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Accepted by R. Matzke-Karasz: 21 Jan. 2015; published: 17 Feb. 2015
ZOOTAXA
ISSN 1175-5326 (print edition)
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1175-5334
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Copyright © 2015 Magnolia Press
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Article
http://dx.doi.org/10.11646/zootaxa.3919.2.4
http://zoobank.org/urn:lsid:zoobank.org:pub:4FF8CDC9-68BC-4985-8646-206F59056D7A
A new late Eocene Bicornucythere species (Ostracoda, Crustacea)
from Myanmar, and its significance for the evolutionary history of the genus
TATSUHIKO YAMAGUCHI
1,7
, HISASHI SUZUKI
2
, AUNG-NAING SOE
3
, THAUNG HTIKE
4
,
RITSUO NOMURA
5
& MASANARU TAKAI
6
1
Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, Kochi Prefecture, 783-8502, Japan.
E-mail: tyamaguchi@kochi-u.ac.jp
2
Faculty of Literature, Otani University, Koyama-Kamifusacho, Kita-ku, Kyoto 603-8143, Japan
3
Department of Geology, Defence Services Academy, Pyin Oo Lwin, Myanmar
4
Department of Geology, Shwebo University, Shwebo, Myanmar
5
Faculty of Education, Shimane University, Matsue 690-8504, Japan
6
Primate Research Institute, Kyoto University, ?Inuyama, Aichi 484-8506, Japan
7
Corresponding author
Abstract
The ostracode genus Bicornucythere (Ostracoda, Crustacea) is abundant in modern-day eutrophic marine bays, and is
widely distributed in estuaries and inner bays throughout East Asia, including in China, Korea, Japan, and the Russian Far
East. The evolutionary history of Bicornucythere is poorly understood. Here, we report on a new species of Bicornucythere
(Bicornucythere concentrica sp. nov.) from the upper Eocene Yaw Formation in the Central Myanmar Basin. The oldest
previously known Bicornucythere taxon, Bicornucythere secedens, was reported from lower Miocene strata in India, al-
though a molecular phylogeny suggests that the genus first appeared in the Late Cretaceous. Bicornucythere concentrica
sp. nov. is at least 10.9 million years older than the earliest known B. secedens. The new species occurs with Ammonia
subgranulosa, a benthic foraminifer, an association that is representative of brackish water conditions in modern Asian
bays. Our findings indicate that extant genera have inhabited Asian bays since the late Eocene. The paleobiogeography of
Bicornucythere indicates that the taxon was dispersed onto Indian coasts during the collision between the Indian and Eur-
asian plates.
Key words: Asia, fossil, Yaw Formation, Central Myanmar Basin, paleobiogeography
Introduction
Bicornucythere bisanensis (Okubo, 1975) (Ostracoda, Crustacea) is widespread in the inner bays of East Asia, and
is currently distributed along the coasts of China, Korea, Japan, and the Russian Far East at latitudes of 22°N to
43°N (e.g., Zhao & Wang 1988; Zenina & Schornikov 2008; Irizuki & Seto 2004; Irizuki et al. 2009). The species
exhibits a high tolerance for variable conditions (Ikeya & Shiozaki,1993), and generally dwells in mud substrates
with salinities higher than 20 psu; it can also survive in dysoxic conditions (dissolved oxygen concentrations of
~0.7 ml/l; Irizuki et al. 2003). At the heads of bays and in the central basins of enclosed inner bays, B. bisanensis
may account for over 90% of the ostracode assemblage (e.g., Ikeya & Shiozaki 1993). The abundance of the
species in Japanese bays has been increasing since the 1960s, following intensified eutrophication related to
accelerated industrialization and urbanization (Yasuhara et al. 2003, 2007). Bicornucythere bisanensis is
commonly associated with Ammonia beccarii, a benthic foraminifer (e.g., Yasuhara and Yamazaki 2005).
In Japan, the oldest record of B. bisanensis (B. cf. bisanensis; Iwatani & Irizuki 2008; Iwatani et al. 2009) is
from late Pliocene strata (3.7–2.8 Ma), and the species appears to have invaded the Japanese Islands at this time
(Ishizaki 1990). However, the evolutionary history of Bicornucythere has not yet been discussed. Fossil records of
the genus are sporadic, and the oldest previously known Bicornucythere species, B. secedens, is from lower
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EOCENE BICORNUCYTHERE (OSTRACODA) FROM MYANMAR
Miocene strata in India (e.g., Bera & Banerjee 1996). A molecular phylogeny suggests that Bicornucythere first
appeared in the Coniacian–Campanian (Late Cretaceous; 89–81 Ma) (Tinn & Oakley 2008). Thus, there is a gap of
at least 58 Myr between the earliest fossil record and the origination age suggested by the molecular phylogeny.
