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The paleoecological significance of nerineoid mass accumulations from the Kimmeridgian of the Swiss Jura Mountains

¹ University of Fribourg, Department of Geosciences, Pérolles, Chemin du Musée 6, CH-1700 Fribourg, Switzerland;
² University of Basel, Institute of Geology and Paleontology, Bernoullistrasse 32, CH-4056 Basel, Switzerland;
³ Natural History Museum Basel, Augustinergasse 2, CH-4001 Basel, Switzerland richard.waite{at}unifr.ch

The Kimmeridgian shallow-water carbonates of the Swiss Jura Mountains display stratigraphic levels containing mass accumulations of nerineoidean gastropods. The macro- and microfacies of the rocks with these nerineoidean assemblages suggest that their occurrence was related to water depth, physical energy within the habitat, sedimentation rate, substrate stability, and food supply. Typical nerineoidean shells are high spired and exhibit unique spiral laminae (folds) on the internal walls. Mass accumulations consisting of large, heavy individuals are believed to represent a semisessile, suspension-feeding community that utilized an external food source. During times of enhanced precipitation and run-off from the hinterland, terrestrial organic matter may have been supplied into the shallow sea, thus forming a source of food for the nerineoids. Alternatively, relative sea-level change during transgression may have led to large-scale coastal erosion and flooding of the platform, also potentially leading to a mobilization of organic matter. Size and morphology of the high-spired shells seem to indicate a trend toward maximal external volume. Development of folds within the shell is interpreted as a modification to maintain a small internal volume. The faunal and floral composition of the strata indicates a suspension-rich, high-energy environment with a low-sedimentation rate and a stable substrate. This allowed mass colonization by epifaunal nerineoids, which, in turn, formed a frameworklike structure. The heavy shells acted as a grid, trapping the sediment in between. In this way, a reinforced carbonate accumulation formed.