Silurian ostracods from the Nyalam region,southern Tibet,China and their implications on palaeoenvironment and palaeobiogeography

Jun-Jun Song , Wen Guo , Ji-Yun Hung ,Yi-Chun Zhng , Zhong-Yng Chen , Yu-Cong Sun , Jun M ,Wen-Kun Qie ,

a State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology and Center for Excellence in Life and Paleoenvironment,Chinese Academy of Sciences,Nanjing 210008,Jiangsu Province, China

b University of Chinese Academy of Sciences, Beijing 100049, China

c State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, Hubei Province, China

Abstract The overall occurrence, stratigraphical distribution, palaeoenvironmental and palaeobiogeographical significances of the ostracod faunas from the Silurian Pulu Formation in the Yalai West II section,Nyalam region, southern Tibet, China, are documented for the first time. Thirty-two species belonging to 19 genera are identified and figured herein.The ostracod fauna in the Pulu Formation indicates an age of the late Llandovery-Pridoli. The ostracods belong to a podocope-rich association, which suggests an offshore environment for the Yalai West II section.The ostracods of the Pulu Formation have strong affinities with the South China taxa and also share some similarities with those from Baltica.This implies that ostracods could probably benefit from sea-level changes to facilitate faunal exchanges between peri-Gondwana and Baltica.Geographical isolation and global sea-level changes are proposed as the primary factors controlling the palaeobiogeographical distribution of ostracods during the Silurian.

Keywords Ostracods,Silurian,Pulu Formation,Tethyan Himalaya terrane,South Tibet,Paleobiogeography

1.Introduction

Ostracods are microcrustaceans that first appeared in the Early Ordovician and are still developing today(Siveter,2008).It has been widely acknowledged that the Silurian time marks a special time interval displaying rich ostracod faunas globally that contains diverse representatives of all the major taxa (Siveter, 1984, 1989).Palaeocopida and Podocopida species were particularly abundant, whereas myodocopes and the possible ostracod group Leperditicopida were relatively scarce(Siveter, 1984, 1989, 2009; Perrier and Siveter, 2013).Silurian ostracods are well studied in European, North American and Asian regions and proved utility in biostratigraphy (e.g., Siveter 1989, 2009; Hairapetian et al.,2011; Wang, 2015; Perrier et al., 2019; Siveter et al.,2019).Seven pelagic myodocope biozones,ranging from the upper Wenlock Series to the Ludlow Series,had been established in Europe (Perrier et al., 2019). Benthic palaeocopids and podocopids provide the best means of interregional correlation using Silurian ostracods for the palaeocontinents of Laurentia(Berdan,1983,1990)and Baltica (Siveter, 1978, 1980, 2009; Hansch and Siveter,1994;Perrier and Siveter,2013).Moreover,Paleozoic ostracods play an important role in palaeoecological and palaeobiogeographical reconstructions due to their sensitivity to ambient environmental conditions such as salinity, temperature, oxygenation, hydrodynamics and nutrients (Hairapetian et al., 2011; Perrier and Siveter,2013; Casier, 2017; Song et al., 2017, 2019a, 2019b,2020;Song and Gong,2019,2020).

The Silurian strata of the Nyalam region, southern Tibet,China were originally briefly documented by the Tibetan Scientific Expedition Team of Chinese Academy of Sciences (Mu et al., 1973). Since then, some progresses on lithostratigraphy and biostratigraphy have been made in the Nyalamregion(e.g.,Lin and Qiu,1983;Mu and Chen,1984;Yang,1985;Lin,1989;Xia and Liu,1997; Zhu et al., 2018), and important fossil groups with significant biostratigraphical value, such as conodonts and graptolites(e.g.,Mu and Ni,1975;Qiu,1985,1988; Chen et al., in press), have been constantly reported and subsequently described in detail.However,comparatively, there is no detailed study about the benthic ostracod assemblages of the Tethyan Himalaya terrane and further their connection with ostracods from other plates during the Silurian.Therefore,in this paper we aim to: 1) document the Silurian ostracod fauna from the Tethyan Himalaya terrane for the first time;2)identify the palaeoenvironmental implications of the fauna; and, 3) discuss the palaeobiogeographic significance of the ostracods,especially their affinities with faunas from other plates.