While studying the ostracodes of Myanmar, we discovered a new species of Bicornucythere in the upper
Eocene Yaw Formation. Here, we describe the new species and infer its habitat based on co-occurring benthic
foraminifer and information about the sedimentary environment of the fossil-bearing host strata. We also discuss
the paleobiogeography of Bicornucythere.
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.
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FIGURE 2. A, Geological map in the vicinity of the Kyauktakha section (B and C). B and C, traverse maps and sample
localities.
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EOCENE BICORNUCYTHERE (OSTRACODA) FROM MYANMAR
Geological setting and paleogeography of the Central Myanmar Basin. The Central Myanmar Basin is
located in the southwest of the Myanmar terrane, a Paleozoic–Mesozoic Asian continental block (Metcalfe 1991)
(Fig. 1A). The basin is surrounded by metamorphic rocks that form the Indo-Burman Ranges, the Sino-Burman
Ranges, and the Shan Plateau (present-day elevations of >1000 m). The Indo-Burman Ranges comprise a
metamorphic complex formed from Mesozoic–Cenozoic deep-sea sediments and flysch (turbidites). The Mogok
Metamorphic Belt and volcanic rocks of the Burma terrane, which are exposed on the western margin of the Sino-
Burman Ranges and the Shan Plateau, are in contact with the Central Myanmar Basin. The eastern margin of the
Central Myanmar Basin consists of the Sagaing strike-slip fault (e.g., Bender 1983; Morley 2012). The Central
Myanmar Basin is bounded by the Kabaw strike-slip fault (Maung 1987; Khan & Chakraborty 2005) and/or a
metamorphic belt (Searle et al. 2007; Mitchell et al. 2012; Morley 2012) on the east of the Indo-Burman Ranges. A
convergent boundary between the Eurasian and Indian plates is located to the west of the basin.
The tectonic history and the succession of depositional environments in the Central Myanmar Basin has been
affected by motions of the Indian and Indochina plates and the expansion of the Andaman Sea (e.g., Curray 2005;
Morley 2009, 2012; Hall 2012). Migrations of the Indian plate have deformed regions of Asia throughout the
Cenozoic. Central Myanmar was deformed during northward migration of the Indian Plate and an approximately
20°–30° clockwise rotation of the Indochina Plate relative to South China (e.g., Morley 2004). The Indo-Burman
Ranges represent an active west-verging accretionary complex formed during the subduction of the Indian Plate
beneath the Burma continental block (e.g., Bender 1983; Mitchell 1993). Uplift of the Indo-Burman Ranges dates
to the late Eocene (~37 Ma) to middle Miocene (Allen et al. 2008). However, the exact timing of uplift is not
known. Through the Oligocene to the middle Miocene, the Myanmar Basin developed and rotated in an extensional
regime accompanied by strike-slip motion of the Sagaing fault and associated with formation of the Andaman Sea
(Curray 2005; Khan & Chakraborty 2005).
According to Licht et al.’s (2013) paleogeography (Fig. 1B), the Central Myanmar Basin was located on the
margin of Eurasia and was open to the Indian Ocean during the late Eocene (37.8–33.9 Ma). Uplift of the Indo-
Burman Ranges had not yet occurred at this time and sediments (including turbidites) now found in the ranges were
accumulating in a deep-sea basin in the southwest Central Myanmar Basin. The Central Myanmar Basin expanded
in a NW–SE direction, and the basin became situated in a fore-arc position. Deltas prograding southwestward into
the basin conveyed sediments seaward into the basin.
Lithostratigraphy and geological age of the Yaw Formation. The Yaw Formation, distributed in the Central
Myanmar Basin (Figs 1 and 2), is ~650 m thick, and consists mainly of claystone and sandstone layers (Cotter
1914). The formation conformably overlies the Pondaung Formation, which is ~2000 m thick and comprised of
alternating beds of mudstone, sandstone, and conglomerate. The Yaw Formation is conformably overlain by the
Shwezetaw Formation, which is ~400 m thick and comprised of medium-grained sandstone.
