Ostracods ( Crustacea ) as shelf to basin indicators : evidence from Late Devonian Yangdi and Nandong sections in Guangxi , South China

Forty-eight ostracod species belonging to 28 genera from the Late Devonian of Guangxi in South China are described and figured. The ostracod assemblage from the Nandong section can be attributed to the rhenana–linguiformis conodont zones. The ecological assemblages of ostracods from the Yangdi section correspond to a smooth-podocopid association accompanied by some pelagic entomozoids. This mixed assemblage is indicative of an environment of carbonate platform to slope during an ongoing regression. The ostracod faunas from the Nandong section are on the contrary composed of both pelagic and benthic ostracods and pelagic forms dominate in number of specimens suggesting a basin environment.

Systematic biostratigraphy and palaeoecology of the Early and Middle Devonian ostracods in South China have been well studied (e.g.Wang, 1979Wang, , 1986Wang, , 1987Wang, , 1988;;Zhang, 1986Zhang, , 1995;;Wang et al., 1992;Wang and Liu, 1994).An entomozoacean zonation was established by Wang (1987Wang ( , 2009) ) and a sinoleperditiine biostratigraphical sequence for the Devonian was summarized by Wang and Peng (2005).However, there are only a few studies on the Late Devonian ostracods from South China (Casier et al., 1997;Wang and Ma, 2007).This paper aims to provide a systematic description of some Late Devonian ostracods from the Yangdi and Nandong sections (Guangxi, South China) and discuss their biostratigraphical and palaeoecological values.

Geological setting and section descriptions
During the Late Palaeozoic, the South China Block has a 90 • anticlockwise position comparative to today.The palaeogeographic frame of South China consisted of the Yangtze-Cathysia continent and the southern South China Sea.The South China Block was subject of two transgressions from the southwest to the northeast during the Devonian.From the  Huang and Gong, 2016;Ma et al., 2016) with the locations of the studied sections.
late Emsian to the Famennian, the basement of the epicontinental sea of South China Block was cut by varied intersected rifts, developing into a complex palaeogeography of shallowwater platforms separated by deep-water basins (Dong, 1982;Ma and Bai, 2002) (Fig. 1).This special palaeogeographic pattern controlled the distribution of bio-and lithofacies (Ji, 1989;Hou et al., 1988).The basin or slope facies are distributed in a rift belt to the northeast and northwest and crosscut the carbonate platform facies.Benthic organisms (such as brachiopods, ostracods and corals) dominated in the shallow carbonate platform facies, while pelagic organisms (e.g.conodonts, ammonoids and some allegedly pelagic ostracods) preponderated in the deep basin facies (Ma, 2004).
The Yangdi section (24 • 58.2 N, 110 • 22.8 E) is located along the road from Baoan to Yangdi, about 35 km southeast of Guilin city, Guangxi.It is a well-outcropped section with continuous deposition of the Late Devonian, which has been studied in detail for biostratigraphy, lithostratigraphy, chemostratigraphy and cyclostratigraphy (Ji, 1989;Gong et al., 1997Gong et al., , 2001Gong et al., , 2005;;Wang, 1990;Ma and Bai, 2002;Chen et al., 2013).A reliable and high-resolution conodont biostratigraphy has been established in this section by Huang and Gong (2016).The Yangdi section displays a continuous sedimentation from the Frasnian to the Famennian with, from the bottom to the top, the Fuhe, Lazhutai, Xiangtian, and Wuzhishan formations (Ji, 1989;Ma and Bai, 2002).The lower and middle Frasnian is represented by two strati-graphic units, i.e. the Fuhe Formation and the Lazhutai Formation (see detailed descriptions in Ji, 1989;Ma and Bai, 2002).The Xiangtian Formation (28 m thick), which is dated to the late Frasnian, can be divided into three parts.The lower part is composed of grey to dark-grey thin-bedded wackestones intercalated with banded cherts (Beds 0-11); the middle part is about 2 m thick and characterized by darkgrey thick-bedded brecciated limestones (Bed 12); the upper part consists of grey argillaceous limestones intercalated with nodular limestones and dark-grey thin-bedded calcareous turbidites (Beds 13-41).The Wuzhishan Formation is mainly characterized by light-grey thick-bedded nodular limestones (Fig. 2).
The Nandong section is well exposed along a river close to Nandong Village, in Wuxuan County, Guangxi (23 • 46 42.98 N, 109 • 41 57.63 E).This section has been studied previously for lithostratigraphy, biostratigraphy and geochemistry (Bai et al., 1994;Gong and Li, 2001;Huang, 2015).The Nandong section exhibits a continuous deposition from the Liujiang Formation to the Wuzhishan Formation (Fig. 3).The Liujiang Formation is made of grey thin-bedded siliceous rocks intercalated with argillaceous limestones.The lower part of the Wuzhishan Formation is characterized by grey nodular limestones, and the upper part is composed of dark-grey thin-bedded siliceous rocks.

