Bull. Tohoku Univ. Museum. No. 20, pp. 1‒7, 2021
© by The Tohoku University Museum

Late Silurian orthocerid cephalopods from the SuberidaniGroup, Tokushima Prefecture, Southwest Japan
Shuji Niko
Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama,
Higashihiroshima City, Hiroshima 739-8521, Japan
Abstract: Orthocerid cephalopods are described for the first time from the Ludlow (early late
Silurian) limestone of the Suberidani Group in the Kurosegawa Belt. The examined specimens
occur from float brocks collected in the Katuura area, Tokushima Prefecture, Southwest Japan.
Four species, Michelinoceras (Michelinoceras) alticameratum Kobayashi, 1984, Kopaninoceras
kobayashii Niko, Hamada and Yasui, 1989, Orthocycloceras sp., and Arionoceras tokushimaense
sp. nov., are identified. The Suberidani assemblage belongs to the Kopaninoceras Fauna developedin the northern margin of Gondwana. This discovery is new evidence suggesting a Gondwana originof the Kurosegawa Belt.

Introduction
The Silurian to Devonian Suberidani Group (Hirayama
et al., 1956) is an unmetamorphosed constituent of theKurosegawa Belt that crops out in the Katuura area ofTokushima Prefecture, Southwest Japan (Figures 1.A,
1.B). It is divided into the lowest limestone dominantunit (10−19 m thick) and the main clastic unit (morethan 475 m thick), which consists mostly of sandstone
with intercalations of volcanic and volcaniclastic rocks(Hamada, 1959; Hamada, 1965; Wakamatsu et al., 1990).
Although limestone of the lowest unit yields diverse marine
fauna, they have been undocumented with the exceptionof some tabulate corals including halysitids (Yamashita,1946), aulocystids (Niko, 2001), and pachyporids (Niko,
2003). Among the unestablished taxa, the present paper
focuses on cephalopods for the first time and discusses
their paleobiogeographic significance. The examinedfossil specimens herein occur in float brocks of limestonecollected from the river-bed of the Miyaga-dani Valley (Loc. 1)
and an abandoned quarry in the Chouzuga-dani Valley (Loc.
2). Detailed geographic positions of the both localities are
given in Figure 1.C. Coral data indicate that the limestone
dominant unit can be correlative with the Ludlow (lower
upper Silurian) G3 Member of the Gionyama Formation(Yamashita, 1946; Niko, 2001, 2003).
Except for preliminary reports, such as Hamada (1961;
Geisonocerina? sp. from the Gionyama Formation) and

Hirata (1965; Orthoceras sp. from the Yokokurayama
Group), the first reliable study about Silurian cephalopods
in Japan was conducted by Kobayashi (1983), in which
the presence of six Ludlow species of orthocerids fromthe Yokokurayama Group was revealed. The specificcomposition of the assemblage subsequently compensatedby Kobayashi (1894) and Niko et al. (1989, 2017) is asfollows: Michelinoceras (Michelinoceras) alticameratum
Kobayashi, 1984 (? = M. mizobuchii Kobayashi, 1984),
Kopaninoceras kobayashii Niko, Hamada and Yasui,
1989, Orthocycloceras gomiense Kobayashi, 1983, O.
aff. gomiense, Leurocycloceras? sp. indet., Arionoceras
densiseptum Kobayashi, 1983, Protokionoceras
fessicancellatum Kobayashi, 1984, and geisonoceratid gen.
et sp. indet. These strata, as with the Suberidani Group,
belong to the Kurosegawa Belt. Additionally, the otherstructural belts, South Kitakami and Hida-Gaien, containing
Middle Paleozoic rocks, also yield Silurian cephalopods.
Of those, Niko (2016) described only a single species,
Bandoceras gifuense Niko, 2016, from the Ludlow Hitoegane
Formation belonging to the Hida-Gaien Belt. It can be said
that our knowledge on Japanese Silurian cephalopods is stillinsufficient; therefore, the present new discovery from the
Ludlow unit of the Suberidani Group is noteworthy.
Repository.―All specimens are stored in the Tohoku
University Museum (Institution abbreviation: IGPS = Instituteof Geology and Paleontology, Faculty of Science, Tohoku
University, Sendai).

