Floral change during the Taghanic Crisis: Spore data from the Middle Devonian of northern and south-eastern Poland

Abstract

Middle Devonian spore successions from northwestern and southeastern Poland show a rapid reduction of taxonomic diversity that occurs close to the boundary between the Middle and Late Givetian. Similar diversity decline was recorded from European Russia and Belarus where it is well marked, and from northern France and Scotland where less taxa are thought to be involved. The assemblages from Poland are assigned to the local ‘Geminospora’ extensa (Ex) and Geminospora aurita (Aur) Biozones. The Ex 3 Sub-biozone, the highest of the three sub-units of the Ex Biozone, is the time equivalent of the Taghanic Crisis interval or most of it. Spore assemblages of the lower part of the Ex 3 Sub-biozone that are recorded from the lithostratigraphic members reflecting the initial transgressive pulse of the T–R cycle, show no diversity reduction. But the aneurophyte spores Rhabdosporites langii became less frequent in that unit while Aneurospora extensa, also belonging to the aneurophytes, flourished. The diversity reduction occurs in the higher part of the Ex 3 Sub-biozone where as many as nine well established species disappear. The impoverished assemblages of the Aur Biozone are dominated by archaeopterid spores and they lack aneurophyte spores. It is supposed that during the Taghanic Crisis, the initial sea level rise caused a serious loss of habitats for aneurophytes — the producers of R. langii. The following, repeated shoreline shifts were the important modifier of the ecosystem pattern and they speeded up the spread of highly competitive archaeopterids. Those shifts were especially extensive in cratonic, low-relief areas in the eastern part of Laurussia. The dryness of climate extinguished a high number of plants, and had a particularly adverse influence on aneurophytes.