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Sauropod Postures, and Low-Slung Diplodocids

Posted by tigerhawkvok on May 31, 2009 17:08 in biomechanics , sauropods , biology , research , paleontology

Recently, Taylor, Wedel, and Naish published a paper on sauropod postures (SV-POW, TetZoo), which challenges a paradigm established by Stevens and Parrish's paper on DinoMorph modeling which states that based on the way the cervical vertebrae articulate together, certain postures are prohibited and thus you get the current model of low-slung necks for the majority of diplodocids. This works out nicely with authors who worry about the blood pressures required to pump blood up to a neck that is elevated so high off the ground. Now, given that only a few posts ago I talked about phylogenetic bracketing and its usefulness, it's appropriate that I talk about the problems in overusing it, and step into dangerously clichéd territory while talking about the paper I am working on.

< !-- Sauropod up image --> Neck held up

First, right off the bat, I want to say that I think this is an excellent piece of work. I think it has a good place in the literature, and that more than studies of giraffe blood pressure is needed to be convincing about the blood supply issues for diplodocids (I am pretty sure I've talked about this before, but if nothing else, let me reiterate that mammals are not necessarily a good model for archosaurs). The crux of Taylor et al's argument is that extant tetrapods from all groups have strongly inclined cervical vertebrae, and that in modern animals, yes, the most favorable position is in fact a horizontal neutral one. However, soft tissues mean that this is actually not the most neutral position, and only using the vertebrae is misleading. Absolutely true, good work, and I'm amazed this hasn't been looked at before. I've even worked with Matt Wedel in writing up my paper (though I'm sure he doesn't recall by now), and I value his opinion.

So, the argument goes, based on phylogenetic bracketing, you would expect sauropod necks to not be held horizontal, regardless of what the cervical vertebrae show. While this might be largely true, I will attempt to briefly, in this blog entry, illustrate why this doesn't have to be true, and give a bit of a preview into my work-in-progress (post-editor revisions) to demonstrate why I don't think this is true for diplodocids (without spoiling my paper. Sadly, something I must take care not to do).

First, it is important to note that phylogenetic bracketing can never tell the whole story. We are the only extant tetrapod that is fully bipedal with an entirely erect vetebral column, and possibly the only one that has yet evolved. No number of examining outgroups will tell you that Homo sapiens bones should be this way; this has to be inferred from our morphology. This is a fact of essentially all novel traits. Just relying on phylogenetic bracketing prohibits you from inferring novel postures based on morphology that have no extant representatives. Second, it's possible that there was something completely bizzarre going on that we just don't know about. As Matt's SV-POW entry very clearly demonstrates, finding the fossil of, say, a budgie 200 MY from now, with no birds, you might guess it has a crazy neck like a flamingo. Sometimes, you just can't tell. That is not to say it is a very good guideline, that is very often right and instrumental in a lot of work; but it is not perfect. They even address this fact:

Can the habitual posture of extant amniotes be expected to apply to sauropods? Phylogenetic bracketing strongly supports this hypothesis as the neck posture described by Vidal et al. (1986) is found in both Aves and Crocodylia, the nearest extant outgroups of Sauropoda, as well as in the increasingly remote outgroups Squamata, Testudines and Lissamphibia.

However, some authors have postulated that the necks of sauropods, rather than representing an extreme development of mechanisms found in other vertebrates, were anomalous structures maintained using novel mechanisms. If this were so, then it would not be surprising if the habitual posture of sauropod necks was different from that of other vertebrates.