Licht et al. (2013) inferred on the basis of a sedimentary facies analysis that the Yaw Formation represents
deltaic deposits, and that stratigraphic changes in sedimentary facies represent shifts from distal prodelta to delta
front and delta plain environments. The sedimentary structures indicate a westward flow of currents, and mineral
and neodymium and strontium isotopic compositions of the Yaw sediments indicate a source area to the east and/or
north of the basin.
The geological age of the Yaw Formation is the late Eocene (Priabonian; 37.8–33.9 Ma), as determined by
benthic foraminifer and molluscs (Bender 1983); this age is supported by the middle to late Eocene age of the
underlying Pondaung Formation (Benammi et al. 2002; Tsubamonto et al. 2002; Zaw et al. 2014). Tuff layers in
the Pondaung Formation have been dated at 37.2 ± 1.3 Ma using fission-track methods (Tsubamonto et al. 2002).
Benammi et al. (2002), using paleo-magnetic data, assigned the Pondaung Formation to Chron C17n.1n
(37.753–36.969 Ma; Gradstein et al. 2012), as constrained by the radiometric ages of Tsubamoto et al. (2002). Zaw
et al. (2014) dated the tuff layers of Tsubamoto et al. (2002) at 36.9 ± 1.2 Ma to 42.6 ± 1.1, based on U–Pb results
from zircons.
Material and methods
Material. Nine rock samples were collected from outcrops of the Kyauktakha section, near the road connecting the
towns of Pauk and Kyauktu, Magway region (Figs 2 and 3); the section includes outcrops of the Pondang, Yaw, and
Shwezetaw formations (Suzuki et al. 2006).
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FIGURE 3. Columnar section of the Yaw Formation in the Kyauktakha section (Fig. 2B and 2C) and horizons of the examined
samples.
To extract ostracode and foraminifer specimens, we decomposed 41 to 91 g of sample using a saturated sodium
sulfate solution and naphtha. The decomposed samples were washed through a 250-mesh (63-μm) sieve. Larger
fractions of processed samples were dried in an isothermal chamber drier. The ostracode and foraminifer specimens
were picked from fractions coarser than 125 μm; we collected as many ostracode specimens as possible, and ~100
foraminifer specimens.
We obtained 23 ostracode and 119 benthic foraminifer specimens from 91 g of sample 69-1 (Fig. 4, Table 1).
We recognized seven ostracode taxa, including Bicornucythere new species and Ammonia subgranulosa, a benthic
foraminifer taxon. The sample bearing the microfossils was collected from a siltstone unit, grey in color and 45 m
thick (Fig. 3); the unit does not display any sedimentary structures. Overall, the microfossil specimens were poorly
preserved. All specimens were reddish and white in color, and were partially broken, scratched, deformed, and
dissolved. Secondary carbonate mineral precipitates were present on the surfaces of the specimens. All of the
valves were filled internally with minerals and clay sediments (Fig. 4).
We also collected specimens of B. bisanensis of Pleistocene age to determine instars of the Eocene specimens.
The Pleistocene sediment samples bearing B. bisanensis were obtained from outcrops in central Japan. Details of
the localities are described in the Appendix. The sediment samples were washed through the sieve without any
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EOCENE BICORNUCYTHERE (OSTRACODA) FROM MYANMAR
chemicals. We collected 110 fossilized valves of B. bisanensis, which were picked from the fraction coarser than
250 µm.
TABLE 1. Ostracode and benthic foraminifera taxa from sample 69-1. Abbreviations: N = number of specimens, L = left
valve, R = right valve, C = carapace.
Methods. To identify taxa, we observed morphological features of specimens using a binocular microscope at
70 times magnification and a scanning electron microscope (SEM), JSM-6500F (JEOL Ltd.), at the Kochi Core
Center (KCC), Kochi University. To describe the new taxon, we captured and analyzed SEM images of selected
specimens. We also measured the type specimens at 200 times magnification, using a Keyence VH-8000 digital
microscope system that combines a VH-Z25 zoom lens and a VH-D800 LCD monitor at KCC.
