Aggregates of the acritarch Dilatisphaera laevigata: faecal pelletization, phytoplankton bloom or defence against phagotrophy?

Abstract. Monospecific aggregates of 2–7 individuals of the Silurian acritarch Dilatisphaera laevigata Lister are described. Each generally consists of a central collection of vesicles surrounded by elongate, radiating processes. Acritarchs could aggregate by chance during sample processing, or they may have formed within a sporangia-like structure, although such structures are considered unlikely for D. laevigata. Analogies with modern algae suggest that the aggregates of D. laevigata could have formed by faecal pelletization in the surface waters, or by coagulation of individuals during phytoplankton blooms. In this latter instance the baculate/spinose vesicle ornament and digitate-like branching of the processes may have increased the chances of adhesion. It is also plausible that the aggregates may represent a morphological defence against predation or parasitic infection.