The Ordovician acritarch genus Rhopaliophora: Biostratigraphy, palaeobiogeography and palaeoecology

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Highlights

  • We review the Ordovician acritarch genus Rhopaliophora.

  • The complete literature of the genus Rhopaliophora is revised.

  • New material of Rhopaliophora from South China allows a taxonomical revision.

  • The biostratigraphy of Rhopaliophora is constrained.

  • The genus Rhopaliophora is a typical indicator of offshore marine habitats.

Abstract

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.

Introduction

Acritarchs are widely used for biostratigraphical purposes for international correlations. Among the numerous acritarch morphotypes, attributed to individual morphogenera and morphospecies, many are long ranging and widespread, but a few are biostratigraphically very useful and have a great potential for Ordovician stage boundaries and for international correlation (e.g., Li et al., 2002a, Vecoli and Le Hérissé, 2004, Molyneux et al., 2007, Li et al., 2010). Several of the Ordovician acritarch taxa with biostratigrapical and/or palaeogeographical significance have been investigated in detail, including taxonomical revisions, detailed analyses of their biostratigraphical range (including their First Appearance Datum, FAD), and documentation of their palaeogeographical distribution. The Early–Middle Ordovician taxa that have so far been revised in detail include, in alphabetical order: Ampullula (Yan et al., 2010), Arbusculidium (Fatka and Brocke, 1999), ArkoniaStriatotheca (Servais, 1997), Aureotesta (Brocke et al., 1997), Coryphidium (Servais et al., 2008), Dicrodiacrodium (Servais et al., 1996), Frankea (Servais, 1993), Pachysphaeridium (Ribecai and Tongiorgi, 1999), Peteinosphaeridium, Liliosphaerdiium, Cycloposphaeridium (Playford et al., 1995), Sacculidium (Ribecai et al., 2002), and Veryhachium (Servais et al., 2007).

Another important Ordovician acritarch taxon that has not yet been revised is Rhopaliophora. This genus was first described by Tappan and Loeblich (1971) from the Upper Ordovician Eden Formation of Indiana (United States) as representing acritarch morphotypes with ‘club’-shaped processes, i.e., short, stout processes of many shapes and forms, ranging from simple bulbous to pointed ones, to others that have irregularly shaped distal terminations. The genus was emended by Playford and Martin (1984) who investigated Early and Middle Ordovician material from Australia and who added many details to the generic diagnosis. Subsequently, Rhopaliophora became a widely cited acritarch taxon, with a total of eight species attributed to the genus. Intraspecific (and also intrageneric) variability is important and the boundaries between the different species (and even the morphologically similar genera) are not clear. It is therefore important to revise the genus Rhopaliophora and its species, in particular because the genus is very useful for biostratigraphy, palaeobiogeography and palaeoecology.

The aim of this study is to revise the genus Rhopaliophora in detail, and to discuss its relationship to morphologically similar taxa such as Peteinosphaeridium, Asketopalla, Loeblichia and Tenuirica, Papilliferum and some species of Pachysphaeridium. All data from published literature have been examined in addition to an analysis of large numbers of new material from the Yangtze Platform, South China. All occurrences of Rhopaliophora are plotted on a palaeogeographical reconstruction to show its palaeobiogeographical distribution. The biostratigrapical distribution of the genus is reviewed, showing a first appearance in the middle Tremadocian, whereas the palaeoecological distribution is also revised, indicating an almost pandemic distribution.

Section snippets

Previous investigations in South China

In the present study large populations of Rhopaliophora are investigated from the Early and Middle Ordovician of the South Chinese Yangtze Platform. Since the late 1980s different authors described several species of Rhopaliophora from several Ordovician sections in South China.

Lu (1987) was the first to report the genus Rhopaliophora from the Dawan Formation of the Huanghuachang section of Yichang. This section later became the locality of the Global Stratotype Section and Point (GSSP) of the

Historical evolution of the concept of Rhopaliophora

Rhopaliophora was first erected in 1971 during a time when most acritarch morphologies still needed to be described and when taxonomical splitting was common. Tappan and Loeblich (1971) described the genus on the basis of well-preserved assemblages from the Upper Ordovician Eden Formation of Indiana, United States. Rhopaliophora, as other new genera of that period, was described with the aim to better distinguish the different morphologies within “waste basket” genera, such as Baltisphaeridium

Biostratigraphy of Rhopaliophora

The genus Rhopaliophora is recognized to be a very important taxon for Early–Middle Ordovician biostratigraphy. As pointed by Molyneux et al. (2007) Rhopaliophora has the potential to correlate the upper Tremadocian time-slices between the Gondwanan margin, Baltica and Laurentia.

Palaeogeography

Fig. 5 shows the global palaeobiogeographical distribution of Rhopaliophora in the Ordovician plotted on a palaeogeographical reconstructions, while Fig. 6 shows that the individual species of Rhopaliophora have different distribution patterns in the different areas. To date, six species have been described from Baltica, three species have been reported from Laurentia, and six species are found on the Gondwanan margin.

Rhopaliophora has possibly the most diversified distribution in South China,

Conclusion

Rhopaliophora is one of most frequently recorded acritarch genera from the Ordovician. The genus is highly variable and transients exist not only at the intraspecific level, but also with other genera, such as Peteinosphaeridium. The taxonomic revision indicates that eight species belong to the genus: Rhopaliophora brevituberculatum, Rhopaliophora florida, Rhopaliophora foliatilis (type species), Rhopaliophora impexa, Rhopaliophora mamilliformis, Rhopaliophora membrana, Rhopaliophora palmata,

Acknowledgments

We acknowledge the two referees, Claudia Rubinstein (Mendoza, Argentina) and Reed Wicander (Mount Pleasant, Michigan, USA) for useful comments and corrections. This paper results from a research project between the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS) and the palaeontology research team of the UMR 8217 Géosystèmes (Lille1 University-CNRS). Li Jun and Yan Kui are grateful for NSFC research grants 41072001, 41272012, 41221001 and 41290260. This is a

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