New species of Miocene cytheracean Ostracoda from the Pohang Basin, SE Korea

Seven new species of ostracods are described from the Miocene deposits of the Pohang Basin in southeastern area, Korea. Baffinicythere paiki is only the ihird record of the genus Baffinicythere in the world. In this study, the following new species are erected: Callistocythere kyongjuensis, C. seojeongriensis, Baffinicythere paiki, Urocythereis pohangensis, Trachyleberis leei, T. praeniitsumai and Acanthocythereis koreana.

Recently. studies on the Ostracoda o f the Pohang Basin have been undertaken by Huh (1991,1994), Huh and Paik (1992a, b) and Huh et al. (1994). Most o f these studies have been focused mainly on F)alaeoen\iironmentaI investigations of the Pohang Basin. This is the first detailed taxonomic study on new ostracods of the Pohang Basin.

GEOLOGICAL SETTING
'The Pohang Basin, the largest sedimentary basin along the eastern coas~ o f the Korean Peninsula. includes fossiliferous Miocene sedimentary rocks which are up t o 10 km thick (Chough, 1983). The Miocene sediinentary sequences o f the I'ohang Basin consist o f the Yeonil tiroup which unconformahly overlies Cretaceous and Eocene sedimentary and volcanic rocks., and consists largely of clastic sediments including conglomt:rate, sandstone and mudstone. The age o f this group is generally considered t o be Middle Mioccne and probably early to mid-Middle Miocene. Conglomerates are abundant in the western part o f the Pohang Basin and are intercalated with sandstone and mudstone, which are abundant in the centre of the basin. Most of the samples which yielded ostracods are from the middle part of the Yeonil Group, where the unconsolidated sandstoncs referred to in the taxonomic part are best developed.
'1-he Yeonil Group comprises two units: a lower unit. mainly derived by rnass flow deposits forming Gilbert-type fan delta. alluvial fan and steep-faced slope systems (C'hough el d., 1993), and an upper unit, composed of hemipelagic to pelagic sediments. The stratigraphical division of the sequence is conventionally divided into several formations, the boundaries and the exact age of which are still debatable (Um et ul., 1964;Kim, 1965: Yoon, 1975Yun, 1986: Choe & Chough, 1988: Hwang, 1993 unpublished doctoral thesis, Seoul University).

MATERIAL AND METHODS
A total of nine sections from six areas containing fossil Ostracoda were collected and measured. The collected areas are given in detail with available palaeontological information and with the columnar sections in Huh (I991 unpublished doctoral thesis, Korea University) and Huh Kr Paik (1992a). The studied areas and sections are as follows: Daejeonri (section DJ). Seojeongri (sections SJ I , SJ2), Danguri (Section D G ) , Hakjeondong (section HJ). Mulcheonri (sections MCI, MC2) and Ododong (sections ODI. OD2). In these areas, 63 samples were collected and examined. but only 24 samples yielded ostracods (Fig. 1).
The stratigraphical relationships o f the various samples to one another are depicted in the following outline: Lower - Most ostracods occurred in the massive, fine t o medium sandstone bed which also yields foraminifera, oysters and shell fragments. About 300 g portions of the dry samples werc processed. Each dried sample was treated using the saturated sodium sulphate solution and naphtha method.

