Bicornucythere concentrica, Yamaguchi, Tatsuhiko, Suzuki, Hisashi, Soe, Aung-Naing, Htike, Thaung, Nomura, Ritsuo & Takai, Masanaru, 2015

Yamaguchi, Tatsuhiko, Suzuki, Hisashi, Soe, Aung-Naing, Htike, Thaung, Nomura, Ritsuo & Takai, Masanaru, 2015, A new late Eocene Bicornucythere species (Ostracoda, Crustacea) from Myanmar, and its significance for the evolutionary history of the genus, Zootaxa 3919 (2), pp. 306-326 : 312-316

Bicornucythere concentrica sp. nov.

( Figs 4 A–I and 6)

Type material. Holotype, UMUT-CA31128, carapace; paratype, UMUT-CA31129, right valve, UMUT-CA31130, left valve, UMUT-CA31131, carapace.

Diagnosis. A species of Bicornucythere characterized by a medium-sized carapace and concentric reticulation with polygonal fossae and sharp muri.

Description. Carapace robust and medium in size (length, 569–654 µm). Left valve slightly larger than right. In external view: lateral outline subrectangular; anterior margin rounded; posterior margin bluntly angular; dorsal margins slightly sinuated; ventral margin curved. Maximum length along middle of carapace; maximum height through anterodorsal point; maximum width across carapace, one-third of the distance from posterior to anterior ends.

Surface ornamented with concentric reticulation formed by polygonal fossae and sharp muri. Polygonal fossae present on upper half of carapace; elongated fossae exist on lower half. Muri parallel to anterior and ventral margins in both anterior and ventral areas. Spine projecting in posteroventral area. Marginal denticles present along anterior margin. Eye tubercle present below anterodorsal point.

In dorsal view: anterior margin narrow at one-sixth the distance from anterior to posterior ends, and tapering toward anterior end; posterior margin bluntly angular; lateral margins broadly curved with weakly indented near middle of margin. On hingement of right valve, triangular teeth present in both anterior and posterior elements.

In anterior view: ovate outline; angular dorsal margin, forming apex; ventral margin angular, flattened on bottom; lateral margins gently curved.

In internal view: anterior marginal infold broad; hingement amphidont-type. In right valve of hingement: anterior element with colonial tooth and a socket behind the tooth; posterior element with colonial tooth.

Measurements. UMUT-CA31128, holotype, L = 569 µm, H = 326 µm; UMUT-CA31129, paratype, L = 654 µm, H = 353 µm; UMUT-CA31130, paratype, L = 651 µm, H = 325 µm; UMUT-CA31131, paratype, L = 580 µm, H = 349 µm.

Type locality. The horizon of sample 69-1 is 464 m above the base of the Yaw Formation in the Kyauktakha section, Myanmar (21°26.693′N, 94°18.853′E) ( Figs 1 and 2 ).

Etymology. “Concentric” in Latin. Named after its concentric reticulation, which is a diagnostic trait for this species.

Remarks. We identified A, A-1, and A-2 instars based on our measurements of 110 values of B. bisanensis ( Fig. 5 ). During molting from the A-1 to the A stage, the mean L and mean H of B. bisanensis increased 1.31 and 1.16 times, respectively, corresponding to an increase in the valve area of 1.52 times (the product of 1.31 and 1.16). The 95% CI of the WM/L ratio in the A stage is 9.3 × 10–2 to 10 × 10–2, and in the A-1 stage is 4.5 × 10–2 to 4.9 × 10–2. The permutation test indicates that the WM/L ratios of adults and juveniles are significantly different (see Appendix). Specimens UMUT-CA31129 and UMUT-CA31130 are both considered adults. The WM/L ratios of specimens UMUT-CA31129 and UMUT-CA31130 are 8.7 × 10–2 and 9.7 × 10–2, respectively; the ratios are almost within the 95% CI of adult B. bisanensis ( Fig. 5 C). The holotype is smaller than specimen UMUT-CA31129, suggesting that the holotype may be a juvenile. However we consider the holotype to be an adult, because the difference in valve size between the paratype and the holotype is smaller than the size difference between instars. According to Kesling (1953), who proposed a method to identify instars of fossil ostracodes, ostracodes increase their valve area by ~1.59 times after molting. In our measurement of B. bisanensis , the valve area of the A is 1.52 times that of the A-1 instar. Specimen UMUT-CA31129 is 1.15 times longer and 1.09 times taller than the holotype and has a valve area 1.25 times larger than the holotype. Thus, the differences in size between the type specimens are considered to represent size variations within an instar stage.

