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Chapter 74. Preliminary notes on the Japanese Miocene Ostracoda

Chapter 74. Preliminary notes on the Japanese Miocene Ostracoda

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1074



M.YAJIMA

130



135



140



145



TEXT-FIG.

l-Localities of the Japanese Miocene ostracod samples.



from the Shukunohora Sandstone (Blow’s N 8 zone) of Mizunami in the East Palaeo-Setouchi

Province, and stated (1977) that “some species of living warm water ostracods, such as Hemicytherura kajiyamai. Aurila sp. and Schizocythere kishinouyei, which settled in Japan in the Middle Miocene, have maintained a status quo in phenotype up to the present”. By 1977, 47 species of 28

genera were reported from the Japanese Miocene. Among these, 13 species of 11 genera are extant.

While investigating Pleistocene shallow warm water ostracods, questions arose concerning their

origin and their history. To clarify these, I studied Miocene ostracods from Shobara (lower sand

of the Bihoku Group), Tsuyama (Yoshino Formation), Mizunami (Shukunohora Sandstone),

Sekinohana (Togi Mud Formation), Sugota (Sugota Formation), and Kamikoani (Kamikoani

Formation) (Text-figs. 1, 2).



EARLYMIDDLE

MIOCENE-SOUTHWEST

JAPAN

The Palaeo-Setouchi (Palaeo-Inland Sea) sediments are distributed in the Chugoku area. During

the Early Middle Miocene, this area was covered by a shallow warm sea with many islands.

Shobara

In the Shobara Formation of the Bihoku Group, the only ostracods present are very rare

Cytherelloidea sp.



Tsuyama

The Yoshino Formation at Tsuyama is about 15 m thick, light brown sand in the lower part,



Japanese Miocene Ostracoaia 1075



low ( 1 9 7 9 )



Shobara



Ma



Mizunarni



Kanazawa

a



Sendai



0%sugota



Sekinohana



Kadonosava

Xamikoanj



8

i16



9



.115



10



11



I1 4



-



L



w

rn

m



Y



u



u

e

n



12



L



a



E



v13



.a



8

13



v1 i

N11



-



11



e

a

Y

0



I



g



lt



5



N



!

16



I

i



1;



TEXT-FIG.

2-Stratigraphical correlation of ostracod-bearing Miocene sediments in the Japanese Islands. Solid

circles show ostracod occurrences.



greenish mud in the middle part, and gravels and sand in the upper part, including many shallow

warm-water molluscus.

Ostracods are rare in the Y2 and Y5 fossil horizons of Taguchi (personal communication) and

very scarce in the Y3 and Y4 fossil horizons. The Y2 horizon is in the lower sand of the Yoshino

Formation and includes the molluscan fossils Phacosoma, Solidcorbula, Vasticardium, Crassostrea, Turbo, and Siratoria. The Y5 horizon is a thin sand layer intercalated in the middle mud of

the Formation and includes molluscan fossils such as Phacosoma and Vasticardium.

Ostracods are very rare in the Yoshino Formation (Table 1). Most specimens are articulated,

but the carapaces are compressed laterally or dorso-ventrally. Among 14 species, Trachyleberis

sp. and Spinileberis? sp. are common. Single valves of Trachyleberis sp. are very similar to Recent

Trachyleberis scabrocuneata, but articulated valves differ a little in ornamentation from Recent

ones. Cletocythereis rastromarginata is represented by only one valve. It is a tropical shallow water

species. Falsobuntonia taiwanica is very rare. Spinileberis quadriaculeata is represented by only one

broken valve. The ostracod assemblage is a shallow warm-water assemblage very similar to the Recent warm-water assemblages along the Pacific coast of Japan.



Japanese Miocene Ostracoda 1077



Mizunami

The Mizunami area is situated at the East end of the Palaeo-Setouchi Sea in Central Japan,

(Text-fig. 1). The transgression into the Palaeo-Setuouchi area occurred in three steps, and

after the third, the seaway continued from the Japan Sea through the main part of the PalaeoSetouchi area in the Chugoku area to east Setouchi (Itoigawa, 1981).

TABLE LIST OF EARLYMIDDLE

MIOCENE

OSTRACOD

SPECIES FROM THE TSUYAMA AREA.

Tsuyama (Yoshino Formation, Blow’s N 8 zone)

Y-5 horizon



Callistocythere sp.



* Cythere omotenipponica Hanai, 1959

Spinileberis? sp.

Trachyleberis sp.

