A Soft-Bodied Endosymbiont in Serpukhovian (Late Mississippian, Carboniferous) Rugose Corals from South China
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TO L O N O G E I L C A A P I ' T A A T L E I I A Bollettino della Società Paleontologica Italiana, 59 (3), 2020, 235-245. Modena C N O A S S. P. I. A soft-bodied endosymbiont in Serpukhovian (late Mississippian, Carboniferous) rugose corals from South China Wei Lin & Hans-Georg Herbig* W. Lin, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, East Beijing Road 39, 210008 Nanjing, China; [email protected] H.-G. Herbig, Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49a, D-50674 Köln, Germany; [email protected] *corresponding author KEY WORDS - Late Palaeozoic, Lower Yangtze Platform, endosymbiosis in corals, skeletal distortion, bioclaustration. ABSTRACT - An endosymbiotic relation between the solitary rugose corals ?Yuanophyllum and ?Dibunophyllum and a large soft-bodied worm-like organism is described from the Hezhou Formation (Serpukhovian), Lower Yangtze Platform, South China. The endosymbiont lived in a U-shaped tube, probably with a horizontally sideways bent base connecting the vertical shafts. A membranous epidermis and relicts of a probable cuticulo-muscular tube are preserved. The corals reacted with skeletal encasement of the infesting organism and irregular, in part dense growth of additional skeletal elements adjacent to it. This is the first description of such an endosymbiotic, parasitic or commensal relation of solitary rugose corals after the Frasnian-Famennian Boundary Event. Similarities between the Devonian association of pleurodictyoform tabulates and the ichnotaxon Hicetes are remarkable. Moreover, the association proves the extraordinarily rare persistence of bioclaustrations in corals after the Hangenberg Event at the Devonian-Carboniferous boundary during the Permo-Carboniferous period of arrested endosymbiont development. INTRODUCTION (Palmer & Wilson, 1988), and provides direct evidence of symbiotic activities. Bioclaustrations have been often Symbiosis, in its wide sense, is defined as the intimate studied and described in the manner of ichnology and association of two organisms “living together.” According can be used as proxies for the health of reef ecosystems to Darrell & Taylor (1993), it ranges from parasitism of the past (Tapanila, 2005; Vinn et al., 2018). through commensalism and amensalism to mutualism, The oldest records of endosymbiosis between macro- but the kind of relationship between the two organisms organisms and corals are from the Late Ordovician, with is already difficult to evaluate in modern, and even more corals either as hosts (Elias, 1986) or as endosymbionts in fossil examples (Smith & Douglas, 1987; Fagerstrom, (Vinn et al., 2016). Abundance and diversity of 1996; Taylor, 2015). If there are great size differences endosymbiosis in corals during the Palaeozoic peaked in between both symbionts, the larger one is called host, the the Middle Devonian. Both dropped dramatically in the smaller one symbiont (Smith & Douglas, 1987). Late Devonian (Darrell & Taylor, 1993; Tapanila, 2005; In spite of the limitations, evidence of symbiotic Zapalski, 2009). In fact, the extreme drop of the coral interactions of organisms through time helps us to better diversity caused by the Kellwasser Event at the Frasnian- understand the evolutionary history of the ecosystem, Famennian boundary also almost wiped out the symbiotic exploitation of ecospace (Bambach et al., 2007), and dependant ecosystem (Tapanila, 2005; Vinn, 2017b) and, formation of specific niches. Palaeozoic corals, as a major as a consequence, the Famennian was almost devoid of group of marine calcareous skeleton producers, dwell endosymbiosis in corals. Hitherto, the youngest record of in a wide range of habitats from reefs to bottom-level bioclaustration was from the late Famennian (“Strunian”) communities (Aretz, 2010). Together with other skeletal tabulate coral Yavorskia (Zapalski et al., 2008). colonial organisms like stromatoporoids and bryozoans, Despite the fact that diversity and abundance of corals they were the most favoured hosts for symbiotic recovered in the Mississippian, bioclaustrations caused organisms. Endosymbiotic interactions between corals by endosymbiotic worms and tentaculitoid tubeworms (Tabulata and Rugosa) and other macro-organisms in the were missing and the ecospace apparently was vacant. Palaeozoic have been frequently reported (e.g., Sokolov, An exception outside of corals is the tentaculitoid 1948; Plusquellec, 1968; Oekentorp, 1969; Oliver, 1983; genus Streptindytes, which infested Pennsylvanian Zapalski et al., 2008; Zapalski, 2009; Stewart et al., chaetetid sponges (Tapanila, 2005; Vinn, 2010, 2016). 