PUTTING 3D MODELS TO THE TEST: QUANTIFYING THE VARIABILITY OF MINERALIZED LOPHOPHORE SUPPORT STRUCTURES IN EXTANT BRACHIOPODA (NEOARTICULATA)

The calcareous structures that support the lophophore (e.g. crura; loops) are important characters phylogenetically, taxonomically, and functionally. Within Neoarticulata (Brachiopoda), a peramorphic evolutionary transformation from crura to short loops to long loops has been hypothesized, based on the stratigraphic first appearance of groups possessing successively more complex mineralized lophophore supports. Many descriptive studies of these structures have focused on their morphology, ontogeny, and ultrastructure. Nonetheless, a much-needed quantitative framework for studying crura and loop morphology and variability is lacking. This study aims to understand how crura, short loops, and long loops relate to one another ontogenetically or phylogenetically, with respect to transformations in morphology over time. Three-dimensional X-ray Computed Tomography (CT) has become an accessible tool for imaging the small, complex, and fragile internal structures of brachiopods, offering non-destructive, high-resolution 3D reconstructions. We imaged adult and juvenile individuals from 18 species using CT and microCT. Three-dimensional geometric morphometric analysis was performed using computer-generated shell reconstructions to quantify intraspecific and interspecific variability of crura and loops. Landmark schemes were established based on putative homologous points, covering the crura, loop, and cardinal area. Any variation in landmarks due to size, translation, or rotation was first eliminated using a Generalized Procrustes Analysis. Multivariate statistical analyses were used to quantify shape variation among the three mineralized lophophore supports and among ontogenetic sequences. These data, along with phylogenetic hypotheses of Recent brachiopods, provide a foundation for investigating hypotheses of heterochronic change in lophophore supports. Preliminary analyses of shared landmarks in 40 adult individuals from 12 species indicate that the three types of lophophore supports can be clearly differentiated using geometric morphometrics. We predicted that short loops and long loops would be more similar to one another; however, these analyses suggest crura and short loops share more similarities.