Junjun Song

Sylvie Crasquin
email: sylvie.crasquin@upmc.fr

Yiming Gong
email: ymgong@cug.edu.cn

Late Devonian benthic ostracods from the western Junggar, NW China: Implications for palaeoenvironmental reconstruction

Keywords: Ostracods, palaeoecology, palaeobiogeography, Late Devonian, western Junggar

This study focuses on palaeoecology and palaeobiogeography of Late Devonian ostracods based on more than 4500 carapaces collected from sections in the western Junggar, NW China. According to the ostracod faunas from the Hongguleleng Formation, three ecological assemblages for an open oceanic island arc are proposed, i.e., OA1 (Ostracod Assemblage-1), OA2 (Ostracod Assemblage-2) and OA3 (Ostracod Assemblage-3). They characterize foreshore, nearshore and offshore, respectively. Water energy should be the first environmental factor controlling the composition of the ostracod assemblages from the Lower Member of the Hongguleleng Formation, and salinity fluctuations could be also involved. Many cosmopolitan species were present in the diversified ostracod faunas of the Hongguleleng Formation, suggesting possible biogeographic relationships between the Kazakhstan Plate and South China, North America and Russian plates. The ostracods from Late Devonian of the Kazakhstan Plate might migrate to the South China Block and Laurentia-Baltica by ocean currents and sea-level changes, which also implies that neritic faunal exchanges were possible along the Palaeo-Tethys during the Late Devonian.

Introduction

Ostracoda are microcrustacea appeared in the Early Ordovician and still developed today [START_REF] Horne | Taxonomy, morphology and biology of Quarernary and living Ostracoda[END_REF][START_REF] Siveter | Ostracods in the Palaeozoic?[END_REF]. They are very sensitive to the ambient environment conditions and variations, such as salinity, bathymetry, temperature, hydrodynamics, oxygenation and nutrients supply, etc. [START_REF] Casier | Middle Frasnian (Devonian) ostracods from the Frasnes railway section (Dinant Synclinorium, Belgium): taxonomy, biostratigraphy, paleoecology[END_REF][START_REF] Olempska | Hydrothermal vent myodocopid ostracods from the Eifelian (Middle Devonian) of southern Morocco[END_REF][START_REF] Jones | Latest Devonian (Strunian) Ostracoda from the Buttons Formation, Bonaparte Basin, Northwestern Australia: Biostratigraphy, Palaeoecology and Palaeozoogeography[END_REF][START_REF] Racheboeuf | New data on the Silurian-Devonian palaeontology and biostratigraphy of Bolivia[END_REF][START_REF] Maillet | Ecophenotypic variation of the Devonian benthic ostracod species Cavellina rhenana Krömmelbein, 1954: A paleoenvironmental proxy for the Ardenne (France-Belgium) and Rheno-Hercynian realm[END_REF]. Ostracods may offer special insights into the characteristics and evolution of the Devonian global bio-environments (Lethiers, 1983;[START_REF] Lethiers | Les crises du Dévonien supérieur par l'étude des faunes d'ostracodes dans leur cadre paléogéographique[END_REF][START_REF] Becker | Devonian ostracods from western Canada-palaeozoogeographic implications[END_REF]Song and Gong, 2017).

Until recently, most studies on palaeoecology of the Devonian ostracods have been conducted on continental margins or in epicontinental basins, primarily in Europe and North America (e.g., [START_REF] Becker | Palaeocopida (Ostracoda) aus dem Mitteldevon der Sötenicher Mulde (N-Eifel)[END_REF][START_REF] Becker | Podocopida (Ostracoda) aus dem Mitteldevon der Sötenicher Mulde (N-Eifel)[END_REF][START_REF] Becker | Paleoecology of Middle Devonian ostracods from the Eifel region, Germany[END_REF][START_REF] Becker | Progress in mid Palaeozoic palaeoceanographical studies from Ostracoda-from local to global importance (a review)[END_REF][START_REF] Bandel | Ostracoden aus paläozoischen pelagischen Kalken der Karnischen Alpen(Silurium bis Unterkarbon)[END_REF][START_REF] Becker | Cypridinellidae (Ostracoda) aus dem Oberdevon Hessens (Unterer Kellwasser-Kalk; Lahn-Dill-Gebiet und ostliehes Sauerland, Rechtrheinisches Schiefergebirge)[END_REF][START_REF] Casier | The mode of life of Devonian entomozoacean ostracods and the MyodocopidMega-Assemblage proxy for hypoxic events[END_REF][START_REF] Casier | Guide de l'excursion: Les ostracodes du Dévonien Moyen et Supérieur du Synclinorium de Dinant[END_REF][START_REF] Casier | Ecology of Devonian ostracods: application to the Frasnian/Famennian boundary bioevent in the type region (Dinant Synclinorium, Belgium)[END_REF][START_REF] Maillet | Ecophenotypic variation of the Devonian benthic ostracod species Cavellina rhenana Krömmelbein, 1954: A paleoenvironmental proxy for the Ardenne (France-Belgium) and Rheno-Hercynian realm[END_REF], as well as South China [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF][START_REF] Wang | Palaeozoic Entomozoacea and Leperditicopida (ostracoda). Fossil Ostracoda of China[END_REF]. [START_REF] Casier | The mode of life of Devonian entomozoacean ostracods and the MyodocopidMega-Assemblage proxy for hypoxic events[END_REF][START_REF] Casier | Guide de l'excursion: Les ostracodes du Dévonien Moyen et Supérieur du Synclinorium de Dinant[END_REF] proposed a new ostracod assemblage denomination, i.e. Eifelian Mega-Assemblage (=Assemblages 0-III), Thüringian Mega-Assemblage (=Assemblage IV) and Myodocopid Mega-Assemblage (=Assemblage V), which express the palaeoenvironments from the nearshore to deep basins. Similarly, five ostracod associations were recognized in the Palaeozoic strata of South China [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF][START_REF] Wang | Palaeozoic Entomozoacea and Leperditicopida (ostracoda). Fossil Ostracoda of China[END_REF] i.e., the leperditiid, palaeocopid, smooth-podocopid, spinose-podocopid and entomozoacean associations. In contrast, the palaeoecology of ostracods in an open oceanic setting far from continental-derived sediments remain poorly known. In this paper, all the samples of ostracods were collected from the western Junggar, NW China, featured by the archipelago during the Palaeozoic [START_REF] Gong | Paleozoic stratigraphic regionalization and paleogeographic evolution in western Junggar, Northwestern China[END_REF].

