THREE-DIMENSIONAL GEOMETRIC MORPHOMETRIC ANALYSES OF THE RECENT TEREBRATULIDINA (BRACHIOPODA, TEREBRATULIDA) LOOP

Within Neoarticulata, the mineralized support structures of the lophophore are important characters phylogenetically, taxonomically, and functionally. A peramorphic evolutionary transformation from crura to short loops to long loops has been hypothesized, based primarily on the stratigraphic first appearance of groups possessing successively more “complex” mineralized lophophore supports. Three pieces of information are required to test this hypothesis: shape change relative to size change; ontogenetic age; and phylogenetic relationship. Recent morphological and molecular phylogenetic analyses both support this peramorphic pattern; however, a comprehensive, quantitative framework for studying crura and loop morphology over time is still lacking. We previously quantified 3D morphologic variability of the crura of Recent Rhynchonellida adult and juvenile individuals. Here we evaluate the short loops of five Recent terebratulidine species ontogenetically and phylogenetically, using geometric morphometric techniques on 3D models generated from microCT scans, to establish the relationship between loop size and shape over ontogeny. Three-dimensional Cartesian coordinates were collected for nine homologous landmarks which, along with semi-landmarks, define the dimensions and curvature of the loop and the surrounding hinge area. We then analyzed the coordinate data using a Generalized Procrustes Analysis and multivariate statistical analyses. Due to similarities in loop morphology, we predicted significant overlap in morphospace of all adult individuals. However, preliminary multivariate statistical analyses suggest that conspecific adult individuals cluster together, but separate from adults of different species. Additionally, a distinct ontogenetic trend is exhibited from the smallest to largest juvenile to adult of each species. Given our hypothesis of a peramorphic pattern among mineralized lophophore support structures, we further predict that juvenile short loops will be morphometrically similar to adult crura types. Quantitatively documenting the relationship between loop size and shape over ontogeny and phylogeny will allow us to test hypotheses of heterochrony in the evolution of the mineralized lophophore support structures of Neoarticulata.