To distinguish between adults and juveniles in Eocene specimens, we compared the widths of the marginal
infold in the valves of Eocene specimens with those of the Pleistocene B. bisanensis. A broad marginal infold is
found on the inside of adult podocopid valves, while a narrow marginal infold occurs in juvenile valves (e.g.,
Yamada 2007) (Fig. 5A). A broad marginal infold is a key trait of the adult stage. However, previous studies have
not indicated a change in the width of the marginal infold through molting.
We first measured the length (L), height (H), and width of the marginal infold (WM) in specimens of B.
bisanensis and the Eocene new taxon (Fig. 5B), using the digital microscope system. The precision (1σ) of the
measurements was ±0.67 µm (relative error of 0.067%), as estimated by 72 repeated measurements using a Zeiss
stage micrometer (length,1 mm). We recognized instars of B. bisanensis based on the broad marginal infold inside
of the valve and by L–H clusters on a scatter plot. Ostracode growth occurs by molting, which is accompanied by
an increase in the valve length of 1.18–1.35 times (Anderson 1964). Ostracode molt stages are expressed as A
(adult) instars A-1, A-2, A-3, etc., in order of decreasing size and younger ages. The podocopid ontogeny usually
constitutes nine instars (eight juvenile stages and one adult stage) (e.g., Horne et al. 2002), and on an L–H scatter
plot, instars form clusters of data points. Second, we calculated the WM/L ratio (to normalize WM to L), as WM
depends on the taxon and the carapace size. In addition, we calculated 95% confidence intervals (CIs) of the WM/
L ratio, L, and H for each instar of B. bisanensis, using a bias-corrected and accelerated bootstrap approach (Efron
1987) with 10,000 replicates. To test the significance of differences in the WM/L ratio between juveniles and
adults, we ran permutation tests for the ratio with 10,000 repetitions. We performed the statistical calculations and
tests using the software R, version 3.1.0 (R core team 2014), and its software packages “perm” (Fay & Shaw 2010)
for the permutation test and “boot” (Canty & Ripley 2014) for calculating the 95% CIs. The measurements and
results of the permutation tests are shown in the Appendix. Finally, we determined instars of the Eocene specimens
by reference to the WM/L ratios of instars of B. bisanensis.
Taxonomic description
For the description of Bicornucythere new species, the morphological terminology follows the scheme reviewed by
Athersuch et al. (1989) and Horne et al. (2002).
The division of sizes is as follows: “small” indicates lengths less than 450 µm; “medium” indicates lengths of
Taxon N C L R
Ostracoda Bicornucythere concentrica Yamaguchi sp. nov. 13 2 4 7
Cytheromorpha ? sp. 1 0 0 1
Eucythere sp. 1 0 1 0
Buntoninae gen et sp. indet 2 2 0 0
Hemicytheridae gen et sp. indet 1 1 0 0
Trachyleberididae gen et sp. indet. 1 4 2 1 1
Trachyleberididae gen et sp. indet. 2 1 1 0 0
Benthic Ammonia subgranulosa (Jacob & Sastri, 1950) 119
Foraminifera
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450–750 µm; and “large” indicates lengths that exceed 750 µm. The type specimens are housed at the University
Museum, University of Tokyo (UMUT), Japan.
Trachyleberididae Sylvester-Bradley, 1948
Bicornucythere Schornikov & Shaitarov, 1979
Bicornucythere concentrica sp. nov.
(Figs 4A–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. 5C). 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.
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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-CA31128, holotype, carapace, left external view (A), right
external view (B), dorsal view (C), and anterior view (D). E–H, UMUT-CA31129, paratype, right valve, external view (E),
internal view (F), dorsal view (G), and hingement (H). I, UMUT-CA31131, 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.
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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.
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 Vene r i cyt h ere . The
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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.
TABLE 2. Comparison of morphological characters in Ruggeria, Keijella, Bicornucythere, Pistocythereis,
Borneocythere, and Venericythere. Abbreviations: LV = left valve, RV = right valve.
Genus Ruggieria Keij, 1957 Keijella Ruggieri, 1967 Bicornucythere Schornikov &
Staitarov, 1979
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)
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continued.