Studies of sedimentary petrology and trace element geochemistry indicate that the Late Devonian Hongguleleng Formation was deposited with intermittent periods of volcanic activities [START_REF] Carmichael | A new model for the Kellwasser Anoxia Events (Late Devonian): Shallow water anoxia in an open oceanic setting in the Central Asian Orogenic Belt[END_REF][START_REF] Carmichael | Climate instability and tipping points in the Late Devonian: Detection of the Hangenberg Event in an open oceanic island arc in the Central Asian Orogenic Belt[END_REF][START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF].

Thus, the main aim of this study is to assess the influence of palaeoenvironmental changes in the open oceanic island arc on the distribution and composition of ostracod assemblage, and discuss their special significance in palaeogeography of Palaeo-Tethys during the Late Devonian.

Geological setting and section descriptions

The western Junggar region (WJR) is located in the northwest of Xinjiang Uygur Autonomous Region (referred as Xinjiang), NW China, where the Kazakhstan, Siberia and Tarim plates join. WJR is an important component of the Central Asian Orogenic Belt (CAOB) during the Palaeozoic [START_REF] Cocks | Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic[END_REF][START_REF] Windley | Tectonic models for accretion of the Central Asian Orogenic Belt[END_REF][START_REF] Gong | Paleozoic stratigraphic regionalization and paleogeographic evolution in western Junggar, Northwestern China[END_REF]. The study area belongs to the Late Devonian volcanic arc setting [START_REF] Xiao | A review of the western part of the Altaids: a key to understanding the architecture of accretionary orogens[END_REF], with widespread Palaeozoic sedimentary rocks and various types of volcanic produces (Fig. 1). Mostly Late Devonian Hongguleleng Formation were deposited in the back-arc basin of the Zharma-Saur arc in the Shaerbuerti Mountains (Fig. 1), where yielded abundant and diversified faunas (e.g., brachiopods, bryozoans, conodonts, trilobites, plants, etc.) [START_REF] Xu | The Hongguleleng Formation in Western Junggar and the boundary between Devonian and Carboniferous[END_REF]Xia,1996;[START_REF] Windley | Tectonic models for accretion of the Central Asian Orogenic Belt[END_REF][START_REF] Chen | Late Devonian (latest Frasnian-Famennian) faunas from the "Hongguleleng Formation" and the F-F boundary in northern Xinjiang, NW China[END_REF][START_REF] Suttner | Stratigraphy and facies development of the marine Late Devonian near the Boulongour Reservoir, northwest Xinjiang, China[END_REF][START_REF] Wang | Conodont and carbon isotope stratigraphy near the Frasnian/Famennian (Devonian) boundary at Wulankeshun, Junggar Basin, NW China[END_REF] and trace fossils [START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF].

In our study, fieldworks had been carried out in four sections, namely the Wulankeshun, Bulongour, Gennaren and Emuha sections (Fig. 2A). The Late Devonian Hongguleleng Formation crops out well with abundant fossils in the first three sections, which have similar lithology, especially in the Wulankeshun and Bulongour sections (Fig. 2). Unfortunately, most of the Upper Devonian is missing in the Emuha section [START_REF] Song | Ostracods from the Emuha section of Devonian-Carboniferous transition in Western Junggar[END_REF]. Therefore, this paper focused on the ostracods collected from the Hongguleleng Formation in the Wulankeshun and Bulongour sections, and the results of systematic palaeontology has already been published (Song et al., 2017).

The Wulankeshun section (N 46°46′05.12″; E 84°54′43.98″, hereinafter referred as WLS) is located approximately 65 km west of Hebukesar Mongol Autonomous County (Figs 1, 2). It exhibits continuous outcrop from the Late Devonian to Early Carboniferous [START_REF] Wang | Late Devonian Conodonts and Chondrichthyes from Taketai Formationin Western Junggar, NW China[END_REF]: series of the Zhulumute, Hongguleleng to Heishantou formations without major tectonic deformation (Fig. 2). The Hongguleleng Formation (372.4 m thick) could be divided into three members: the Lower Member is made of grey, grey-yellow bioclastic limestones; the Middle Member consists of variegated fine-grained volcaniclastic rocks and the Upper Member is composed of grey-green, grey yellow bioclastic limestones and calcareous-pelitic clastic rocks (Fig. 2).

The Bulongour section (N 46°45′10.26″; E 86°08′20.70″, hereinafter referred as BLS) is located in the north of Hoxtolgay, about 1.5 km northwest of the Bulongour Reservoir (Figs 1, 2), and about 90 km east to the WLS. This is the type section of the Hongguleleng Formation and has been extensively studied previously (e.g., [START_REF] Xu | The Hongguleleng Formation in Western Junggar and the boundary between Devonian and Carboniferous[END_REF][START_REF] Chen | Late Devonian (latest Frasnian-Famennian) faunas from the "Hongguleleng Formation" and the F-F boundary in northern Xinjiang, NW China[END_REF][START_REF] Suttner | Stratigraphy and facies development of the marine Late Devonian near the Boulongour Reservoir, northwest Xinjiang, China[END_REF]. In this study, the sampling is concentrated the Lower Member of the Hongguleleng Formation in both sections for well-preserved ostracods fossils in the limestones.