A carapace ornamented with reticulation and posteroventral spines is present in Ruggieria Keij, 1957 , Keijella Ruggieri, 1967 , Bicornucythere Schornikov & Shaitarov, 1979 , Borneocythere Mostafawi, 1992 , and Venericythere Mostafawi, 1992 ( Table 2 ). These genera are reported from modern marine sediments in Asia (e.g., Whatley & Zhao 1988; Mostafawi 1992). The lateral outline of the new taxon is different from that of Venericythere . The absence of a ventral longitudinal carina in the new species distinguishes it from Ruggieria . Because Keijella and Borneocythere have a swelled tooth and two teeth in the anterior hinge element of the right valve, the new species cannot be assigned to either of these genera. The new taxon possesses a posterior margin without marginal denticles, as in Bicornucythere bisanensis . Hence, we identified the genus of the new taxon as Bicornucythere . A subrectangular carapace with concentric reticulation is present in Pistocythereis Gou in Gou et al. 1983, which is also distributed in Asia ( Table 2 ). However, Pistocythereis possesses marginal denticles on the posterior margin and lacks postero-ventral spines.

Genus Ruggieria Keij, 1957 Keijella Ruggieri, 1967 Bicornucythere Schornikov &

Staitarov, 1979

continued.

Genus Pistocythereis Gou, 1983 Borneocythere Mostafawi, 1992 Venericythere Mostafawi,

1992

Type species Echinocythereis bradyi Keijiella paucipunctata Whatley Cythere darwini Brady, 1868

Ishizaki, 1968 & Zhao, 1988

Bicornucythere concentrica sp. nov. differs from B. secedens (Lubimova & Guha in Lubimova et al. 1960) in having concentrically arranged fossae in lateral view and a narrow anterior margin one-sixth of the distance from the anterior end in dorsal view. Bicornucythere secedens was originally described as a species of Cytheretta Müller, 1894 , based on specimens from lower Miocene deposits of Kutch, western India. This new species is different from B. bisanensis ( Okubo, 1975) and the Bicornucythere sp. of Yasuhara & Irizuki (2001) in having a smaller carapace and reticulation with polygonal fossae and sharper muri. These species are commonly found in Japanese enclosed bays (e.g., Irizuki et al. 2009).

Bicornucythere concentrica sp. nov. is similar to Keijella mutata (Lubimova & Guha, 1960 in Lubimova et al. 1960), Keijella reticulata Whatley & Zhao, 1988 , and Borneocythere papuensis ( Brady, 1880) in the shape of the lateral outline and the distinctive reticulation. The new species is distinguished from these taxa by having a smaller carapace covered with concentric reticulation, reticulation with narrower muri and polygonal fossae, while the described taxon possesses reticulation with distinct longitudinal muri and rectangle fossae. Keijella mutata was originally described from lower Miocene deposits in Kutch. Keijella reticulata and Boreocythere papuensis are found in modern sediments in Southeast Asia, such as in Papua and Borneo (e.g., Whatley & Zhao 1988; Mostafawi 1992).

Figures

FIGURE 4. SEM images of ostracodes and benthic foraminifer from sample 69 - 1. The arrows point anteriorly. A – I, Bicornucythere concentrica Yamaguchi sp. nov. (A – D), UMUT-CA 31128, holotype, carapace, left external view (A), right external view (B), dorsal view (C), and anterior view (D). E – H, UMUT-CA 31129, paratype, right valve, external view (E), internal view (F), dorsal view (G), and hingement (H). I, UMUT-CA 31131, paratype, carapace, left external view. J, Trachyleberididae gen. et sp. indet. 1, right external view of carapace. K, Buntoninae gen. et sp. indet., left external view of carapace. L, Cytheromorpha? sp., right valve, external view. M – O, Ammonia subgranulosa (Jacob and Sastri, 1950), spiral view (K), umbilical view (L), and apertural view (M). Scale bars = 100 µm.

