DETAILED FAUNAL CHANGE

The general pattern of radiolarian faunal change through the diatom interval is a change in the nassellarian:spumellarian ratio; there is an increase in nassellarian shells above the diatom maximum (Fig. F5). In detail, the radiolarian families reacted as follows (Fig. F6):

1. Actinommids increased above the diatom maximum to a point where spumellarians and nassellarians are equal during the latest stage of the event.
2. Higher contents of theoperids in the upper sections of the diatom interval indicate a minor decline in artostrobids.
3. The artostrobids show two maxima: a smaller peak at the base of the diatom event and a second peak above the diatom maximum but in the diatom zone. The peak at the base of the interval probably represents a generally high artostrobid content, mostly represented by D. mongolfieri (E.), which is interrupted by one sample with fewer specimens. The upper peak could also be seen as a sedimentologic effect but is located at the top of a gradual increase of artostrobids during the diatom event.

There is no dramatic change in the radiolarian composition during the Zone RP-15 diatom interval. The major groups, such as coccodiscids and theoperids, display a long-term trend that does not seem to be influenced by the diatom occurrences. It is difficult to interpret these findings. Actinommids comprise the only group that clearly contributes to the composition of the radiolarian assemblage as a reaction to the diatom event and probably represent faunal elements that originated in the upper parts of the water column (cf. Casey, 1993). Actinommids probably show minimum occurrences because of the diatom impact in the uppermost part of the water column. Other radiolarian groups, which are not affected by the diatom event, belong to tropical warm-water assemblages that, especially in the Eocene, were located in deeper bathymetric zones. The artostrobids seem to react in colder and deeper zones of the water column. In the literature, robust forms of this group are interpreted as belonging to cold-water upwelling faunal assemblages (Casey, 1993); therefore, it is of interest to recognize them at both the base and top of the diatom interval. Consequently, the artostrobids together with the diatoms could be interpreted as representatives of a cooling event, but they dominated only for short periods of time.