Extensive works on the Hongguleleng Formation had been carried out by our research group forehand, especially in the WLS for conodont biostratigraphy [START_REF] Wang | Conodont and carbon isotope stratigraphy near the Frasnian/Famennian (Devonian) boundary at Wulankeshun, Junggar Basin, NW China[END_REF], sedimentary [START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF] and geochemistry [START_REF] Wang | Late Devonian Conodont Biostratigraphy and Carbon and Oxygen Isotopic Composition in Western Junggar, NW China[END_REF]. In this paper, the palaeoenvironment of the Lower Member of the Hongguleleng Formation summarized the results of [START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF] and [START_REF] Wang | Late Devonian Conodont Biostratigraphy and Carbon and Oxygen Isotopic Composition in Western Junggar, NW China[END_REF]. [START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF] gave an overview on sedimentary structure and trace fossils from both WLS and BLS, [START_REF] Wang | Late Devonian Conodont Biostratigraphy and Carbon and Oxygen Isotopic Composition in Western Junggar, NW China[END_REF] studied the conodont biofacies and microfacies from WLS. Totally, four palaeoenvironments could be recognized from the bottom to top in the Lower Member of the Hongguleleng Formation, i.e., foreshore (bed 18 of WLS and bed 1A of BLS), nearshore (bed 19 and beds 22-23 of WLS, bed 1B and bed 3 of BLS) and offshore (beds 20-21 of WLS and bed 2 of BLS).

Materials

213 samples were collected by our working group from the Wulankeshun, Bulongour and Gennaren sections from 2011 to 2016. More than 4500 carapaces, valves and fragments of ostracods were extracted from limestones by hot-acetolysis [START_REF] Lethiers | Comment extraire des microfossiles à tests calcitiques de roches calcaires dures[END_REF][START_REF] Crasquin-Soleau | Ostracods as markers of the Permian/Triassic boundary in the Khuff Formation of Saudi Arabia[END_REF]. Among them, about 2500 and 1900 specimens were obtained from the BLS and WLS, respectively. 53 species belonging to 31 genera were recognized: 45 species belonging to 29 genera from the BLS and 49 species belonging to 28 genera from the WLS (Song et al., 2017). The ostracod faunas from all the Lower Member of the Hongguleleng Formation are dominated by Metacopina (i.e., Bairdiocypridoidea, Healdioidea and Quasillitoidea), Podocopida (i.e., Bairdioidea), Palaeocopida (e.g., Hollinoidea, Primitiopsoidea and Paraparchitoidea), Platycopida (Kloedenelloidea) and Myodocopida (Entomozoidea) (Song et al., 2017; Table 1).

Beyrichioidea 0 0 1 0 0 0 0 0 Hollinoidea 5 4 2 4 6 4 5 3 Primitiopsoidea 1 1 1 1 1 1 1 1 Kirkbyoidea 0 1 1 1 1 0 1 0 Cryptophyllidae 1 0 0 0 0 0 0 0 Paraparchitoidea 2 2 2 3 2 1 1 1 Youngielloidea 1 1 1 0 1 1 1 1 Platycopida Kloedenelloidea 3 4 2 2 2 3 3 2 Podocopida Bairdioidea 1 11 5 1 10 10 8 6 Metacopina Bairdiocypridoidea 3 7 10 1 8 8 9 7 Quasillitoidea 0 0 1 1 0 0 0 0 Healdioidea 2 4 3 2 3 4 3 2 Myodocopida Entomozoidea 0 1 0 0 0 1 1 0

Ecology of ostracod assemblages from the Hongguleleng Formation

Ecological assemblages of ostracods

The ostracod assemblages from the Lower Member of the Hongguleleng Formation vary with environments, and three ecological assemblages of ostracods could be defined as follows (Figs. 3,4).

OA1 (Ostracod Assemblage-1) is located in the foreshore (Fig. 4), characterized by ostracods from the bed 18 of the WLS and the bed 1A of the BLS. The ostracod fauna is rather rare with few species (18 species in the WLS; 15 species in the BLS) were discovered in mostly small number. The assemblage is dominated by Hollinidea Swartz, 1936, including of two families, Hollinidae and Pribylitidae. Specifically, the Palaeocopida, i.e., Hollinoidea (Hollinella Coryell, 1928, Parabolbinella Adamczak, 1968, Pribylites Pokorný, 1951), Paraparchitidae (Guerichiella Adamczak, 1968),

Primitiopsidae (Paraparchites Ulrich & Bassler, 1906, Samarella Polenova, 1952), Amphissitidae (Amphissites Girty, 1910), Youngiellidae (Moorites Coryell & Billings, 1932) and Cryptophyllidae (Cryptophyllus Levinson, 1951) show a significant advantage on OA1 and represent more than 50% and 70% of total number of species in the WLS and BLS, respectively. Among them, the first five families/superfamilies were occurred in two sections, while Crytophyllidae only appeared in the BLS.