SYSTEMATIC DESCRIPTIONS
The following abbreviations are employed in the descriptions: J =juvenile, C' = carapace, V = valve. RV = right valvc, I,V = left valve, L = length, H = height, W = width. All   Remarks. This species somewhat resembles CLIllisrocythrrr reticrilrrfrr Hanai. I957 in carapace outline and surface ornamentation, but differs in the finer details o f ornamentation and the pattern of anterior and posterior ribs.
Distribution. Abundant at sample horizon MC2-I , t o which it is confined.
Cullistocy there seojeongriensis s p . n ov .
(PI. 1, figs 7-10) Derivation of name. From Seojeongri village, the type locality of the species. Diagnosis. A species of Callistocythere characterized by its finely reticulate surface ornament and numerous, various ribs, especially strong posterior marginal ribs. Holotype. Female RV (CNU-0-505). Material. 11 carapace, 45 adults. Type locality and horizon. Sample horizon SJ2-3, Seojeongri area of Yeongil-gun, near Pohang. Unconsolidated sediment with foraminifera and numerous shell fragments; massive fine-grained sandstone. Description. Carapace small, subrectangular, highest at the anterior cardinal angle. Sexual dimorphism strong: female broader than male in posterocentral area, male slightly narrower than female in lateral outline. Anterior margin obliquely rounded; posterior margin truncated in upper half, gently rounded in lower half. Posterior cardinal angle distinct. Dorsal margin broadly arched; ventral margin broadly concave. Surface ornament with finely reticulation over most of the central part of the carapace. Two anterior marginal ribs prominent: outer rib weakly, very short, nearly vertical: inner rib narrow, distinct, broadly rounded, extending from lower terminal to upper terminal of anterior margin. Two posterior marginal ribs strongly prominent; outer rib subcircular, extending from the posterior fifth of dorsal margin to the ventral sinuosity: inner rib subrounded, distinct, starting from the posterior fourth of dorsal margin and terminates at mid-length. Numerous small ribs occur, and a sinuous posteroventral rib is especially distinct. Eye spot distinct. Hingement pseudoentomodont and of the Callisrocythere juponicn type. Calcified inner lamella broad anteriorly: moderate posteriorly. Anterior vestibulum narrow. Snap-knob and snap-pit distinct. Four adductor muscle scars in a vertical row and a frontal muscle scar in a row. Normal pore small, few, simple.
Distribution. Abundant at sample horizon SJ2-3 and rare at SJ2-1, SJ2-2, SJ2-4, HJ-2, OD1-2, OD1-3 arid ODI-4. Anterior margin well rounded without distinct cardinal angles: posterior margin rather narrowly terminated; upper half broady arched, lower half convex. Dorsal margin virtually straight and inclined towards the posterior. or broadly convex: ventral margin more or less sinuous, ;interior third slightly concave, behind midlength slightly convex. Surface coarsely, regularly reticulate with distinct ribs forming triangle in postero-lateral area but slightly lesh prominent than that of BqfJinicyrhcw lzowei. Less prominent submarginal curved rib extending from in front of cyc tubercle, converging with anteroventral margin, and bending upwards posteriorly to meet anterior corner of postcrovcntral triangle o f ribs. Slightly prominent postcrodorsal and posterovcntr;il node, extending to nearly vertical ribs. Subcentral tubercle pre\ent but not prominent. Eye tubercle sin al I, prom i ne n 1. Ma rgi n ; i I pore cii 11 a1 s n umcrous. s w ol I e n at mid-length. Small anterior and posterior vestibulac. Hinge robustly holaniphidont. with posterior tooth o f the right valve reniforni. Ocular sinus conspicuous. Normal pore canals scattered. sieve type. Adductor muscle scars; a single subrounded dorsal scar. two rounded dorso-median scars, an elongate ventro-median scar (in some cases, the right half is absent) and an elongate ventral scar. Two rounded frontal scars. Strongly sexual dimorphism; male more slender and shorter than female. Remarks. This species is superficially similar in general appearance to BoffirzicyrhwcJ rwiurgirirrra (Sars, 1866) (Hazel. 1967), but differs in details of ornament, carapace outline, carapace size and frontal muscle scar pattern. This species is distinguished from Hcifinicyrlirrr liowci Hazel, 1967(Hazel, 1967Horne & Whittakcr. 198.1) in its different muscle scar pattern. size, sexual dimorphism and the distribution pattern o f its ribs. Also, this species is distinguished from Mrritlioticrli~yrheua ciiscopl7orrr (Skogsberg, 1928) from the S. W. Atlantic (Whatley c v ul., 1087) by its adductor muscle scar pattern and the nature of its surface ornament. This species seems to belong to Hrr,fjinicyhrve by virtue o f its undivided ventro-median adductor muscle scar, strong sexual dimorphism and the triangle formed by ribs in the postero-lateral area. This species is also closely allied t o the genus l-loriiicytl~c~rc~ in h;iving two frontal scars, but Hazel (1967) observed that the lower frontal muscle scar tcnds t o be easily overlooked.
Distribution. Abundant at sample horizon SJ2-3 and rare at SJ2-2. SJ2-4, H.1-2 and MC'2-1.  fig. 7. Derivation of name. From Pohang, the Tertiary sedimentary basin in Korea where the type locality o f the species is situated. Diagnosis. An inflated, elongate species of Uvocytlicrc~is characterized by its coarselv reticulate surface ornament with numerous intramural spines. two subp;ir;illel vcntrolateral ribs and prominent whcciitral tubercle.

