Gondolelloid multielement conodont apparatus (Scythogondolella) from the Lower Triassic of Jiangsu, East China, revealed by high-resolution X-ray microtomography
Introduction
Natural assemblages (bedding plane assemblages) and/or fused clusters preserve original information on the number, morphology and position of elements in the conodont apparatus. Therefore, they display a great role in conodont paleobiology, such as apparatus architecture, functional morphology, suprageneric classification and evolution among conodont taxa (Aldridge et al., 1995; Purnell and Donoghue, 1997, 1998; Tolmacheva and Purnell, 2002; Goudemand et al., 2011; Huang et al., 2019a, 2019b). Despite more than 1000 conodont natural assemblages have been discovered (Purnell and Donoghue, 1997; Huang et al., 2019a, 2019b and their references therein), few were known from Triassic deposits (Goudemand et al., 2012), and even less from the Triassic of China. Huang et al. (2010) for the first time reported three conodont clusters of the genus Nicoraella from the middle Anisian (Pelsonian) Guanling Formation of the Dawazi section, Yunnan. Later on, Huang et al. (2019a, 2019b) exhaustively revealed this gondolelloid multielement conodont apparatus based on another 41 fused conodont clusters, five of which were three-dimensionally reconstructed using synchrotron X-ray microtomography (Huang et al., 2019b). Goudemand et al. (2011) for the first time applied synchrotron microtomography on a partial cluster of Novispathodus from the lowermost Spathian Luolou Formation of the Tsoteng section (Tiandong County, Guangxi) and built a dynamic reconstruction of the conodont feeding apparatus through biomechanical analysis. Later, Goudemand et al. (2012) confirmed and partially revised the reconstruction of Novispathodus apparatus in Orchard (2005) under the additional material. More recently, Zhang et al. (2017) described six partial clusters of Hindeodus parvus from the lowermost Feixianguan Formation of the Shangsi section, Sichuan, China.
Outside China, Triassic conodont natural assemblages have been known from the Induan of Japan (Hindeodus parvus and Hindeodus typicalis, Agematsu et al., 2015, 2017; Clarkina, Takahashi et al., 2019); the Olenekian of Japan (Ellisonia triassica, Koike et al., 2004; Neostrachanognathus tahoensis, Agematsu et al., 2008) and Oman (Neostrachanognathus sp., Agematsu et al., 2008); the Ladinian of Slovenia (Pseudofurnishius murcianus, Ramovš, 1977, 1978; Kolar-Jurkovšek et al., 2018), Switzerland (Neogondolella, Rieber, 1980) and Italy (Metapolygnathus mungoensis, Mietto, 1982). The Ladinian Neogondolella natural assemblages discovered by Rieber (1980) from the Monte San Giorgio of Switzerland were later restudied and proposed as the standard 15-element architectural template for gondolelloid apparatuses by Orchard and Rieber (1999). Given such a template, Orchard (2005) presented multielement reconstructions of 26 Triassic conodont species representing 26 genera within the superfamily Gondolelloidea based on empirical data. However, only Novispathodus and Nicoraella apparatuses have been tested by their corresponding natural assemblages (Huang et al., 2010, 2019a, 2019b; Goudemand et al., 2011, 2012).
So far, the apparatus architecture of the genus Scythogondolella has only been known from Orchard's (2005) hypothesis based on the supposed discrete elements of Scythogondolella mosheri. The genus Scythogondolella was originally erected by Kozur (1989) to include segminiplanate P1 elements with strongly sculptured upturned platform margins that were ornamented by irregular transversal ribs or/and marginal denticles from Dienerian of Induan to Smithian of Olenekian, Early Triassic (Orchard, 2007). The type species S. milleri is the diagnostic species of the latest Smithian and displays an almost worldwide distribution: already recorded from Siberia (Dagis, 1984; Konstantinov et al., 2013), North America (Idaho, Utah, Nevada: Müller, 1956; Clark, 1959; Sweet et al., 1971), western Canada (the British Columbia and the Canadian Arctic: Mosher, 1973; Orchard, 2008; Orchard and Zonneveld, 2009), Svalbard (Nakrem et al., 2008), Afghanistan (Sweet, 1970), Nepal (Fuchs and Mostler, 1969), Japan (Nogami, 1968), Timor (Berry et al., 1984), Spitzbergen (Nakrem et al., 2008), Southern Primorye (Bondarenko et al., 2015), Spiti of India (Krystyn et al., 2007), and China (e.g., Nyalam of Tibet, Maduo of Qinghai, Tewo of Gansu, Chaohu of Anhui, and Longtan and Jurong of Jiangsu: Tian, 1983; Liang et al., 2011; Fang et al., 2015; Li et al., 2019; Qiu et al., 2019; Liu et al., 2020). We herein report four conodont natural assemblages of Scythogondolella milleri from the Lower Qinglong Formation (latest Smithian, Early Triassic) at the Longtan section, Jiangsu, East China, and scan them using high-resolution X-ray microtomography to reveal number, morphology and positional homology of the component elements in these natural assemblages and thus to test multielement reconstruction of the gondolelloid Scythogondolella apparatus previously proposed (Orchard, 2005).
Section snippets
Geological setting
All specimens described in this paper were collected from claystones of the uppermost Lower Qinglong Formation at the Longtan section (32°10′10″N, 109°03′57″E) in the Longtan, Nanjing, Jiangsu (Fig. 1A; Liu et al., 2020). In the Early Triassic, Longtan section was located in a quiet, open-marine basin on the northern margin of the South China Craton (Fig. 1B; Song et al., 2013). Stratigraphically, it straddles the upper Lower Qinglong Formation and the basal Upper Qinglong Formation where the
Materials and methods
Fossil-bearing claystones were split into thin layers, and thus the split bedding surfaces were carefully checked under magnifying lens for fossils. Because all specimens are preserved on the bedding surface or partially buried in the matrix, the details of the natural assemblages cannot be completely observed under binocular or scanning electron microscopes. In order to exhaustively interpret the pattern of elements in the apparatus, these natural assemblages were scanned using nondestructive
Conodont assemblages of Scythogondolella
Four natural assemblages were herein described, of which Specimen GMPKU-P-3351 (Fig. 2) preserved the expected complement of 15 elements like other ozarkodinid apparatuses, including two pairs of P elements, one pair of M elements and nine S elements. Although all elements are somewhat fragmentary, this specimen displays a bilaterally symmetrical or nearly symmetrical array of elements so that it provides the precise number and original arrangement of elements in the apparatus. Elements of the
Conclusions
Four conodont natural assemblages were collected from the Lower Qinglong Formation of the Longtan section (latest Smithian, Early Triassic) of Nanjing, East China and imaged using X-ray microtomography. They revealed the original topological homologies of elements in the apparatus of Scythogondolella, including element morphologies, arrangement, and number of elements in the apparatus. Accordingly, an octomembrate apparatus with 15 elements is presented for Scythogondolella, composed of alate
Acknowledgements
This study was financially supported by National Natural Science Foundation of China (No. 41920104001, 41876124, 41572008, 41972003), the Strategic Priority Research Program of the Chinese Academy of Science (No. XDA14010404, XDB26000000), Ministry of Science and Technology (No. 2016YFC0503301), State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (No. 123107, 143108, 173123, 20182107), and China Geological Survey (No.
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