2

Shuji Niko

Figure 1. Maps showing distribution of the Kurosegawa Belt (KB) and location of Tokushima Prefecture in Southwest
Japan (A), distribution of the Kurosegawa Belt and location of studied area in Tokushima Prefecture (B), detailed locationsof collecting sites of cephalopods in the Katuura area (C). Used topographic map is “Ai” scale 1:25,000 published by the
Geospatial Information Authority of Japan.

Systematic Paleontology
Order Orthocerida Kuhn, 1940
Superfamily Orthoceratoidea M’Coy, 1844
Family Orthoceratidae M’Coy, 1844
Subfamily Michelinoceratinae Flower, 1945
Genus Michelinoceras Foerste, 1932
Type species.―Orthoceras michelini Barrande, 1866.
Subgenus Michelinoceras Foerste, 1932,
emend. Kiselev and Gnoli, 1992

Michelinoceras (Michelinoceras) alticameratum
Kobayashi, 1984
Figures 2.A−2.D, 2.GMichelinoceras sp. α, sp. nov., Kobayashi, 1983, p. 293, fig. 1.
Michelinoceras alticameratum Kobayashi, 1984, p. 245, pl. 3,
fig. 1; 1988, p. 1.
Description.―Longiconic large orthocones with circularcross section; conch expansion is gradual, approximately 4°
in angle; the largest specimen (IGPS coll. cat. no. 112448)→
Figure 2. A−D, G. Michelinoceras (Michelinoceras) alticameratum Kobayashi, 1984. A, IGPS coll. cat. no. 112446,
longitudinal polished section: B, IGPS coll. cat. no. 112447, longitudinal (dorsoventral) polished section, venter on right:
C, IGPS coll. cat. no. 112449, transverse polished section, venter down: D, G, IGPS coll. cat. no. 112448; D, longitudinal
polished section; G, longitudinal thin section, showing siphuncular structure, arrow indicates septal neck. E, F, H.
Kopaninoceras kobayashii Niko, Hamada and Yasui, 1989. E, IGPS coll. cat. no. 112455, longitudinal (slightly off conch
center) polished section: F, H, IGPS coll. cat. no. 112452; F, longitudinal (dorsoventral) polished section, venter on right; H,
longitudinal thin section, arrow indicates septal neck. I−K. Orthocycloceras sp., IGPS coll. cat. no. 112456. I, longitudinal thinsection: J, partial enlargement of I, arrow indicates septal neck: K, longitudinal polished section. Scale bar is 15 mm in A, B, D,
K; 6 mm in C, E−G, I; 3 mm in H, J.

Late Silurian orthocerid cephalopods Southwest Japan3

4

Shuji Nikoattains 20 mm in conch diameter; shell surface probablysmooth; apex and body chamber are not preserved.
Camerae long to relatively long with form ratios (maximum
width/length) of 1.0−1.4; septa deep; sutures directlytransverse; siphuncle narrow and slightly subcentral in
position; siphuncular wall consists of orthochoanitic septal
necks and cylindrical connecting rings; length of septal
neck is very long, attaining 2.5 mm at approximate conch
diameter of 13 mm. Cameral deposits episeptal-mural.
Material examined.―IGPS coll. cat. nos. 112443−112450.
Occurrence.―Locs. 1 (IGPS coll. cat. nos. 112443−112449)
and 2 (IGPS coll. cat. no. 112450).
Discussion.―The present material including large
specimens from the Suberidani Group is assigned toMichelinoceras (Michelinoceras) alticameratum fromthe Yokokurayama Group in the Yokokurayama area,
Southwest Japan, based on angles of conch expansion andslightly subcentral siphuncular position. Because M. (M.)
alticameratum was represented by an only known holotype
whose adoral end has 9 mm in diameter, the new specimens
indicate more adoral shell characters.
Genus Kopaninoceras Kiselev, 1969
Type species.―Orthoceras jucundum Barrande, 1870.
Kopaninoceras kobayashii Niko,
Hamada and Yasui, 1989
Figures 2.E, 2.F, 2.H
Kopaninoceras kobayashii Niko, Hamada and Yasui, 1989,
p. 61, 63, figs. 2.A, 2.B.
Description.―Longiconic orthocones with circular crosssection; conch expansion is relatively rapid, approximately 9°in angle; the largest specimen (IGPS coll. cat. no. 112455)
attains 8 mm in conch diameter; shell surface probablysmooth; apex and body chamber are not preserved. Cameral
length varies from short to relatively long with form ratios(maximum width/length) of 1.4−3.5; septa relatively deep;
sutures slightly oblique; siphuncle narrow and subcentral in
position; siphuncular wall consists of orthochoanitic septal
necks and cylindrical connecting rings; length of septal neckis long, 1.2 mm at conch diameter of 5 mm; septal necks
converge faintly adapically and indicate narrow funnelshape, where siphuncle shallowly constricted. Camera filledwith calcite that may be recrystallized cameral deposits.
Material examined.―IGPS coll. cat. nos. 112451−112455.
Occurrence.―Locs. 1 (IGPS coll. cat. nos. 112451−112453)
and 2 (IGPS coll. cat. nos. 112454, 112455).
Discussion.―The author cannot detect differences in mostof the important taxonomic criteria, including general conchshape, cameral length, siphuncular position and nature of