Material and methods
Three periods of fieldwork, from 2012 to 2015, have been carried out in South China to sample the Yangdi and Nandong sections.In total, 131 and 81 samples were collected respectively from the two studied sections (Yangdi and Nandong).The so-called "hot-acetolysis" method was used to extract ostracods from limestones (Lethiers and Crasquin-Soleau, 1988;Crasquin-Soleau et al., 2005).For siliceous rocks, the ostracods were extracted after dilute hydrofluoric acid (HF) (2 to 5 %) processing, the same method used to extract radiolarians from cherts (Pessagno Jr. and Newport, 1972).About 1000 and 1500 specimens were thus obtained from the Yangdi (including single valves and carapaces) and Nandong (mostly single valves) sections, respectively.In total, 48 species belonging to 28 genera were recognized: 31 species belonging to 19 genera in the Yangdi section and 19 species belonging to 15 genera in the Nandong section (Figs. 2, 3).All specimens figured in this paper are deposited in the palaeontological collections of the Museum of the China University of Geosciences (Wuhan, People's Republic of China) numbered from YD15001 to YD15034 (Yangdi section) and ND15001 to ND15022 (Nandong section).

Palaeoenvironmental analysis
Five ostracod associations were recognized in the Palaeozoic strata of the South China by Wang (1988) i.e. the leperditiid, palaeocopid, smooth-podocopid, spinose-podocopid, and entomozoacean associations, which represent the palaeoenvironments from the nearshore to deep basins (Fig. 5).Following Wang (1988), the smooth-podocopid association was generally characterized by a rich bairdiacean fauna with smooth carapaces and is indicative of offshore environments.The entomozoacean association characterized by entomozoid and/or cyprinoid ostracods is indicative of deep-water basin environments.
The ostracod faunas from the Nandong section are dominated by podocopids (65 % of total number of species) and myodocopids (25 % of total number of species).Palaeocopids and platycopids are rare, with each having just 5 % of the total number of species.The ostracod assemblage is represented by smooth podocopids (i.e.Bairdioidea and Bairdiocypridoidea), which is similar to the ostracod faunas in the Yangdi section.However, thin-shelled entomozoaceans show a significant preponderance (more than 65 % of total number of specimens) in the Nandong section.Moreover, most ostracods species are represented by relatively small carapaces (< 1 mm), thin valves, and simple hinge structures, which may be a result of specialization in a low-energy and anoxic deep-water environment (Wang, 1988).Therefore, the bathymetry was deeper during the deposition of the Liujiang and Wuzhishan formations in the Nandong section than during the contemporaneous deposition in the Yangdi section.The ecological assemblages of ostracods in the Nandong section indicated a basin environment (Fig. 5).

Systematic palaeontology
The taxonomic classifications of Becker (2002) and Liebau (2005)  Description: Equivalve carapace.DB straight and long; CA obtuse.AB rounded with a large radius of curvature and a maximum convexity located between first third of H and mid-H ; VB regularly curved; PB with a relative small radius of curvature and a maximum convexity located below mid-H .Surface smooth.
Remarks: In lateral outline, the specimens resemble Aparchites productus Polenova, 1960 from the Middle Devonian of the Russian Platform (Polenova, 1960).The specimens figured by Polenova (1960, Plate 1, figs. 2a, b), as well as our specimens, do not posses the adventral structure characteristic of the genus.Therefore, our species is assigned to Aparchies with doubt.
Description: DB straight to slightly curved; AB and PB rounded; VB regularly concave; left valve slightly overlaps the right along the free margins.Surface smooth.

