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Category: Communication

Description: For over a century, it has been assumed that the plane of the lateral semicircular canal of the inner ear is parallel to the earth horizontal when the head is at rest. This has long been used by paleontologists to reconstruct head posture in extinct species. Thought this hypothesis has been repeatedly questioned, it has never been tested on a large sample size and at a broad taxonomic scale in mammals. This study presents a comprehensive test of this hypothesis in more than a hundred “ungulate” species. Using CT scanning and manual segmentation, the orientation of the skull was reconstructed as if the lateral semicircular canal of the bony labyrinth was aligned with the earth's horizontal plane. This reconstructed cranial orientation was statistically compared to the actual head posture of the corresponding animals using a dataset of 10,000 photographs and phylogenetic regressions. A statistically significant correlation between the reconstructed cranial orientation and head posture is found, although the plane of the lateral semicircular canal significantly departs from the earth's horizontal plane. We thus caution against the use of the lateral semicircular canal as a proxy to infer the horizontal plane on dry skulls and in extinct species. Diet (browsing or grazing) and head-butting behavior are significantly correlated to the orientation of the lateral semicircular canal, but not to the actual head posture. Head posture and the orientation of the lateral semicircular canal are both strongly predetermined by phylogeny.