Comparing size of morphospace occupation among extant and cretaceous fossil freshwater mussels using Elliptical Fourier Analysis

Background. Freshwater mussels of superfamily Unionoidea are a diverse group with an evolutionary history of at least 360 million years. Many fossil and modern species exhibit a generally unsculptured, roughly elliptical shell outline. Such morphology results in difficulties when attempting to identify or classify fossil material, as only hard parts are preserved. Several latest-Cretaceous fossil localities over a small geographic area in southwestern North Dakota contain poorly preserved edentulous freshwater mussels that effectively lack all but shell outlines. This paper discusses methods that were used to attempt to determine how many fossil taxa were present in one of these assemblages. Methods. Elliptical Fourier Analysis was performed on two-dimensional shell outlines of both edentulous fossils and modern taxa to create Fourier scores representing outline shape. Morphospace occupation, or the amount of variation within a sample of shell outlines, was calculated using two methods: within-group dispersion and sum of variance. Morphospace occupation was compared among each of the modern taxa (at both generic and specific levels) and fossil assemblage using confidence intervals, ANOVA, and Tukey’s HSD tests. Results. The amount of outline variation within the fossil assemblage tested is more than some modern genera and species and less than others. Morphospace occupation and confidence intervals are defined for the modern taxa (nine genera and 24 species) that were examined. Discussion. Although results were inconclusive, discussion points are presented to drive future research. Methodological improvements are suggested including choice of extant (comparative) genera, ontogeny and size, morphological plasticity, phenotypic convergence, taphonomic deformation, and general geometric morphometrics.