siphuncular wall, between the Suberidani specimens and thetype series of Kopaninoceras kobayashii, that was collectedfrom the Yokokurayama Group (Niko et al., 1989).
Subfamily Leurocycloceratinae Sweet, 1964
Genus Orthocycloceras Barskov, 1972
Type species.―Orthocycloceras alayense Barskov, 1972.
Orthocycloceras sp.
Figures 2.I−2.K
Description.―A single longiconic specimen of annulated
orthocone was available for study, whose cross section iscircular and adoral part represents body chamber; conch
expansion is gradual, approximately 3° in angle; conch
diameter attains 10 mm; there are 4−5 annuli in length ofcorresponding conch diameter; apex and peristome are notpreserved. Camerae short to very short with approximate
form ratios (maximum width/length) of 3−6 in usual camerae
and attaining 15 in the last camera; sutures slightly oblique;siphuncle relatively large for the family and central in
position; siphuncular wall consists of orthochoanitic septal
necks and weakly inflated connecting rings; length of septal
neck is short, 0.4 mm at conch diameter of 8 mm. Cameral
deposits episeptal-mural.
Material examined.―IGPS coll. cat. no. 112456.
Occurrence.―Loc. 2.
Discussion.―This poorly preserved specimen shows close
similarities to Orthocycloceras gomiense Kobayashi, 1983,
from the Yokokurayama Group, but differs by its shorter
camerae and weakly inflated connecting rings. Because of
insufficient material, it is left in open nomenclature.
Family Arionoceratidae Dzik, 1984
Genus Arionoceras Barskov, 1966
Type species.―Orthoceras arion Barrande, 1866.
Arionoceras tokushimaense sp. nov.
Figures 3.A−3.I
Diagnosis.―Species of Arionoceras with depressed conchcross sections; camerae short; cameral form ratios ranges
from 3.0 to 4.9; sutures slightly incline toward dorsum;
siphuncle nearly central in apical and subcentral in moreadoral shells, thus its position ratios decrease to 0.4; septal
necks orthochoanitic and very short, 0.19−0.63 mm.
Description.―Conchs longiconic orthocones withmoderate to relatively rapid expansion, whose angle
ranges from 8° to 10°; cross sections of conch are
dorsoventrally depressed oval with approximate form ratio(lateral diameter/dorsoventral diameter) of 1.3; the largest
specimen of fragmentary phragmocone (holotype) has 15

Late Silurian orthocerid cephalopods Southwest Japan

5

Figure 3. Arionoceras tokushimaense sp. nov. A. Paratype, IGPS coll. cat. no. 112458, longitudinal (slightly oblique)polished section; venter on right. B, F. Holotype, IGPS coll. cat. no. 112461: B, longitudinal (dorsoventral) polished section,
venter on left: F, longitudinal (dorsoventral) thin section, showing ventral siphuncular wall, arrow indicates septal neck. C,
E. Paratype, IGPS coll. cat. no. 112463: C, longitudinal (slightly off conch center) polished section: E, transverse polishedsection, venter down. D, H. Paratype, IGPS coll. cat. no. 112462: D, longitudinal (slightly oblique) polished section: H,
longitudinal (slightly oblique) thin section, arrows indicate septal necks. G, I. Paratype, IGPS coll. cat. no. 112457: G,
longitudinal (dorsoventral) thin section: I, partial enlargement of G to show details of septal neck. Scale bar is 6 mm in A−E; 0.6
mm in F, I; 2.1 mm in G, H.