Now, it is my personal opinion that Taylor et al. is probably right in the majority of the cases. Among other things, the construction of, say, Brachiosaurus would suggest strongly inclined necks, and I suspect that all sauropods would be able to list their heads like this, at least for moderate periods — it seems the obvious, niche-opening thing to do. Even in diplodocids, it seems that a completely flat neck is not necessarily correct, and I personally favor a slightly cantilevered position (this partially addresses their comments about the orientation of semicircular canals, by coincidence). However, according to their paper:

In all four sauropodomorphs figured by Sereno et al. (2007: fig. 1G), the occipital condyle is directed postero-ventrally when the HSCCs are horizontal. If the HSCCs were inclined upwards, as in most birds and mammals, the down? ward tilt of the occipital condyles would be even greater. Therefore, even if the cranio-cervical joints were held in ONP, the anterior part of the neck would be inclined in all four taxa.If the cranio-cervical joints were flexed as in extant terrestrial amniotes, the anterior portion of the neck would need to be even more steeply inclined in order to hold the HSSC horizontal, and would possibly have approached vertical in Camarasaurus and Diplodocus (Fig. 4B, C). Taylor et al. 2009

< !-- My image --> Suspension Model

My own paper works on estimating the sizes of diplodocids, with biomechanical parameters based on the assumption that they held their necks roughly horizontal, as estimated by Stevens and Parrish's work in line with the accessible ranges in Stevens and Parrish's work. Most accurately, the level of the "bridge" is the same as the level of the acetabulum (thanks to Matt for pointing out the error in this statement). The upshot of this is, when you assume this for diplodocids, you get the correct length popping out of the math. This is very strong evidence, in my opinion, that for at least that clade the neutral position was holding the neck horizontal. This model, in fact, pulls within 4.3% of of current restoration lengths.

Now, I really want to write more — but it probably lives somewhere in that mystical realm where Bad Ideas come from. I'm slowly working on an extensive rewrite of my opening, which does not lead to quick work! But with luck, the pace will pick up soon. I should talk to Matt and see if he is interested in taking a look at what I have so far — and if Darren or Mike is interested, as well. Hopefully I can revisit this in a few months, and talk about it more!

 

Teaser Post

Posted by tigerhawkvok on May 28, 2009 21:15 in biomechanics , sauropods , biology , research , paleontology

A teaser post ....

Suspension Model

Image from a preprint of my paper. CC-BY-NC-ND 3.0

VS

Neck held up

Copyright Taylor, Wedel, Naish 2009.

Via TetZoo and SV-POW. Amusingly, I know one of the authors personally, and I am really interested in what his feeling will be when my paper gets to review.

Commentary on Sauropod Neck Posture

Posted by tigerhawkvok on April 30, 2009 00:24 in biomechanics , dinosaurs , sauropods , biology , paleontology

If you look at a (yikes! month old) Science "Letters", there an interesting, if brief, back-and-forth about sauropod neck posture between RS Seymour and PM Sander.

Up front, I'd like to say like Sander, I don't necessarily disagree with Seymour's conclusion; my own work (still undergoing revision that is halting its review process) strongly suggests that the necks were mostly held laterally, due to energetics and biomechanical concerns. However, I do take issue with the blanket scaling argument used on several points.

DSCF4647

Ara ararauna (Blue and yellow macaw), Hawaii

First, it is important to note that the work done by Seymour is based on mammalian modeling. I have no comment as to whether it strengthens or weakens his argument this way; however, as a saurischian dinosaur, a bird would be a much more convincing model point. Second, their morphology is wildely divergent even from birds, the closest living species to them. The anology, even to birds, would be as problematic for me as the biomechanical study that based T. rex on Gallus gallus (chicken). Merely being a close relative does not ensure analogy; the musculature distribution, posture, and body shape in both cases are significantly different from the model animal. The results might very well be interesting, but they are not sure be relevant at all.

While I also have some issue with the "plug and chug" nature of the blood pressure calculations, those you can't really get around — though I'd at least like to know if it was based on mammals or birds.

Well, that's my 2 cents for now. More later.

Fossil finds, oh my!

Posted by tigerhawkvok on March 17, 2009 03:24 in dinosaurs , sauropods , news , paleontology

Seems that the news has been an bit heavy on the fossil finds! How about a rundown.

Hesperonychus claw, hotlinked via National Geographic. The claw is on a Canadian penny.

