Mem . 15, pp Fac. Sci., Shimane Univ. 89-108 Dec. 20, 1981 Fossil Molluscs from the Mioce Group at Okuda, Masuda City Molluscan Fossils from Various Localiti in Shimane Prefecture, Part 2 Katsumi TAKAYASU Department of Geology, Faculty of Science, Shima (Received September 5, 1981) Introduction Abundant fossils of Vicarya and other molluscs jounior high school at Okuda in the south of (Textfig. l). Recently the details were reported covery, many specrmens have been collected on the for students of Shimane University and some excu twenty species of molluscs have been identified by specimens and TANAKA'S collections. In this paper The author wishes to express.his hearty thanks Junior High School, Muika-ichi-cho, Shimane Prefect Textfig. l+ / * H amada f' ~~:~;(~,/ Masuda _>-- ~ ,: . ~, jt -- ;~~~~~~'~:~:~ _ { ~ - '1~ T suw ano ¥/ ~ ~_/_ ;lA~; l ¥t ¥ ~ - ! 5km ¥~._lJ ¥J 1 . Map showing the fossil locality. 'elwamryokota", I : 25,000 in scale) (Taken from the topographical
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Mem . 15, pp
Fac. Sci., Shimane Univ.
89-108 Dec. 20, 1981
Fossil Molluscs from the Miocene Masuda
Group at Okuda, Masuda City
Molluscan Fossils from Various Localities
in Shimane Prefecture, Part 2
Katsumi TAKAYASU Department of Geology, Faculty of Science, Shimane Unrversity
(Received September 5, 1981)
Introduction
Abundant fossils of Vicarya and other molluscs were found by the students of a
jounior high school at Okuda in the south of Masuda City in December of 1977
(Textfig. l). Recently the details were reported by TANAKA (1981). After this dis-
covery, many specrmens have been collected on the chance of the geological training
for students of Shimane University and some excursions at the site. Here to fore,
twenty species of molluscs have been identified by the author's examination of these
specimens and TANAKA'S collections. In this paper, they will be described
The author wishes to express.his hearty thanks to Mr. Ikutaro TANAKA of Kuraki
Junior High School, Muika-ichi-cho, Shimane Prefecture, for providing fossil specimens
Textfig.
l+ / *
H amada f'
~~:~;(~,/
Masuda _>-- ~ ,: .
~, jt -- ;~~~~~~'~:~:~ _ { ~
- '1~ T suw ano ¥/ ~ ~_/_ ;lA~;
l ¥t ¥ ~ - ! 5km ¥~._lJ ¥J
1 . Map showing the fossil locality.
'elwamryokota", I : 25,000 in scale)
(Taken from the topographical map
90 Katsunu TAKAYASU and Dr. JunJi ITOIGAWA of Nagoya University, Professor Masahiro OKUBO and Dr.
Takao TOKUOKA Of Shimane University for their helpful advice
Geologic Outline
The Miocene marine sediments distributed in Masuda City are named the Masuda
Group and is divided into a lower unit, the Toyoda Formation, and an upper unit,
the Yasuda Formation (FUJITA, 1964). The Toyoda Formation is about 1 10 meters
in thickness, and consists of conglomerates, sandstones and alternating beds of sand-
stone and shale. The Yasuda Formation exceeds 80 meters in thickness, and is re-
presented by black shale interbedded with sandstones and tuffs. Fourteen fossil
localities were shown in FUJITA (1964) and he listed 21 species of mollusca, 4 species
of plant, some crabs and fish scales. On the basis of such fossil molluscs as Batillaria
tateiwai, Cardium ogurai and Placopecten cf. nomurai, he regarded the Masuda Group
as the early Middle Miocene in age and correlated it with the Bihoku Group of the
Setouchi province.
The fossil locality of Okuda newly reported by TANAKA (1981) belongs to the lower
part of the Toyoda Formation. As shown in Textfig. 2, the strata of about 23 meters
thick at this locality consist mostly of dark colored sandy shales, but some beds of
breccra of the underlying Sangun metamorphic rocks are contained in the basal part
o
Textfig. 2. (~ieologic column_ of the fossil locality
LithicSection cha-racters Zonule
・ ■■● ■・
m・ 一 一一 一 一 一
(Weathered2◎・
一 一 一 一一 一 一 一
ZOne)■ ‘ ‘ ‘ 一
一 ・ ‘ 一 一 一 ■ 一 ‘ 一 一 一 一 一 一 一.