Ambostracon ikeyai Yajima, 1978

* Cletocythereis rastromarginata (Brady, 1880)

Falsobuntonia taiwanica Malz, 1982

Aurila sp.

Cornucoquimba moniwensis (Ishizaki, 19660)

Semicytherura sp.

Loxoconcha sp.

Y-4 horizon

Trachyleberis sp.

Y-3 horizon

Trachyleberis sp.

Y-2 horizon (sample 1)

* Spinileberis quadriaculeata (Brady, 1880)

S.? sp.

Trachyleberis sp.

“Buntonia” sp.

Ambostracon ikeyai Yajima, 1978

Loxoconcha sp.

Y-2 horizon (sample 2)

Spinileberis? sp.

Trachyleberis sp.

* Acunthocythereis munechikai Ishizaki, 1972

Falsobuntonia taiwanica Malz, 1982



R

R

R

C

R

R

R

R



R

R

R

R

R

R

C

C

R

R

R



C

C

R

R



C=common; R=rare; *=extant species.

PLATE1-Early Middle Miocene Ostracoda from Mizunami (Shukunohora Sandstone).

Fig. 1. Truchylberissp. 1. Left valve, CA 17734. x 52. Figs. 2,5. Trachyleberis sp. 2. Fig. 2. Left valve view of

male carapace, CA 17735. X 52. Fig. 5. Right valve view of female carapace, CA 17736. X 52. Fig. 3. Loxoconcha

pulchra Tshizaki, 1968. Right valve, CA 17737. x 62. Fig. 4. Ambostracon ikeyai Yajima, 1978. Female left valve,

CA 17689. x 67. Fig. 6. Cornucoquimba moniwensis (Ishizaki, 1966). Right valve view of carapace, CA 17690.

x 63. Fig. 7. Falsobuntonia taiwanica Malz, 1982. Left valve, CA 17691. x 68. Fig. 8. Pontocythere subjaponica

(Hanai, 1959). Femaleleft valve, CA 17692. x68. Fig. 9. Callistocythere sp. Right valve view of carapace, CA

1769). ~ 8 9 Figs.

.

10,13. Hirsutocythere? sp.-Fig. 10. Female left valve, CA 17694. ~ 4 2 Fig.

.

13. Maleleft

valve, CA 17695. ~ 4 2 Fig.

.

11. Semicytherura miurensis Hanai, 1957. Left valve, CA 17696. X 107. Fig. 12.

Semicytherura henryhowei Hanai and Iekya, 1977. Right valve view of carapace, CA 17697. x 107. Fig. 14.

Schizocythere kishinouyei (Kajiyama, 1913). Left juvenile valve, CA 17698. x 82. Fig. 15. Loxocorniculum sp.

Right valve, CA 17699. X 89. Fig. 16. Loxoconcha sp. Left valve, CA 17700. x 86. Fig. 17. Kangarina sp. Right

valve, CA 17701. x 104. Fig. 18. Hemicytherura cuneata Hanai, 1957. Right valve, CA 17702. X 120. Fig. 19.

Aurila sp. Left valve, CA 17703, x 62. Fig. 20. Spinileberis qua&iaculeata (Brady, 1880). Right valve, CA 17704.

.

illustrated specimens are

x 62. Fig. 21. Neonesidea sp. Right valve view of carpace, CA 17705 ~ 4 0 (All

deposited in the collection of the University Museum, University of Tokyo (UMUT). Specimen numbers are

prefixed CA for Cenozoic Arthropoda).



Japanese Miocene Ostracoda 1079



TABLE

2-LIST OF EARLYMIDDLEMIOCENE

OSTRACOD SPECIES FROM THE MIZUNAMI

AREA.

Mizunami (Shukunohora Sandstone, Blow’s N 8 zone)

Neonesidea sp.

Propontocypris sp.

*Pontocythere subjaponica (Hanai, 1959)

P. sp.

*Parakrithella pseudadonta Hanai, 1959

Munseyella sp.

Callistocythere sp.

*Cythere omotenipponica Hanai, 1959

*Schi.zocythere kishinouyei (Kajiyama, 1913)

*SpiniIeberis quadriaculeata (Brady, 1880)

Trachyleberis sp. 1

T. sp. 2

Hirsutocythere? sp.

Falsobuntonia taiwanica Malz, 1982

Aurila sp.

Cornucoquimba moniwensi? (Ishizaki, 1966)

Ambostracon ikeyai Yajima, 1978

*Hemicytherura cuneata Hanai, 1957

Kangarina sp.