FIGURE 5. Measurements of Bicornucythere bisanensis and Bicornucythere concentrica sp. nov. A, SEM images of B. bisanensis in the A, A- 1, and A- 2 stages. Scale bars = 100 µm. B, Measurements used in the study: valve length (L), valve height (H), and width of marginal infold (WM). Scale bar = 100 µm. C, The 95 % confidence intervals of L, H, and WM / L of B. bisanensis and B. concentrica sp. nov. The bars in the A and A- 1 stages indicate 95 % confidence intervals of the measurements, while the bars in the A- 2 stage indicate ranges of the measurements. The open diamonds indicate mean values. The horizontal dotted lines indicate the WM / L ratios of B. concentrica sp. nov. “ n ” indicates the number of measurements. The measurements and detailed information about B. bisanensis specimens are given in the Appendix.

FIGURE 1. A, Geological map and the study area. Modified after Morley (2009) and Mitchell et al. (2012). B, Late Eocene geography of Myanmar, simplified from Licht et al. (2013). Abbreviations: Kb = Kabaw fault, IBR = Indo-Burman Ranges.

FIGURE 2. A, Geological map in the vicinity of the Kyauktakha section (B and C). B and C, traverse maps and sample localities.

Tables

TABLE 2. Comparison of morphological characters in Ruggeria, Keijella, Bicornucythere, Pistocythereis, Borneocythere, and Venericythere. Abbreviations: LV = left valve, RV = right valve.

Type species	Cythere micheliniana Bosquet, 1852	Cythere hodgii Brady, 1866	Legminocythereis bisanensis Okubo, 1975
Lateral outline	Ovate	Ovate	Rectangluar to ovate
Anterior margin	Obliquely round	Obliquely round	Obliquely round, larger than the posterior margin
Posterior margin	Tapering near the middle, upturned posterior end	Tapering near the middle, upturned posterior end	Truncated in the upper part
Dorsal margin	Slightly arched	Slightly arched	Straight in the middle part
Ventral margin	Convexly curved in the middle part	Convexly curved in the middle part	Slightly curved
Anterior marginal denticle	Present	Present	Present
Posterior marginal denticle	Present	Present	Absent
Surface ornaments	Partly or entirely reticulation and/or longitudinal carinae	Partly or entirely reticulation and/or longitudinal carinae	Partly or entirely reticulation with round fossae; winding longitudinal muri
Ventral longitudinal carina	Present	Absent	Absent
Postero-ventral spine	Present	Present	Present
Eye tubercle	Not defined; often prominent	Not defined	Often present
Sexual dimorphism	Male more slender than female	Male more slender than female	Male longer and more elongated than female
Anterior hinge element	In RV, a conical tooth and a socket; in LV, a socket and a conical tooth	In RV, an elongated tooth with swelling posteriorly and a socket	In RV, a round tooth and a socket
Median hinge element	Crenulate, straight	Crenulate	Crenulate
Posterior hinge element	In RV, an ovate, smooth or obscurely lobed tooth; in LV, a socket	In RV, a crenulate tooth	In RV, a round, crenulate tooth
Vestibulum	Absent	Present, narrow	Present, narrow
Marginal pore canal	Simple, wavy	Simple, slightly curved	Simple, straight
Frontal muscle scar	V-shaped	V-shaped	V-shaped
Adductor muscle scar	Four elongated	Four ovate	Upper two ovate; lower two semicircle
Pattern of adductor muscle scars	Vertical row	Slightly oblique row	Arcuate row: Lower two arranged anteriorly at angle of 40° to the upper two
Reference	Keij (1957)	Ruggieri (1967), Doruk (1973)	Schornikov & Staitarov (1979)