First up is Hesperonychus ("western claw") — as yet, the smallest theropod found in North America. At the time of this writing, the PNAS paper isn't up yet, but in particular PhysOrg has a pretty good synopsis. As the name suggests (via a linguistic kinship to Deinonychus), Hesperonychus elizabethae is a diminuitive dromaeosaurid, approximately 2 kg and 50 cm tall "beating" Albertonykus borealis for title of "smallest North American non-avian dinosaur". It retains the trademark "sickle claw" of the dromaeosaurids, and the hip configuration was one of the primary diagnostic traits used to pin it to dromaeosauridae. The pelvis is also fused, suggesting that this was a mature adult. It's dated to about 75 million years ago (which the PhysOrg article accidentally writes as 45 million years ago, obviously in error at 20 MY after the K-T event).

Picture of the Svalbard excavation, from the New York Times.

The next big find announced today was of an unnamed, massive pliosaur found in the Svalbard region. I'm outright going to say I wish Kit had something to say about this, as I vaguely recall him mentioning this almost a year ago, and he knew some basics about it already. Never mind how he knows, considering it hasn't been published yet — the NYT article is already a pre-publishing press release.

At any rate, this guy was massive. This pliosaur had a skull measuring over 3m long, and the entire animal was probably longer than 15m at over 40 tonnes. Its "nickname" for now is "Predator X". Particularly novel about this find is that it might represent a new family of pliosaurs, which would be rather significant, paleontologically. Initial biomechanical work suggests its bit force would be 2-4 times that of T. rex and more than 10 times that of any extant animal. It was estimated to live about 150 MYA.

Figure 1 from Mud-trapped herd captures evidence of distinctive dinosaur sociality by Varricchio et al. 2008. (property of the authors)

Sinornithomimus dongi (Chinese bird mimic)is the next guy to hit to press. It has the slightly dubious distinction of being discovered when a large immature herd got stuck in a mud trap. It is siginificant in that the herd was apparently all juveniles, and in the number of animals found. With 25 individuals found (aged 1-7), this promises to make Sinornithomimus particularly well described. This was a bit of a late reporting on a December 2008 paper by Varricchio et al., which I've not had a chance to go over yet. I might update this with more detailed information once I get the chance.

Finally, SV-POW has an interesting piece up on Brachiosaurus. I'd strongly suggest you read it. And, for kicks, I adapted Matt's image into something nice and widescreen-background sized:

I swear, I will post Higgs and the mouse post ...

Paper Creep

Posted by tigerhawkvok on May 29, 2008 17:13 in General , biomechanics , dinosaurs , sauropods , computers

As I've been working on the sauropod paper (aside from feeling a bit neglectful of Kepler), I've definitely felt the wish to "feature creep". For those unfamiliar with the term, its one I borrowed from computer program development; its the wish to include ever more "new" stuff into a program that you're writing, often to the detriment of timeliness. Aside from finding ever more species to want to add to my analysis, I find that I am having a difficult time finding the line where I say "enough -- I've explained this to my satisfaction." There are some legitimate holes that are left over from its "essay" form, such as comparisons on different ways of getting the taper constant, ways of extrapolating neck length and accuracy, and even I can't remember what I did to find the trunk length.

I also was a bit lax about my citing in the initial essay (oh, the usefullness of BibTeX -- I wish I knew it sooner. Kudos to Kit for helping me out there), I also really want it to look nice.

So, with its current state of revisions, it is being looked at by Sara W, Sarah W, Sarah K, and Andrew. Hopefully, it will be ready for submission by the end of June, July at the latest. Sarah (K) brought up a good point though -- where to submit it? Its hardly Nature or Science quality, but that still leaves:

1) Journal of Paleontology
2) Paleontology
3) Historical Biology

As the prime candidates at the moment. Hm. I'm leaning towards Historical Biology, but we'll see.

Hopefully this will be out of my system by the time Hawaii is over -- then I can do some solid astronomy. Helps that that's the one I get paid for, too.