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ABSTRACT

The stratigraphic distribution of benthonic Foraminifera in a regressive sedimentary sequence ranging from the latest middle Eocene throughout most of the late Eocene was investigated and interpreted paleoecologically. The greater part of the species show distinct ranges within this sequence. The most important can be divided into groups of species which are diagnostic for certain intervals of the section. Ten such groups are distinguished: Gyroidinoides soldanii, Hoeglundina eocaenica, Nuttallides truempyi, Clavulina parisiensis, Vulvulina lacera, Cyclammina acutidorsata, Bulimina midwayensis, Quinqueloculina juleana and Pararotalia audouini groups as well as a group of larger Foraminifera. Based on the combined ranges of these groups five assemblage zones are established. They are interpreted to be mainly paleobathymetric zones. Estimated values for the water depth of the assemblage zones are at around 1,000 m for zone 1, between approximately 1,000 and 600 m for zone 2, between approximately 600 and 150 m for zone 3, between 150 and 30 to 40 m for zone 4, and between 30 to 40 and almost 0 m for zone 5.

The distribution and development of some representatives of the genus Bolivina were studied in particular. Bolivina nobilis, which is present mainly in the lower part of the section, grades into B. gracilis in the upper part. This transition is thought to be ecologically controlled. A similar development is noted in the group of Bolivina antegressa, where three subspecies are recognized. In this case, however, we may be dealing with an evolutionary sequence at least in the earlier part of the sequence, whereas the occurrence of one or the other of the intergrading later forms is probably ecologically controlled.

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