Mio–Pleistocene Ostracoda from the Zhada Basin (western Tibetan Plateau)

We present a list of Ostracoda (Crustacea) from stratigraphic sections of Mio–Pleistocene lacustrine deposits from Zhada Basin, western Tibetan Plateau. In this area, almost no taxonomical studies were carried out so far, and, aiming to a future use of ostracods as palaeoenvironmental proxy for this sector of the Tibetan Plateau, a documentation of several species was performed. The taxa Leucocytherella sinensis Huang, 1982, ?Leucocythere dorsotuberosa Huang, 1982, Leucocythere postilirata Pang, 1985, Ilyocypris spp., Eucypris cf. zandaensis Yang, 1982, ?Prionocypris sp., Paraeucypris sp. and Leucocytherella dangeloi sp. nov. were found and classified. The taxon Ilyocypris spp. probably represents three different species; other taxa in open nomenclature are Paraeucypris sp., Eucypris cf. zandaensis and ?Prionocypris sp. The reported taxa from the Zhada Basin are mainly lacustrine species, and their diversity is comparable to those of other Neogene and Quaternary basins located on the Tibetan Plateau.


Introduction
There are many examples for the prominent role of Ostracoda (Crustacea) in different fields of geosciences and their use as palaeoenvironmental, palaeoclimatic and biostratigraphic indicators (Boomer et al. 2003;Horne 2003Horne , 2007. Their sensitivity to environmental changes and their wide distribution in all types of water bodies call for their good documentation also in less studied areas. In contrast to the large number of geological and palaeontological studies on the Tibetan Plateau, research on ostracods in this area is rather rare and improved only in the last decade Frenzel et al. 2010;Mischke 2012). Investigations were mainly conducted in the more easily accessible northern and eastern part of the plateau (Zhang et al. 1989(Zhang et al. , 1994(Zhang et al. , 2006Mischke et al. 2010), and only a few studies improved the available knowledge of the local ostracod fauna of its central and western part (Li et al. 1991;Zhu et al. 2010;Guo et al. 2016;Song et al. 2017;Alivernini et al. 2018a, b). Furthermore, ostracod studies in the central and southern parts of the Tibetan Plateau focussed on Holocene and Late Pleistocene faunas whereas the precursors of these mainly endemic species are not known. Investigations on Mio-Pleistocene ostracods from the Tibetan Plateau are restricted to its northeastern part so far (Sun 1988;Yang et al. 1997;Mischke et al. 2006Mischke et al. , 2010Wu et al. 2011;Lu et al. 2019) where they are a valuable tool for biostratigraphy in hydrocarbon exploration.
This work focuses on Mio-Pleistocene ostracods of the Zhada Basin located in the western Tibetan Plateau (Fig. 1). Previous works carried out in this area concern 1 3 its tectonic origins (Wang et al. 2004(Wang et al. , 2008Saylor et al. 2010a) and environmental history (Saylor et al. 2010b) using mostly sedimentological and pollen analyses. Kempf et al. (2009), who investigated petrographic and sedimentological properties, were the first to describe also elements of the ostracod fauna in this area. They found some typical endemic taxa like Leucocytherella sinensis and several not identified species. In this work, we present the Mio-Pleistocene ostracod assemblage recovered from 105 samples of Joel Saylor's stratigraphic "South Zhada" ("SZ") section (Fig. 2), located in the southern part of the Zhada Basin and already sedimentologically analysed and Fig. 1 Location of the Zhada Basin on the Tibetan Plateau (a) and focus of the sampling area (b) (Geomapapp image modified). The correspondence between sectors and sample list is explained in Table 1 dated by Saylor (2008) to improve the taxonomic database on ostracods of this area for future palaeoecological and potentially stratigraphical studies.

Study area
The Zhada Basin is the largest late Cenozoic sedimentary basin in the Tibet Autonomous Region. It is located north of the high Himalayan ridge crest in the western part of the orogen (~ 32° N, 82° E; Fig. 1). The basin is at least 150 km long and 60 km wide, and the current outcrop extent of the basin fill covers at least 9000 km 2 (Saylor et al. 2010a). It is bounded by the South Tibetan detachment system to the southwest, the Indus suture to the northeast, and the Leo Pargil and Gurla Mandhata gneiss domes to the northwest and southeast, respectively (Saylor et al. 2010b). The Zhada Basin contains a thick sequence of late Neogene and early Quaternary fluvial and lacustrine deposits (Kempf et al. 2009) which allows the reconstruction of long-term climate history.

