Late Frasnian-Early Famennian entomozoacean ostracod assemblages from the Płucki section in the Holy Cross Mountains were studied to establish the effect of the "Kellwasser bio-event" on the planktonic biodiversity and faunal content. The composition of ostracod assemblages changes from a moderately diverse (10 species) Entomoprimitia-Richterina- Nehdentomis-Nandania dominated "background" assemblage characterising a pre-event interval, to an Entomoprimitia-assemblage during the event interval, and finally to a Franklinella-dominated post-event assemblage in the Middle Palmatolepis triangularis conodont Zone. The Frasnian-Famennian extinction caused substantial losses among entomozoacean lineages. In the Płucki section it occurred in two closely spaced steps within the Palmatolepis linguiformis conodont Zone. The first step, at the base of the dark cephalopod limestone (Upper Kellwasser Horizon), reduced the abundance and the species diversity of entomozoaceans to only two Entomoprimitia species. The vacant niche was then filled by the new, immigrant species Entomoprimitia (Entomoprimitia) kayseri which is dominant in the Upper Kellwasser interval. All these species were lost at the second step within the Upper Kellwasser Horizon. The entomozoaceans remained virtually absent during a long time interval between the end-Frasnian crisis and the Middle Pa. triangularis Zone. They reappear as new species from refugia lineages (Franklinella, Nehdentomis) and became widespread, indicating favourable ecological conditions. Some 13 species have been identified and assigned to seven genera. Rabienella? lagowiensis sp. nov. is proposed.

Key words: Ostracoda, Entomozoacea, Frasnian, Famennian, Upper Kellwasser Event, extinction.

Ewa Olempska [olempska@twarda.pan.pl], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL−00−818Warszawa, Poland.