*Semicytherura henryhowei Hanai and Ikeya, 1977

*S. miurensis Hanai, 1957

S. wakamurasaki Yajima, 1982

Cytheropteron sp.

*Loxoconchapulchra Ishizaki, 1968

L. sp.

Loxocorniculum sp.

Xestoleberis sp.

A=abundant; C-common; R=rare; *, extant species.



A

R

R

R

R

R

C

R

C

R

C

R

R

R

A

C

C

C



R

R

R

R

R

C

R

R

R



The Shukunohora Sandstone of the Mizunami area is a thin (10 m), ill-sorted sandy silt with

the foraminifer Myogipsina kotoi and many shallow warm-water molluscs.

The Shukunohora Sandstone contains abundant well-preserved ostracods (Table 2). Most of the

ostracod valves are transparent and good for study of internal features. The sieve type pores of

Schizocythere kishinouyei and Spinileberis quadriaculeata are well preserved.

The ostracod assemblage consists exclusively of shallow warm-water species (Plate 1). Among

PLATE2-Miocene



Ostracoda from Tsuyama, Kamikoani, and Sekinohana.

Figs. 1-3. Tsuyama, Yoshino Formation, Early Middle Miocene (Blow’s N 8 zone). Fig. 1. Spinileberis? sp.,

Right valve view of carapace, CA 17706 (Y-2 horizon), x 80. Fig. 2. Cletocythereis rastrornarginata(Brady,

1880), Right valve view of carapace, CA 17707 (Y-5 horizon), x 82. Fig. 3. Trachyleberissp., Left valve view of

male carapace, CA 17708 (Y-5 horizon), x 58. Figs. 4-7. Kamikoani, Kamikoani Formation, Middle Miocene

(Blow’s N 15 zone). Fig. 4. Finmarchinella japonica (Ishizaki, 1966), Right valve, CA 17709. ~ 8 1 Fig.

;

5.

Finmarchinella japonica (Ishizaki, 1966), Juvinile left valve, CA 17710, x 87. Fig. 6. Hemicythere? sp., Right

valve, CA 17711, x 80. Fig. 7. Hemicytherura cuneata Hanai, 1957, Right valve, CA 17712, X 130. Figs. 8-16.

Sekinohana, Togi Formation, Middle Miocene (Blow’s N 9 zone). Fig. 8. Palmenella limicola (Norman,

1865), Left valve view of juvenile carapace, CA 17713, x 82. Fig. 9. Finmarchinella hanaii Okada, 1979, Right

.

10. Finmarchinella hanaii Okada, 1979. Left valve, CA 17715

valve, CA 17714 (sample Togi D), ~ 8 1 Fig.

(sample Togi D), x 80. Fig. 11. Munseyella sp., Left valve, CA 17716 (sample Togi C), x 82. Fig. 12. Cornucoquimba saitoi (Ishizaki, 1963), Right valve, CA 17717 (sample Togi D), x79. Fig. 13. Ambostracon ikeyai

Yajima, 1978, Female left valve, CA 17718 (sample Togi D), x 81. Fig. 14. Acanthocythereis? munechikai

Ishizaki, 1982, Left valve, CA 17719 (sample Togi D), x58. Fig. 15. Bradleya sp.. Right valve, CA 17720

(sample Togi D), x 58. Fig. 16. Hirsutocythere? nozokiensis (Ishizaki, 1963), Left valve, CA 17721 (sample

Togi D), x58.



1080



M.YAJIMA



27 species of 21 genera, 9 species are extant. Neonesidea sp. which is very similar to the Recent

Neonesidea oligodentata, and Aurila sp. which is very similar to the Recent Aurila punctata are

abundant. Callistocythere sp., Schizocythere kishinouyei, Trachyleberis sp., Cornucoquimba

moniwensis, Ambostracon ikeyai, Hemicytherura cuneata and Loxoconcha pulchra are common.

Pontocythere subjaponica, Parakrithella pseudadonta, Cythere omotenipponica, Spinileberis

quadriaculeata, Semicytherura henryhowei, and S. miurensis are rare but identical to the species

presently living along the Pacific coast of Japan. Tropical species of Kangarina sp., Falsobuntonia

taiwanica and Loxocorniculum sp. are rare.