Besides the domination superfamily Hollinoidea (about 34% of total number of species), Paraparchitidae shows relatively high abundance and diversity (about 15% of total number of species On the whole, OA1 is dominated by Palaeocopida without any Bairdioidea, which similar to the 'Eifelian Mega-Assemblage-1' from Europe and North America (e.g., [START_REF] Becker | Paleoecology of Middle Devonian ostracods from the Eifel region, Germany[END_REF][START_REF] Becker | Progress in mid Palaeozoic palaeoceanographical studies from Ostracoda-from local to global importance (a review)[END_REF][START_REF] Bandel | Ostracoden aus paläozoischen pelagischen Kalken der Karnischen Alpen(Silurium bis Unterkarbon)[END_REF][START_REF] Casier | The mode of life of Devonian entomozoacean ostracods and the MyodocopidMega-Assemblage proxy for hypoxic events[END_REF][START_REF] Casier | Guide de l'excursion: Les ostracodes du Dévonien Moyen et Supérieur du Synclinorium de Dinant[END_REF] and the 'Palaeocopid association' from South China [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF][START_REF] Wang | Palaeozoic Entomozoacea and Leperditicopida (ostracoda). Fossil Ostracoda of China[END_REF] Although the smooth-podocopids dominated the ostracods of OA2, the percentage of Bairdioidea (about 40% of podocopid species) is much smaller than that in 'smooth-podocopid association' from South China [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF]) (Table 2), which is 80%-90% of total numbers of species of podocopids. Moreover, the domination taxa of OA2, some genera of Bairdiocyprididae, e.g., Praepilatina Polenova, 1970

and Pseudobythocypris Shaver, 1958 could appeared in deep water along with 'spinose-podocopids association' in South China [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF] and Thüringian Mega-Assemblage in Europe and North America [START_REF] Becker | Progress in mid Palaeozoic palaeoceanographical studies from Ostracoda-from local to global importance (a review)[END_REF][START_REF] Casier | The mode of life of Devonian entomozoacean ostracods and the MyodocopidMega-Assemblage proxy for hypoxic events[END_REF] more frequently than in the shallow-water environment as the Lower Member of the Hongguleleng Formation.

OA3 (Ostracod Assemblage-3) is located in the offshore (Figs. 3,4), marked by ostracods from the beds 20-21 of the WLS and the bed 2 of the BLS. The ostracods of OA3 were discovered in relatively large number in both two sections, especially the WLS, where 43 species belong to 26 genera were recognized. The ostracod faunas from OA3 are dominated by smooth Podocopida and Metacopina (about 60%-65% of total number of species), including of Bairdioidea (Bairdia McCoy, 1844, Fabalicypris Cooper, 1946, Acratia Delo, 1930and Famenella Polenova, 1953), Bairdiocyprididae (Bairdiocypris Kegel, 1932, Healdiacypris Bradfield, 1935, Praepilatina Polenova, 1970and Pseudobythocypris Shaver, 1958) and Healdioidae (Bairdiocypris Kegel, 1932, Praepilatina Polenova, 1970and Pseudobythocypris Shaver, 1958) with high diversity and abundance, as well as a few of Bufinidae (i.e., Aurigerites Roundy, 1926). Among them, the Bairdioidea presented in a large number that more than 60% of total number of podocopids species. Compared with the ostracods of OA2, the Bairdioidea is more abundant in OA3, while Bairdiocypridoidea is lower in abundance. Palaeocopida make up 30% of total number of species, which consists of Hollinoidea, Paraparchitidae, Primitiopsidae, Amphissitidae, Cryptophyllidae and Youngiellidae. Hollinoidea is still dominating in the Palaeocopida of OA3 as that in OA1, including of four genera (Hollinella Coryell, 1928, Parabolbinella Adamczak, 1968, Pribylites Pokorný, 1951and Parasargentina Zheng, 1982). Platycopida (including 3 genera, Sargentina Coryell & Johnson, 1939, Quasiknoxiella Tschizova, 1977and Knoxiella Egorov, 1950) and Myodocopida are rare in the OA3, just 5%-10% and 5% of total number of species.

Overall, the ostracod faunas of OA3 are characterized by Bairdioidea and Bairdiocyprididae with large-sized (length of carapaces is more than 1.40 mm; Beck, 1971) and smooth shell, which suggest to be benthic. This is a little similar to the 'Eifelian Mega-Assemblage-2-3' [START_REF] Casier | The mode of life of Devonian entomozoacean ostracods and the MyodocopidMega-Assemblage proxy for hypoxic events[END_REF][START_REF] Casier | Guide de l'excursion: Les ostracodes du Dévonien Moyen et Supérieur du Synclinorium de Dinant[END_REF]) and the 'smooth-podocopid association' [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF][START_REF] Wang | Palaeozoic Entomozoacea and Leperditicopida (ostracoda). Fossil Ostracoda of China[END_REF]) (Table 2). Nevertheless, what is noticeable is that some Entomozoidea were realized with relatively high abundance in the OA3 from both WLS and BLS, though only one species (Franklinella latesulcata (Paeckelmann), 1921; Fig. 5). Moreover, more species of Entomozoidea (2 species belonging to 2 genera; Fig. 5) were discovery in relatively large number from the same environment in the Gennaren section, which is not far away from BLS. Previous studies have indicated that entomozoaceans (pelagic 'finger-print' ostracods) prefer swimming in low-energy deeper waters (Bandel and Beker, 1975;[START_REF] Casier | Les ostracodes au passage Eifelien/Givetien a Glageon (Avesnois, France)[END_REF][START_REF] Becker | Cypridinellidae (Ostracoda) aus dem Oberdevon Hessens (Unterer Kellwasser-Kalk; Lahn-Dill-Gebiet und ostliehes Sauerland, Rechtrheinisches Schiefergebirge)[END_REF][START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF][START_REF] Lethiers | The uppermost Frasnian (Upper Kellwasser) ostracodes from Coumiac (Montagne Noire, France)[END_REF][START_REF] Olempska | The Late Devonian Upper Kellwasser Event and entomozoacean ostracods in the Holy Cross Mountains, Poland[END_REF][START_REF] Casier | The mode of life of Devonian entomozoacean ostracods and the MyodocopidMega-Assemblage proxy for hypoxic events[END_REF][START_REF] Casier | Guide de l'excursion: Les ostracodes du Dévonien Moyen et Supérieur du Synclinorium de Dinant[END_REF], reflecting an environment of deep-water basin in general [START_REF] Wang | Late Paleozoic Ostracode Associations from South China and Their Paleoecological Significance[END_REF]. As for biocoenoses, mixtures between the Eifelian and the Entomozoacean assemblages had never been observed in Europe or South China [START_REF] Becker | Biotope indicative features in Palaeozoic ostracods: a global phenomenon[END_REF]. Villozona Gründel, 1965 appeared in OA3 from the WLS (Fig. 5), which was only known from basinal environments (Thuringia, Morocco, South China) previously [START_REF] Gründel | Zur Ausbildung und taxionomischen Bedeutung der Narben der zentralen Muskelgruppe in der Unterklasse Ostracoda[END_REF][START_REF] Becker | Progress in mid Palaeozoic palaeoceanographical studies from Ostracoda-from local to global importance (a review)[END_REF], and never reported in shallow marine yet.