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
Genus Pistocythereis Gou, 1983 Borneocythere Mostafawi, 1992 Venericythere Mostafawi,
1992
Type species Echinocythereis bradyi
Ishizaki, 1968
Keijiella paucipunctata Whatley
& Zhao, 1988
Cythere darwini Brady, 1868
Lateral outline Subrectangular to subovate In LV, ovate; in RV,trapezoidal Trapezoidal
Anterior margin Round Obliquely round Round
Posterior margin Tapering Short caudal process Crooked round
Dorsal margin Slightly arched In LV, convex; in RV, straight,
backward sloping
In LV, straight
Ventral margin Slightly curved, parallel to
dorsal margin
Convexly curved Slightly arched
Anterior marginal denticle Present Present Present
Posterior marginal denticle Present Present Present
Surface ornaments Concentric reticulation with
deep polygonal fossae;
occassionally spines on muri
Punctae in the central area;
elongated fossae arranged in
three row in the postero-ventral
area; foveolae in the posterior
area
Reticluation with longitudinal
muri and fossae
Ventral longitudinal carina Occassionally present Absent Absent
Postero-ventral spine Absent Present Present
Eye tubercle Present Present Present
Sexual dimorphism Not defined Male longer and shorter than
female
Not defined
Anterior hinge element In RV, an elongated, crenulate
tooth and a round socket; in
LV, an elongated socket and
an elongated tooth
In RV, two conical teeth and a
small socket
In RV, a conical smooth tooth
and a small socket
Median hinge element Crenulate Crenulate Crenulate
Posterior hinge element In RV, rectangle tooth; in LV,
elongated socket
In RV, a crenulate tooth with
nine notches
In RV, a conical smoothtooth
Vestibulum Absent Present Present
Marginal pore canal Simple and straight Twenty in anterior; four in
posterior
Straight
Frontal muscle scar Circle V-shaped U-shaped
Adductor muscle scar Four ovate Four elongated Four superimposed
Pattern of adductor muscle
scars
Vertical row Vertical row Vertical row
Reference Gou et al. (1983) Mostafawi (1992) Mostafawi (1992)
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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).
FIGURE 6. Interpretative drawing of the hingement of the right valve (Fig. 4H). Scale bar = 100 µm.
Discussion
The massive claystone of the Yaw Formation, in which the new species was found, lacks sedimentary structures,
indicating that the environment was not influenced by waves, tides, or storms. Ammonia subgranulosa, which co-
occurs with the new species, is similar to brackish forms of A. beccarii, such as the Ammonia beccarii forma 1 of
Nomura & Seto (1992) and the molecular type T6 of Hayward et al. (2004). Ammonia subgranulosa and the
brackish form of A. beccarii share a moderate-sized test (<0.7 mm) and a small umbilical boss. Ammonia
subgranulosa differs from the typical open-marine form of A. beccarii in having a smaller test without strong
ornamentation on the spiral and umbilical sides and without a distinct groove on the umbilical side. The brackish
form abounds in bayheads and central basins under brackish conditions (15–20 psu) in Japanese bays, accounting
for over 80% of the benthic foraminifer assemblage (e.g., Nomura & Seto 1992; Hayward et al. 2004). An
Ammonia-dominated foraminifer assemblage in modern bay/estuarine environments is indicative of water depths
of less than 10 m (e.g., Scott et al. 2001). We consider that the claystone, which contains extremely abundant A.
subgranulosa, constitutes deposits in a central basin or mudflat in a bay/estuarine environment, at water depths of
less than 10 m and with brackish conditions. Licht et al. (2013) suggested that sediments of the Yaw Formation
represent river discharge on deltas (Fig. 1B). As mentioned above, Bicornucythere dwells in brackish conditions.
The two brackish taxa are an evidence to support the fluvial discharge. The ostracodes, including B. concentrica
sp. nov., dwelled in mud substrates under brackish water conditions at water depths of less than 10 m in a bay/
estuarine environment. Our findings also indicate that Ammonia and Bicornucythere have co-inhabited Asian bays
since the late Eocene.
Bicornucythere concentrica sp. nov. is currently the oldest recorded species of the genus, and except for our
data, Bicornucythere has never been reported in pre-Miocene strata (e.g., Gramann 1975; Singh 1988; Keen et al.
1994) (Fig. 7). The late Eocene ranges from 37.8 to 33.9 Ma and the early Miocene ranges from 23.0 to 15.97 Ma
(Gradstein et al. 2012). The new taxon is at least 10.9 Myr older than the oldest previously reported Bicornucythere
species, B. secedens. Tinn & Oakley (2008) indicated that Bicornucythere separated from the
Hemicytheridae–Thaerocytheridae clade at 89–81 Ma (Late Cretaceous), based on molecular phylogenetic data.
