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

Abstract

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.]

Quantifying shell outline variability in extant and fossilLaqueus(Brachiopoda: Terebratulida): are outlines good proxies for long-looped brachidial morphology and can they help us characterize species?

“…From our previous studies (López Carranza and Carlson 2019), we concluded that we can quantitatively and reliably discriminate extant species based on long-loop brachidial morphology. However, delicate internal morphology, including brachidia, is rarely preserved in fossils.…”

“…7), indicating that these two species are more similar in shape than either is to L. vancouveriensis , even though L. erythraeus and L. vancouveriensis are both eastern Pacific species. Although they occur on opposites sides of the Pacific Ocean, L. erythraeus and L. rubellus were collected from similar latitudes and possibly similar temperature regimes (López Carranza and Carlson 2019). The similarity in outlines between L. erythraeus and L. rubellus is also visible in their mean outline shapes, shown in Figure 6.…”

“…Our previous research (López Carranza and Carlson 2019) demonstrated that we can reliably differentiate among species of the genera Laqueus , Terebratalia , and Dallinella through the use of 3D geometric morphometric analysis of internal morphology, focusing on the long-looped brachidia. However, the relationship between valve shape and brachidial shape has not been quantified previously, and it is not clear if the two structures grow simultaneously in an integrated fashion with one another or grow in a more modular fashion, relatively independently of one another.…”

“…The questions we focus on in this study ask whether valve outline alone can clearly distinguish morphological clusters that proxy for biological species in Laqueus , a genus of terebratulide brachiopod. If so, how do valve outline morphometrics compare with long-looped brachidial morphometrics (López Carranza and Carlson 2019) in the delineation of morphological clusters? If we can answer yes to the first question, our confidence in using valve outline morphometrics to delineate biological species, both extant and extinct, will be greatly increased and, fortunately, can be tested further with DNA sequence data from extant individuals (López Carranza and Carlson 2018; N.L.C.…”

“…Quantitative assessments of the differences and similarities between these species have been limited, however. López Carranza and Carlson (2019) tested the validity of species groups within Laqueus , demonstrating that Laqueus species could be reliably distinguished based on 3D geometric morphometrics of brachidial and cardinalia morphology. Yet, because internal structures such as the brachidia are not commonly preserved in fossils (MacKinnon et al 2008), analyses of delicate internal morphology can be less useful for paleontological inference, particularly if only partial individuals are preserved.…”