The Next Step

While discussing lips in theropod dinosaurs is easy, and a bit dinosaur-centric (what about lips in nondinosaurian archosaurs?!), there are two major elements of the dinosaury that bear understanding: First, that ornithischians present an additional hurdle to overcome when discussing facial skin and its extent (i.e., the issue of “cheeks”) and sauropods present the interesting issue of what to do with large open spaces in the jaw. The latter issue is easier to deal with and represents the next step.

But before I get to it, I will present part of that here as a “filler” post of sorts. This is because I am taking on the most problematic sauropod head to reconstruct when discussing facial skin, Nigersaurus taqueti. Now, Tyler Keillor (in conjunction with Paul Sereno and Jeff Wilson) has already produced an excellent head reconstruction:

This reconstruction is life-sized, highly detailed, and highlights one of the difficulties in such a reconstruction: There’s a lot of empty space being borne in the jaw between upper and lower, filled here with extra heapings of tissues from the back of the upper tooth row to a sort of fleshy flap halfway along the jaw length. The head is also oriented in “standard” pose. This was reconstructed while study on the skull was underway, which culminated in a paper by Sereno et al. (2007) including Larry Witmer and their work attempting to correct the head postures based on a variety of sources, including the same ones used by Taylor et al. (2009) to correct for neck postures in extinct animals. These data indicate that the head should be held in a range of 50° (30° above and 20° below Earth horizontal) when measuring the mean horizontal of the horizontal semicircular canal. This also roughly corresponds to the basal endocranial margin, which runs between the foramen magnum and the sella turcica within the braincase, and as I suggested for some taxa, it appears to correspond to the mean long axis of the jugal, although in sauropods that’s right out (it’s very tiny and highly modified due to preorbital cranial bones just being all there is to much of a sauropod skull).

Sauropodomorphan head posture variation, from Witmer et al. (2007).

One may note a strong trend in sauropodomorph evolution here from a largely level skull (based on a crushed specimen of Massospondylus kaalae) to Nigersaurus taqueti, which shows the skull deviation declined considerably from the horizontal. An additional element to the head orientation is that Dujim (1951) proposed that an average range of motion can be as much as 20-30° above or below the orientation of the horizontal semicircular canal, based on the average attitude of animals holding their heads. This is only for “alert” posture, and that tends not to involve feeding or resting postures, which can differ remarkably among animals.

Sauropodomorphan head posture variation, from the guys at SV-POW!, modifed by them from the Witmer et al. illustration above.

Nigersaurus taqueti is not just unusual in its head posture, but also its jaw morphology, which presents a broad, T-shaped aspect in cranial view (in the above orientation) and a strongly rostrally restricted array of dentition. For many reasons, including the work of John Whitlock (a co-author of the Nigersaurus paper) who reconstructed a potential ecological restraint on sauropod feeding habits based on skull shape geometry and microwear analyses of the teeth, Nigersaurus provides many unique opportunities to test the ideas of sauropod head- and neck-attitude and the means through which we evaluate posture, be it “average” or “standardized” in any form, and what “alert,” “feeding,” and “resting” style postures have on the average at all. For not only does Nigersaurus present an unusual head shape, with the rostrum strongly down-turned, but the head-neck angle is very strongly down-turned with respect to the long axis of the skull, quite different from that of typical sauropods, and this would mean general neck orientation would favor a skull at the end of a low, gently sloping neck, possibly near horizontal, or with a raised, more-S-shaped neck, and quite different from the ramrod-straight necks that are typically envisioned for “long-necked” sauropods like Diplodocus.

Some elements of the discussion, including the arrangement and effect of jaw muscles in various reconstructions, have direct bearing on this effect, and the oddity of sauropod dental wear even more so. These, it may be suggested, can heavily influence how we reconstruct even neck posture in sauropods (as Mike Taylor of SV-POW! [here] concedes, but is extremely — and understandably — cautious about).

Duijm, M. 1951. On the head posture in birds and its relation to some anatomical features. II. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, Series C 54:260–271.
Sereno, P. C., Wilson, J. A., Witmer, L. M., Whitlock, J. A., Maga, A. Ide, O. & Rowe, T. A. 2007. Structural extremes in a Cretaceous dinosaur. PLoS ONE 2(11):e1230.
Taylor, M. P., Wedel, M. J. & Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica 54(2):213–220.
Whitlock, J. A. 2011. Inferences of diplodocoid (Sauropoda: Dinosauria) feeding behavior from snout shape and microwear analyses. PLoS ONE 6(4):e18304.

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