STRATEGY

The reliability of biostratigraphic events (biohorizons) is a much-discussed topic (e.g., Gradstein et al., 1985; Hills and Thierstein, 1989; Rio, Fornaciari, et al., 1990; Rio, Raffi, et al., 1990; Bralower et al., 1989). In this paper, an event is considered as reliable when it is easily reproducible among different researchers and when it keeps the same ranking and spacing in different successions (Gradstein et al., 1985). The reliability of a biohorizon is dependent upon

1. The clear taxonomy of species that defines the event (index species),
2. The relative abundance of the species in the proximity of the event that the species defines,
3. The mode of occurrence (subtle, abrupt, etc.) of the change in distribution pattern of the index species, and
4. The preservation potential of the species defining the event.

Therefore, the rank of biostratigraphic reliability of an index species is evaluated on the following basis:

1. Unambiguous taxonomy,
2. Mode of occurrence of the event (= morphology of the event),
3. Consistency of the relative position with respect to the other biohorizons, and
4. Position of the events vs. the chronomagnetostratigraphy.

To this purpose, the presence and abundance of those species that have been proposed as biostratigraphically useful were monitored, establishing their distribution patterns with the quantitative and semiquantitative counting methods outlined below. On the basis of such distribution patterns, the morphology of the various biohorizons was evaluated, thus obtaining the first fundamental information on their reliability. Afterward, the ranking of the various biohorizons was checked within the study area and with respect to other regions. When available, magnetostratigraphy was used for constructing age-depth plots and inferring age estimates to be compared with the ages reported in the literature.