EARLYMIDDLEMIOCENE-NORTHEAST

JAPAN

Kanazawa

Ishizaki studied Miocene ostracods from the Sunakosaka Member (Blow’s N 8 zone) in

1963. The ostracod assemblage is very small but resembles that from the Tsuyama area because

Trachyleberis is abundant.



Sendai

Miocene ostracods from the Moniwa Formation (Blow’s N 8 zone), Sendai, were reported by

Ishizaki (1966). Ostracods are very rare. Abundance of Neonesidea oligodentata, Trachyleberis

scabrocuneata, Cornucoquimba saitoi, and Schizocythere kishinouyei shows that the assemblage is

similar to the Mizunami and Tsuyama faunas.



MIDDLEMIOCENE-NORTHEAST

JAPAN

Seikinohana

The Sekinohana area is situated on the Noto Peninsula, near Kanazawa along the Japan

Sea. The Togi Mud Formation in Sekinohana is a very thin (2 m), greenish, massive, silty

mudstone containing abundant foraminifers, and rare radiolarians, diatoms, and ostracods. Suda

and Ishigaki (1984) reported the presence of Groborotalia peripheroronda and correlated the sediments with Blow’s N 7 to N 9 zones, a little later than the earliest Middle Miocene transgression.

Ostracods are rare (Table 3). The fauna is dominated by Neonesidea sp., Hirsutocythere? nozokiensis, and Acanthocythereis? munechikai (Plate 2). Only one immature carapace of Palmenella

limicola is present. This is a typical arctic species. Finmarchinella hanaii, which is common in

shallow Pleistocene sediments along the Japan Sea coast, is common Rarely found Bradleya sp. is

very similar to the Recent deep sea species of Bradleya albatrossia. The extant species Pontocythere subjaponica, Hanaiborchella miurensis, and Semicytherura henryhowei are rare. Occurrences of

other species are hitherto confined to Miocene sediments.



Sugota

The Sugota Formation is a 180 m thick, fine argillaceous sandstone found around Sugota

near Akita, in Northeast Japan. The basal part of the Sugota Formation is dark greenish, poorly

sorted medium to coarse sandstone with abundant fragments of barnacles, molluscs, and benthonic

foraminifers. Molluscs show that the sediments were deposited in temperate water, and planktonic

foraminifers present correlate with Blow’s N 9 zone or higher. Ostracods are very rare. Only one

broken valve of Hemicythere was found. The Middle part of the Formation is dark grey to dark



Japanese Miocene Ostracoda 1081



TABLE3-LIST OF MIDDLE

MIOCENE

OSTRACOD SPECIES FROM THE SEKINOHANA AREA.

Sekinohana (Togi Mud Formation, Blow’s N7-9 zones)

Togi C

Munseyella sp.

Schizocythere sp.

*PaImeneIIa Iimicola (Norman, 1865)

FinmarchineIIa hanaii Okada, 1979

Cornucoquimba moniwensis (Ishizaki, 1966)

C. saitoi (Ishizaki, 1963)

*Hirsutocythere? nozokiensis (Ishizaki, 1963)

*Acanthocythereis munechikai Ishizaki, 1972

BradIeya sp.

Cytheropteron sawanense Hanai, 1957

Paracytheridea neolongicaudata Ishizaki, 1966

Togi D

Neonesidea sp.

*Pontocythere subjaponica (Hanai, 1959)

*Hanaiborchellamiurensis (Hanai, 1970)

*PaImeneIIa Iimicola (Norman, 1865)

Finmarchinella hanaii Okada, 1979

Ambostracon ikeyai Yajima, 1978

Caudites? posterocostatus (Ishizaki, 1966)

Cornucoquimbasaitoi (Ishizaki, 1963)

Hirsutocythere? nozokiensis (Ishizaki, 1963)

*Acanthocythereis munechikai Ishizaki, 1972

Bradleya sp.

Eucytherura neoalae (Ishizaki, 1966)

*Semicytherura henryhowei Hanai and Ikeya, 1977

Loxocorniculum kotoraformum Ishizaki, 1966

A=abundant; C=common; R=rare; *=extent species.



R

R

R

C

.C

R

A

A

R

C

R

C

R

R

R

C

R

C

R

C

C

R

R

R

R



brownish-grey, fine argillaceous sandstone with autochthonous cold water molluscs and foraminifers. Ostracods are very rare, only a few specimens of Callistocythere were found.