6Shuji Nikomm in length and 8 mm in adoral dorsoventral diameter;
shell surface probably smooth; apex and body chamber are
not preserved. Camerae short with form ratios (maximum
dorsoventral width/length) of 3.0−3.9 in a paratype (IGPS
coll. cat. no. 112457, approximately 3 mm in dorsoventral
diameter) representing apical shell and 4.1−4.9 in theholotype; sutures straight and slightly incline toward dorsum;
siphuncle narrow; diameters of siphuncle are 0.21 mm
in apical shell and 0.73 mm in the holotype; siphuncular
position nearly central in apical shell, then it becomessubcentral with 0.4 in position ratios (distance between
central axis of siphuncle and surface of ventral shell surface/corresponding dorsoventral conch diameter) at more adoralshells; siphuncular wall consists of orthochoanitic septal
necks and cylindrical connecting rings; length of septal neckis very short, 0.19 mm in apical shell and 0.63 mm in theholotype; connecting rings thin, undifferentiated. Cameral
deposits episeptal-mural; no endosiphuncular deposits
developed.
Material examined.―Holotype, IGPS coll. cat. no. 112461.
Paratypes, IGPS coll. cat. nos. 112457, 112458, 112462,
112463. In addition, four poorly preserved specimens, IGPS
coll. cat. nos. 112459, 112460, 112464, 112465, were also
examined.
Occurrence.―Locs. 1 (IGPS coll. cat. nos. 112457−112463)
and 2 (IGPS coll. cat. nos. 112464, 112465).
Etymology. ―The specific name is derived from
Tokushima Prefecture.
Discussion.―Arionoceras tokushimaense sp. nov. issomewhat similar to A. densiseptum Kobayashi, 1983, but
the new species has shorter camerae and more eccentricsiphuncular position in the adoral shells than those of thepreviously known species from the Yokokurayama Group.
The type species of the genus, A. arion from the Prague
Basin in Bohemia, differs from the new species by its circularcross sections of the conch and nearly central siphuncular
position even in the adoral shells. Arionoceras mahsuri
Niko, Sone and Leman, 2017, from the Langkawi Islands,
Malaysia, clearly differs from the new species by its much
longer camerae.

SignificanceThe orthocerid cephalopod assemblage of the SuberidaniGroup described here is composed of Michelinoceras(Michelinoceras) alticameratum, Kopaninoceras kobayashii,
Orthocycloceras sp., and Arionoceras tokushimaense sp.
nov. It has greater affinity with that of the Yokokurayama
Group. Michelinoceras (M.) alticameratum and K. kobayashii
are common to both assemblages. In addition, there are
related species in the Yokokurayama assemblage for a
species of Orthocycloceras left in open nomenclature and A.

tokushimaense, i.e., O. gomiense and A. densiseptum. Nikoet al (2017) posted the Kopaninoceras Fauna and indicatedthat it includes a local cephalopod assemblage from the
Yokokurayama Group. Consequently, the Suberidani
assemblage also belongs to the Kopaninoceras Fauna.
Because the main territory of the Kopaninoceras Fauna was
the northern margin of Gondwana including North Africa,
Variscan Europe, southern Afghanistan, western Tibet, andMalaysia (Niko et al., 2017), this new discovery provides
additional paleontological evidence supporting Hada andYoshikura’s (1999) theory that the Kusosegawa Belt has a
Gondwana origin.

Acknowledgements
The author is indebted to Yoshihito Senzai and Noboru
Okitsu for providing the important cephalopod specimensfrom their personal collections, on which this study was
based. Furthermore, they kindly helped in my field work. The
manuscript benefited from the helpful review of Masayuki Ehiro.

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