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.

腕棒（crura）是小嘴贝型腕足动物（rhynchonellide brachiopods）总担（lophophore）的钙质支持结构，长期以来被作为小嘴贝亚目高阶分类的依据，并用于揭示其内部的主要演化支系。目前已报道17种腕棒类型，并根据整体结构与横截面形态的差异划分为4个类群，但相关研究尚未以定量或全面的方式开展评估。异时发育（heterochrony）假说认为其在部分类型的演化过渡中发挥作用，但目前尚未建立可用于验证这些假说的结构、发育及系统发育研究框架。本研究采用三维几何形态测量学（three-dimensional geometric morphometric techniques）技术，对现生成年小嘴贝亚目物种全部6种腕棒形态型（crural morphs）的形态差异进行量化；研究旨在确立更客观的腕棒形态型识别与对比标准，最终验证解释个体发育与系统发育过程中形态转变的相关假说。研究人员通过X射线显微计算机断层扫描（X-ray computed microtomography，microCT）对7种现生小嘴贝亚目物种的腕棒进行成像，采用标记点（landmarks）与半标记点（semi-landmarks）定义腕棒及其周围铰合区域的尺寸与弯曲度。本研究对普氏叠加标准化（Procrustes-standardized）后的标记点坐标进行主成分分析（principal component analysis, PCA），以验证各腕棒形态型及已命名类群的离散性，并识别腕棒构型中差异最显著的特征。研究结果表明，显微CT成像技术为研究小型结构的形态特征提供了全新途径，而这类特征若采用传统成像技术则难以实现量化分析。尽管研究人员此前预测腕棒形态型在三维形态空间中存在重叠，但主成分分析结果显示，6种腕棒形态型中有5种彼此间存在显著差异。部分（而非全部）此前划定的腕棒类群呈现出形态一致性。本研究为探究腕棒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，进而可实现异时发育假说的验证。