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Re: Sauropod necks (Re: DinoMorph Strikes Back!... or does it?)



On Jan 13, 2006, at 6:11 AM, Renato Santos wrote:

Overall, as David said a very interesting and enlightening rant. Guess
we can scrap DinoMorph's conclusions for the most part.

who's "we"? And why couple the two methodologies (2D digital photo- compositing and 3D modeling)? And why "scrap conclusions" regarding the scientific utility of reconstructing osteological neutral poses (see specifics below)?


1) Some published silhouette drawings, while dimensionally accurate as far as major dimensions such has overall bone lengths, are morphologically inaccurate, simplified, or idealized. This is a matter that can be verified, and should not be caught up in endless arguments and rants.

2) We have just learned that, for one artist at least, the lateral view depictions of the body plan are NOT necessarily intended to show osteological neutral pose (ONP) along the axial skeleton. Instead, they are constrained only to showing some osteologically-achievable posture. I have only created 2D digital photo-composites of the axial skeleton in the osteological neutral pose (ONP). That I believe, and I expect most others believe, is unbiased starting point for attempting to reconstruct an overall bauplan and to subsequently explore the range of motion of an extinct vertebrate. If someone chooses to depict (by silhouette art) a pose other than the ONP, then it would be important to make that clear (visually by taking the effort to indicate the concomitant displacement at the intervertebral joints, and perhaps by an accompanying annotation in the text or title of the artwork). I mentioned the _Apatosaurus_ mount at the Yale Peabody to attempt to deflect some readers from dismissing my words merely as a rant about a few silhouette drawings, or about a particular artist. The issue is one of maximizing the scientific utility of a reconstruction (be it 3D, as in a physical mount that requires lots of plaster, or 2D as in a pen-and-ink illustration).

3) I agree that the utility of a 2D digital photo-reconstruction (as opposed to a 2D pen-and-ink illustration) ultimately depends on the original artwork. Again, I caution to first get the scale correct, as one can otherwise naively end up with pretty silly reconstructions of gopher-hunters and, on that basis, advocate giving up on ONP as a scientifically useful concept. Regarding what to do with any obvious distortion I suggest interpolating the geometric mean of the curvatures (i.e. pairwise tangents defined by the long axis dimensions) of the vertebrae adjacent to the distorted vertebra. For an isolated vertebra that doesn't fit with its neighbors, that fact should be visually obvious for what it is: the given vertebra is distorted, and that I thought goes without saying. If there are multiple, subtly distorted vertebrae in a region, then this approach will not yield useful results, hence my explicitly annotating the reconstruction of _Diplodocus carnegii_ (CM 84) where that occurs, plus discussion to that effect in the text.

4) Regarding the relationship between osteological neutral pose (ONP) and the habitual (whatever that means) pose, the field will be assisted by more workers attending to this issue. Regarding hartebeest and wildebeest, it would be scientifically of value to start from the osteology, and reconstruct the ONP of each, and see if it reflects any difference in the habitual poses. Someone has got to do that before jumping conclusions. ONP will remain a useful starting point regardless, as it then would be a standard on which to quantify differences among related taxa in habitual pose. Think about it a moment, please: how otherwise can one describe neck posture in a useful manner than to base it on some osteological neutral point? Think freezing point of water: a useful neutral point for anchoring temperature scales. We likewise need a means for describing degrees of dorsiflexion or ventriflexion, and the zero point would be the ONP. It's not meant to be an inflammatory or radical notion. Just scientifically useful.

5) Regarding whether pleurodiran turtles really disarticulate their zygapophyses, I'll contact someone who did a Ph.D. dissertation on those very necks. Based on his personal communication to Mike Parrish and myself when we visited him (soon after a DML comment to this effect), this is a myth. But let's look at it dispassionately and carefully. It is of significant importance to find cases where vertebrates voluntarily (and repeatably) disarticulate their zygapophyses. If there are indeed any such vertebrates. It is not the case regarding camels; they seem to bend so far back that the zygapophyses must disarticulate, but they remain in articulation, and are prevented from doing so by an osteological adaptation.