There may be two interpretation to explain the uncommon melange. One is that some species of Entomozoidea and Villozona might be able to expand to the shallow water from slop or deep basin under particular environment. The other is that the complicated back-arc basin environment with short shelf and sharp slope may offer possibility to the mixtures of ostracods from shallow and deep water. In the study, we tend to the second hypothesis because of the sedimentary data and geological setting [START_REF] Gong | Paleozoic stratigraphic regionalization and paleogeographic evolution in western Junggar, Northwestern China[END_REF][START_REF] Wang | Late Devonian Conodont Biostratigraphy and Carbon and Oxygen Isotopic Composition in Western Junggar, NW China[END_REF]. Actually, the uncommon mixtures reflect that the ecology of ostracods is more flexible and complicated from open oceanic island arc than that from continental margins.

Totally, the common species in the three ostracod assemblages are Guerichiella cf. pulchra, Moorites copelandi and most of endemic taxa (e.g., Pribylites wulankeshunensis, Pribylites junggarensis, Arcuaria hebukesarensis, Cribroconcha honggulelengensis and Microchelinella hoxtolgayensis) (Fig. 5). All of them are thick shells, small-sized and smooth or simple ornamented (e.g., Guerichiella cf. pulchra, Moorites copelandi and Pribylites wulankeshunensis), suggesting to be crawlers or burrowers and survived in the high-energy and great turbulence waters, alike the Hongguleleng Formation that was totally affected by the storm as depositing [START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF].

Main factors controlling the distribution of ostracods from the Hongguleleng

Formation Becker (1969[START_REF] Becker | Paleoecology of Middle Devonian ostracods from the Eifel region, Germany[END_REF]) considered water salinity, temperature, depth, turbulence, oxygen content, and food supply in analyzing the ostracod distribution in the Middle Devonian of Eifel Region, Germany and concluded that water turbulence was the most important factor controlling ostracods distribution. Previous works (e.g. [START_REF] Olempska | Middle to Upper Devonian Ostracoda from the Southern Holy Cross Mountains, Poland[END_REF]Beck and Bless, 1990) have shown that the Devonian ostracods increased both in diversity and frequency when water turbulence and turbidity decreased, especially specimens with large-sized smooth carapaces such as Bairdia.

In most cases, increased turbidity conditions would favour the deposit-feeding ostracods, which mainly consisted of Podocopida (e.g., Bairdiocypridoidea and Bairdioidea; [START_REF] Lethiers | The use of Ostracoda to reconstruct the oxygen levels of Late Palaeozoic oceans[END_REF] and a few of Palaeocopida (i.e., Beyrichioidea and Paraparchitoidea; [START_REF] Olempska | Soft body-related features of the carapace and the lifestyle of Paleozoic beyrichioidean ostracodes[END_REF], because they could catch coarser particles easily by developed limbs [START_REF] Horne | The Platycopid Signal deciphered: responses of ostracod taxa to environmental change during the Cenomanian-Turonian Boundary Event (Late Cretaceous) in SE England[END_REF].

Based on what discussed above, the palaeoenvironement of the Lower Member of the Hongguleleng Formation should be livable with adequate oxygen, rich food and suitable temperature for yielding abundant and varied body and trace fossils [START_REF] Wang | Late Devonian Conodont Biostratigraphy and Carbon and Oxygen Isotopic Composition in Western Junggar, NW China[END_REF]. Therefore, in this study, we agree that hydrodynamic condition could be the first factor controlling the distribution of the ostracod assemblages from the Hongguleleng Formation. Compared with the environments of offshore, the nearshore is higher water-energy and more turbid due to strongly storm. Additionally, volcanic activity was intense and frequent in the volcanic island, especially in the upper part of the Lower Member and the Middle Member of the Hongguleleng Formation. For example, a number of volcanism-influenced deposits were preserved, such as the layered euhedral feldspars in medium-to coarse-grained tempestites on the bottom of the Middle Member of the Hongguleleng Formation [START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF]. Thus, the water turbidity increased significantly as storm bursting with volcanic debris and flash falling, causing lower abundance and diversity of ostracods in the OA2 (from nearshore) than that in the OA3 (from offshore), especially the Bairdioidea. However, the deposit-feeding of OA2 (more than 75% of total species) is more dominant, which is about 65% of total species in OA3.

