The Ordovician acritarch genus Rhopaliophora: Biostratigraphy, palaeobiogeography and palaeoecology

Rhopaliophora Tappan and Loeblich, 1971 emend. Playford and Martin, 1984 is one of the most commonly recorded Ordovician acritarch genera. Originally described in the early 1970s from Laurentia, it had subsequently been found on most other palaeocontinents with 8 species attributed to the genus. This paper is a revision of the genus based on the analyses of published literature and on observations of large populations of new material from the South Chinese Ordovician. Our review indicates that the following species belong to the genus: Rhopaliophora brevituberculatum (Kjellström, 1971) Martin, 1983, Rhopaliophora florida Yin et al., 1998, Rhopaliophora foliatilis Tappan and Loeblich, 1971 (type species), Rhopaliophora impexa Tappan and Loeblich, 1971, Rhopaliophora mamilliformis Lu, 1987 emend. Tongiorgi et al., 1995, Rhopaliophora membrana Li, 1987, Rhopaliophora palmata (Combaz and Péniguel, 1972) emend. Playford and Martin, 1984, Rhopaliophora pilata (Combaz and Péniguel, 1972) emend. Playford and Martin, 1984. However, we consider that Rhopaliophora granulata Yin, 1995 is a junior synonym of R. pilata, whereas Rhopaliophora reticulata Uutela and Tynni, 1991 is considered as a junior synonym of R. foliatilis. Intraspecific variability is great and the boundaries between individual species are sometimes not clear. At the genus level, Rhopaliophora shows some transitional forms with Peteinosphaeridium Staplin et al., 1965 emend. Playford et al., 1995 and with some species of Pachysphaeridium Burmann, 1970 emend. Ribecai and Tongiorgi, 1999, whereas its relation to the morphologically similar genera Asketopalla Loeblich and Tappan, 1969 emend. Loeblich and Tappan, 1971, Loeblichia Playford and Wicander, 1988, Tenuirica Playford and Wicander, 1988 and Papilliferum Yin, 1994 needs to be clarified. Rhopaliophora first appears, together with Peteinosphaeridium, in the middle Tremadocian (first stage of the Lower Ordovician). After its original description from Laurentia and later from Australia, in low latitude warmer water environments, it has subsequently been found in intermediate latitudes in Baltica and South China, and few findings are also reported from high latitude areas of Gondwana, indicating a pandemic distribution in warm and temperate water masses. In terms of palaeoecology, the genus is rarely found in nearshore palaeoenvironments, but it is a typical indicator of offshore marine habitats, being abundantly present on carbonate shelf platforms.