The Ordovician acritarch genus Rhopaliophora: Biostratigraphy, palaeobiogeography and palaeoecology
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), Arkonia–Striatotheca (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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2020, Review of Palaeobotany and PalynologyCitation Excerpt :Accordingly, a critical systematic revision of acritarch taxa is necessary. Many acritarch taxa have already been investigated in detail, for taxonomic revision as well as biostratigraphic and paleogeographic analyses (e.g., Albani et al., 2007; Fatka et al., 1997; Fatka and Brocke, 1999; Li et al., 2014; Moczydłowska and Stockfors, 2004; Navidi-Izad et al., 2020; Servais, 1997; Servais et al., 2007, 2008; Vanguestaine, 2002; Vecoli et al., 1999; Wang et al., 2015; Yan et al., 2010, 2017). The present study represents a continuation of these works on the description and revision of Paleozoic microphytoplankton with the aim to enhance the taxonomic knowledge of acritarchs and their biostratigraphic correlation before conducting paleobiodiversity and macroevolutionary studies.
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2020, Review of Palaeobotany and PalynologyCitation Excerpt :Some of the First Appearance Datums (FADs) of significant acritarch taxa have shown a great potential for the definition of Ordovician stage boundaries and long-distance correlation (Li et al., 2002; Molyneux et al., 2007; Li et al., 2010; Servais et al., 2018). Some Ordovician acritarch taxa with biostratigraphic, but also paleogeographic, significance have been reviewed in recent years, for example, Ampullula (Yan et al., 2010), Arbusculidium (Fatka and Brocke, 1999), Arkonia–Striatotheca (Servais, 1997), Aureotesta (Brocke et al., 1997), Barakella (Yan et al., 2017), Coryphidium (Servais et al., 2008), Dactylofusa velifera (Wang et al., 2015), Dicrodiacrodium (Servais et al., 1996), Frankea (Servais, 1993; Fatka et al., 1997), Pachysphaeridium (Ribecai and Tongiorgi, 1999), Peteinosphaeridium, Liliosphaeridium and Cycloposphaeridium (Playford et al., 1995), Rhopaliophora (Li et al., 2014), Sacculidium (Ribecai et al., 2002) and Veryhachium (Servais et al., 2007). These revisions allow precise correlations that challenge graptolite and chitinozoan biostratigraphy (Servais et al., 2018) and provide detailed biostratigraphic information from strata where other fossils are absent (e.g., Kroeck et al., 2019).