Moreover, the salinity of foreshore in the Hongguleleng Formation would fluctuate because of runoff and it yields some euryhaline faunas (e.g., bivalves and serpulas) [START_REF] Wang | Late Devonian Conodont Biostratigraphy and Carbon and Oxygen Isotopic Composition in Western Junggar, NW China[END_REF]. Some Platycopina and Paraparchitoidea ostracods are interpreted as euryhaline [START_REF] Bennett | Ostracods from freshwater and brackish environments of the Carboniferous of the Midland Valley of Scotland: the early colonization of terrestrial water bodies[END_REF], which corresponds with their occurrence in foreshore (OA1), nearshore (OA2) even offshore (OA3) in the Hongguleleng Formation. In addition, a few large species of the advantage Hollinoidea (e.g., Hollinella Coryell, 1928 andParabolbinella Adamczak, 1968) in the OA1 can characterize low-salinity environments such as interdistributary bays, prodelta and interdeltaic embayments [START_REF] Melnyk | Ostracode biostratigraphy of the permo-Carboniferous of Central and North-Central Texas, part I: paleoenvironmental framework[END_REF][START_REF] Olempska | Silicified shallow-water ostracodes from the Early Carboniferous of South China[END_REF][START_REF] Crasquin-Soleau | Ostracods as markers of the Permian/Triassic boundary in the Khuff Formation of Saudi Arabia[END_REF]. So salinity fluctuations may be also a factor to influence the composition apart from the hydrodynamic condition, especially to the ostracods of OA1.

Palaeobiogeographical relationships indicated by ostracods

Study of some marine fossils, such as brachiopods [START_REF] Zong | Productide, Athyridide and Terebratulide Brachiopods Across the Devonian-Carboniferous Boundary in Western Junggar, Northwestern China[END_REF][START_REF] Zong | Comparative study of Late Devonian (Famennian) brachiopod assemblages, sea level changes, and geo-events in northwestern and southern China[END_REF] and conodonts [START_REF] Wang | Conodont and carbon isotope stratigraphy near the Frasnian/Famennian (Devonian) boundary at Wulankeshun, Junggar Basin, NW China[END_REF] from the Hongguleleng Formation in the western Junggar showed the isolation of the region during the early Famennian, which is also supported by the endemic ostracods (Song et al., 2017). However, many cosmopolitan species were present in diversified ostracod faunas of the Hongguleleng Formation, suggesting possible biogeographic relationships between the Kazakhstan Plate and South China, North America, Europe and Russian plates.

The ostracods from the Hongguleleng Formation have a particularly close connection to the faunas of South China. The species, which are common in the western Junggar, appeared earlier or simultaneously in South China (from Early to Late Devonian). For example, Fabalicypris sundarijanata [START_REF] Wang | Devonian Ostracods from Dacaozi Section of Ninglang, Yunnan -Podocopids[END_REF] Cryptophyllus solotensis [START_REF] Krandyevsky | On the first occurrence of the ostracod genus Eridoconcha in Silurian deposits of the USSR[END_REF] were reported in the Lower Devonian of Yunnan, South China [START_REF] Wang | Devonian Ostracods from Dacaozi Section of Ninglang, Yunnan -Podocopids[END_REF][START_REF] Wang | Late Silurian and Early Devonian Ostracodes from the Qujing area, Yunnan[END_REF]; Bairdiocypris sichuanensis [START_REF] Wei | Palaeontological Atlas of Southwest China-Micropalaeontology Volumes-Ostracods[END_REF]Kummerowia suboblonga Wei, 1988 also occurred in the Lower and Middle Devonian of Longmen Mts. [START_REF] Wei | Palaeontological Atlas of Southwest China-Micropalaeontology Volumes-Ostracods[END_REF][START_REF] Wei | Systematic palaeontology-Ostracoda[END_REF]; the genus of Parasargentina Zheng, 1982 just appeared in the Silurian and Devonian of South China previously [START_REF] Zheng | Some Silurian and Devonian Ostracodes from Jiangyou, Sichuan[END_REF][START_REF] Wang | Late Silurian and Early Devonian Ostracodes from the Qujing area, Yunnan[END_REF]. So we deduce that, one ostracod assemblages of South China might gradually have migrated northward to the Kazakhstan plate through the Devonian (Fig. 6), which shows that the South China block moved northward to the Kazakhstan plate during the Late Devonian [START_REF] Scotese | The Paleomap Project[END_REF]Song and Gong, 2017).

In addition, the ostracod faunas from the Hongguleleng Formation reveal links between the faunas of the Russian Platform as well as the Euramerica Plate. For instance, Acratia buregiana [START_REF] Egorov | Frasnian Ostracods of the Russian Platform. 2. -Bairdiidae, Hollinidae, Kirkhyidae[END_REF], A. ivanovoensis Egorov, 1953and Hollinella valentinae Egorov, 1953, occurred in the Frasnian-Famennian transition of the Russian Platform [START_REF] Egorov | Frasnian Ostracods of the Russian Platform. 2. -Bairdiidae, Hollinidae, Kirkhyidae[END_REF], where Cryptophyllus solotensis [START_REF] Krandyevsky | On the first occurrence of the ostracod genus Eridoconcha in Silurian deposits of the USSR[END_REF] were described in the Lower Devonian [START_REF] Krandyevsky | On the first occurrence of the ostracod genus Eridoconcha in Silurian deposits of the USSR[END_REF][START_REF] Abushik | Practical Manual on Microfauna of USSR[END_REF].

Some species in the Hongguleleng Formation are same or similar to those have been reported in the western Canada [START_REF] Lethiers | Ostracodes du Devonien terminal de la Formation Big Valley, Saskatchewan et Alberta[END_REF][START_REF] Lethiers | Uppermost Devonian Ostracodes from the Western Canada systematic, biostratigraphy and palaeoecology[END_REF] [START_REF] Lethiers | The uppermost Frasnian (Upper Kellwasser) ostracodes from Coumiac (Montagne Noire, France)[END_REF]; Praepilatina adamczaki [START_REF] Olempska | Middle to Upper Devonian Ostracoda from the Southern Holy Cross Mountains, Poland[END_REF][START_REF] Olempska | Lower Carboniferouso stracodes of the Holy Cross Mountains, Poland[END_REF][START_REF] Olempska | Changes in benthic ostracod assemblages across the Devonian-Carboniferous boundary in the Holy Cross Mountains, Poland[END_REF] and Parabolbinella postaculeata [START_REF] Adamczak | Palaeocopa and Platycopa (Ostracoda) from Middle Devonian rocks in the Holy Cross Mountains, Poland[END_REF][START_REF] Adamczak | Palaeocopa and Platycopa (Ostracoda) from Middle Devonian rocks in the Holy Cross Mountains, Poland[END_REF]) usually occurred in Holy Cross Mts. of Poland; Nehdentomis schmidti Matern, 1929 and Franklinella latesulcata (Paeckelmann),1921 were reported in the Late Devonian of Germany [START_REF] Matern | Die Ostracoden des Oberdevons[END_REF]Paeckelmann,1921;Rabien,1954).