Kamikoani

The Kamikoani Formation in the Kamikoani area is a calcareous fine sand. It is considered to

have been deposited 10 to 8 Ma., based on its stratigraphical position. Ostracods are very abundant

and well-preserved (Table 4). Schizocythere kishinouyei, Aurila sp. 1, Finmarchinella japonica,

Hemicytherura cuneata, and Cytheroteron sawanense are abundant. Palmenella limicola, a typical

arctic species, is rare. Among ostracods found in this formation, Munseyella hokkaidoana,

Finmarchinellajaponica, F. nealei, Howeina neoleptocytheroidea, and Cytheropteron sawanense are

species common in the Pleistocene sediments and Recent shallow water fauna along the Japan

Sea coast. A specimen which probably belongs to the genus Hemicythere, a cold water genus,

is also present. Many species such as Cornucoquimba moniwensis, Hirsutocythere? nozokiensis,

Paracytheridea neolongicaudata, Loxoconcha nozokiensis, and Loxocorniculum kotoraformum, are

confined to the Miocene.



Sendai

Ishizaki’s report (1966) also included the ostracod fauna from the Hatatate Formation. This

formation is correlated with Blow’s N 9 to N 15 zones, and the formation is considered to have

been deposited under the influence of cold currents on the basis of the molluscan fossils. Ishizaki



1082 M. YAJIMA



TABLE4-LIST OF MIDDLEMIOCENE

OSTRACOD SPECIES PROM THE KAMIKOANI

AREA..

Kamikoani (Kamikoani Formation, 10-8 Ma).

Munsyella hokkaidoana (Hanai, 1959)

*Callistocythere japonica Hanai, 1957

*C. ruogsoforma Hanai, 1957

*Cythere uranipponica Hanai, 1959

Schizocythere sp.

*Palmenella limicola (Norman, 1865)

Aurila sp. 1

A. sp. 2

Finmarchinellajaponica (Ishizaki, 1966)

*F. nealei Okada, 1979

Hemicythere? sp.

Caudites? posterocostatus (Ishizaki, 1966)

Cornucoquimba moniwensis (Ishizaki, 1966)

*C. tosaensis (Ishizaki, 1968)

Trachyleberis sp.

Hirsutocythere? nozokiensis (Ishizaki, 1963)

Eucytherura sp.

*Hemicytherura cuneata Hanai, 1957

*Semicytherura henryhowei Hanai and Ikeya, 1977

*Howeina neoleptocytheroidea Hanai, 1957

H . sp.

Cytheropteron sawanense Hanai, 1957

Paracytheridea neolongicaudata Ishizaki, 1966

P. sp.

Loxoconcha nozokiensis Ishizaki, 1963

Loxocorniculum kotoraformum Ishizaki, 1966

A=abundant; C=common; R=rare; *=extant species



R

R

R

R

C

R



A

R

C

R

R

R

R

R

R

C

R



A

R

R

R

A

C

R

R

C



pointed out that the ostracod fauna of “the Hatatate Formation is not essentially different from

that of the Moniwa Member, although it is more or less different in productivity” (1966, p. 135).

I think that the occurrence of Palmenella limicola and Finmarchinella japonica in the Hatatate Formation demonstrate cold water influence.



FAUNAL

CONSIDERATIONS

Palaeogeographical maps (Chinzei, 1986) of the Japanese Islands during the Early Middle Miocene (Blow’s N 8 zone) and Middle Miocene (Blow’s N 9 to N 15 zones) are shown in Text-figs.

3 and 4. Recent palaeomagnetic studies show that in the Early Middle Miocene the Japanese Islands

were situated closer to the Asian continent and the Japanese Island arc was convex toward the

northwest, opposite to the trend of the present arc. The Tsushima Strait between the Korean

Peninsula and Kyushu Island opened and tropical water flowed into the Palaeo-Inland Sea and the

Japan Sea up to Northeast Japan. Cold water influence was very weak and only extended into the

Hokkaido area. Ostracods from Shobara, Tsuyama, Mizunami, Kanazawa, and Sendai, during

the age of Blow’s N 8 zone are shallow, tropical water species, similar to the warm-water ostracods

presently living along the Pacific coast of Southwest Japan. Ostracods from the Shukunohora

Sandstone are very similar to Recent ones. The Shukunohora Sandstone was probably deposited

a little after the deposition of the main part of the Palaeo-Inland Sea sediments. Among the Early

Middle Miocene ostracod faunas, the one from the Shukunohora Sandstone may have persisted

up until Recent times.



1083



TEXT-FIG.

3-Palaeogeographical map of the

Japanese Islands during the age of

Blow’s N 8 zone, Early Middle Miocene.

