Wrench in the Works of Head Attitude

The orientation of the head for standardized anatomical comparison is important for biological and paleontological analysis. It influences anatomical direction, description, the relationship of soft-tissue, study of behaviors such as feeding by the orientation of muscles versus gravity, and so forth. In the past, most researchers have focused on the passive observation of animal posture to determine “static” poses, leaning towards the “at rest” and “alert” or “active” postures, whereby the animal is in a static, but attentive posture. In the 40s, 50s and 60s, several studies of avian, mammalian, crocodilian, and squamate rest/alert postures attempted to find static anatomical features that could translate to fossils for the purpose of finding a “holy grail” of head attitude. Most of this focused on the orientation of the lateral semicircular canal (LSC) relative to the skull. I covered portions of this here and here, and through those links you will find more extensive discussion.

Fig. 1 from Marugán-Lobón et al. (2013), showing basal and LSC-calibrated postures for a human and a stork (Ciconia ciconia).

Recently, a new paper published in the open access journal PeerJ by Jesus Marugán-Lobón, Luis Chiappe, and Andy Farke has suggested that the variance in orientation indicated of the LSC in living animals is not very well constrained. This was made even more dramatic when assessed against the shapes of the skull in a wide variety of taxa, although the sample was restricted to Archosauria. Ordinarily, phylogenetically similar animals with similar skulls should have similar finer details of the skull; it is the further you get from a group that variance in these fine details should increase. The bias that the braincase varies least among regions of the skull is itself not a proven hypothesis, and as such the braincase is under selection just as much as the surrounding bones. So when the orientation of the LSC relative to the shape of the skull varies widely, it becomes more difficult to place the orientation on a pedestal as a proxy for natural cranial orientation. Skull attitude, braincase attitude, and head carriage may all vary independently, and this means the baseline orientation of a feature within these structures may have little congruity across these many disparate species.

Remarkably, this showed that in some taxa, among them birds and the bizarre sauropod Nigersaurus tacqueti, the strongly ventrally rotated occipital plate does not seem to influence high congruence of cranial morphology: Nigersaurus, particularly, has a skull remarkably similar in overall shape to that of other sauropods, despite its ridiculous appearance and possible vertical orientation. These things may spell problems for the use of the LSC orientation as a metric for cranial orientation, and this is true for me. Recall, if you will, that I based my cranial reconstructions off of the basal jugal margin, which itself seems to have a general congruence to the LSC and basal floor of the endocranium. You’d think this large series of similarities would be useful, but this isn’t the case, and it is only a baseline from which to test orientation. But suddenly, I am tempted to toss these orientations and go for the basic “most horizontal” perspective most other skull illustrators prefer. Hmm. food for thought.

Marugán-Lobón, J., Chiappe, L. M. & Farke, A. A. 2013. The variability of inner ear orientation in saurischian dinosaurs: Testing the use of semicircular canals as a reference system for comparative anatomy. PeerJ 1:e124.

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This entry was posted in Biological Comparison, Biology, Biomechanics, Morphometrics, Paleobiology, Science Reporting and tagged , . Bookmark the permalink.

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