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.
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 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!
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freedom mentor | 05/03/2015 06:46
freedom mentor | 05/03/2015 06:43