(after Chinzei, 1986).



130



135



140



145



TEXT-FIG.

GPalaeogeographical map of the Japanese Islands

during the age of Blow’s N9-15 zones, Middle Miocene.

(after Chinzei, 1986).



1084 M.YAJIMA



During the Middle Miocene (Blow's N 9 to N 15 zones), Southeast Japan was emergent except

for a few localities along the Pacific coast. The Japanese Islands moved to nearly the same position

as they occupy today. The Tsushima Strait closed, and the influence of warm water ceased, while

Northeast Japan came under cold water influence. Ostracods from Sekinohana, Sendai, Sugota,

and Kamikoani show cold water influence and are very similar to Pleistocene and Recent ostracod

faunas distributed along the Japan Sea and Pacific coast of Northeast Japan.



ACKNOWLEDGEMENTS

Most samples were obtained personally, but some were provided by Messrs E. Taguchi of Hiroshima University (Tsuyama samples), S. Ito of Shizuoka University (Kamikoani samples), Y. Saito

of JAPEX (Sugota samples), Y. Okumura of Mizunami Fossil Museum (Mizunami samples), and

Dr. Ikeya of Shizuoka University (Sekinohana samples). I thank Drs. T. Hanai and K. Chinzei of

the University of Tokyo for helpful discussion and Drs. H. Malz of Forschungs-Institut Senckenberg and P. Frydl of Mobil Oil Co. for coxments on the manuscript.



REFERENCE^

BLOW, W.H. 1969. Late Middle Eocene to Recent planktonic foraminifera1 biostratigraphy. In BRONNIMANN, P. and

RENZ, H.H. (eds.). Proc. 1st Internat. Conf: Planktonic Microfossils, Genova, 1967, 1, 199-422, pls. 1-54.

CHINZEI, K. (1986). Faunal succession and geographic distribution of Neogene molluscan faunas in Japan. In

KOTAKA T. (ed.) Origin and migration of the Japanese Cenozoic mohscs. Palaeontol. Soc. Japan. spec. pap.,



29, 17-32.

1957. Studies on the Ostracoda from Japan, 111. Subfamilies Cytherurinae G.W. Miiller (emend, G.O.

Sars, 1925) and Cytheropterinae n. subfam. J. Fac. Sci. Univ. Tokyo, sec. 2, 11, pt. 1, 11-36, pls. 2-4.

-1977. Future problems for systematic study of Japanese ostracods. Univ. Mus. Univ. Tokyo, Bull. no. 12,8486.

ISHIZAKI, K. 1963. Japanese Miocene ostracods from the Sunakosaka Member of the Yatsuo Formation, east of

Kanazawa city, Ishikawa Prefecture. Japan J. Geol. Geogr., 34 (l), 19-34, pi. 2.

- 1966. Miocene and Pliocene ostracods from the Sendai area, Japan. Sci. Rept. Tohoku Univ., 2nd ser. (Geol),

37 (2), 131-163, PIS. 16-19.

ITOIGAWA, J. 1981. Stratigraphic outlines of selected Neogene sequences. Pacific side areas. 8. Mizunami area. I n

TSUCHI R. (ed.) Neogene of Japan. 62-64. Kurofune Printing Co. Shizuoka.

SUDA, A. and ISHIGAKI, T. 1984. Foraminifers from Togi Mudstone in Togi-cho, Central Noto Peninsula, Japan. J.

Geof. SOC.Japan, 90(6), 417-420, 1 pl.

TSUCHI, R. (ed.). 1981. NeogeneofJapan. IGCP-114 National Working Group of Japan, 140 pp. Kurofune Printing

Co. Shizuoka.

HANAI, T.



DISCUSSION

Cronin : Are the Miocene species of Finmarchinella and Cornucoquimba still extant?

Yajima: Three cold water species of Finmarchinella and one warm water species of Cornucoquimba are extant. Two species of Cornucoquimba are confined to the Miocene as far as we

know at present.

Hazel : I noticed that you had Ambostracon ikeyai as old as N7 or N8 in the Miocene. This seems

to be a long range for a hemicytherid. Would you care to comment?

Yajima : Morphologically, Ambostracon ikeyai from Mizunami (Shukunohora sandstone, N8

zone) is the same as that from Chiba (Kioroshi Formation, 143,000yr BP). I think that many species not only of Ambostracon, but also of other hemicytherids from the Miocene are still extant.



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