2.Geological setting and section description

The Qinghai-Tibetan Plateau can be subdivided,by several suture zones representing past oceans from south to north, into the Tethyan Himalaya terrane,Lhasa, South Qiangtang and North Qiangtang blocks,which were located in the peri-Gondwana during the Silurian(e.g.,Boucot et al.,2013;Zhang et al.,2013).The Nyalam region was part of the Tethyan Himalaya terrane,which was located to the south of South China Plate through the Silurian (Torsvik and Cocks, 2017).The Silurian succession of the Nyalam region is almost continuous, including the Llandovery Shiqipo Formation and the Wenlock-Pridoli Pulu Formation (Lin,1989; Zhu et al., 2018). Among them, the Shiqipo Formation is made up of mostly graptolitic silty shales and the Pulu Formation consists of thin-bedded limestones.

The Yalai West II section(28°24′17′′N,86°5′48′′E)is located to the west of the Yalai Town,about 32 km to the north of Nyalam County,Tibet Autonomous Region(Fig. 1A). The studied section mainly crops out the Wenlock-Pridoli Pulu Formation, which is 13.7 m in thickness. This formation is composed of limestones with some interbeds of bioclastic limestones. The lower boundary of the Pulu Formation conformably overlies the Shiqipo Formation, but the upper boundary was covered by Quaternary sediments (Fig. 1B).Other fossils such as cephalopods,conodonts,crinoids and trilobites can also be recognized in the studied section (Fig. 2).

3.Materials and methods

A total of 42 samples were collected from the Pulu Formation in the Yalai West II section in years 2018 and 2019, and each sample is about 1000 g in weight. The methodology, known as ‘hot-acetolysis’, was used to extract ostracods from the limestones (Lethiers and Crasquin-Soleau, 1988). About 1050 specimens were thus obtained, including both single valves and carapaces.Totally,32 species belonging to 19 genera were recognized (Table 1; Figs. 3 and 4). The ostracod faunas from the Pulu Formation are dominated by Podocopida (28 species belonging to 16 genera) and Palaeocopida (4 species belonging to 3 genera). All specimens figured in this study are deposited in Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, and they are numbered from YL2019001 to YL2019044.

Fig.1 A)Location of the Yalai West II section in the Nyalam region,southern Tibet,China(the insert map of China is referring the standard map available on the official website of Ministry of Natural Resources of China:http://bzdt.ch.mnr.gov.cn/);B)View of the Pulu Formation in the Yalai West II section; note the circled part by dashed line shows the view of the lower part of the Pulu Formation.

Fig. 2 Ostracod distribution in the Pulu Formation of the Yalai West II section, Nyalam region, southern Tibet, China. Abbreviations: L. -Llandovery; T. - Telychian; Shein. - Sheinwoodian; s.l. - sensu lato.

Fig. 3 Ostracods from the Silurian Pulu Formation in the Yalai West II section, Nyalam region, southern Tibet, China. A) Brevidorsa cf.huanghuaensis (Sun, 1988). Left lateral view of complete carapace; collection number YL2019001; B) Brevidorsa sichuanensis Wang (2015).Left lateral view of complete carapace; collection number YL2019002; C) Primitia cf. monstrata Shi et Wang (1985). Right lateral view of complete carapace;collection number YL2019003;D)Primitia cf.monstrata Shi et Wang(1985).Dorsal view of complete carapace;collection number YL2019004; E) Bollia sp. Right lateral view of complete carapace; collection number YL2019005; F) Bairdia cf. erlangshanensis Li(1989). Right lateral view of complete carapace; collection number YL2019006; G) Bairdia cf. similelongata Wang et Liu (1992). Right lateral view of complete carapace; collection number YL2019007; H) Bairdia cf. altitumida Wang (2015). Right lateral view of complete carapace; collection number YL2019008; I) Acratia sp. Right lateral view of complete carapace; collection number YL2019009; J) Microcheilinella subindistincta Wang et Liu (1992). Right lateral view of complete carapace; collection number YL2019010; K) Microcheilinella subindistincta Wang et Liu (1992). Dorsal view of complete carapace; collection number YL2019011; L) Microcheilinella xuanheensis Wang(2015). Right lateral view of complete carapace; collection number YL2019012; M) Microcheilinella xuanheensis Wang (2015). Dorsal view of complete carapace; collection number YL2019013; N) Microcheilinella sp. Right lateral view of complete carapace; collection number YL2019014; O) Microcheilinella sp. Dorsal view of complete carapace; collection number YL2019015; P) Microcheilinella cf. vinei (Jones,1887). Right lateral view of complete carapace; collection number YL2019016; Q) Microcheilinella cf. vinei (Jones, 1887). Dorsal view of complete carapace; collection number YL2019017; R) Microcheilinella vinei (Jones, 1887). Right lateral view of complete carapace;collection number YL2019018; S) Microcheilinella vinei (Jones, 1887). Dorsal view of complete carapace; collection number YL2019019; T)Poniklacella cf.reniformis Wei(1988).Right lateral view of complete carapace;collection number YL2019020;U)Healdianella cf.subovata Zheng (1982).Right lateral view of complete carapace; collection number YL2019021;V) Bairdiocypris sichuanensis Wei et al. (1983).Right lateral view of complete carapace; collection number YL2019022. Scale bars = 200 μm.

