Oxytoxaceae are prorocentralean rather than peridinialean dinophytes and taxonomic clarification of heterotrophic Oxytoxum lohmannii (≡ "Amphidinium" crassum) by epitypification.

During evolution of Dinophyceae, size reduction of the episome has occurred in several lineages (including unarmoured Amphidiniales and armoured Prorocentrales). One such species is Amphidinium crassum, whose taxonomic identity is elusive though showing morphological similarities with Oxytoxaceae (currently placed in armoured Peridiniales). Plankton samples were taken at the type locality of A. crassum in Kiel Bight (Baltic Sea) in order to establish monoclonal strains. The protist material was examined in detail using light and electron microscopy, and a long (2984 bp) ribosomal RNA sequence gained was part of a taxon sample comprising 206 specimen vouchers and representing the known molecular diversity of Dinophyceae. Cells of A. crassum were ovoid and exhibited a plate pattern po, 4', 1a, 6'', 5c, 4s, 5''', 1''''. In the molecular phylogeny, the species seemed to belong neither to Amphidiniales nor to Peridiniales but to Prorocentrales and clustered with other representatives of Oxytoxaceae. The morphological diversity of Prorocentrales appears thus expanded, and the group may include a number of previously unrecognised representatives unusually having five postcingular and only a single antapical plate. The taxonomic identity of A. crassum is clarified by epitypification, and the species notably exhibits both an apical pore and an additional epithecal pore.