THE EARLY PALAEOZOIC DIVERSIFICATION OF THE PLANKTON, ITS POSSIBLE CAUSES AND PROBABLE CONSEQUENCES

The phytoplankton is considered a major part of the basis of the modern marine food chain. Today, it is composed of three major groups that diversified in the Mesozoic: the dinoflagellates, coccolithophorids and diatoms. Only the dinoflagellates have a possible Palaeozoic equivalent: the acritarchs, a group that is generally interpreted as representing organic-walled cysts of different algal groups.

Our revision of the Early Palaeozoic literature indicates that after the appearance in the Proterozoic, acritarchs were only moderately diversified during the Early Cambrian. Their diversity continuously increased during the Cambrian, with a significant increase in the Late Cambrian and Early Ordovician, and highest values in the Middle Ordovician, when over 400 species recorded.

It can be assumed that the Early Palaeozoic phytoplankton radiation paralleled a long-term increase in sea level with an accompanying expansion of flooded continental shelf areas, that were at a maximum during the Middle Ordovician. The rapid diversification of the acritarchs in the Late Cambrian was possibly related to the important SPICE δ13Ccarb excursion, that is considered by some authors as an indication of an oxygenation event in the Late Cambrian oceans.

Although we have no data on the pico- and bacterioplankton, and although the biodiversity of the acritarchs cannot be related to bioproductivity, we assume that the increased quantities of phytoplankton in the Late Cambrian and Early Ordovician allowed a synchronous radiation of the zooplankton, including radiolarians, chitinozoans and graptolites, but also the development of planktotrophy and the radiation of benthic suspension feeders that used the phytoplankton as a food source.