Plio/Pleistocene Candonidae (Freshwater Ostracoda) from boreholes in The Nanning Area, Guangxi Province, Southern China

Six cored boreholes in Guangxi Province, southern China yielded a fauna of Cypridacea as well as diverse Limnocytheridae which will be the subject of a separate publication. This paper deals only with the Cypridacea. Four species of Candona were present of which three are new, one previously described species of Candoniella (a controversial genus which is probably a synonym of Candona) and one new species of Pontoniella.


INTRODUCTION
Six out of nine cored boreholes drilled in the area round Nanning in Guangxi Province, southern China yielded nonmarine faunas in the form of cypridacean and limnocytherid ostracods. This paper deals with the cypridacean part of the fauna. Preservation was generally good although, notably in the lower part of the X4 borehole between 120 and 126 metres, the calcified matrix in which these delicate valves were found made the ascertainment of the finer morphological details difficult. Seven species of candonids were found including three new species of Candona and one of Pontoniella. One previously described species came from southwest China and the other from Central Asia. In view of the large number of species described from Europe it was somewhat disappointing that no European species could be recognised.
The Tiandong Borehole (X9), which lies some 150 kilometres from the other boreholes yieldedonly Candoniellaperisena Yongning area and contain good faunas except for the limited material of Yongning No. 1 (X7). Candona nanningensis sp. nov. occurs in all five boreholes and is wide ranging. Pontoniella praeceps sp. nov. occurs in four of the five holes and, apart from an isolated occurrence at 12 l m in Nanning No. 1, occurs in the upper part of the sections. Candoniella perisena Jiang 1983 occurs in the upper and middle parts of Nanning Nos. 1 , 2 and 3. A species compared with C . perisena is found in the lowest parts of Nanning Nos. 1 and 2 and low down in Yongning No.
2 but preservation problems make detailed analysis of this form difficult and it would be unwise to place too much emphasis on it as an arbiter of the lower horizons. Candona compressaeformis Mandelstam 1963  Due to their fragility the faunas are not well enough developed to recognise more than tentatively an upper, middle and lower part in the sections based on the evidence outlined above. Upper, middle and lower parts may also be recognised, however, in the case of the Limnocytheridae where the fauna is more diverse and the correlation potential correspondingly greater.
The age of the material is suggested by a number of lines of evidence. Of the two previously described species, Candona compressaeformis Mandelstam 1963 was found in the Pliocene of Kazakhstan and Candoniella perisena Jiang 1983 in the Pleistocene of Southwest China. The new species are not helpful in this respect although their general aspect suggests PliocenePleistocene affinities. The nature of the fauna provides some supplementary evidence. Candona is confined to the northern hemisphere and favours cooler, more temperate waters. Its presence so far south suggests that conditions may have been cooler in the area at that time. This may be linked with the cooling that took place during the Pliocene and Pleistocene leading to the onset of glaciation in large parts of the northern hemisphere. Taken altogether the evidence suggests that the material is of PliocenePleistocene age.
The environment was freshwater to oligohaline. All material is deposited in the collections of the Academia Sinica, Institute of Geology in Beijing to which the specimen numbers refer.

SYSTEMATIC DESCRIPTIONS
Superfamily Cypridacea Baird, 1845 Family Candonidae Kaufmann, 1900Subfamily Candoninae Kaufmann, 1900 Genus Candona Baird, 1845 Candona compressaeformis Mandelstam, 1963 (Pl.  It is not close to any of the Early Tertiary coastal forms from the Bohai Region and although C. aequalis Li & Lai (1978, pl. 44, figs. 2-4) is the nearest, the latter differs in its much steeper posterior margin and far greater size (length 1330pm, 1500 The Chinese specimens are fairly close to C. annae Mehes, 1913 as figured by Keyser (1976, pl. 7, figs 6,7) from SW Florida although not so concave ventrally and less narrow in the anterior part which is also not set at such a marked angle to the length. Keyser's material is also larger (0 960-1060 pm; 0 1000-1 10 pm).
The present material differs from C. (Cryptocandona) vavrai (Kaufmann), as figured by Sywula (1974, pl. IV, a,b) in his work on the Polish fauna, in its less angular antero-dorsal margin. Sywula's specimens are slightly larger (850-930 pm). Candona trieheli Krstic 1972 is somewhat similar but is more elongate in proportion to the length, is not so high anteriorly and is larger (length 1090-1360pm).
The Nanning specimens are fairly close to C. kirgizica Mandelstam 1963 from the Xining and Minhe basins (Hao et al., 1983,pl. 12,figs 17a, 18)although thelattertapers toomuch in side view. The present material differs much more from the material of C. kirgizica figured by Mandelstam (1 963,pl. XXII,figs 8,9)  Other species are not close.

