A Revision of the Genus Sanyuania Zhao and Han, 1980 Ostracoda, Crustacea) with the Description of New Species from the Late Cainozoic of China

The endemic genus Sanyuania, one of the most important elements of the Cainozoic brackish water faunas unique to China, is redescribed, and 3 new species, S. wangi, S. cuneata and S. sublaevis are established, based on Quaternary material from the eastern coast of China. The evolution of the genus is discussed and a general trend involving a reduction in the strength of carapace ornamentation with time is established, as is a general migration seawards exhibited by subsequent chronological species.


INTRODUCTION
Sanyuania was first recorded in manuscript by Huang in 1970's from the Pliocene and early Pleistocene of the Fenwei Basin of Shanxi Province, central China. It has subsequently proved to be widespread in Quaternary sediments of eastern China, along the coasts of the Yellow and East China Seas (Zhao and Han, 1980;Zhao et al., 1986;Hou et al., 1982;Wang, 1982;Zhang, 1985;Yang et al., 1988;and others). The significance of this genus in the paleoenvironmental analysis of these sediments has been briefly mentioned by one of us (Zhao and Han, 1980;Zhao et al., 1986) and its appearance in Quaternary sediments of coastal areas is considered as a reliable indicator of marine-continental transitional facies. Sanyuania was once thought to be monotypic. Additional material, however, have been collected in recent years during our studies of the late Cainozoic and Recent Ostracoda of eastern China. The purpose of this paper is to describe these new species of Sanyuania and to emend its diagnosis, and also to comment on its significance as one of the most important elements of late Cainozoic brackish water faunas of China.

Sanyuania psaronius
Surface reticulation with multiangular fossae and delicately punctate solae. A small but prominent, punctate, rounded tubercle developed posteroventrally. Sieve-type normal pore canals rounded and small. Inner lamella moderately wide with narrow vestibulae anteriorly and posteriorly.
Some 12 radial pore canals anteriorly and 8 posteriorly. Adductor scars in an oblique row.. Hinge of LV with a bar anteriorly bearing 3 large teeth; posteriorly the terminal element is a quadriloculate socket; median element a smooth bar with weak denticles anteriorly and posteriorly. Material. More than 20 specimens. Distribution. Pleistocene and Holocene, coastal plain of easternchina (Fig.2). It always occurs withthe twoeuryhaline marine ostracods Sinocytlieriden impressn, Leptocytherc ventriclivosa Chen and non-marine ostracods such as Cnndonn spp. and Cnndonielln spp.. The present species seems to be extinct but in the past to have lived in inland brackish water or supratidal environments of low salinity. Description. Surface ornamented with weak concentric ribs and intercostal punctae which tend to merge into reticulation  centrally. All other characters as for genus except that the anterior socket of the RV has a delicate anti-slip bar ventrally.  (Fig.2).
Description. Cardinal angles distinct in RV. Surface nearly smooth centrally but with a few feeble ribs anteriorly and posteriorly. Sieve-shaped normal pore canals large, rounded and very evident. Carapace in dorsal view with gently arched lateral margins and acutely pointed ends; greatest width centrally. Inner lamella moderately wide with relatively large terminal vestibulae; radial pore canals short, straight and simple, 10-12 anteriorly and 8-9 posteriorly. Other characters as for genus. Remarks. This species is characterized by its virtually smooth surface, which is shares with Sanyuania abei (Choe), 1988. The two species differ, however, in that the latter is more elongate and more pointed posteriorly.
central inland basin to the eastern coastal areas, while the other 3 Chinese species are limited to the eastern coastal areas. S. abei is apparently confined to Recent shallow seas off the southern coast of Korea. It is probable that S. psaronius is the ancestral species of the genus, although' relationships to an ancestral genus are unknown. The probable speciation patterns within the Sanyuania lineage is shown in Fig.3: S. wangi evolved from S. psaronius in the early Pleistocene, and subsequently inthelatePleistocene,S. cuneata and S. sublaevis evolved from either S. wangi or S. psaronius. The former is more likely given the observed similarity in ornament between these species. S. abei was almost certainly derived from S. sublaevis, probably late in geological time.
The evolutionary sequence S. psaronius -S. wangi -S. cuneata -S . sublaevis -S. abei illustrated in Fig. 3, is also a morphological series illustrating a general reduction in strength of ornament with time from the robustly reticulate/ punctate S. psaronius to thecompletely smooth S. abei.
During the Pliocene -Recent evolution of this genus, a trend of seaward mirgration through time takes place. In the Pliocene and early Pleistocene, S. psaronius occurred only in the inland brackish water basin of central China (Lin et al., 1982), but since the mid-Pleistocene all 4 Chinese species have been restricted to the coastal areas of eastem China where they have become one of the most common inhabitants of marginal marine environments, such as estuaries, lagoons and tidal marshes (Zhao & Han, 1980;Zhao et al., 1986). The Recent Korean species, S. abei, has become a typical marine form living in shallow seas with a depth range from 21.5 to 75m (Choe, 1988). S. abei is probably confined to the shallow Korea. Of the five 'pecies reported above, S. psaronius is the oldest and the only one from the Pliocene-early Pleistocene. The first appearance of S. wangi is dated as early to mid-Pleistocene; and that of S. cuneata and S. sublaevis islatel'leistocene. S. abei isknownonly from the Recent. S. psaronius has a wide distribution from the water part of this range and those specimens found by Choe at greater depths were post-mortem transported seawards.
The late Cainozoic brackish water ostracod faunas of east-exclusion was caused by the similar ecological requirements of Cyprideis and its Chinese endemic competitors.
em China are characterized by Albileberis, Cocoonocythere, Sinocythere, Sanyuania, Sinocytheridea and Neosinocythere. These genera occur either only in China (the former 3) or mainly in China (the latter 3) (Zhao and Wang, 1988;Zhao and Whatley,1989;Ishizaki, 1990). The ecological distribution of these genera is summarized in Fig. 4. Sinocytheridea is the most typical member of the fauna being the most widespread ecologically and geographically, and is abundant and ubiquitous in late Cainozoic brackish water faunas (Zhao and Han, 1980;Zhaoet al., 1986;Zhao, 1985;Zhaoand Wang, 1988;Li, 1985;Wang et al., 1988;and others). Sanyuuniu is one of the important elements of brackish water faunas especially in the Pliocene and early Pleistocene of central China where it often is the dominant genus. Compared with other regions of the world, the brackish water faunas of eastern China are unique. The most notable features is the absence of the worldwide euryhaline genus Cyprideis (Jones) from both fossil and modern brackish water faunas in eastern China. Cyprideis has only been found in western and central China where it occurs in brackish formations of Miocene toHolocene age and in modem salt lakes (Yang, 1981(Yang, ,1988Lin et al., 1982;Hao and Zhen, 1984;Xi et al.,). The Chinese late Cainozoic brackish water ostracod faunas, therefore, can be divided into two types: the western type characterized by the presence of Cyprideis, and the eastern type where Cyprideis is replaced by Sinocytheridea and other Chinese endemic genera. The boundary between the two types is located at the central part of China (Fig.5). The absence of the otherwise ubiquitous euryhaline genus Cyprideis from both fossil and Recent faunas of eastern China has been noted by Wang et 01. (1985); Zhao and Wang (1988). They argued that mutual