Manual landmark data collection on large samples takes a long time. Several options exist to speed the process, though each introduces problems or raises concerns of its own. For bilaterally symmetric structures (e.g., crania), some recent papers recommend limiting landmark collection to one side and the anatomical midline, then approximating the true bilateral configuration by merging the hemi-form and its reflection at the midline. However, where the midline is narrow relative to the bilateral anatomy, net midline landmark deviations from the mid-sagittal axis or plane will distort this “mirror-reflected” configuration. Here, I use a sample of bilaterally landmarked human mandibles to evaluate whether these distortions are a substantive concern at the scale of real biology. Through simulation, I introduce small mediolateral errors at the midline landmarks of the mean mandible form, then mirror-reflect one side in order to quantify and visualize the effect of midline error on mirror-reflected outcomes. I also test how faithfully mirror reflection and other reduced-landmarking strategies preserve shape and size relationships among observations characterized by the full, bilateral complement of landmarks. In both analyses, mirror reflection is shown to produce striking distortions. Mirror reflection is clearly inappropriate for these data and is likely suspect in all cases of narrow midline morphology. I also demonstrate that bilateral shape and size can be reasonably well recovered with a reflected-relabeling strategy where the landmark data consists of hemi-form landmarks plus a small number of landmarks from the specimen’s opposite side.

中文翻译：

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偶尔发生反射（横跨中线；在几何形态计量学中）

对大样本进行手动地标数据收集需要很长时间。尽管有每种选择都会带来问题或引起人们的关注，但存在多种选择可以加快该过程。对于双边对称结构（例如，cr骨），一些最新论文建议将界标收集限制在一侧和解剖学中线，然后通过合并半形及其在中线处的反射来近似真实的双边配置。但是，如果中线相对于双侧解剖体较窄，则中线相对于矢状轴中轴或平面的净偏差将使这种“镜面反射”构型变形。在这里，我使用一个具有双边标志性的人类下颌骨的样本来评估这些畸变在实际生物学范围内是否是实质性问题。通过模拟，我在平均下颌骨形状的中线界标处引入了较小的经前外侧误差，然后镜面反射一侧，以便量化和可视化中线误差对镜面反射结果的影响。我还测试了如何忠实地使用镜面反射和其他减少地标的策略来保持以地标的完整，双边互补为特征的观测结果之间的形状和大小关系。在这两种分析中，均表明镜面反射会产生明显的畸变。镜面反射显然不适用于这些数据，并且可能在所有中线形态狭窄的情况下都令人怀疑。