Table 1 Ostracods from the Silurian Pulu Foramtion in the Yalai West II section.

4.Results

4.1. Age comparative analysis

A few ostracod species from the Pulu Formation in the Yalai West II section suggest an older age than the Silurian,e.g.,Longiscula clara and Nikitinella glabella were described in the Upper Ordovician of Hubei Province,South China(Table 1;Sun,1987,1988).Most of the ostracods from the Yalai West II section are typical taxa of the Silurian, i.e., from the late Llandovery to Pridoli. For example, Brevidorsa sichuanensis, Longiscula modica and Microcheilinella xuanheensis are common in the Telychian of Llandovery in Shaanxi and Sichuan provinces and the latter species also appeared in Guizhou Province (Wang,2015). Microcheilinella vinei occurred in both the Llandovery of South China (Wang, 2015) and the Wenlock of England (Jones, 1887). Microcheilinella subindistincta was first reported in the Miaogao Formation of Ludlow age from Yunnan Province (Wang et al., 1992; Wang, 2015). Besides, Bairdia cf.erlangshanensis and Silenis cf. subkaugatomus are close to B. erlangshanensis and S. subkaugatomus,respectively, both of which are common in the Telychian of Llandovery in the Erlangshan of Sichuan Province (Li, 1989; Wang, 2015). Bairdiacypris cf. singensis is similar to B.singensis,which was endemic in the Ludlow of Southern Tien Shan, Central Asia(Mikhailova, 1977; Mikhailova and Siveter, 2021).Healdianella cf. subovata is close to H. subovata,which appeared in the Ludlow of Sichuan Province(Zheng,1982).Longiscula cf.arcuaris and Rectella cf.aequalis are close to Longiscula arcuaris and R.aequalis, respectively, and they are common in the Wenlock of the Russian Platform(Neckaja,1958). It is noteworthy that the genera of Longisculidae Neckaja,1966, such as Silenis Neckaja, 1958, Longiscula Neckaja, 1958, Rectella Neckaja, 1958, Rectalloides Neckaja, 1966 are abundant and various in the Pulu Formation of the Yalai West II section, and the similar ostracod faunas were found in the Wenlock of the Russian Platform and may range to the Pridoli(Neckaja, 1958).

The late Llandovery-Pridoli age of the fauna is supported by the presence of index fossil of conodonts as well.For example,Chen et al.(in press)established five conodont biozones/interval biozones, i.e., the Ozarkodina sagitta Biozone, the Kockelella crassa Biozone,the Kockelella variabilis Interval Biozone,the Polygnathoides siluricus Biozone, and the “Ozarkodina” eosteinhornensis s.l. Interval Biozone in the Yalai West II section (Fig. 2), which indicates a late Telychian of Llandovery to Pridoli age for the Pulu Formation (Chen et al., in press).

