Effects of taphonomic deformation on geometric morphometric analysis of fossils: a case study using the dicynodont Diictodon feliceps (Therapsida, Anomodontia)

Taphonomic deformation, the distortion of fossils as a result of geological processes, poses problems for the use of geometric morphometrics in addressing paleobiological questions. Signal from biological variation, such as ontogenetic trends and sexual dimorphism, may be lost if variation from deformation is too high. Here, we investigate the effects of taphonomic deformation on geometric morphometric analyses of the abundant, well known Permian therapsid Diictodon feliceps. Distorted Diictodon crania can be categorized into seven typical styles of deformation: lateral compression, dorsoventral compression, anteroposterior compression, 'saddle-shape' deformation (localized collapse at cranial mid-length), anterodorsal shear, anteroventral shear, and right/left shear. In simulated morphometric datasets incorporating known 'biological' signals and subjected to uniform shear, deformation was typically the main source of variance but accurate 'biological' information could be recovered in most cases. However, in empirical datasets, not only was deformation the dominant source of variance, but little structure associated with allometry and sexual dimorphism was apparent, suggesting that the more varied deformation styles suffered by actual fossils overprint biological variation. In a principal component analysis of all anomodont therapsids, deformed Diictodon specimens exhibit significant dispersion around the 'true' position of this taxon in morphospace based on undistorted specimens. The overall variance associated with deformation for Anomodontia as a whole is minor, and the major axes of variation in the study sample show a strong phylogenetic signal instead. Although extremely problematic for studying variation in fossil taxa at lower taxonomic levels, the cumulative effects of deformation in this study are shown to be random, and inclusion of deformed specimens in higher-level analyses of morphological disparity are warranted. Mean morphologies of distorted specimens are found to approximate the morphology of undistorted specimens, so we recommend use of species-level means in higher-level analyses when possible.

埋藏变形（Taphonomic deformation）指地质作用引发的化石扭曲现象，这给利用几何形态测量学（geometric morphometrics）解答古生物学相关问题带来了诸多难题。若变形带来的变异程度过高，诸如个体发育趋势、两性异形这类生物变异所携带的信号可能会丢失。本研究针对丰富且广为人知的二叠纪兽孔目（Therapsida）类群二齿兽（Diictodon feliceps），探究了埋藏变形对其几何形态测量分析的影响。变形后的二齿兽头骨可划分为七种典型变形样式：侧向挤压、背腹向挤压、前后向挤压、“鞍状”变形（saddle-shape deformation，即头骨中部局部塌陷）、前背向剪切、前腹向剪切以及左右剪切。在纳入已知“生物”信号并施加均匀剪切变形的模拟形态测量数据集中，变形通常是变异的主要来源，但多数情况下仍可恢复准确的“生物”信息。然而在实际标本数据集中，变形不仅是变异的主导来源，且几乎未呈现出与异速生长、两性异形相关的结构特征，这表明真实化石所经历的更多样化变形样式掩盖了生物变异。对所有缺齿亚目（Anomodontia）兽孔目进行主成分分析后发现，变形的二齿兽标本在形态空间中，围绕基于未变形标本确定的该类群“真实”位置出现了显著弥散。整体而言，缺齿亚类的埋藏变形所带来的总变异幅度较小，研究样本的主要变异轴反而呈现出较强的系统发育信号。尽管在较低分类阶元上开展化石类群的变异研究时，埋藏变形极具挑战性，但本研究证实变形的累积效应为随机效应，因此在更高阶元的形态差异度分析中纳入变形标本是合理的。研究还发现，变形标本的平均形态与未变形标本的形态近似，因此我们建议在可行时，高阶元分析中优先采用物种级平均形态。