Remarks:
In outline, the specimens resemble Aparchites circularis Wei, 1983 from Early Devonian of Sichuan, South China (Wei et al., 1983), but poor preservation makes further identification impossible.Description: DB slightly curved; AB rounded with a large radius of curvature and a maximum convexity located a little below mid-H ; VB nearly straight on left valve and regularly convex on right one; PB with a relative small radius of curvature and a maximum convexity located at mid-H .Left valve overlaps slightly the right one with a maximum at  (Wei, 1983).Right valve, YD15008, Frasnian, Yangdi section (Bed 1).(9, 10) Paraparchites subcircularis (Wang and Shi, 1982).Description: Small equivalve carapace with subrectangular outline in lateral view.DB straight and long; ACA obtuse and bigger than PCA.AB rounded with a large radius of curvature and a maximum convexity located below mid-H ; VB regularly curved; PB with a relative small radius of curvature and a maximum convexity located at lower third of H . Surface finely reticulated.

Remarks:
The present specimens are assigned to Paraparchites circularis (Wei et al., 1983) from the Early Devonian of Sichuan, South China.Wei (1988) changed the generic attribution, certainly due to the lack of adventral structure and referred the species to Antiparaparchites Coryell & Rogatz, 1932.This genus is considered as a junior synonym of Paraparchites since the revision of Paraparchitoidea by Sohn (1971Sohn ( , 1972)).
Remarks: In lateral outline, the specimens resemble Glyptopleura venosa (Ulrich, 1891) from the Late Devonian to the Early Carboniferous of the Illinois, USA (Ulrich, 1891), but broken valves forbid further assignation.
Description: DB long and straight; CA obtuse; AB with a relatively large radius of curvature, and with a maximum curvature located at mid-H ; VB regularly rounded with a maximum curvature located at posterior part of VB; PB rounded with a large radius of curvature and a maximum located at mid-H .A short and deep subcentral sulcus almost reaches the dorsal border.

Remarks:
The specimens resemble Knoxiella tuqiaoensis Wei, 1988 from the Middle Devonian of Longmen Mts., Sichuan, South China (Wei, 1988) in outline, but poor preservation of the surface ornamentation forbids further assignation.Remarks: In outline, the specimens are close to Sargentina arcuata (Egorov, 1950) sensu Wei et al., 1983 from the Early Devonian of Sichuan, South China (Wei et al., 1983).But S. arcuata is more elongate and has a larger radius of curvature at PB.Our specimens may belong to a new species.The poor preservation could not allow a precise determination.Description: DB long and straight; CA obtuse; AB flattened with a relatively large radius of curvature and with a maximum located a little below mid-H ; PB flattened with a large radius of curvature with maximum located at mid-H ; VB straight to slightly curve.S 2 developed and nearly reach to DB.Right valve slightly overlaps the left one with a maximum at VB.

Remarks: The specimens resemble Westmontia devilensis
Casier & Lethiers, 1997 from the Frasnian-Famennian (Late Devonian) of Nevada, USA (Casier and Lethiers, 1997) Description: DB slightly convex; AB with a relatively large radius of curvature and with a maximum located a little below mid-H ; PB with a small radius of curvature and with a maximum located at mid-H ; VB regularly curved with a maximum convexity located at between first third of L and mid-L.Overlap of left valve on right one.Surface smooth.
Description: DB nearly straight; AB and PB with a relatively small radius of curvature and with a maximum located at mid-H ; VB slightly curved to straight.Overlap of left valve on right one with a maximum at DB. Surface smooth.
Remarks: In outline, our specimens resemble Acratia mossolovica Egorov, 1953 from the Late Devonian of the Russian Platform (Egorov, 1953).But A. mossolovica is more elongate and has a larger radius of curvature at AB and PB.Our specimens may belong to a new species.
Description: Carapace diamond-shaped in lateral outline; DB regularly arched; ADB and PDB arched; AB and PB with small radius of curvature and with maxima located at mid-H ; VB regularly concave.Left valve slightly overlaps the right with a maximum at VB. Surface smooth.
Remarks: In lateral outline, these specimens resemble Acratia mayselae Egorov, 1953 from the Late Devonian of the Russian Platform (Egorov, 1953).They differ by a more tapered AB.
Description: DB regularly convex; AB with a large radius of curvature and with a maximum located between first third of H and mid-H ; PB with a relative small radius of curvature and with a maximum located at lower third of H ; VB nearly straight.Left valve slightly overlaps the right one along the free margins with a maximum at DB and VB.Surface smooth.
Remarks: In lateral outline, the specimens resemble Acratia zhongyingensis Wang, 1978 from the Late Permian of Guizhou, China (Wang, 1978).It differs by being more elongate.
Description: DB slightly convex; AB with a relatively small radius of curvature and with a maximum located at lower third of H ; PB with a large radius of curvature and with a maximum located at mid-H ; VB slightly concave.Overlap of left valve on right one with a maximum at DB. Surface smooth.Dimensions: L = 0.58-0.65 mm, H = 0.32-0.38mm, W = 0.20-0.24mm.

