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

publication ID

https://doi.org/ 10.11646/zootaxa.3919.2.4

publication LSID

lsid:zoobank.org:pub:4FF8CDC9-68BC-4985-8646-206F59056D7A

DOI

https://doi.org/10.5281/zenodo.5674265

persistent identifier

https://treatment.plazi.org/id/1F6D879F-FFCC-FFB4-68B0-C2DCFE87DD7D

treatment provided by

Plazi

scientific name

Bicornucythere concentrica
status

sp. nov.

Bicornucythere concentrica sp. nov.

( Figs 4 View FIGURE 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 View FIGURE 1. A and 2 View FIGURE 2. A ).

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 View FIGURE 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 View FIGURE 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 View 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 View TABLE 2 ). However, Pistocythereis possesses marginal denticles on the posterior margin and lacks postero-ventral spines.

Genus Ruggieria Keij, 1957 View in CoL Keijella Ruggieri, 1967 View in CoL Bicornucythere Schornikov & View in CoL

Staitarov, 1979

continued.

Genus Pistocythereis Gou, 1983 View in CoL Borneocythere Mostafawi, 1992 View in CoL Venericythere Mostafawi, View in CoL

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) View in CoL and the Bicornucythere View in CoL 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 View in CoL , 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 View in CoL and Boreocythere papuensis are found in modern sediments in Southeast Asia, such as in Papua and Borneo (e.g., Whatley & Zhao 1988; Mostafawi 1992).

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 View in CoL Legminocythereis bisanensis Okubo, 1975 View in CoL
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)

Kingdom

Animalia

Phylum

Arthropoda

Class

Ostracoda

Order

Podocopida

Family

Trachyleberididae

Genus

Bicornucythere

Loc

Bicornucythere concentrica

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

Borneocythere

Mostafawi 1992
1992
Loc

Venericythere

Mostafawi 1992
1992
Loc

Keijella reticulata

Whatley & Zhao 1988
1988
Loc

Pistocythereis

Gou 1983
1983
Loc

bisanensis

Okubo 1975
1975
Loc

B. bisanensis (

Okubo 1975
1975
Loc

Keijella

Ruggieri 1967
1967
Loc

Ruggieria

Keij 1957
1957
Loc

Cytheretta Müller, 1894

Muller 1894
1894
Loc

Borneocythere papuensis (

Brady 1880
1880
Loc

Cythere hodgii

Brady 1866
1866
Loc

Cythere micheliniana

Bosquet 1852
1852
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