8th International Nannoplankton Association Conference


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Giuliana Villa, Davide Persico:
Morphology of Eprolithus and comparison to Micula decussata

We examine, in great detail, the morphologic and structural features of some species belonging to the Family Polycyclolithaceae, in particular those of Eprolithus floralis, Eprolithus octopetalus, Eprolithus eptapetalus and Micula decussata.

The stratigraphic range of these species is fairly well defined, and there is general agreement on their distribution. Nevertheless, some discrepancies are reported among different authors. In particular, some papers report a first occurrence of Micula decussata in the Early Turonian, while most authors assign it an Early Coniacian age. A taxonomic problem could be the base of this discrepancy, and which could lead to misinterpretation of the chronostratigraphy.

This work is part of a study on the Basal Complex of the Helminthoid Flysch in the N Apennines, which is represented by Cenomanian-Turonian turbidite sandstones. The discovery of forms similar to Micula, but overlapping in range with older species, such as Eprolithus eptapetalus and Corollithion kennedyi, led us to focus on the morphology and structure of the two genera, along with the stratigraphic distribution of these important markers.

The Polycyclolithaceae contain those taxa which possess two element cycles. The number of elements in each cycle varies from three to 12, and is a fundamental taxonomic feature. The side view of Eprolithus in the light-microscope (LM) is reminiscent of Micula decussata, and has a rectangular outline crossed by an X; this is the interference figure that could be erroneously interpreted as M. decussata (Figure 1). Here we make a comparison of the same specimens with the LM and the scanning electron microscope (SEM).

In side view, the SEM images of Eprolithus floralis, Eprolithus octopetalus, Eprolithus eptapetalus show two cycles of elements arranged side by side, to form a double crown that spirals slightly in two opposite directions (Figure 2). It is then evident in the LM that the specimens, which could easily be confused with Micula decussata, are nothing more than side views of Eprolithus in which the X is the result of the side view of four elements under the same focus.

Our analyses allow us to overcome such a misinterpreting attribution, also at the LM the two genera can be recognised using the quartz-plate or phase-contrast. This taxonomic work elucidates the structural features responsible for the optical interference figures that make side views of Eprolithus similar to Micula decussata, which can lead to misinterpretation and the wrong biostratigraphic assignment.

Figure 1: Eprolithus eptapetalus side view, LM. Sample ODP 1050C-21-1-42.
Figure 2: Eprolithus eptapetalus oblique side view, SEM. Sample ODP 1050C-21-1-42.


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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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