to be state of neutral deflection. Sigh.

So, regarding laws, perhaps there should be (if there is not an unwritten one already) a law stating that if an axial skeleton is formally drawn (or mounted) for scientific purposes, as depicting OTHER THAN the "perfect neutral osteological articulation" then said state of deflection should be accurately and clearly apparent by examination of the articular surfaces. There should also be documentation, perhaps, suggesting that the artist intends on depicting the given axial skeleton in a state of deflection out of the neutral pose, if that is not made sufficiently clear by the intervertebral joint facets. Such cautions are not needed, of course, with the appendicular skeleton: the limbs are often drawn as in mid-stride, without the potential of misleading the viewer.

Since so many colleagues, when referring to silhouette drawing in order to derive an overall understanding of an extinct animal's bauplan, tacitly assume that the depiction of the axial skeleton reflects its characteristic curvature (as is the way skeletons of giraffe, horse, or deer, etc. are traditionally drawn or mounted), there should indeed be a law. Thanks for suggesting it.

Kent