pm).
Candona (Fahaeformiscandona) nanningensis sp. nov (Pl. I , figs 3-6) Derivation of name. In reference to its occurrence in the Nanning borehole. Diagnosis. A species of Candona with greatest length at midheight. Dorsal and ventral margins parallel with only slight concavity in the middle of the ventral margin and very straight postero-dorsal margin which begins at two-thirds length. Anterior margin evenly rounded. Remarks. C. nanningensis shows similarities to a number of species already described. In 1978 Guan (p. 193,pl. 49), figs 5,6) described Candona cyrtoformis from Oligocene deposits of South Central China. This agrees well in size (875-900 pm) and general shape but has a shorter, straighter  fig. 4) from the Xining and Minhe basins is fairly close in shape although the correspondence is not exact. In Hao's species the maximum length (junction of straight posterodorsal margin and curved postero-ventral margin) is higher but on the whole very near to the situation in our species. However, in his material the dorsal margin is more vaulted, the shell is less elongate and is much smallerlength 680pm. Hao's original material was even smaller (length 600pm). In the present species the height also tends to be maintained more anteriorly.
C . nanningensis shows considerable affinities with the Candona fahaeformis group and is fairly close to C. fahaeformis of the Polish fauna figured by Sywula (1974,283, pl. V, figs a-c) but the present species tapers less anteriorly. Candona (Fahaeformiscandona) aff. krstici Petkovski 1969 as figured by Krstic (1972, p. 93-4, pl. XXIX, (figs 5,6) is probably closest and with its given length of 880-960 pm it also agrees well in size. In Petkovski's species the postero-ventral margin appears to be more broadly rounded.

Diagnosis.
A species of Candona with the greatest height at the mid-length, posterior of which the straight dorsal margin falls gently to the posterior cardinal angle at three-quarters length and then steeply to the posterior termination at about one-third (PI. I , fig. 7) height. Anterior half of dorsal margin straight, sloping gently anteriorly where it joins the evenly rounded anterior margin. A gentle concavity occurs in the middle of the ventral margin. Holotype. A Right Valve No. 9.11 Length 1100pm, Height 550pm. Type Horizon. Depth 87-88m, Borehole X4, Nanning No. 1. Other Horizons. 8 1-82m, 123-124m, Borehole X4, Nanning No. 1.

Dimensions of figured specimen
Length Height Holotype, Right Valve 9.11 (PI. 1, fig. 7) 1 lOOpm 550pm Other material varies in length from 750 pm to 1015 pm. Remarks. The present species is close to Candonaprorensa Li & Lai (1978 pl. 38, fig. 13). In the present material the posterior margin has a straighter and steeper slope and the highest part of the valve lies in the middle of the dorsal margin, whereas in C. protensa the highest part occurs at the posterior cardinal angle. It is also quite close to C . bellula Yang 1982in Hao et al. (1983 fig. 19) from the Xining and Minhe basins but C. bellula has a longer, steeper fall antero-dorsally, the postero-dorsal margin is less straight and it tends to be higher at the posterodorsal comer.
The closest comparison in material figured from the Cretaceous-Quaternary of Jiangsu (1982) appears to be Candona (Lineocypr-is) lubrica Chen & Ho (pl. 19, fig. 9) whose length ranges from 920 to 1090 pm. The resemblance is probably accidental since it does not compare closely with the other figured specimens and especially with their pl. 19, figs 1-6. In general C. lubr-ica has the greatest height at the posterior cardinal angle whereas in our species it lies at the mid-length.
None of the species figured by Krstic (1972) in her monograph on the genus Candona show any resemblance to this species . 57-58m, Borehole X5, Nanning No. 2.

