On Sauropod “Cheeks”

As SVP looms, and its contents include much interesting things, especially in several valuable studies on the soft-tissue anatomy of sauropod necks and heads — but which I won’t further discuss outside of the authors’ own discussions — I figured I might share some additional reconstruction variations. The first one I illustrated in this post, for an ankylosaur. This time, it’s a little more specific, the sauropod Nigersaurus taqueti.

Skull and endocranium and head posture of Nigersaurus taqueti, from Sereno et al., 2007.

The preceding models represent just four possible reconstructions, each plausible to some degree, and each at least somewhat testable (for instance, Chiappe et al., 1998, reported on the finding of facial skin impressions of sauropod embryos from Auca Mahuevo in southern Argentina, which indicate detailed squamous integument around the oral margins, at least partially). But we are not so settled on how this can vary from sauropod to sauropod. Some models may work better for other types of sauropod, while some sauropods can exclude models directly. It is interesting to note that to achieve dental occlusion in Diplodocus longus, more than half of the oral margin of the mandible must slide medial to the upper jaw, and indeed even further dorsal than the ventral margin of the maxilla-jugal.

Reconstruction of CM 11255, juvenile skull referred to Diplodocus, species unclear. From Whitlock et al., 2010.

This association doesn’t occur in “brachiosaurs,” or likely even in titanosaurs such as Rapetosaurus krausei (Curry-Rogers & Forster, 2004). The clustering of dentition towards the mesial (rostral) ends of the jaws may also play a role in inferring extent of the soft tissues, for unlike ornithischians (which, like mammals, include taxa where the dentition proceeds medial and caudal to  regions where muscles attach) there is no “crossing” of the muscular and dental components of the jaw. This implies one can sectorialize the jaw into distinct mesio-distal regions, just as one can in theropods, where the muscles are always distal (caudal) to the tooth bearing portions of the jaw. But there is evidence that the intervening regions represent a sort of third, intermediate sector, a region with no teeth, and no muscle, whose functional relationship to the jaw’s appearance is relatively unknown. In Nigersaurus taqueti, this region seems to occupy over half the effective jaw length, and the upper jaw forms a broad, high arch (Sereno et al., 2007). It seems questionable whether this region was entirely without coverage … and yet, no analysis of the direct associated tissue structures of the jaw has been performed.

Chiappe, L. M., Coria, R. A., Dingus, L., Jackson, F., Chinsamy, A. & Fox, M. 1998. Sauropod dinosaur embryos from the Late Cretaceous of Patagonia. Nature 396:258-261.
Curry-Rogers, K. A. & Forster, C. A. 2004. The skull of Rapetosaurus krausei (Sauropoda: Titanosauria) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 24(1):121–144.
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.
Whitlock, J. A., Wilson, J. A. & Lamanna, M. C. 2010. Description of a nearly complete juvenile skull of Diplodocus (Sauropoda: Diplodocoidea) from the Late Jurassic of North America. Journal of Vertebrate Paleontology 30(2):442—457. [PDF]