Our finding therefore bridges the gap between the fossil evidence and inferences based on molecular phylogenies.
During the late Eocene, Bicornucythere dwelled on the southeast margin of Asia. By the early Miocene, it had
spread to Indian coasts (Fig. 7). Bicornucythere secedens has been found in lower Miocene shallow-marine strata
of India (Khosla 1978; Singh 1988; Khosla & Nagori 1989; Bera & Banerjee 1996; Fig. 7A). Since the Cretaceous,
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FIGURE 7. Early Miocene (A) and late Eocene (B) localities of Bicornucythere are represented by solid black symbols. The
paleogeography was generated using the online service of the Ocean Drilling Stratigraphic Network (http://www.odsn.de),
accessed on September 11, 2013. The late Eocene and early Miocene paleogeographies represent configurations at 35 Ma and
22 Ma, respectively. The grey shading indicates plate fragments. The black line shows today’s shoreline. Open white circles
represent localities of fossil ostracodes without Bicornucythere. Numbers are references to the localities: 1, Gramann (1975); 2,
Bera & Banerjee (1996); 3, Khosla & Nagori (1989); 4, Khosla (1978), Singh (1988); 5, Ahmad et al. (1991); 6, Yeşilyurt et al.
(2009); 7, Nazik (1993); 8, Honigstein et al. (2002); 9, Bosboom et al. (2011); 10, Bhandari (1991); 11, Bhandari (1992); 12,
This study.
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the Indian plate has moved northeastward, collided with the Eurasian plate, and deformed southeastern Asia (e.g.,
Ali & Aitchison 2008; Van Hinsbergen et al. 2012). During the collision, the Tethyan Seaway shallowed and
vanished. During the late Eocene, the Indian plate was already sutured to the Eurasian plate (Morley 2009; Hall
2012). We suggest that Bicornucythere dispersed from southeast Asian coasts onto Indian coasts during the
collision. Dispersion was possibly facilitated by the presence of contiguous coastlines between the Indian and
Eurasian continents. Previous biogeography studies have principally addressed terrestrial biotic exchanges between
India and Asia during the collision event (e.g., Hausdorf 2000; Briggs 2003; Li et al. 2013). Our finding suggests a
marine biogeographic connection between the Indian and Eurasian coasts that were affected by the continental
collision.
Acknowledgements
We are grateful to Dr. Eugene I. Schornikov (A.V. Zhirmunsky Institute of Marine Biology of the Far Eastern
Branch of the Russian Academy of Sciences) for his advice on the taxonomy of Bicornucythere at the 17th
International Symposium on Ostracoda, to Dr. Simone Brandão (Universidade Federal do Rio Grande do Norte,
Brazil) for her assistance in collecting relevant literature, to Mr. Takuya Matsuzaki (Kochi University) for his help
in operating the SEM and the digital microscope, to Drs Takenori Sasaki and Yasuhiro Ito (University Museum,
University of Tokyo) for their help in depositing the type specimens, to the members of the Myanmar–Japan Joint
Fossil Expedition Team for their help in fieldwork, and to Dr. Toshiaki Irizuki (Shimane University, Japan) for his
suggestion on the taxonomy at the 2014 meeting of the Palaeontological Society of Japan. We are also indebted to
Drs Mark Warne (Deakin University, Australia) and Ashraf M.T. Elewa (Minia University, Egypt) for their critical
reviews. This study was financially supported by a Grant-in-Aid for Scientific Research to M.T. (No. 20405015).
Stallard Scientific Editing Ltd. corrected English errors.
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APPENDIX. Measurements of B. bisanensis and B. concentrica sp. nov.
The specimens of Bicornucythere bisanensis were collected from two exposures (Locations 2 and 7) of muddy fine-grained
sandstone of the Pleistocene Omma Formation, in Kanazawa Prefecture, central Japan. The Omma Formation consists of thin
tuff layers and muddy fine-grained sandstone bearing fossil molluscs and ostracodes (e.g., Ozawa & Kamiya 2001; Kitamura &
Kimoto 2006). Location 2 (36.52947°N, 136.68397°W) is 0.6 m above the base of the O3 tuff layer, while Location 7
(36.52912°N, 136.68383°W) is 8.8 m below the base of the O3 tuff layer.
Previous studies have reported variable morphotypes of B. bisanensis (e.g., Abe 1988; Irizuki & Seto 2004; Ozawa 2009).
