Abstract
In contrast to species ofCyprideis, Hemicyprideis, and Neocyprideis, noding in the genusSerrocytheridea Carbonnel, 1982 was up to now an unknown phenomenon. In the “Zone fossilifere a Bryozoaires”, i.e. the middle part of the Middle Pechelbronn Beds (first Rupelian transgression, Ru 1 TVR-cycle) of the Mainz Basin, the development of nodes was observed in connection with a shift from marine to brackish palaeoenvironmental conditions down to estimated salinities between 5 and 7 ‰. This development can be deduced in the Wallau B98-BK5 borehole from the strong reduction in species diversity towards a monospecific assemblage ofHemicyprideis pechelbronnensis (Stchepinsky, 1960), a specific phenotypic shift within this species, visible up-sequence via increasingly stronger ornamentation with admarginal bends and nodes, as well as from indications of other fossil groups. Four morphs could be discriminated which, in connection with their different ecostratigraphical ranges in the borehole profile, point to a pro-gradational parasequence. The studied Wallau section gives an example in which noding helps to recognize offlaps in sedimentary sequences and can be used in sequence stratigraphical analyses.
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