Sacculidium gen. nov. (Acritarcha), a new representative of the Ordovician Stelomorpha–Tranvikium plexus

Eisenack first described peteinoid acritarchs bearing a membranous, lobate outgrowth (“Auswuchs”) opposing the pylome from Lower to Middle Ordovician marine strata of Öland, Sweden and from Estonian Ordovician erratics. We demonstrate that the ‘lobate outgrowth’ invariably encloses a pseudopylome and represents the principal diagnostic character of the distinctive acritarch genus Sacculidium gen. nov. characterised by a spheroidal, pylomate vesicle bearing partly or entirely laminate processes. Many acritarch species previously assigned to Ammonidium Lister 1970, Baltisphaeridium Eisenack 1958 ex Eisenack 1959, Multiplicisphaeridium Staplin 1961 emend. Lister 1970, and Peteinosphaeridium Staplin et al. 1965, are attributable to Sacculidium gen. nov. [type species Sacculidium macropylum (Eisenack 1959) comb. nov., emend.]. The presence of a membranous, lobate extension is a basis for taxonomic revisions and innovations, as follows: institution of the new genus Sacculidium; new combination, and emendation of Sacculidium acaciaense (Playford and Martin 1984), Sacculidium aduncum (Playford and Martin 1984), S. macropylum (Eisenack 1959), and Sacculidium tenuibarbatum (Eisenack 1976); institution of three new species, Sacculidium eisenackii, Sacculidium inornatum, and Sacculidium peteinoides; informal description of a further taxon (Sacculidium sp. A). In addition, a new species of Tranvikium Tynni 1982 is also established (Tranvikium persculptum). Two other species may be assigned to Sacculidium: Multiplicisphaeridium cacteum Uutela and Tynni 1991, and Ammonidium furtivum Playford and Martin 1984. The geographic and stratigraphic distribution of all species judged to be authentic or likely representatives of Sacculidium are documented. Known occurrences of Sacculidium include Baltica (Baltoscandia and the East European Platform), southern Gondwana (Tunisia, North Africa), and northeastern Gondwana (South China and Australia). Stratigraphically, Sacculidium is restricted to the Lower to Middle Ordovician, in strata equivalent to the Baltic Volkhov and most of the Kunda stages. The morphological characters of Sacculidium are critically compared with analogous features of Tranvikium and of Stelomorpha Yin 1994. Together with Sacculidium, the last named acritarch genera comprise a distinctive, internally consistent plexus, which – based on process morphology – is assumed to have a common origin from a possible peteinoid ancestor. The FADs of Stelomorpha, Tranvikium, and Sacculidium suggest a possible scenario for the phylogenetic history of the entire plexus. Based on comparisons with some modern dinoflagellates, a hypothetical phagotrophic and parasitic life style for species of the Stelomorpha–Tranvikium–Sacculidium plexus is discussed.