Fieldwork
A total of 124 sediment samples from seven, relatively thick lake beds were collected from Saylor's "South Zhada" section (Saylor 2008) in 2012 (Table 1). The selected lake beds are distributed more or less evenly over the 820-m-thick sediment sequence to enable the investigation and comparison of ostracods from stagnant-water deposits formed over the last ca. 8 million years as estimated by Saylor et al. (2010a). Sediment samples from individual lake beds were collected at ca. 0.5-m intervals ( Table 1). The seven selected lake beds are located between 31.46538° N and 79.72865° E as the lowermost and northernmost position and 31.36556° N and 79.75152° E as the uppermost and southernmost position.

Micropalaeontological analysis
All 124 sediment samples were prepared for micropalaeontological analysis. The samples were treated with H 2 O 2 (ca. 5-10% for about 1-2 h) to separate aggregates of mud, and they were subsequently sieved with water through a 200-µm-sieve to remove fine-grained particles. For quantitative ostracod analysis, the sieve residues were split into sub-samples using a microsplitter. Sixteen samples were barren of ostracods. The species proportions and the relative ostracod abundances were calculated considering all ontogenetic stages (juvenile and adult valves). To assess water turbulence (Boomer et al. 2003) and the possible removal of thinner and smaller juvenile valves by dissolution, the adult/ juvenile ratio was determined. Identification was performed primarily with a low-power binocular microscope and was occasionally supported by a scanning electron microscope (SEM) as well as a Keyence Digital Microscope. The valves were classified and taxonomically attributed, where possible, by comparison with previous studies of ostracods from the Tibetan Plateau Mischke et al. 2010;Akita et al. 2016) and using Chinese literature (Huang 1982;Hou et al. 2002;Hou and Gou 2007). In addition, an amended description of the valves was added.

Presence of organism remains and preservation
In total, 105 sediment samples contained ostracod valves. From these samples, 6722 ostracod valves were recovered. Most valves are disarticulated, and adult valves are dominant. Juvenile valves, especially from silty to fine sandy sediments, are often deformed. Besides ostracod valves, gyrogonites of charophytes and mollusc shells, mostly fragments of Gastropoda, were found frequently in the samples.

Ostracod taxonomy
We recorded at least eight ostracod species in the 124 samples from the Zhada Basin (Table 2). The most abundant species is L. sinensis (Huang 1982) which often occurs in association with ?Leucocythere dorsotuberosa and L. postilirata (Table 2). Other abundant taxa are Paraeucypris sp. and Leucocytherella dangeloi sp. nov. A systematic overview of the ostracod taxa of the Zhada Basin follows below. The synonymy lists contain selected papers as first description, emendations and other taxonomically important references. The systematic description is adopted from Martin and Davis (2001) and complemented from Fürstenberg et al. (2015).
Original description. (Huang 1982). Valve of female rectangular in lateral view, anterior end higher than posterior, two transverse sulci anterodorsally, radial pore canal zone moderately broad, with slender, straight and sparse radial pore canals. Hinge of left valve consists of an anterior small reniform tooth, a posterior small triangular one and middle shallow groove. Valve of male rather long, both ends nearly equivalently high. Valve of juveniles rather short, anterior end higher than posterior. For a detailed description of the valves of recent specimens (adult and juveniles, males, females,) on the Tibetan Plateau see Fürstenberg et al. (2015).

Stratigraphic, ecological and geographic distribution. From
Miocene to recent (Huang 1987). Leucocytherella sinensis is ubiquitous and endemic on the Tibetan Plateau above 4000 m above sea level (Akita et al. 2016). Valves of L. sinensis were found in lakes, ponds, rivers, and lagoon-like and estuary-like water bodies at lake shores in salinities of 0.08-12.81 psu. Specimens live on mud, sand, sandy gravel and in phytal habitats in permanent fresh to brackish lacustrine waters, preferentially in Ca 2+ depleted waters. The nodes on the calcitic valves are more numerous and pronounced at low salinity and can be used as a proxy for palaeosalinity (Fürstenberg et al. 2015).
Leucocytherella dangeloi Alivernini sp. nov. Figure 4a-g Etymology. The name "dangeloi" was given to commemorate the death of Fabio D'Angelo, a young micropalaeontologist at the beginning of his academic career who died in 2012.