Remarks:
The specimens are close to Acratia buregiana Egorov, 1953 from the Late Devonian of the Russian Platform (Egorov, 1953) in outline, but they differ by a sharper ADB and a rounder AB.
Description: DB slightly curve; AB with relatively a large radius of curvature and with a maximum located at mid-H ; PB with a small radius of curvature and with a maximum located at lower third of H ; VB nearly straight.Slight overlap of left valve on right one; maximum at VB. Surface smooth.
Acratia aff.badwildungensis Casier & Lethiers, 1999 (Plate 2, fig. 6) Material: Five carapaces.Description: DB straight; AB with relatively a large radius of curvature and with a maximum located at mid-H ; PB with a small radius of curvature and with a maximum located at lower third of H ; VB regularly curved.Left valve slightly overlaps the right one with a maximum at DB. Surface smooth.

Remarks:
In lateral outline, the specimens strongly resemble Acratia badwildungensis Casier & Lethiers, 1999 from Frasnian-Famennian (Late Devonian) of the Schmidt quarry, Germany (Casier et al., 1999).They differ by a more elongate carapace and an AB with a larger radius of curvature.
Description: DB straight, almost equal and parallel to VB; AB and PB taping.The left valve slightly overlaps the right one.Surface smooth.
Occurrence: Late Devonian of western Junggar, NW China, and Guangxi (Wuzhishan Formation of Yangdi section), South China.
Remarks: In outline, the specimens resemble Acratia tanaica Egorov, 1953 from the Late Devonian of the Russian Platform (Egorov, 1953), but broken valves forbid further assignation.
Description: DB nearly straight, ADB and PDB straight; AB with a small radius of curvature and with a maximum located below mid-H ; PB with a relative large radius of curvature and with a maximum located at lower third of H ; VB nearly straight.Left valve slightly overlaps the right one along the free margins with maxima at DB and VB.Surface smooth.
Description: DB straight to slightly convex; AB with a relatively large radius of curvature and with a maximum located below mid-H ; PB with a small radius of curvature and with a maximum located at mid-H ; VB straight to slightly curved.Left valve overlaps the right one with a maximum at VB. Surface smooth.
Remarks: In outline, the specimens resemble Famenella postkairovaensis Lethiers & Casier, 1996 from the Late Devonian of Coumiac (France) (Lethiers and Casier, 1996), but differ by bigger radius of curvature of PB and AB.Description: DB slightly convex; AB with a relatively large radius of curvature and with a maximum located at mid-H ; PB with a small radius of curvature and with a maximum located at mid-H ; VB straight to slightly curved.Left valve overlaps the right one with a maximum at VB. Surface smooth.
Remarks: These specimens are assigned to the genus Famenella Polenova, 1953 based on their outline, but poor preservation makes impossible further assignation.Description: DB slightly convex; AB with a large radius of curvature and with a maximum located at mid-H ; VB nearly straight; PB with a relative small radius of curvature and with a maximum located at the lower third of H ; left valve slightly overlaps the right one.Surface smooth.
Remarks: These specimens are assigned to the genus Famenella Polenova, 1953, for their outline.But this one has a smaller radius of curvature of PB by comparison with other species belonging to the genus.It may be a new species; however, to date, the material is insufficient to establish a new species.Remarks: In outline, the specimens are close to Bairdia dushanensis Shi, 1964, from the Middle and Late Devonian of Guizhou, South China (Shi, 1964).The difference is the larger radius of curvature of PB in our specimens and the maximum located higher.They may belong to a new species.
Description: DB slightly convex; AB with a relatively large radius of curvature and with a maximum located at mid-H ; PB with a small radius of curvature and with a maximum located at below mid-H ; VB straight to slightly curved.Left valve slightly overlaps the right one.Surface smooth.
Description: DB straight; AB with a relatively large radius of curvature and with a maximum located below mid-H ; PB with a small radius of curvature with a maximum located at the lower third of H ; VB slightly convex.Left valve overlaps the right one with a maximum at DB and AB.Surface smooth.
Genus Fabalicypris Cooper, 1946 Fabalicypris pseudoillustris Lethiers, 1974 (Plate 2, fig. 16) 1974 Fabalicypris pseudoillustris Lethiers: 1616, pl. 1, figs. 3, 4. Material: Four carapaces three valves.Description: DB nearly straight; AB with a large radius of curvature and with a maximum located at mid-H ; PB with a relative small radius of curvature and with a maximum located at the lower third of H ; VB slightly curved.Left valve slightly overlaps the right one with maximum at VB. Surface smooth.
Remarks: The outline of our specimens is similar to Rectobairdia wuxuantianensis Jiang, 1983 from the Middle Devonian of South China (Wei et al., 1983) Material: Two carapaces and three valves.
Description: DB regularly arched and ADB and PDB almost equally inclined; AB with a small radius of curvature and with a maximum located at lower third of H ; PB with a relatively larger radius of curvature and with a maximum located at mid-H ; VB slightly convex.Left valve overlaps right one with a maximum at VB. Surface smooth.