Candona (Sirmiella?) posteroconica sp. nov.
(PI. 1, figs 9,11) Derivation of name. A reference to its distinctive shape posteriorly in side view. Diagnosis. A species of Candona with subparallel dorsal and ventral margins, evenly rounded anterior margin, only slight concavity in the median part of the ventral margin but with very distinctive posterior termination, the gently convex postero-Explanation of Plate 1 Paired stereographic photographs. External lateral views. All material from Borehole X4, Nanning No. I , Nanning, Guangxi Province, China.    Pl. 1, fig. 9) 775pm 395pm Right Valve 9.5 (Pl. 1, fig. 11) 755pm 370pm Description. See Diagnosis and stereographic figures. Remarks. In her detailed workon the candonids, Krstic (1972) recognised a number of new subgenera including Sirmiella which she differentiated from Caspiolla by a number of features including their generally smaller size. The present material agrees fairl-y well with her interpretation of this taxon. However, the posterior termination of our species is less downturned than in typical Sirmiella and so is only tentatively referred to this subgenus. Among Krstic's species our species is closest to C. (S.) sirmica Krstic (1972, pl. VII, figs 6-9) but differs from the latter in its sharper and slightly higher posterior termination of the shell in side view. Candona (Lineocypris) pallida Chen & Ho sp. nov., in Hou etal. 1982 (pl. 20, figs 1-8) is similar but differs in having the greatest height at the anterior, not the posterior cardinal angle. The right valve also shows a considerable resemblance to Lienenklaus's C. recta Lienenklaus 1905 from the Lower Miocene of Germany especially in the nature of the posterior termination and its size (800pm but our specimens are more rounded anteriorly and not quite so accentuated in height at the posterior cardinal angle. It is also similar to the Upper Oligocene Candona candidula being virtually identical in size, but the left valve in that species is very different with a much lower posterior termination. Another somewhat similar species, C. balatonica Daday afinis Zalanyi 1959 also has a lower posterior termination and is much bigger (1 120pm). The large (1 220pm) OliocenePleistocene C. chosei. Dickinson & Swain 1967 is much more concave ventrally. Other species are not close.
Genus Candoniella Schneider, 1956 This was regarded as a doubtful genus in the Treatise (Moore 1961) and the type species was firmly placed as a synonym of Candona by Hartmann & Puri (1974). Jiang's species represents a well known group of species which are usually placed in Schneider's genus. This group should probably be recognised as a subgeneric grouping in Candona. Meanwhile we have here retained Jiang's original combination. Jiang, 1983 1983 Candoniellaperisena Jiang (sp. nov.) inLi Yu-Wenetal., 93, pl. 28, figs 5,6.

