QUANTIFYING LONG LOOP VARIABILITY IN RECENT TEREBRATULIDE BRACHIOPODS AND ITS IMPLICATIONS FOR SPECIES DELIMITATION IN THE FOSSIL RECORD (Invited Presentation)

In brachiopods, long loops—the calcareous supporting structures of the lophophore—are important morphological characters, particularly in phylogenetic, taxonomic, and ontogenetic analyses. Nonetheless, long loop morphology and variability have only been studied using a descriptive, qualitative approach. Given the complex geometry of these structures and their functional significance, a quantitative framework is necessary. In order to better understand species delimitation in Recent terebratulide brachiopods, the objective of this research is to quantify intra- and interspecific variability in long loops through the use of 3D reconstructions and geometric morphometric analyses. We used X-ray computed tomography to scan adult specimens from ten species with a North Pacific biogeographic distribution to create 3D models. Homologous landmarks were placed on the 3D reconstructions of the shells—focusing on the loop and cardinal area—and were analyzed using geometric morphometrics. To eliminate variation due to size, translation, or rotation, a Generalized Procrustes Analysis was implemented. After obtaining Procrustes-fitted coordinates, variation was simplified through a Principal Components Analysis. To determine if size and shape are related to one another, a multivariate regression using centroid size and principal components was calculated. Finally, in order to statistically test shape differences among groups (e.g. different species or localities), a multivariate analysis of variance was performed. Based on preliminary results, it is possible to recognize that extant named species of brachiopods correspond well with geometric morphometric studies of their lophophore support structures; i.e. we are able to differentiate among named species—some with problematic taxonomic histories—and distinct populations using loop and cardinal morphology. Since species boundaries in fossil brachiopods have never been assessed relative to the boundaries that currently delimit the fewer than 500 extant brachiopod species, this study—along with molecular phylogenetic analyses of living terebratulides and shell outline analyses of extant and fossil specimens—will allow us to test whether brachiopod species are delimited in the fossil record in a manner comparable to extant species.