Remarks:
In outline the specimens resemble Bythocypris fabalis Cooper, 1941, from the Early Carboniferous of Illinois, USA (Cooper, 1941) Material: Eight carapaces and three valves.
Description: DB regularly convex; AB with a relatively large radius of curvature and with a maximum located at mid-H ; PB with a small radius of curvature and with a maximum located at lower third of H ; VB nearly straight.Left valve overlaps the right one with maxima at DB and AB.Surface smooth.Description: DB regularly convex; AB with a relatively large radius of curvature and with a maximum located a little below mid-H ; PB with a small radius of curvature and with a maximum located at lower third of H ; VB nearly straight.Left valve strongly overlaps the right one with maxima at DB and ADB.Surface smooth.

Remarks:
The specimens strongly resemble Bairdiocypris cracenis Jiang, 1983 from the Middle Devonian of Yunnan, South China (Wei et al., 1983) (Wang and Ma, 2007), but they differ by a more curved PVB.Our specimens perhaps belong to a new species, but at present the material is insufficient to establish it.
Description: Carapace arched.DB regularly curved; both AB and PB rounded with a small radius of curvature and a maximum convexity located at lower third of H . VB straight.Slight overlap of left valve on right one with a maximum at DB. Surface smooth.(Adamczak, 1976).(9) Right lateral view of complete carapace, YD15029; (10) Dorsal view of complete carapace, YD15030; Frasnian-Famennian, Yangdi section (Bed 68).(11) Kummerowillina cf.oblonga (Wei, 1988).Right lateral view of complete carapace, YD15031, Frasnian, Yangdi section (Bed 15).(12) Kummerowillina suboblonga (Wei, 1988).Right lateral view of complete carapace, YD15032, Frasnian, Yangdi section (Bed 15).Description: DB regularly arched; AB with a small radius of curvature and with a maximum of curvature located at lower third of H ; PB with a relative large radius of curvature and with a maximum located at mid-H ; VB nearly straight to slightly convex.Left valve overlaps the right one with maxima at VB and PVB.Surface smooth.

Remarks:
The specimens resemble Kummerowia oblonga Wei, 1988 from the Middle Devonian of Longmen Mts., South China (Wei, 1988) in outline.It differs by its straighter VB and a smaller overlap on PB.
Kummerowllina suboblonga (Wei, 1988) (Plate 3, fig.12 Description: DB regularly convex; AB with a relatively large radius of curvature and with a maximum located at mid-H ; PB with a small radius of curvature and with a maximum located at lower third of H ; VB nearly straight.The right valve strongly overlaps the left one with a minimum at PB.Some small nodules distribute irregularly on the surface.