Dimensions of figured specimen
Length Height Left Valve 9.13 (Pl. 1, fig. 12) 650pm 305 pm Remarks. The present material (length 650-670 pm) agrees well in size and shape with Jim's Pleistocene species (630-650 pm). As figured by Mandelstam & Schneider (1963, pl. XXVII, fig. 6) C. Pellucida Schneider differs in the posterior termination of the shell as seen in lateral view and in its greater size (750 pm). Candoniella extensa Shi & Yan (1978, pl. 47, fig. 17) from the Oligocene of the coastal region of Bohai differs in its more rounded posterior termination and greater size (length 870 pm). Candoniella tianzhuangtaiensis Shi & Si 1978 (pl. 48, fig. 8) also from the Oligocene of the coastal region of Bohai is somewhat similar but differs posteriorly; its greatest length is below mid-height and the species is slightly larger (length 760 pm).
Jiang's original specimens came from the Pleistocene Yangling Formation of Songmin, Yunnan Province. Jiang, 1983 In the lower part of Boreholes X4, X5 and X8 delicate valves occur in a calcified matrix. This makes it difficult to ascertain their shape and morphology accurately. They probably belong in Jiang's species but at present they are only tentatively referred here. Horizons. Depths 120-121m, 123-124m, 125-126m, Borehole X4, Nanning No. 1. 62-63m, 63-64m, 64-65m, Borehole X5, Nanning No. 2. 96-97m, Borehole X8, Yongning No. 2. Subfamily Disopontocypridinae Mandelstam 1956 Genus Pontoniella Mandelstam 1956 Mandelstam established this genus with Paracypria acuminata Zalanyi 1929 as the type species. Zalanyi (1929,57-61, figs 23,24) gave good figures of this showing the typical elongate triangular shape and the main muscle scar pattern which consists of a rosette of five scars with an elongate scar above. The hinge is simple, the dorsal margin of the right valve fitting into a groove below the margin of the left valve. Anteriorly the left valve margin expands slightly to form a characteristic small overhanging flange well figured by Mandelstam & Schneider (1963,85, fig. 81). Our specimens agree well with all these features and fit well in this genus. The genus Subulacypris Schneider 1957, which agrees quite well in shape with the Guangxi material and whose type species comes from the Quaternary of northern China, was considered but rejected on account of its very wide inner lamellae and the lack of a marginal expansion of the left valve dorsal margin anteriorly to fonn a small flange. Krstic (1971) treats Pontoniella as a subgenus of Candona and lists the species which she considers to belong in it. The differences from Candona appear to justify the full generic status which is adopted here.

cf. Candoniella perisena
The genus is typically Pannonian (late Miocene) to Recent in age and best known from the Pontian of Europe.
Derivation of name. praeceps L. steep, in reference to the steep anterior margin in side view. Diagnosis. A species of Pontoniella whose anterior margin is steep and very widely rounded, meeting the antero-dorsal margin in a distinct, obtuse angle. Greatest height lies at about one-third the length and posteriorly the valve ends in a blunt point giving the greatest length at about one-sixth height. Holotype. A Right Valve No.,9 . 8

Description.
The inner lamella is fairly broad anteriorly and forms a smaller, crescentic vestibule postero-ventrally. The marginal areas are narrow with fine, straight, densely-packed marginal pore canals which number about 50 anteriorly. The central muscle scar pattern consists of a compact cluster of four or five scars with an elongate scar dorsally. Some specimens appear to show an additional elongate scar above the latter. External features are adequately covered by reference to P1. 1, fig. 10.

Dimensions of figured specimen
Length Height Holotype Right Valve 9.8 (Pl. .1, fig. 10) 660pm 305pm Remarks. The closest European species is Candona (Pontoniella) paracuminata Krstic (1968), a new taxon set up to accommodate the Upper Pontian material from the Pejinovic areaof Serbia placed in Paracypria acuminata by Zalanyi (1929). Krstic's figures (1971, pl. 1, figs 9-1 1) show a good general correspondence but a more prominent posterior cardinal angle and sharper posterior point in side view. The Caucasus specimens figured by Mandelstam & Schneider (1963, pl. VI, figs. 1,2) as Pontoniella acuminata (Zalanyi) are also close but again have a sharper posterior termination in side view. Candona (Pontoniella) glabra Krstic (1969,728-9, pl. 1, figs 6 7) from the Pontian of Kladovo differs most notably in its greater height at the posterior cardinal angle. Krstic's line drawings give a good representation of the internal structures.
Differences from the genus Subulacypris have been dealt with above but some forms included in this genus resemble the ( Pl. 1, fig. 10) present species and the genus is well known from the Miocene, and especially Pliocene and later rocks in China and the USSR. Among Soviet species, S. gubkini Mandelstam 1962 (in Mandelstam & Schneider) from the Upper Sarmatian (Upper Miocene) of the Caucasus is the closest. This differs in being higher in proportion to the length and in having the anterior margin more rounded in lateral view.