働 吋ε
O⑩ o
15一Salndy sha1e ~
ミ
1刺~何
’ 勺~
qCalcareous
刊
1
COnCretiOnS呵
q10・
.H
H藪麹 oS~mdy shaユe
)
o載
←”05tre∂”bed 附0∂rga
油養藻直召一丁∂亡e立waia
Sandy sha1e 一Cr∂ssOS亡カe∂5□
.乙雑1鎚 BrecCia口ψ S㎝dy sha1e
公.・.ぺ.. Breccia&ss. 0ヱassoβ亡rea
鱗瀦蝸於ムSaハd.y sha1e
BreCCia∩ 口
Fossil Molluscs from the Miocene Masuda Group at Okuda, Masuda City 91
Notes on the Molluscan Assemblage
The molluscan assemblage of this locality can be divided into three zonules which
are the C,rassostrea zonule, Vicarya-Tateiwaia-Crassostrea zonule and Cyclina-Anadara-
Macoma zonule in ascending order (Textfig. 2). The Crassostrea zonule is composed
exclusively of C. gravitesta which is swarmed in a few beds. The Vicarya-Tateiwaia-
Crassostrea zonule is composed mainly of Vicarya japonica, Tateiwaia tateiwai and
Crassostrea gravitesta with minor amount of Tateiwaia yamanarii. Vicarya and Tateiwaia are swarmed in a bed of about I .5 meters thick. Bivalves except Cl'assostrea
are scarcely found in this zonule. The Cyclina-Anadara-Macoma zonule is represented
by Cyclina? hwabongriensis, Anadara (Hataiarca) daitokudoensis and Macoma (Maconla)
izurensis, and contains other various species of mollusca. In this zonule, Crassostrea
does not swarm but is contained sporadically, and Vicarya is hardly found
The upward sequence of the representative molluscs mentioned above indicates
the environmental change from the brackish embayment to the coastal water area near
the bay-mouth.
Systematic Description
Class GASTROPODA
Order ARCHAEOGASTROPODA Family TROCHIDAE
Subfamily CALLIOSTOMlNAE Genus Tristichotrochus IKEBE 1 942
Tristichotrochus sp.
(Pl. I, fig. 10)_
Description : Shell small trochiform. Post nucleous whorls five in number. Ex-
ternal surface of the body whorl sculptured with fine, slightly beaded spiral threads
Shell base flat and with about 10 spiral grooves, Umbilicus closed
Dimensions :
DGSU* coll. cat. no.
T1800 Height (mm) Diameter (mm)
16.5+ 20.4
Remarks : The spire is crushed axially, and owing to the poor preservation, the
surface sculpture is not clear. This specimen somewhat resembles Tristichotrochus
* Abbreviation for Department of Geology, Faculty of Sclence Shimane Unrversrty Matsue
92 Katsunu TAKAYASU takeharai ITOIGAWA and SHIBATA (1976) from the Miocene Mizunami Group in outline
but differs from the latter i.n its large and rather solid shell
Order MESOGASTROPODA Family POTAMIDIDAE
Genus Tateiwaia MAKIYAMA, 1936
Tateiwaia tateiwai (MAKIYAMA, 1 926)
(P1. I, figs. 6, 7)
Batillaria tateiwai MAKIYAMA, 1926, pp. 147-148, P1. 12, figs. 5, 6; OTUKA, 1934, p. 624, Pl. 49,
Remarks : Young form of the present species is somewhat allied to Anadara (Scaphaca) abdita MAKIYAMA, a Miocene Anadara frecuently associated with Anad.
daitqkudoensis, but the present species is distinguished by its convex shell and its
small number of radial ribs.
Subfamily STRIARClNAE
Genus Striarca C0.NRAD, 1 862
Striarca uetsukiensis (HATAI and NISIYAMA, 1 949)
(Pl. II, figs. 7a-c)
Barbatia (Barbatia) uetsukiensis HATAI and NISIYAMA, 1 949, p. 89, P1. 23, figs. 6-7 ; UOZUMI and FuJIE,
Phaxas cf. izumoensis (YOKOYAMA). MASUDA, 1955, Pl. 19, figs. 14a-b.
Description : An inner mould and a fragmental specimen are obtained. Shell medium in size, compressed, transversely elongated ; beak situating anteriorly ; post-
erior side about four times as 16ng as the anterior side ; anterior margin broadly
Remarks : The present specimen very resembles C. japonica KAMADA (1952) in outline, but the former is distinguished from the latter by its small slze and rts very
weak surface sculptures.
Cyclina ? hwabongriensis YooN and NoDA, 1 976
(Pl. 111, figs. Ia-b, 3-8b.) -
Cyclina (Cyclina?) hwabongriensis YOON and NoDA, 1 976, PP. 29-3 1 , figs. 2-5 ; TAGUCHI, ONO and
OKAMOTO, 1979. Pl. 2, figs. 19-23.
Description : Shell moderate to large in size, suborbicular, generally a little longer
than high, moderately inflated ; beak small, but not so much pointed ; anterodorsal
margin slightly concaved, Ionger than posterodorsal margin ; posterodorsal margm
broadly rounded, forming a blunt angle with posterior margin. Sculpture consrstmg
of fine numerous, concentric growth-lines and weak radial ribs on the anterior surface
Anterior part of the interior margin crenated ; the other part of the interior margm
smooth. Paliial sinus triangula~, round~d at end.