2010; Vinn & Wilson, 2015; Vinn et al., 2017; also see The few records for macro-symbiosis of corals in the the reviews by Darrell & Taylor, 1993; Tapanila, 2005; Carboniferous refer to intergrowth or encrustation Vinn, 2017a, b). They are most easily recognised when (Darrell & Taylor, 1993; Tapanila, 2005; see review in organisms caused changes in the growth of the coral Vinn, 2017b). The roles that different taxonomic groups skeleton. In many cases, the organisms settling within a of corals played in the symbiotic associations also living coral were embedded in the skeleton of the host differed. Tabulate corals had been more preferable hosts during growth, with apertures connected to the outside, than rugose corals. In the latter group, records as hosts or were totally overgrown, resulting in internal tubes are rare, even more for solitary taxa (Darrell & Taylor, or cavities. Such process is termed “bioclaustration” 1993; Tapanila, 2005). ISSN 0375-7633 doi:10.4435/BSPI.2020.16 236 Bollettino della Società Paleontologica Italiana, 59 (3), 2020 Two specimens of solitary rugose corals retrieved with calcareous mudstone, topped by a seven meters from the upper Mississippian (lower Carboniferous) from thick dolomite, sometimes being referred as a separate South China bear structures of endosymbiotic origin, lithostratigraphic unit named Laohudong Formation. shortly described by Lin & Herbig (2019). The aim of this The Gaolishan and Hezhou formations yielded relatively contribution is to detail these structures that are extremely abundant rugose corals (Wang et al., 2010; Lin, 2013). scarce in the Carboniferous, and to compare them with The specimens bearing the endosymbiotic structures were older endosymbiotic ichnofossil types. retrieved from the Hezhou Formation. In the Gaolishan Formation, the rugose coral fauna is dominated by solitary corals, including Arachnolasma, GEOLOGICAL SETTING AND LOCALITY Dibunophyllum and Koninckophyllum, associated with a few colony fragments of Siphonodendron pauciradiale During the Carboniferous, the South China block (McCoy, 1844). The coral fauna in the lower and middle was located in the northeastern Palaeotethys near the part of the Hezhou Formation is more diverse than in palaeoequator. Its northeastern part was constituted by the the formation below. It comprises Koninckophyllum, Lower Yangtze Platform, which was bound by the northern Yuanophyllum, Bothrophyllum, Arachnolasma, margin of the Yangtze Land to the south and modern Dibunophyllum, Axophyllum, Siphonophyllia and cf. day Tanlu Fault Zone and Qingling-Dabie Orogen to the Empachyphyllum, with more numerous occurrences north. The Mississippian strata of this region are largely of the colonial coral Lithostrotion. In the upper part of composed of carbonate rocks (mudstones, wackestones, the Hezhou Formation, just below the dolomite, only packstones, dolostones), sandstones, and siltstones two fragments of Aulina rotiformis Smith, 1917 were intercalated with coal-bearing strata (Wang et al., 2013). recovered. The Wangjiacun section (Fig. 1, GPS coordinates: Further corals from the Hezhou Formation were N 31°31’00”, E 117°48’18”) is located close to the described by Xia & Xu (1980) some 20 km north of eastern banks of Chaohu Lake, Yingping County, Anhui Wangjiacun from the Fenghuangshan section, which is Province. Geologically, the section is located near the a parastratotype of the Hezhou Formation (Li & Jiang, northern margin of the Lower Yangtze Platform. Two 1997). lithostratigraphic units can be recognised: the lower part The coral assemblage of the Wangjiacun section in represents the upper part of the Gaolishan Formation, general is typical of the Yuanophyllum Zone of South which mainly consists of calcareous mudstone with a China (Yu, 1931; Wu, 1964; Tan, 1987), which has a thin layer of sandstone (about 20 cm thick) near the top; range from middle Visean to Serpukhovian. Aulina the upper part represents the Hezhou Formation, which rotiformis, which occurs in the top of the section, is a mainly consists of argillaceous limestone intercalated characteristic latest Visean to early Serpukhovian marker Fig. 1 - (color online) Location of the Wangjiacun section east of Chaohu Lake, Anhui Province. W. Lin & H.-G. Herbig - Soft-bodied endosymbiont in Serpukhovian rugose corals 237 in South China (e.g., Poty et al., 2011; Lin et al., 2012), opposed to western Europe, where it enters at the base of the Serpukhovian (Rugose coral Zone RC9; Poty et al., 2006, 2011). However, based on foraminifers, and refining the earlier stratigraphic assignment of Xia & Xu (1980) and Cai & Xi (1985), Sheng et al. (2018) defined the Tarussian Plectomillerella tortula Interval Zone and placed the complete Hezhou Formation into the Serpukhovian. This relies on the occurrence of the foraminifers Plectomillerella tortula (Zeller, 1953) (= Millerella tortula