Holotype.
A male right valve (0.70 mm). Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Collection number: 171681.     Studied material. 506 valves (adults and juveniles, females and males, including specimens with complete carapaces).
Diagnosis. Typical Leucocytherella species but with smooth surface and posterior part relatively higher compared to anterior part than in L. sinensis, more evident in the left valve. Lophodont hinge.
Description and comparison. Carapace nearly rectangular. The posterior part of valves of L. dangeloi sp. nov., as well as of L. sinensis, is more rounded and higher than the anterior one, but this difference is more pronounced in L. dangeloi sp. nov. with an even higher rounding ( Fig. 5; Table 3). Valves smooth and less pitted than those of L. sinensis. Weak dorsomedial sulcus at half-length of carapace. Protuberance in the anterodorsal part of the carapace. Valves are un-noded or weakly noded (Table 3). Four adductor muscular scars arranged in an almost vertical and slightly inclined row, located slightly anteriorly from the centre of the valve. Marginal pore canals thin, from weakly curved to straight and not numerous. Similar to L. sinensis, the hinge of the right valve consists of a pit on both ends and a ridge in between; the hinge of the left valve presents an anterior small rounded tooth, a small triangular posterior one and a shallow groove in between. Inner posterior lamella is broad. Dorsally, the carapace is sexually dimorphic, males are larger and more expanded posteriorly than females.

Attributes
Carapace form Nearly rectangular Nearly rectangular Posterior part/anterior part The beginning of the postero-ventral marginal rounding is significantly closer to the anterior margin than to the posterior one Posterior part more rounded and higher than the anterior one The beginning of the postero-ventral marginal rounding is significantly closer to the posterior margin than to the anterior one. Posterior part higher rounded than in L. sinensis Original description (Huang 1982: 335). Male valve rectangular, anterior end higher than posterior, dorsal margin nearly straight, ventral margin distinctly concave in the middle. Valves with reticulation. Two transverse sulci anterodorsally, and an alar protuberance extending posteroventrally to medioventrally, and a tubercle in posterodorsal position.
Marginal pore-canal zone broad, comprising 10-11% of the length of carapace, marginal pore-canals slender, not numerous, several are furcated, anteriorly with 19 marginal pore-canals. Hinge of the left valve consists of sockets in both sides and a shallow ridge in between; hinge of the right valve consists of elongated teeth at both ends and a groove in between. Valve of male is longer than that of female, posterior bulgy. Juvenile valve short, anterior broadly rounded, dorsal margin slightly rounded. Hinge of male, female and juvenile are similar. Valves are transparent. Carapaces sub-rectangular in lateral view. Remarks. The found valves of ?L. dorsotuberosa have a lophodont hinge. Wrozyna et al. (2009) and Danielopol et al. (1989) doubted that ?L. dorsotuberosa belongs to the genus Leucocythere because of the different hinge, lophodont in ?L. dorsotuberosa instead of the typically anterior significantly smaller tooth than the posterior one and a crenulated hinge bar of the genus Leucocythere.
Stratigraphic, ecological and geographic distribution. Pliocene to recent (Huang 1982). Living ?L. dorsotuberosa occur mainly in brackish lakes (phytal and muddy substrate) and its marginal lagoon-like water bodies on the Tibetan Plateau. Living individuals have also been found in freshwater, but in low numbers only. Empty valves of ?L. dorsotuberosa were found in higher proportions at larger water depth of modern Tibetan lakes (Akita et al. 2016).
Valve of male of elongated kidney shape. Anterior slightly higher and/or has the same height as posterior. Both ends curved. Dorsal margin is elongated and almost straight, mediodorsal slightly curved. The anteromediodorsal area is compressed. Two transverse sulci anterodorsally, the more anterior sulcus shorter than the more posterior one. A rounded node is located between the sulci, another more pronounced bulge behind the posterior sulcus and a third at the end of both sulci. A distinctive anterodorsal carina occurs where the dorsal margin meets the anterior one, another carina runs along the central ventral side below the sulci. A third carina lays posteriorly and protrudes the valve outline. The ventral and posterior carinae are not connected to each other. The maximum width lies at ¼ length of the valve.
Valve not curved so much, ornamented with a net of large alveoli. Valves of female shorter than male, kidney shaped. Anterior higher than posterior. Posterior carina and ventral carina weak (modified from Wrozyna et al. 2009).
Further description. As already observed by Wrozyna et al. (2009), the valves present a typical sharp carina running parallel to the ventral margin, more distinct on the right valve. Another more or less developed carina runs parallel to the anteroventral margin. Additionally, a marginal parallel posterior carina following the curvature of the margin can be more or less developed, separated from or fused with the ventral carina. The valves are strongly reticulated.
Remarks. Wrozyna et al. (2009) andFrenzel et al. (2010) regard ?L. postilirata as morphotype of ?L. dorsotuberosa with most pronounced medio-ventral and anterior and often posterior carinae as protruding foldings of the valve. On the basis of the remarkable differences in morphology of both forms, they are discriminated as two species in this paper, as it was done in the original description.
Stratigraphic, ecological and geographic distribution. Living specimens reported from Nam Co and Pumoyong Co;
Remarks. The species of the genus Ilyocypris are often hard to discriminate relying on hard parts only, even in wellstudied regions as Central Europe (Meisch 2000). Hou et al. (2002) list eleven Ilyocypris species for the Tibetan Plateau but many of them are of dubious taxonomic state. The partly poor preservation of our material and impossible attribution of most juvenile valves to adult stages makes it difficult to discriminate and identify Ilyocypris species from the Zhada Basin. All documented valves bear the typical characters of the genus-a rectangular carapace in lateral view, about 1 mm long, with pitted to smooth surface and two conspicuous transverse dorsolateral sulci; the left valve overlaps the right one.
Based on outline and ornamentation, three morphotypes, probably different species, are recognisable: (a) wellrounded anterior and posterior end in lateral view, surface weakly or not pitted, no tubercles; (b) well-rounded anterior and posterior end in lateral view, surface weakly pitted, five distinct tubercles similar to Qinghaicypris subpentanoda Yang, 1982;(c) lateral view with truncated posterior end similar to Ilyocypris inermis Kaufmann, 1900, surface pitted, no tubercles. Left valves of the two well-rounded morphotypes a and b show distinct marginal ripplets on the inner lamella of both ends. This character resembles Ilyocypris bradyi Sars, 1890 and Ilyocypris decipiens Masi, 1905(Mazzini et al. 2014) but the ripplets are more numerous and can be found at the anterior end as well.
Original description. (Hou et al. 2002) Valves big, female elliptical in lateral view, dorsal margin straight and short inclining to the posterior part in lateral view, ventrally slightly concave, highest point at 2/5 of length, network of lines on the valve, marginal pore channels thick and numerous, central muscle scars with four in front of two others, oviduct traces, male valve longer, traces of four loops of testes recognisable.
Remarks. Our material differs from the male adult description in having a slightly more trapezoidal outline of the right valve and being slightly smaller. Unfortunately, only three adult valves were available.
Description. Valves elongated elliptical in lateral view, both ends well rounded, highest point well in front of mid-length, posterior part of right valves more slender than anterior one, dorsal margin along the hinge straight and distinctively inclined towards posterior, ventral margin slightly concave. Surface of valves smooth or fine pitted. Hinge with a simple groove in the left valve and a smooth bar in the right valve. Central muscle scars of the typical cypridid paw-like pattern.