4.2. Palaeoenvironmental analysis

Fig. 4 Ostracods from the Silurian Pulu Formation in the Yalai West II section, Nyalam region, southern Tibet, China. A) Bairdiocypris cf.sichuanensis Wei et al.(1983).Right lateral view of complete carapace;collection number YL2019023;B)Bairdiocypris cf.sichuanensis Wei et al. (1983). Dorsal view of complete carapace; collection number YL2019024; C) Silenites sp. Left lateral view of complete carapace;collection number YL2019025; D) Longiscula clara (Sun, 1987). Right lateral view of complete carapace; collection number YL2019026; E)Longiscula cf.arcuaris Neckaja(1958).Right lateral view of complete carapace;collection number YL2019027;F)Bairdiacypris cf.singensis Mikhailova (1977). Right lateral view of complete carapace; collection number YL2019028; G) Longiscula modica Wang (2015). Right lateral view of complete carapace;collection number YL2019029; H)Longiscula modica Wang(2015).Dorsal view of complete carapace; collection number YL2019030;I)Longiscula cf.angularis(Li,1989).Right lateral view of complete carapace;collection number YL2019031;J)Silenis cf.subkaugatomus Wang (2015). Right lateral view of complete carapace; collection number YL2019032; K) Silenis cf. subkaugatomus Wang(2015). Dorsal view of complete carapace; collection number YL2019033; L) Rectella cf. aequalis Neckaja (1958). Right lateral view of complete carapace;collection number YL2019034;M)Rectella sp.Right lateral view of complete carapace;collection number YL2019035;N)Rectella sp. Dorsal view of complete carapace; collection number YL2019036; O) Rectalloides cf. ningqiangensis Wang (2015). Left lateral view of complete carapace; collection number YL2019037; P) Nikitinella cf. dubia Wang (2015). Right lateral view of complete carapace;collection number YL2019038;Q)Nikitinella glabella(Sun,1988).Right lateral view of complete carapace;collection number YL2019039;R)Nikitinella glabella (Sun, 1988). Dorsal view of complete carapace; collection number YL2019040; S) Auricypris cf. chengxiacunensis Wang(2015). Right lateral view of complete carapace; collection number YL2019041; T) Auricypris cf. chengxiacunensis Wang (2015). Dorsal view of complete carapace;collection number YL2019042;U)and V)Eobufina cf.subelliptica Wang(2015).Right lateral and dorsal views of complete carapace; collection number YL2019043; W) Parathrallella sp. Right lateral view of complete carapace; collection number YL2019044. Scale bars = 200 μm.

Fig. 5 The Pridoli (～420 Ma) palaeogeographical map modified from Ron Blakey (www2.nau.edu/rcb7), showing the location of the study area (red triangle).

The vast majority of Silurian ostracods, i.e., most of palaeocope and podocope species, were probably benthic, living mostly on relatively shallow-water submarine shelves and shelf slopes, as benthic crawlers/swimmers and perhaps as burrowers (Perrier and Siveter, 2013). At higher stratigraphical levels in the Silurian, particularly in the Wenlock, ostracod faunas are dominated by palaeocopes(e.g.,Martinsson,1967;Siveter, 2009). However, the ostracods from the Pulu Formation represent a podocope-rich association(i.e.,28 species belonging to 16 genera) with rare palaeocopes (i.e., 4 species belonging to 3 genera). The 16 genera of podocopes belong to 4 superfamilies (Table 1), including Bairdioidea, Bairdiocypridoidea, Longisculoidea,and Thlipsuroidea.Most of them appear to have relatively wide environmental tolerance, occupying a range of inner to outer shelf carbonate- and mud-dominated environments. For example, Bairdia,

Bairdiacypris, and Bairdiocypris species inhabited a variety of environments from shallow to open marine(Copeland, 1974; Meidla, 1996). For much of the Silurian,such podocope-rich faunas were considered more characteristic of the offshore settings (Siveter, 1984;Hairapetian et al., 2011).

The Pulu Formation comprises bioclastic limestones containing ostracods, cephalopods, crinoids,trilobites, conodonts and some gastropods. The associated fauna suggests a shallow marine environment.Moreover, evidences on lithology and sedimentology indicate that the Pulu Formation in the Nyalam region represents a shallow-water marine environment with slow depositional rates (Rong et al., 2003; Zhu et al.,2018). Overall, the ostracods in the Yalai West II section represent a podocope-rich association of an offshore environment.

4.3. Palaeobiogeographical relationships

Ostracods have high utility in determining faunal provinces and independently testing palaeogeographical reconstructions (e.g., Vannier et al., 1989;Williams et al., 2003; Siveter et al., 2019). Several broadly defined ostracod faunal provinces mainly from the Europe and North America regions, i.e., Appalachian, North American midcontinent, Cordilleran, Bohemian and European faunal regions, have been recognized for the Wenlock-Pridoli (e.g., Siveter,1989; Perrier and Siveter,2013).

