The Micrhystridium amd Veryhachium complexes (Acritarha: Acanthomorphitae and Polygonomorphitae): a taxonomic reconsideration

The acritarch genera Micryhstridium Deflandre 1937 and Veryhachium Deunff 1954a are re-examined in light of the many revisions in their diagnoses and reassessed in relation to morphologically similar genera. Principal criteria utilized are the number of spines or other processes; their character and situation in one or several planes; whether the spines or other processes are solid or hollow; and, if hollow, whether their cavities are or are not linked to the vesicle interior

The diagnosis of Micrhystridium is revised and the genera Brachiprojectidium, Lecithodinium, Microbaculidinium, Nannobarbophora and Odontothrix, erected by Habib and Knapp 1982 on the basis of scanning electron microscope studies, are reduced to the status of subgenera of Micrhystridium and emended. The statuses of Eomicrhystridium Deflandre 1968 and Lophosphaeridium Timofeyev 1959 ex Downie 1963 are critically discussed. Emendations are proposed to the generic diagnoses of Buedingiisphaeridium Schaarschmidt 1963, Comasphaeridium Staplin, Jansonius and Pocock 1965 and Filisphaeridium Staplin, Jansonius and Pocock 1965. The genus Solisphaeridium Staplin, Jansonius and Pocock 1965 is treated as a junior synonym of Micrhystridium; Adara Fombella 1977, as a junior synonym of Buedingiisphaeridium; Elektoriskos Loeblich Jr. 1970a, Globosphaeridium Moczydlowska 1991 and Heliosphaeridium Moczydlowska 1991, as junior synonyms of Comasphaeridium; Trichosphaeridium Timofeyev 1966 and Asteridium Moczydlowska 1991 as junior synonyms of Filisphaeridium. New names have been formulated for five species which, upon reassignment to Comasphaeridium or Filisphaeridium, became junior homonyms. The diagnoses of Veryhachium and of the related spinose polygonomorph genera Dorsennidium Wicander 1974, Neoveryhachium Cramer 1970, Polygonium Vavrdova 1966 and Striatotheca Burmann 1970 are emended. Goniosphaeridium Eisenack 1969 and Celtiberium Fombella 1977 are treated as junior synonyms of Polygonium and Exilisphaeridium Wicander 1974 as a junior synonym of Dorsennidium Wicander 1974. The new genus Striatostellula is proposed. Polygonomorph genera having broad-based processes are likewise reconsidered. Pulvinosphaeridium Eisenack 1954, and its type species P. pulvinellum, are emended, the new genus Chalaziosphaeridium being proposed to incorporate other species formerly placed into Pulvinosphaeridium. The genera Palacanthus Wicander 1974 and Estiastra Eisenack 1959 are likewise emended, the new genus Barbestiastra being proposed. Rhiptosocherma Loeblich Jr. and Tappan 1978 is considered a junior synonym of Estiastra and Pulvinomorpha Timofeyev 1966 as a junior synonym of Pulvinosphaeridium. The above generic emendations and rejections involve the reassignment to other genera of over 300.