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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.