Tectorotularia hexagona (Polychaeta, Serpulidae) co-existing with Rhynchostreon suborbiculatum (Bivalvia, Gryphaeidae) in a river-dominated marginal marine environment of Cenomanian–Turonian age in Slovakia: An example of Late Cretaceous amensalism or competition?

The mutual relationship and co-occurrence of the tube-dwelling polychaete Tectorotularia hexagona and the oyster Rhynchostreon suborbiculatum in an instance of fluctuating marginal marine environment, represented by the Orlové Sandstone in the Western Carpathians (Pieniny Klippen Belt), is recorded. As a consequence of climate change during the Cenomanian and early Turonian, changes over time in key ecological factors (e.g., decrease in freshwater inflow and increase in temperature and water mass salinity) led to critical trophic alterations in the marginal marine system studied. The environmental change from polyhaline/eutrophic to euhaline/lower nutrient conditions led to a reduction in the pioneer palaeopopulation of R. suborbiculatum, and Tectorotularia hexagona and other stenohaline marine recliners took over the ecological niches originally occupied by the oyster. In this respect, the relationship between T. hexagona and R. suborbiculatum may be considered a Late Cretaceous example of competition between two groups of sessile-benthic recliners. In the present work, two different tube morphologies of T. hexagona from Hôrka are described, and the tube microstructure of this species is documented for the first time. In addition, the material from Hôrka is compared with that from the type locality, Essen in Germany, and ‘Hamulus’ hexagonus and Tectorotularia ‘westfalica’ are considered to constitute one and the same species. The wider ecological and systematic context of this serpulid species is discussed as well.