Bicornucythere bisanensis in this study is identified as Form A of Abe (1988). In the Omma specimens, the mean L and mean
H of adult male left valves are 816 µm and 418 µm, respectively. The mean values fall within the ranges of L and H in the Form
A morphotype (780–840 µm and 390–420 µm, respectively), as indicated by Abe (1983) from measurements of specimens
from Aburatsubo Bay, Japan.
In the permutation test, we examined the null hypothesis that there is no difference between the WM/L ratios of adult (A)
and juvenile (A-1 and A-2) instars; the null hypothesis was rejected at a significance level of less than 0.01 (p-value = 2.0 ×
10
–3
). Abbreviations: L = length, H = height, WM = width of marginal infold, LV = left valve, RV = right valve.
Taxon Instar Type of L H WM WM/L Locality Remark
valve (µm) (µm) (µm) ( × 10
-2
)
B. bisanensis A LV 788 429 81 10 2 Female
B. bisanensis A LV 832 458 88 11 2 Female
B. bisanensis A LV 779 431 78 10 2 Female
B. bisanensis A LV 782 440 86 11 2 Female
B. bisanensis A LV 842 447 93 11 2 Female, Fig. 5A, B
B. bisanensis A LV 816 398 83 10 2 Male
B. bisanensis A LV 790 407 61 7.7 2 Male
B. bisanensis A LV 804 414 55 6.8 2 Male
B. bisanensis A LV 821 424 67 8.2 2 Male
B. bisanensis A LV 780 437 79 10 7 Female
B. bisanensis A LV 792 452 94 12 7 Female
B. bisanensis A LV 764 437 86 11 7 Female
B. bisanensis A LV 774 443 86 11 7 Female
B. bisanensis A LV 805 375 88 11 7 Male
B. bisanensis A RV 784 421 64 8.2 2 Female
B. bisanensis A RV 767 415 81 11 2 Female
B. bisanensis A RV 750 406 92 12 2 Female
B. bisanensis A RV 788 427 54 6.9 2 Female
B. bisanensis A RV 806 437 76 9.4 2 Female
B. bisanensis A RV 759 413 55 7.2 2 Female
B. bisanensis A RV 781 434 90 12 2 Female
B. bisanensis A RV 759 422 84 11 2 Female
B. bisanensis A RV 737 411 83 11 2 Female
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Continued.
Taxon Instar Type of L H WM WM/L Locality Remark
valve (µm) (µm) (µm) (× 10
-2
)
B. bisanensis A RV 735 430 85 12 2 Female
B. bisanensis A RV 813 384 70 8.6 2 Male
B. bisanensis A RV 836 399 67 8.0 2 Male
B. bisanensis A RV 788 385 79 10 2 Male
B. bisanensis A RV 774 380 54 7.0 2 Male
B. bisanensis A RV 792 392 65 8.2 2 Male
B. bisanensis A RV 813 409 69 8.5 2 Male
B. bisanensis A RV 798 403 67 8.4 2 Male
B. bisanensis A RV 839 431 86 10 2 Male
B. bisanensis A RV 803 422 76 9.5 2 Male
B. bisanensis A RV 761 407 85 11 7 Female
B. bisanensis A RV 756 406 83 11 7 Female
B. bisanensis A RV 780 422 80 10 7 Female
B. bisanensis A RV 786 429 74 9.0 7 Female
B. bisanensis A RV 787 436 86 11 7 Female
B. bisanensis A RV 836 414 83 10 7 Male
B. bisanensis A RV 792 399 83 10 7 Male
B. bisanensis A-1 LV 626 390 14 2.2 2
B. bisanensis A-1 LV 589 342 33 5.6 2 Fig. 5A
B. bisanensis A-1 LV 643 394 30 4.7 2
B. bisanensis A-1 LV 598 360 29 4.8 2
B. bisanensis A-1 LV 599 358 33 5.5 2
B. bisanensis A-1 LV 594 363 29 4.9 2
B. bisanensis A-1 LV 593 350 30 5.1 2
B. bisanensis A-1 LV 615 347 31 5.0 2
B. bisanensis A-1 LV 595 371 34 5.7 2
B. bisanensis A-1 LV 622 368 34 5.5 2
B. bisanensis A-1 LV 598 344 38 6.4 2
B. bisanensis A-1 LV 606 352 30 5.0 2
B. bisanensis A-1 LV 618 370 18 2.9 2
B. bisanensis A-1 LV 626 392 27 4.3 2
B. bisanensis A-1 LV 572 335 37 6.5 2
B. bisanensis A-1 LV 656 386 35 5.3 2
B. bisanensis A-1 LV 670 409 32 4.8 2
B. bisanensis A-1 LV 538 343 24 4.5 2
B. bisanensis A-1 LV 565 320 20 3.5 2
B. bisanensis A-1 LV 607 365 22 3.6 2
B. bisanensis A-1 LV 542 357 20 3.7 2
B. bisanensis A-1 LV 540 353 21 3.9 2
B. bisanensis A-1 LV 622 369 28 4.5 2
B. bisanensis A-1 LV 524 358 23 4.4 2
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Continued.