Figure 1 .
Figure 1.Late Devonian lithofacies and palaeogeography of South China (modified after Huang and Gong, 2016; Ma et al., 2016) with the locations of the studied sections.

Figure 2 .
Figure 2. Distribution of ostracods in the Late Devonian in the Yangdi section, South China (log and conodont zones from Huang and Gong, 2016).

Figure 3 .
Figure 3. Distribution of ostracods in the Late Devonian in the Nandong section, South China (log and conodont zones from Huang, 2015).

Figure 4 .
Figure 4. Comparison of the distribution of entomozoacean ostracod found in the Nandong section (conodont zones from Huang, 2015; scale bars represent 200 µm).

Family
Three valves.Description: DB straight and long; CA obtuse and ACA larger than PCA.Both AB and PB rounded with large radius of curvature and maximum convexity located at mid-H ; VB regularly curved.Valve ornamented by longitudinal narrow ridges.Surface finely reticulate.
Sargentina Coryell & Johnson, 1939 Sargentina sp.(Plate 1, fig.13) Material: Two carapaces and three valves.Description: DB regularly convex; AB with relatively large radius of curvature with maximum located at mid-H ; PB with small radius of curvature with maximum located a little below mid-H ; VB slightly concave.Overlap of right valve on left one with a maximum at VB.A short subcentral sulcus located a little below DB.Surface smooth.Dimensions: L = 0.41-0.48mm, H = 0.28-0.35mm, W = 0.26-0.32mm.Occurrence: Middle Devonian of Yunnan, South China; Late Devonian of Guangxi (Xiangtian and Wuzhishan formations of Yangdi section), South China.

Material:
Four carapaces and two valves.Description: DB strongly convex, ADB and PDB almost symmetrically inclined; AB and PB widely rounded; VB straight to slightly concave.Left valve overlaps right one all around carapace with maxima at VB and AB.Surface smooth.Dimensions: L = 0.46-0.58mm, H = 0.32-0.43mm, W = 0.21-0.33mm.Occurrence: Late Devonian of Holy Cross Mts., Poland, western Junggar, Xinjiang, NW China, and Guangxi (Liujiang and Wuzhishan formations of Nandong section), South China.Praepilatina?sp.(Plate 3, fig.3) Material: Three valves.Description: Carapace rounded in lateral outline.DB strongly curved; PB slight rounder than AB; VB regularly curved.Left valve slightly overlaps the right one along the free margins with maximum at VB. Surface smooth.Dimensions: L = 0.50-0.60mm, H = 0.46-0.55mm.Occurrence: Late Devonian, Guangxi (Xiangtian and Wuzhishan formations of Yangdi section), South China.Remarks: Based on the rounded outline our specimens may be assigned to the genus Praepilatina Polenova, 1970.The specimens in outline resemble Praepilatina subcirculata Wang & Ma, 2007 from the Late Devonian in Xikuangshan, Hunan, China

Remarks:
The specimens close to Paracoelonella saltatoriaWang & Cao, 1997 from the Early Devonian of Yunnan, South China(Wang and Cao, 1997) in outline.However, in our specimens the ADB is sharper and the VB is straighter.Family Entomozoidae Přibyl, 1951 Genus Entomoprimitia Kummerow, 1939 Entomoprimitia cf.kayseri (Waldschmidt, 1885) (Plate 3, fig.15) Material: One carapaces and four valves.Description: Carapace subelliptic.DB straight; AB and PB rounded and nearly equal; VB regularly convex.Surface with 10 concentric ribs, and four to five longitudinal ribs in the middle part of the carapace; a small node is present at the end of sulcal depression.The sulcus is shallow and short.Dimensions: L = 1.00-1.20 mm, H = 0.96-1.08mm, W = 0.35 mm.Occurrence: Late Devonian, Guangxi (Wuzhishan formations of Nandong section), South China.
Material: Two carapaces and two valves.
, but differ by the small radius of curvature of the PB.Subrounded carapace.DB slightly curved; both AB and PB rounded with a large radius of curvature and a maximum convexity located at mid-H ; VB regularly curved.Surface smooth. Description: , in outline, but differ by more obtuse CA.
, but our specimens differ by larger carapaces and a smaller overlap on DB.
, but differ by a smaller radius of curvature at PB.
in outline.However, the broken carapaces forbid further attribution.