International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

TAPHONOMY AND PALEOECOLOGY OF THE LOWER BADENIAN (MIDDLE MIOCENE) MOLLUSCAN ASSEMBLAGES AT GRUND (LOWER AUSTRIA)

Pages: 117 - 128

Authors: MARTIN ZUSCHIN, MATHIAS HARZHAUSER, OLEG MANDIC

Abstract: The typical sandy, shell-rich deposits of artificial outcrops in the Grund Formation were identified as distinctly allochthonous event beds with channel-structures, sharp erosional bases, and graded bedding. They are interpreted as proximal tempestites and contain a densely packed, polytaxic molluscan assemblage. The faunal composition and taphonomic features of shells indicate that transport occurred from wave- or current-agitated nearshore habitats into a dysaerobic, pelitic, inner shelf environment. This pelitic environment was colonized by a single molluscan species, the chemosymbiotic Thyasira michelottii, which occurs in life position, as confirmed by valve articulation and preservation of the inhalant tube and postero-ventral tunnel networks. In contrast to the depauperate autochthonous fauna, the skeletal concentrations contain a highly diverse molluscan fauna. We identified 130 species from more than 4200 individuals, but two bivalve species, Timoclea marginata and Loripes dentatus, strongly dominate each of the five samples from different shell beds. In contrast, the diversity (measured as species richness and heterogeneity diversity) and the frequency distribution of shell sizes differ strongly between the five shell beds. A regression analysis identifies the diversity of the shell beds as a function of shell sorting. Poorly sorted shell beds have higher species richness than well-sorted shell beds. The diversity in Grund is therefore taphonomically controlled, because sorting of the allochthonous shell beds is determined by their transport history.

Keywords: Miocene, chemosymbiosis, diversity, paleoecology, taphonomy, shell beds, Mollusca

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Volume 55 no. 2 / April 2004

Volume 55 no. 2