Evolutionary Studies Institute (ESI), School of Geosciences, University of the Witwatersrand, Braamfontein, Johannesburg, 2050, South Africa.
Jackson School of Geosciences, The University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX, USA.
Sci Rep. 2020 Nov 11;10(1):19602. doi: 10.1038/s41598-020-76757-0.
For over a century, researchers have assumed that the plane of the lateral semicircular canal of the inner ear lies parallel to the horizon when the head is at rest, and used this assumption to reconstruct head posture in extinct species. Although 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 over one 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 horizontally. This reconstructed cranial orientation was statistically compared to the actual head posture of the corresponding species using a dataset of 10,000 photographs and phylogenetic regression analysis. A statistically significant correlation between the reconstructed cranial orientation and head posture is found, although the plane of the lateral semicircular canal departs significantly from horizontal. We thus caution against the use of the lateral semicircular canal as a proxy to infer precisely the horizontal plane on dry skulls and in extinct species. Diet (browsing or grazing) and head-butting behaviour 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 correlated with phylogenetic history.
一个多世纪以来，研究人员一直假设内耳的外侧半规管平面在头部静止时与地平线平行，并利用这一假设来重建已灭绝物种的头部姿势。尽管这一假设一再受到质疑，但它从未在哺乳动物中进行过大样本量和广泛的分类学尺度的测试。本研究在一百多种“有蹄类”物种中对这一假设进行了全面测试。使用 CT 扫描和手动分割，将头骨的方向重建为如果骨迷路的外侧半规管是水平对齐的。使用 10000 张照片的数据集和系统发育回归分析，将重建的颅部方向与相应物种的实际头部姿势进行了统计学比较。重建的颅部方向与头部姿势之间存在显著的相关性，尽管外侧半规管的平面明显偏离水平。因此，我们警告不要将外侧半规管用作在干颅骨和已灭绝物种中推断精确水平平面的代理。饮食（觅食或放牧）和头部撞击行为与外侧半规管的方向显著相关，但与实际头部姿势无关。头部姿势和外侧半规管的方向都与系统发育史密切相关。