Dimenstons ; -
104 Katsumi TAKAYASU
DGSU coll. cat. no.
T171 7 (conjoined)
T1718 (conjoined)
T1719 (conjoined)
T1720 (core)
T1721 (core)
T1722 (conjoined)
T1723 (core)
T1724 (core)
T1782 (core)
T1783 (conjoined)
T1784 (conjoined)
T1785 (conjoined)
T1 786 (conjoined)
Tanaka's specimen-1 (core)
Tanaka's specimen-2 (core)
Height (mm)
28.3
39.0
32.2 (28 .O)
(33.8)
22.9 (21 .5)
(16.8)
(19.6)
ca. 24.4 46 . 3
644.5 5-5.9
(43 . I )
(39.6)
Length (mm)
32.4 43 , 8
35.0
(32.3)
(36.8)
24.4 (,~5.5)
(_17.0+)
(21.8)
28.2 47 . 7
49.5
ca. 52.8 (46.2)
(41 . 7)
Depth (mm)
1 7.2/2
24.5/2
1 2.9/2
(15.7/2)
(17.8/2)
1 1 ・8/2
(ro.5/2)
(9 ・ 3/2)-
(ro.1/2)
1 3・2/2
15.6+/2 26.8/2
31.5/2
(24.0/2)
(22.3/2)
Depth of pallial
sinus (mm)
1 1 .5
10.9 1 1 .O
8.5
8.2
5.8
7.8
13.1
14.8
Remarks : The present specimens closely resemble C. (Cyclinorbis) Iunulata MAKI-
YAMA (1926) ; the original description and the remarks by KAMADA (1952) of lunulata
are almost applic~ple to .them. The lunple-like sculpture characterizing that species
is also observable in some specimens from the Masuda Group. ' However, the speci-
mens at hand are slightly different from lunulata in having rather blunt end of pallial
sinus and weak radial ribs on the anterior surface.
Order MYOIDA
Family CORBULIDAE
Subfamily COLBULlNAE Genus Anisocorbula IREDALE, 1930
FUJIE, T. and UozuMI, S., 1 957; Illustrated Cenozoic Fossils, 25. Changes of the Neogene Fauna
m Hokkaido (Preliminary Report). Pt. I . Outline of Fossil Cominunities and Their Geological
Distribution (in Japanese). Cenoz. Res., no. 23, 499-504, Pl. 24, 2 text-figs
FuJrrA, T., 1964; Geology of the Northem Area of Masuda City, Shimane Prefecture. With Special References to the Neogene Formations (in Japanese with English abstract). Jour
Geol. Spc. Japan, vol. 70, 100-109.
HATAI, K. and NlSIYAMA, S., 1'949 ; New Tertiary Mollusca fron) Japan. Jour. Paleont., vol. 23,
no. 1, 87-94, Pls. 23, 24.
HAYASAKA, S.; 1 969 ; Molluscan "Fauna of the Kukinaga Gtoup' in Tan~-ga-shima, South Kyushu
and SHIBATA, 1 976; Twelve New Gastropos from the Miocene Mizunami Grouop, Gifu Prefp.cture, Japan. Ibid., no. 3, 5-15, Pls. 2, 3
and NISHIMOTO, H., 1 974 ; Molluscan Fossils of the Mizunami Group. (in Japanese). Ibid., no. 1,43-203, Pls. 1-63, 5 text-figs
and Okumura, K., 1 981; Miocene Fossils of the Mizunami Group, Central Japan. 2. Molluscs (in Japanese). Monogr. Mizunami Fossil Mus., no. 3-A, 1-53, Pls
1 -52.
IWAI, T., 1 961 ; The Miocene Molluscan Fossils frdm the Area Southwest of Hirosaki~City, Aomori
Prefecture, Japan. Trans.' Proc. Palaeont. Soc. Japan. N.-S;. no. 41, 1-8, Pl. l; I text-fig
, 1 965 ; The Geological and Paleontological Studies in the Marginal Area of the Tsugaru
WATANABE. K., ARAI, J. and HAYASHI, T., 1 950 ; Tertiary Geology of the Chichibu Basin (in Japanese). Bull. C.hichibu Mus.. Nat. Hist., no. I , 29-92, Pls. 1-6, 16 text-figs
YABE, H. and HATAl:. K., 1938; On the Japanese Species of Vicarya. Sci. Rep. Tohoku Imp. Univ..
2nd Ser., vol. 21, no. 2, 149-172, Pl. 21.
YAMANA, I., 1 966 ; Neogene Fossil Mollusca from Akenobe, Tottori Prefecture, Japan (in Japanese)
Bull. Japan. Mus. Assoc., no. I , 33-37, Pls. 1-2.
YOKOYAMA, M., 1 923 ; On Some Fossil Mollusca from the Neogene of lzumo. Jap. Jour. Geol Geogr., vol. 2, no. 1, 1-9, Pls. 1, 2.