Data from: Three-dimensional morphological variability of recent rhynchonellide brachiopod crura

Crura, the calcareous support structures of the lophophore in rhynchonellide brachiopods, have historically been used to justify higher-level rhynchonellide classification and reveal major evolutionary lineages within rhynchonellides. Seventeen crural types have been described and categorized into four groups based on variation in overall structure and cross-sectional shape, but not evaluated in a quantitative or comprehensive manner. Heterochrony has been hypothesized to play a role in the evolutionary transitions among some types, but the structural, developmental, and phylogenetic context for testing these hypotheses has not yet been established. In this study, we use three-dimensional geometric morphometric techniques to quantify morphological disparity among all six crural morphs in Recent adult rhynchonellides, with the goal of delineating more objective criteria for identifying and comparing crural morphs, ultimately to test hypotheses explaining morphological transformations in ontogeny and phylogeny. We imaged the crura of seven Recent rhynchonellide species, using X-ray computed microtomography. We used landmarks and semi-landmarks to define the dimensions and curvature of the crura and the surrounding hinge area. Procrustes-standardized landmark coordinates were analyzed using a principal component analysis to test the discreteness of the individual crural morphs and named groups of morphs, and to identify features that vary most among the crural configurations. Our results demonstrate that microCT imaging techniques provide novel ways to investigate the morphology of small features that may be otherwise impossible to quantify using more conventional imaging techniques. Although we predicted overlap among crural morphs in the 3-D shape space, the principal component analyses suggest that five of the six crural morphs differ distinctly from one another. Some but not all previously designated crural groups appear to exhibit morphological cohesion. This study establishes a quantitative morphological foundation necessary to begin an investigation of the phylogenetic significance of ontogenetic changes in crura, which will allow hypotheses of heterochrony to be tested.

Crura（腕支）是穿孔贝目腕足动物触手冠的钙质支持结构，长期以来被用于佐证穿孔贝目高阶分类体系的合理性，并揭示该类群内部的主要演化支系。迄今已描述17种腕支类型，并依据整体结构与横截面形态的差异将其划分为4类，但尚未经过定量或系统性的评估。学界曾提出异时发育（heterochrony）假说，认为其在部分腕支类型的演化过渡中发挥了作用，但验证这些假说所需的结构、发育及系统发育研究框架尚未建立。本研究采用三维几何形态测量技术，对现生成年穿孔贝目物种中全部6种腕支形态的形态差异进行量化，旨在确立更客观的腕支形态识别与比较标准，最终验证解释个体发育与系统发育中形态转变的相关假说。我们通过X射线显微计算机断层扫描（microCT）对7种现生穿孔贝目物种的腕支进行成像，并使用标记点（landmarks）与半标记点（semi-landmarks）定义腕支及周围铰合区域的形态与曲率。对普氏叠加（Procrustes）标准化后的标记点坐标开展主成分分析，以检验各腕支形态及预定义形态类群的离散性，并识别腕支构型中差异最显著的特征。我们的研究结果表明，X射线显微计算机断层扫描技术为探究那些采用传统成像手段难以量化的小型结构的形态特征提供了全新途径。尽管我们最初曾预测腕支形态在三维形态空间中存在重叠，但主成分分析结果显示，6种腕支形态中有5种彼此间差异显著。此前划定的部分腕支类群（而非全部）表现出形态一致性。本研究为探究腕支个体发育变化的系统发育意义奠定了必要的定量形态学基础，这将使得异时发育假说得以验证。