Abstract

Shells and opercula of bithyniid gastropods assigned to Gabbiella are found in high abundance in the Pleistocene upper Burgi and KBS Members of the Koobi Fora Formation, Turkana Basin, northern Kenya. The systematic paleontology of the Turkana Basin Gabbiella is revised herein based on morphological comparison with the opercula of other Recent African bithyniids. The fossils from the upper Burgi and KBS Members are here assigned to Gabbiella roseaMandahl-Barth, 1968, a species not known from the Turkana fossil record before, but extant in this lake today. A sampling and taphonomic bias is identified which influences the relative abundance of Gabbiella shells and opercula, as a mesh size of 0.63 mm or less is necessary to capture all opercula preserved in the sediments. Accordingly, opercula were found to be significantly more abundant than shells, indicating a different preservation potential of shells and opercula, as the calcitic operculum is more robust than the aragonitic shell. In contrast to previous arguments that most shellbeds in the Turkana Basin sequence represent undisturbed life assemblages, a taphonomic bias is clearly evident reducing the fidelity of the Turkana Basin mollusk assemblages.

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