Taxon Instar Type of L H WM WM/L Locality Remark
valve (µm) (µm) (µm) (× 10
-2
)
B. bisanensis A-1 LV 599 362 36 6.0 7
B. bisanensis A-1 LV 619 359 37 6.0 7
B. bisanensis A-1 LV 648 369 30 4.6 7
B. bisanensis A-1 LV 641 339 29 4.5 7
B. bisanensis A-1 LV 600 342 32 5.3 7
B. bisanensis A-1 LV 620 378 35 5.6 7
B. bisanensis A-1 LV 617 356 32 5.2 7
B. bisanensis A-1 LV 632 382 42 6.6 7
B. bisanensis A-1 LV 626 372 28 4.5 7
B. bisanensis A-1 LV 598 378 31 5.2 7
B. bisanensis A-1 LV 598 382 36 6.0 7
B. bisanensis A-1 RV 624 364 24 3.8 2
B. bisanensis A-1 RV 639 346 30 4.7 2
B. bisanensis A-1 RV 630 386 24 3.8 2
B. bisanensis A-1 RV 610 362 27 4.4 2
B. bisanensis A-1 RV 626 363 33 5.3 2
B. bisanensis A-1 RV 670 380 38 5.7 2
B. bisanensis A-1 RV 616 350 34 5.5 2
B. bisanensis A-1 RV 594 331 31 5.2 2
B. bisanensis A-1 RV 592 344 19 3.2 2
B. bisanensis A-1 RV 649 399 27 4.2 2
B. bisanensis A-1 RV 613 361 31 5.1 2
B. bisanensis A-1 RV 648 404 30 4.6 2
B. bisanensis A-1 RV 651 395 20 3.1 2
B. bisanensis A-1 RV 525 332 26 5.0 2
B. bisanensis A-1 RV 541 344 27 5.0 2
B. bisanensis A-1 RV 585 340 30 5.1 2
B. bisanensis A-1 RV 545 345 29 5.3 2
B. bisanensis A-1 RV 595 345 29 4.9 2
B. bisanensis A-1 RV 596 344 17 2.9 2
B. bisanensis A-1 RV 576 341 24 4.2 2
B. bisanensis A-1 RV 551 327 27 4.9 2
B. bisanensis A-1 RV 589 309 24 4.1 2
B. bisanensis A-1 RV 526 353 28 5.3 2
B. bisanensis A-1 RV 512 342 18 3.5 2
B. bisanensis A-1 RV 641 361 44 6.9 7
B. bisanensis A-1 RV 607 344 17 2.8 7
B. bisanensis A-1 RV 607 362 29 4.8 7
B. bisanensis A-1 RV 613 374 23 3.8 7
B. bisanensis A-1 RV 606 376 32 5.3 7
B. bisanensis A-1 RV 621 349 21 3.4 7
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Continued.
Taxon Instar Type of L H WM WM/L Locality Remark
valve (µm) (µm) (µm) (× 10
-2
)
B. bisanensis A-1 RV 616 370 21 3.4 7
B. bisanensis A-1 RV 651 384 32 4.9 7
B. bisanensis A-1 RV 617 364 29 4.7 7
B. bisanensis A-2 LV 468 285 24 5.1 2 Fig. 5A
B. bisanensis A-2 LV 493 298 25 5.1 2
B. concentrica
sp. nov. A RV 654 353 57 8.7 69-1 UMUT-CA31129
B. concentrica
sp. nov. A LV 651 325 63 9.7 69-1 UMUT-CA31130
