Unidad de Paleontología, Dpto. Biología, Universidad Autónoma de Madrid , Cantoblanco (Madrid) , Spain ; The Dinosaur Institute, Natural History Museum of Los Angeles County , Los Angeles, CA , USA.
PeerJ. 2013 Aug 6;1:e124. doi: 10.7717/peerj.124. Print 2013.
The vestibular system of the inner ear houses three semicircular canals-oriented on three nearly-orthogonal planes-that respond to angular acceleration stimuli. In recent years, the orientation of the lateral semicircular canal (LSC) has been regularly used to determine skull orientations for comparative purposes in studies of non-avian dinosaurs. Such orientations have been inferred based on fixing the LSC to a common set of coordinates (parallel to the Earth's horizon), given that the orientation to gravity of this sensory system is assumed constant among taxa. Under this assumption, the LSC is used as a baseline (a reference system) both to estimate how the animals held their heads and to describe craniofacial variation among dinosaurs. However, the available data in living birds (extant saurischian dinosaurs) suggests that the orientation of the LSC in non-avian saurischian dinosaurs could have been very variable and taxon-specific. If such were the case, using the LSC as a comparative reference system would cause inappropriate visual perceptions of craniofacial organization, leading to significant descriptive inconsistencies among taxa. Here, we used Procrustes methods (Geometric Morphometrics), a suite of analytical tools that compares morphology on the basis of shared landmark homology, to show that the variability of LSC relative to skull landmarks is large (ca. 50°) and likely unpredictable, thus making it an inconsistent reference system for comparing and describing the skulls of saurischian (sauropodomorph and theropod) dinosaurs. In light of our results, the lateral semicircular canal is an inconsistent baseline for comparative studies of craniofacial morphology in dinosaurs.
内耳的前庭系统包含三个半规管，分别位于三个几乎正交的平面上，用于响应角加速度刺激。近年来，外侧半规管（LSC）的方位经常被用于确定非鸟类恐龙研究中颅骨的方位，以便进行比较。这种方位是基于将 LSC 固定在一组共同的坐标上（与地球的地平线平行）来推断的，因为假设这个感觉系统对重力的方向在分类群中是恒定的。在这个假设下，LSC 被用作基线（参考系统），用于估计动物如何保持头部姿势，并描述恐龙之间的颅面变异。然而，现生鸟类（现存的蜥脚类恐龙）的数据表明，非鸟类蜥脚类恐龙的 LSC 方位可能非常多变，且具有分类特异性。如果是这样，将 LSC 用作比较参考系统会导致对颅面组织的不适当视觉感知，从而导致分类群之间存在显著的描述不一致性。在这里，我们使用了 Procrustes 方法（几何形态测量学），这是一套基于共同标志同形性比较形态的分析工具，结果表明 LSC 相对于颅骨标志的变异性很大（约 50°），且可能无法预测，因此使其成为比较和描述蜥脚类（蜥脚形亚目和兽脚亚目）恐龙颅骨的不一致参考系统。鉴于我们的结果，外侧半规管是恐龙颅面形态比较研究中不一致的基线。
