Ecophenotypic variation of the Devonian benthic ostracod species Cavellina rhenana Krömmelbein, 1954: A paleoenvironmental proxy for the Ardenne (France–Belgium) and Rheno-Hercynian realm

The morphological variation of the carapace of the ostracod species Cavellina rhenana Krِmmelbein, 1954 is explored here quantitatively. This species existed during the Late Givetian to the earliest Frasnian and it is characteristic of the Rheno-Hercynian area. Five morphological characters and several linear measurements of the carapace were analyzed on 260 complete carapaces coming from three different areas, each of them representing a different paleoenvironment of the Devonian Ardennean platform. The original species description and diagnosis are emended based on this newly observed variability in ornamentation, sulcation, curvature of the ventral margin, marginal flattening and asymmetry of valves. It is established herein that the morphological variation expressed in these five characters is independent of the ontogenetic growth. Morphological variation appears to be correlated with different types of facies, reflecting a distinct depositional environment. A high degree of morphological variation is found in facies of inner platform (semi-restricted) environments, while a low degree of variation is found in facies of outer platform (open-marine) environments. Thus, the degree of morphological variation of C. rhenana is an indicator of the stability of the environment in the neritic realm. While environmentally-cued polymorphism is well documented in Mesozoic to Recent ostracods, our study demonstrates the first Devonian case of ecomorphism and suggests that intraspecific polymorphism of the ostracod carapace can be a useful tool for refining paleoenvironmental reconstructions.