On a whole, ostracods from Northern Kazakstan Plate (western Junggar) show higher similarity to South China (9 species in common), as well as Russian Platform (4 species in common), Europe (5 species in common) and North America (4 species in common) (Fig. 6). Therefore, the Late Devonian ostracods in the western Junggar are not endemic apparently due to the common fauna exchange with other plates, which implies the biological world should be in relatively weak provincialism globally during the Late Devonian [START_REF] Boucot | Devonian biogeography: an update[END_REF][START_REF] Briggs | Global Biogeography[END_REF]. We deduce that the ostracods from the Late Devonian of the Kazakhstan Plate might migrate to the South China Block and Laurentia-Baltica by ocean currents and sea-level changes (Song and Gong, 2017). It also illustrates that neritic faunal exchanges may be possible along the Palaeo-Tethys mostly by oceanic circulation during the Late Devonian. For instance, such kind of oceanic currents along the Palaeo-Tethys ocean flowed to middle and high latitudes from low-latitude [START_REF] Smith | Rapid onset of late Paleozoic glaciation on Gondwana: evidence from Upper Mississippian strata of the Mid-continent, United States[END_REF][START_REF] Saltzman | The late Paleozoic Ice Age: Oceanic gateway or pCO2[END_REF] might have actively improved affinity between the South China, Kazakstan and the East Europe plates especially in the Frasnian, which is also supported by palaeogeography of stromatoporoids [START_REF] Stock | Biogeography of the Devonian stromatoporoids[END_REF] and brachiopods [START_REF] Qiao | A quantitative study of global brachiopod palaeobiogeography from Frasnian (Late Devonian) to Serpukhovian (Early Carboniferous)[END_REF]. Casier, 1995;12-Praepilatina adamczaki Olempska, 1979;13-Moorites copelandi Lethiers, 1981;14-Bairdia turbinata Lethiers, 1981;15-Pseudobythocypris rectodorsualis Lethiers, 1981;16-Nehdentomis schmidti Matern, 1929;17-Franklinella latesulcata (Paeckelmann,1921). Black arrows shows possible path of migration and communication for the ostracod faunas. Base map from [START_REF] Boucot | Phanerozoic Paleoclimate: an atlas of lithologic indicators of climate[END_REF]

Conclusions

(1) Three assemblages of ostracods in the open oceanic island arc are first summarized based on ostracod faunas from the Hongguleleng Formation, i.e., OA1

(Ostracod Assemblage-1), OA2 (Ostracod Assemblage-2) and OA3 (Ostracod Assemblage-3) represent foreshore, nearshore and offshore, respectively. They are different with the ecological assemblages in continental margins.

(2) Both of the common species from the Lower Member of the Hongguleleng Formation are small-sized and thick shell with smooth or simple ornamented for living in the shallow water with high-energy and great turbulence.

(3) Hydrodynamic condition is the first environmental factor controlling composition of the ostracod assemblages from the Hongguleleng Formation.

Ostracods are less abundant when water turbulence and turbidity increased, while the deposit-feeding seems more adaptable. Salinity fluctuations may also influence the composition.

Fig. 1 .

 1 Fig.1. Location map of the studied sections and simplified geology in the western Junggar, NW China (modified after Gong and Zong, 2015). a-Quaternary; b-Mesozoic; c-Permian; d-Carboniferous; e-Devonian; f-Silurian; g-Ordovician; h-ophiolite; i-granite; j-fault

Fig. 3 .

 3 Fig.3. Distribution of sedimentary environments and ostracod assemblages from the Lower Member of the Hongguleleng Formation in the WLS and BLS, the western Junggar, NW China. Conodont biofacies from Wang , (2016) ; trace fossils from[START_REF] Fan | Ichnological and sedimentological features of the Hongguleleng Formation (Devonian-Carboniferous transition) from the western Junggar, NW China[END_REF] ; microfacies types (mf1-6) modified after Wang , (2016) , mf1-mudstone with fine-grained bioclasts; mf2-wackestone-packstone with fine-grained bioclasts; mf3-mudstone-wackestone with fine-grained bioclasts; mf4-wackestone with fine-grained bioclasts; mf5-packstone with bioclasts; e. mf6-grainstone with coarse-grained bioclasts.

Fig. 4 .

 4 Fig.4. Palaeoenvironments and ostracod assemblages from the Lower Member of the Hongguleleng Formation, the western Junggar, NW China

Fig. 6 .