Holotype. Male LV (CNIJ-0-510).
Material. 3 carapaces, 84 valves, 22 ,juveniles Type locality and horizon. Sample horizon 9 2 -3 , Scojeongri area of Ycongil-gun. near I'ohnng. I inconsolidated \cdimcnt with foraminifera and shell fragments rich; massive line-grained sand\tonc. 1)escription. Carapace large. iitHatcd, clongatc and subrectangular in lateral view. Maximum height at anterior cardinal angle. Anterior margin well rounded: posterior margin dorsally and ventrally convex and centrally compressed. Posteroventral margin more or less prominent, projecting posteriorly. Dorsal margin nearly straight. slightly inclined posteriorly: ventral margin with oval incurvatine at mid-length. Surface coarsely reticulate with numerous intramural spines. 'Two subparallel ribs extend across the ventro-lateral surface. Slightly depressed postero-lateral area. Prominent subcentral tubercle. Eye tubercle ovate with post-ocular sulcus. Normal pore canals scattered, sieve type. Marginal pore canals numerous and straight. C'alcified inner lamellar of median width. Anterior vestibulum moderately deep. Hinge holamphidont, with reniforni posterior tooth in right valve which exhibits a wide auriline notch. Adductor muscle scar vertical row of four. of which the two middle scars are distinctly divided. Threc frontal scars, of which the middle is distinctly smaller. Strongly sexual dimorphism. male more elonsate and slender than female.  (Irizuki. 1996) in lacking ;I denticulate posterior margin and in its larger size. (lriiuki'y specimens measure Holotype L = 1.219. H = 0.569 and the paratypes range from 1.268 t o 1.336 long, and 0.630 to 0.740 high). The present Ypecies is distinguished from other-I!r-oc.~flic,rc,is species hv its siiriacc ornuiient with intr:rmur;iI s pi 11 cs
Type locality and horizon. Sample horizon SJ2-3, Seojconp i area o f Yeongil-gun, near Pohang. Unconsolidated sediment with foraminifera and rich in shell fragments: massive tine sandstone. Description. Carapace large, subrectangular, tapering posteriorly. Left valve is slightly larger than right valve. Strongly >exually dimorphic: male is longer and narrower than female in lateral view. Greatcst length near mid-height, greatest height at anterior cardinal angle. Anterior inargin broadly and obliquely rounded with tiny spines: posterior margin protruded posteriorly, posterodorsal margin slightly concave or straight. posteroventral margin broadly and obliquely rounded with small spines. Dorsal margin straight.,. with anterior hinge ear; ventral margin with more pronounced oral incurvature in male than in fc male. Surface reticulate ornament with con,junctive spine:; on intersections of lattice. Eye tubercle distinct, prolongcd with ocular rib which extends to the mid anterior riiargin, from whcre it parallels the anterior and ventral niargins. Anterior region compressed Posterior lateral surface almost smooth with some small spines. Marginal denticles spinuse numcrous, small, along anterior and postcroveritral margins. Internal features typical o f genus with distinct snap-knob orally in RV. Dimensions (mm). Remarks. The present species resembles 7rrrc.liylebrris ,~iiiicnrtniicrisi.s Yajima. l9Y2 from Mizunanii i n central Japan in shape and surface ornamentation, but the present species d ffers from thme latter in its weak, shorter ocular rib, the riat ure o f its ;interior ornament and its v-shaped frontal muscle scar. This species differs from most other species of 7'rric./iylf~hr~ri.c. because of its enhanced reticulate ornament. However, it clearly belongs in Trcrclivlt+eris because o f its anterodorsal rib, subrectangular lateral outline. spinose ornament and oblique ocular rib. The type species, 7rric~/iylrhc~ris scrihror~irnrcrta (Brady ). is more elongate, is pustulate rather than spinose and has a very strong, sharp. ocular rib.
at SJ2-1. SJ2-2. SJ2-4. S.12-5, HJ-I, H.J-2, ODI-2, ODI-3,  fig. 12. Derivation of name. From Korea, referring to the first occurrence of the species in the Korean peninsula. Diagnosis. A rather large, thick-shelled species of A~,rrnrhoc,ythcrc,i\ distinguished by its surface ornament with numerous small spines and tubercles superimposed on coarse web-like reticulation and lack of short vertical posterodorsal rib. Holotype. Fenialc, KV ((:NU-0-533). Material. 27 car;tpaces, 159 valves, 2 juveniles. Type locality arid horizon. Sample horizon DJ-1, Daejonri area o f Yeongil-Sun, near Pohang. Ilnconsolidated foraminifera and shell lragmcnt rich; massive mudstone. Description. ('arapacc rather large, thick-shelled, subtrapezoidal. I n lateral view, carapace tapering slightly toward the po\terior end. Sexual dimorphism distinct; male carapace is longer and narrower than female carapace. Maximum height at anterior cardinal angle. Anterior margin broadly rounded and nearly symmetrical with numerous inarginal spines: posterior margin nearly vertical and obliquely rounded with sinall spines. Dorsal margin straight with numerous spines; ventral margin nearly straight in female, but slightly sinuous in male. Surface ornanicnt with numerous, irregularly small spines superimposed on feeble web-like reticulation. Anterior marginal spines arranged concentrically about subcentral lubercle. Subccntral tubercle prominent. Eye tubercle pl. 2, figs 8, 9. distinct, just below anterior end of dorsal margin, and somewhat obliquely elongated. Calcified inner lamella moderately wide. Line of concrescence and inner margin coinciilc throughout the margin. Marginal pore canals numci-om, simple, nearly straight, and numerous at anterior and posterior margins, and slightly inflated near the middle ventral margin. Hinge holamphidont, with slightly crenulate posterior tooth in right valve. Muscle scars consist of a v-shaped frontal scar and a vertical row of tour adductor scars: dorso-median adductor scar clongiitc anteroventrally. Normal pores simple. Remarks. This species resembles Acunthocythereis mut-.suen.\r.s Ishizaki 1971 from the Aomori bay of Japan in general appearance, but differs in the developmental degrci: of spines, the reticulation on the surface, lack of a short vertical posterodorsal rib consisting of small spines and larger carapace size. This species differs from Acan/hocythereis dunelmensis (Norman, 1865) from Quaternary of the Gulf of Alaska (Brouwers, 1993) in its less developed reticulate ornament, and numerous, irregularly small spines, and lack of a short vertical posterodorsal rih.