Testing Species Assignments in Extant Terebratulide Brachiopods: A Three-dimensional Geometric Morphometric Analysis of Long-Looped Brachidia

Species of terebratulide brachiopods have been largely characterized qualitatively on the basis of morphology. Furthermore, species-level morphological variability has rarely been analyzed within a quantitative framework. The objective of our research is to quantify morphological variation to test the validity of extant named species of terebratulide brachiopods, focusing on the lophophore-supporting structures—the “long loops.” Long loops are the most distinctive and complex morphological feature in terebratellidine brachiopods and are considered to be phylogenetically and taxonomically informative. We studied eight species with problematic species identities in three genera distributed in the North Pacific: Laqueus, Terebratalia, and Dallinella. Given how geometrically complex long loops are, we generated 3D models from computed tomography (CT) scans of specimens of these eight species and analyzed them using 3D geometric morphometrics. Our goal was to determine ranges of variation and to test whether species are clearly distinguishable from one another in morphospace and statistically. Previous studies have suggested that some species might be overly split and are indistinguishable. Our results show that these extant species of terebratellidines can be reliably distinguished on the basis of quantitative loop morphometrics. Using 3D geometric morphometric methods, we demonstrate the utility of CT beyond purely descriptive imaging purposes in testing the morphometric validity of named species. It is crucial to treat species described and named from qualitative morphology as working hypotheses to be tested; many macroevolutionary studies depend upon the accurate assessment of species in order to identify and seek to explain macroevolutionary patterns. Our results provide quantitative documentation of the distinction of these species and thus engender greater confidence in their use to characterize macroevolutionary patterns among extant terebratellidine brachiopods. These methods, however, require further testing in extinct terebratellidines, which only rarely preserve the delicate long loop in three dimensions. In addition, molecular analyses of extant terebratellidines will test the species delimitations supported by the morphometric analyses presented in this study. [Species determination; morphological variability; 3D geometric morphometrics; terebratulide brachiopods; long loops.]