 6 Fig. 6. Famennian palaeobiogeography of ostracods from the western Junggar (The numbers 1-17 are common species occurred in the Hongguleleng Formation and other plates. 1-Cryptophyllus solotensis (Krandyevsky, 1958); 2-Bairdia cestriensis Ulrich, 1891; 3-Fabalicypris sundarijanata Wang & Cao, 1997; 4-Acratia cf. voronegiana Egorov, 1953 sensu Wang &Ma, 2007; 5-Bairdiocypris sichuanensis Wei, 1983; 6-Kummerowia suboblonga Wei, 1988; 7-Hollinella valentinae Egorov, 1953; 8-Acratia buregiana Egorov, 1953; 9-Acratina ivanovoensis Egorov, 1953; 10-Parabolbinella postaculeata Adamczak, 1968;11-Samarella coumiacensis Lethiers &Casier, 1995; 12-Praepilatina adamczaki Olempska, 1979; 13-Moorites copelandi Lethiers, 1981; 14-Bairdia turbinata Lethiers, 1981; 15-Pseudobythocypris rectodorsualis Lethiers, 1981; 16-Nehdentomis schmidti Matern, 1929; 17-Franklinella latesulcata (Paeckelmann,1921). Black arrows shows possible path of migration and communication for the ostracod faunas. Base map from[START_REF] Boucot | Phanerozoic Paleoclimate: an atlas of lithologic indicators of climate[END_REF]

( 4 )

 4 Many cosmopolitan species were present in diversified ostracod faunas of the Hongguleleng Formation, which suggest possible and varied biogeographic relationships between the Kazakhstan Plate and South China, North America and Russian plates. The ostracods from the Late Devonian of Kazakhstan Plate might migrate to the South China Block and Laurentia-Baltica by ocean currents and sea-level changes, which also suggests neritic faunal exchanges were possible along the Palaeo-Tethys margins during the Late Devonian.

Table 1

 1 Numbers of species per bed in the studied sections (summarized fromSong et al., 2017)

	Superfamily/		Bulongour		Wulankeshun
	Order
	Family	bed 1	bed 2	bed 3	bed 18 bed 19 bed 20 bed 21 bed 22	bed 23
	Palaeocopida

  ). Metacopina may account for 30%-40% of total number of species (WLS: 33%; BLS: 37%), which is mainly made up of Healdioidae (i.e.,

	Arcuaria Neckaja, 1958, Cribroconcha Cooper, 1941; about 13% of total number of
	species) and a few Bairdiocyprididae (Praepilatina Polenova, 1970) and
	Pachydomellidae (Microchelinella Geis, 1933). Moreover, rare platycopids
	(Kloedenelloidea) of OA1 just appeared in the bed 18 of the WLS, accounting for 8%

of total number of species. Notably, there is no Bairdioidea in the OA1. The endemic taxa

(Song et al., 2017)

(e.g., Pribylites wulankeshunensis Song & Crasquin, 2017, P. junggarensis Song & Crasquin, 2017, Microchelinella hoxtolgayensis Song & Crasquin, 2017, M. bulongourensis Song & Crasquin, 2017, Arcuaria hebukesarensis Song & Crasquin, 2017 and Cribroconcha honggulelengensis

[START_REF] Song | Ostracods of the Late Devonian Frasnian/ Famennian transition from Western Junggar, Xinjiang, NW China[END_REF]

are common and abundant in OA1.

Morphologically, Palaeocopida are characterized by straight dorsal border. They should survive in the high-energy environment as crawlers with small size and thick carapaces. Similarly, some genera of

Metacopina (such as Praepilatina Polenova, 1970 and Microchelinella Geis, 1933)

should be benthic crawler with small-sized (length of carapaces is less than 0.95 mm; Beck, 1971), thick shell and low gravity.

  (Table 2). However, compared with ostracod faunas in the two ecotypes above, most taxa in OA1 are smooth small-sized, and the domination and common taxa are different. For example,

	beyrichiids, cavellinids and leperditiids, which are common and abundant in both
	Eifelian Mega-Assemblage and Palaeocopida association, whereas are absent from the
	OA1.
	OA2 (Ostracod Assemblage-2) is located in the nearshore (Figs 3, 4), characterized by
	ostracods from the bed 19 and beds 22-23 of the WLS and the bed 1B and bed 3 of

the BLS. It is dominated by Bairdiocypridoidea, including of two families, i.e., Bairdiocyprididae and Pachydomellidae. The diversity and abundance of ostracods in OA2 are higher than that in OA1, and 27 species and 29 species presented in the WLS and the BLS, respectively. Bairdiocypridoidea

(Bairdiocypris Kegel, 1932, Healdiacypris Bradfield, 1935, Praepilatina Polenova, 1970 and Pseudobythocypris Shaver, 1958

; about 35% of total number of species) and Bairdioidea

(Bairdia McCoy, 1844, Fabalicypris Cooper, 1946 and Acratia Delo, 1930

; about 25% of total number of species). Palaeocopida (20%-25% of total number of species) is dominated by Hollinoidea

(Pribylites Pokorný, 1951)

and Paraparchitidae (Guerichiella

[START_REF] Adamczak | Palaeocopa and Platycopa (Ostracoda) from Middle Devonian rocks in the Holy Cross Mountains, Poland[END_REF]

) but with lower abundance and diversity. Platycopida (i.e., Kloedenelloidea, about 20% of total number of species) is more abundant than that in OA1.

Table 2 .

 2 Comparison of the ecological assemblages of neritic ostracods in the Palaeozoic from the western Junggar, Europe and South China

  , such as Bairdia turbinata, B.

	cestriensis, Moorites copelandi, Pseudobythocypris rectodorsualis, Bairdia cf.
	fratereleziana, Famenella cf. bisangulata. To the Europe Plate, Samarella
	coumiacensis Lethiers & Casier, 1995 has been described in the late Frasnian in
	Coumiac of France

Acknowledgments

This work was financially supported by the Natural Science Foundation of China (Grant Nos. 41290260, 41472001), China Geological Survey (Grant Nos. 1212011120502, 1212011220245). Gratefully acknowledges Dr.

Ruiwen Zong, Dr. Zhihong Wang, Dr. Ruoying Fan, Tao Jiang, Xinsong Zhang, Kunyuan Ma, Ruochen Li, Dongjian Li, Zhenzhong Xiang and Caohui Dong (all from Wuhan) for their joint field work.