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Despite the peri-Gondwanan setting of the Nyalam region of the Tethyan Himalaya terrane during the Silurian and its geographical isolation from low palaeolatitude Silurian palaeocontinents, the ostracods of the Pulu Formation have strong affinities with South China taxa. For instances, Microcheilinella xuanheensis, M. vinei, Brevidorsa sichuanensis, Longiscula modica and species of Rectalloides, Bollia, and Silenis were common in the Telychian (Llandovery)Wangjiawan and Ningqiang formations of Sichuan,Guizhou and Shaanxi provinces(e.g.,Sun,1987,1988;Wang,2015).Even though most of the strata from the Wenlock to the Pridoli in South China were absent or cropped out poorly (Rong et al., 2019), a few species occurred in the Ludlow of Yunnan Province, such as Microcheilinella subindistincta (Wang et al., 1992;Wang, 2015). The ostracod fauna from the Pulu Formation as a whole shares similarities with that from the Wenlock of the Russian Platform, which was located in the low palaeolatitude Baltica, sharing species of Silenis, Longiscula, Rectella and Rectalloides (Neckaja, 1958). The presence of Microcheilinella vinei in the Pulu Formation indicates a few palaeobiogeographic links with the European Plate(Jones, 1887). However, the Silurian ostracod faunas from the Nyalam region are compositionally different,at both generic and especially species level, from those of the Silurian ostracod biogeographical regions of North America that have been recognized based on podocope and palaeocope faunas (Siveter, 1989),though it must be remembered that the total number of ostracods involved in the study is not large.

The controlling factors on the palaeobiogeographical distribution of ostracods have been explained in terms of climatic zonation, geographical isolation, global sealevel changes, as well as oceanic circulation (Song and Gong, 2017). Generally, lower sea level had favored dispersal in two ways. First, it narrowed the marine seaways between different palaeocontinents.Second,it favored dispersal via outer shelf or mid-ocean islands,which were named‘island-hopping’routes(Fortey,1984;Williams et al.,2003).South China Plate was not far away from the Tethyan Himalaya terrane (the red triangle in Fig.5)during the Silurian(the Pridoli),and the ostracod migration during low sea level periods might be possible.Increased migration of ostracods coadapted with the Ordovician sea-level lowstands was also suggested(Williams et al.,2003).Moreover,sea-level fluctuations during the Silurian might have played an important role in the ostracod migration. Sea level was fluctuating probably as a response to the waxing and waning of a continental-scale Gondwanan ice sheet (Loydell, 1998;Page et al., 2007; Schofield et al., 2009; Hairapetian et al., 2011). The dispersal of ostracods might have been promoted during transgressive pulses along juxtaposedshallowshelvesoftheadjacent terranesofTethyan Himalaya,South China,and Baltica.Therefore,it seems that the primary factors affecting the ostracod palaeobiogeographical distribution during the Silurian should be geographical isolation and global sea-level changes.

5.Conclusions

1) Thirty-two species belonging to 19 genera are identified and figured for the first time from the Silurian Pulu Formation in the Yalai West II section,Nyalam region, southern Tibet, China.

2) The ostracod fauna suggests a probable late Llandovery-Pridoli age for the Pulu Formation in the Yalai West II section.

3) The ostracods represent a podocope-rich association with rare palaeocopes, which implies an offshore environment for the Yalai West II section during the Silurian.

4) Ostracods from the Pulu Formation show a high similarity to the Silurian taxa from South China and also share some similarities with the faunas from Baltica, which implies that ostracods experienced faunal exchanges between peri-Gondwana and Baltica during the Silurian.

Funding

This work was financially supported by the Second Tibetan Plateau Scientific Expedition and Research Program (Grant No. 2019QZKK0706), the National Natural Science Foundation of China (Grant Nos.41802002 and 91855205), and the Strategic Priority Research Program(B)of Chinese Academy of Sciences(Grant No. XDB26000000).

Authors' contributions

JJS conceived the idea of the study and wrote the paper.WG,JYH,YCZ,ZYC,YCS,JM and WKQ discussed the results and revised the manuscript. All authors read and approved the final manuscript.

Availability of data and materials

The datasets used during the current study are available from the corresponding author on reasonable request.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

Special thanks go to Dr.Ruo-Ying Fan from the China University of Geosciences (Wuhan) for her help in improving the original manuscript. Many thanks to the reviewers,Dr.Ai-Hua Yuan and Dr.Rui-Wen Zong(all from the China University of Geosciences,Wuhan),whose suggestions and comments have improved the manuscript.