MICROFOSSILS/Acritarchs 421 Figure Colour photomicrographs of acritarchs: (A) Pterospermella (Â1000); (B) Cymatiosphaera (Â1000); (C) Multiplicisphaeridium (Â1000); (D) Helosphaeridium (Â1000); (E) Pulvinosphaeridium (Â500); (F) Visbysphaera (Â1000) macrofossil migration from shallower into deeper shelf areas which caused bioturbation Although it is possible that some monospecific clusters were derived from production in a sporangia, it is probable that others formed aggregates to make up part of the plankton snow, as a mechanism against predation by zooplankton by increasing the speed of sinking from the surface water to the sediment-water interface