Discussion and conclusion
Our list of ostracod taxa from the Zhada Basin contains at least eight species, several of them are already described for the Tibetan Plateau. Among them, the opportunistic and ubiquitous L. sinensis is the most abundant species. Leucocytherella sinensis is often observed together with the deeper lacustrine species ?L. dorsotuberosa and ?L. postilirata as it is known for recent faunas Akita et al. 2016). Kempf et al. (2009) list only five species from the Zhada Basin. One of them, Candona xizangensis Huang, 1982, was not found in our study. Adding C. xizangensis to our list results in a minimum of nine species, representing a low diversity for the studied area and time. Akita et al. (2016) found eleven species in the recent Tangra Yumco lake system, a number comparable to our count from the Zhada Basin. Considering that other investigated Cenozoic to modern lake basins of the Tibetan Plateau Mischke 2012;Alivernini et al. 2018a) with different salinities and depths have a similar low-diversity fauna, we assume the high altitude and the relative isolation of the Tibetan Plateau as the cause for the low diversity observed in the Zhada Basin. The newly described species L. dangeloi sp. nov. is very interesting for the evolution of the genus Leucocytherella Huang, 1982, endemic to the Tibetan Plateau. All specimens of the genus described so far and studied by Fürstenberg et al. (2015) belong to L. sinensis Huang, 1982. The accompanying taxa of the species L. dangeloi sp. nov. from the Zhada Basin are, similar to the recent ostracod fauna of the Tibetan Plateau, mainly lacustrine species. Thus, the association of L. dangeloi sp. nov. with the lacustrine species points to a lacustrine habitat of the new species as well.
The species assemblage changes recorded in the sampled lake beds of the "South Zhada" section (Table 2) probably reflect changes in the depositional setting including more shallow and deltaic conditions or also deeper environments. However, combined sedimentological and geochemical analyses together with quantitative palaeoecological analysis of ostracod species assemblage data from the sediments of the Zhada Basin based on the presented taxonomical research are required to better understand the Miocene to Pleistocene environmental and climatic history of the western Tibetan Plateau.
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