Rhynchostreon oyster assemblages across the Central European region: Three types of environment and an alternative view in the intraspecific variability of Rhynchostreon suborbiculatum (Lamarck, 1801)

Abstract

The morphological variability in Rhynchostreon suborbiculatum (Lamarck, 1801) is a well-known phenomenon in the Cretaceous palaeobiology. The current study focuses on two morphotypes differentiated by the presence/absence of radial ribs on the apical part of the left valve, while the fusion of palaeoecological and morphological results allows the study of the evolution of R. suborbiculatum under specific environmental conditions. Based on analyses of grain size, concentration of major oxides and oyster taphonomy in five Cenomanian–Turonian oyster-bearing localities, three habitats that differ in substrate, energy level and salinity regime were recognised: the dynamic Cenomanian nearshore environment (environment B), the Turonian hemipelagic environment (environment C) and the Cenomanian–Turonian marginal marine environment influenced by river dynamics (environment A). An analysis of the size distribution and morphotype structures of three palaeopopulations of R. suborbiculatum shows lithofacial dependence in morphotype distribution, indicating substrate granularity and level of environmental energy. The higher prevalence of ribbed morphs in smaller size classes as well as in fine-grained deposits suggests that apical ribbing is an adaptive trait with a stabilising effect, especially during the early growth stages or when colonising low-energy environments with very soft substrates. The distribution of the large size classes, combined with the shell thickness and vaulting, suggest that the compensation provided by the ribs is insufficient in high energy environments, where strong waves, turbulent flow, and higher sedimentation rates are the main selection pressures. Thus, other adaptations, such as body size and massiveness of the shell, become more critical for the survival of individuals.