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.

Rhynchonellida is the stratigraphically oldest and phylogenetically most basal of the extant rhynchonelliform brachiopod orders, yet phylogenetic relationships among rhynchonellides are poorly known. The fourteen named rhynchonellide superfamilies (four of which have extant representatives) were defined primarily on the basis of features of the dorsal cardinalia, particularly crural morphology, but their homology and polarity have not been investigated rigorously. Superfamily monophyly is unclear, as is the evolution of several distinctive rhynchonellide morphological features, such as crura. The purpose of this study is to investigate the phylogenetic relationships among extant rhynchonellide genera using skeletal characters, and to compare the results with the current classification, elucidating the evolution of morphological features in the process. We completed parsimony-based and Bayesian analyses using fifty-eight characters of the interior and exterior of the shell that vary among the nineteen extant genera. Our results are readily interpretable with respect to the classification, and indicate that Hemithiridoidea, Dimerelloidea, and (in some analyses) Pugnacoidea appear to be monophyletic. Species classified in Dimerelloidea and Pugnacoidea, and in certain cases Hemithiridoidea, each form derived subclades that evolve from within a paraphyletic Norelloidea at the base of each subclade. Raduliform crura appear to be the most basal, phylogenetically; five other crural morphologies evolve from the raduliform state. However, morphological characters currently uniting genera in rhynchonellide superfamilies are not clearly diagnostic and exhibit a relatively high degree of homoplasy overall, suggesting that consistency with the classification may be based on a false sense of confidence in rhynchonellide morphology to clearly elucidate evolutionary relationships. Published molecular phylogenetic hypotheses conflict with the morphological topologies, further supporting this possibility. The evolutionary trends among diagnostic characters of Recent rhynchonellides appear to reflect successive juvenilization in adult morphology in several subclades, suggesting that heterochrony may have played an important role in the evolution of the group.