Bull. Tohoku Univ. Museum. No. 23, pp. 1‒10, 2024
© by The Tohoku University Museum

Silurian cephalopods from the Gionyama Formation in the
Kuraoka area, Miyazaki Prefecture, Southwest Japan
Shuji Niko
Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University,
Higashihiroshima 739-8521, Japan
Abstract: Silurian longiconic cephalopods are described for the first time from the Gionyama
Formation in the Kuraoka area, Miyazaki Prefecture, Southwest Japan. They include four orthocerids(Michelinoceras (Michelinoceras) alticameratum Kobayashi, 1984, Kopaninoceras kobayashii Niko,
Hamada and Yasui, 1989, Orthocycloceras sp., Arionoceras densiseptum Kobayashi, 1983) andthree pseudorthoceratids (Haloites gionyamaensis sp. nov., Gordonoceras? sp., Subdolocerassp.). Except for Subdoloceras sp. that occurs from the Wenlock (upper lower Silurian) G2 and the
Ludlow (lower upper Silurian) G3 members, their occurrences are restricted exclusively to the G3
Member. The new species is differentiated from the type species of Haloites, H. bellus Chen in Chenet al., 1982, from the Wenlock in Hubei, Central China by its less eccentric siphuncular position. The
Gionyama cephalopod assemblage belongs to the Kopaninoceras Fauna, whose main distributional
area is the northern margin of Gondwana, as with the Yokokurayama and Suberidani assemblages
in the Kurosegawa Belt.

Introduction
The Gionyama Formation in the Kuraoka area, Miyazaki
Prefecture, Southwest Japan comprises an approximately
1400 m succession of Middle Paleozoic sedimentary rocks.
Hamada (1959a) divided it into four members, namely
G1 (sandstone with thin acidic tuff layers), G2 (limestone
conglomerate with sandy to muddy matrixes), G3 (massive
limestone), and G4 (sandstone, acidic tuff, conglomerate).
Among them, the Wenlock (upper lower Silurian) G2 andLudlow (lower upper Silurian) G3 members occur wellpreserved fossils, including corals (i.e., Hamada, 1958;
Kido, 2010; Niko and Adachi, 2013) and trilobites (Hamada,
1959b; Kobayashi and Hamada, 1987), that typify Silurian
faunas in the Kurosegawa Belt.
This paper describes Silurian cephalopods from theGionyama Formation for the first time. Following three
localities in the Kuraoka area have yielded cephalopods(Figure 1). Locality 1 states the western foot of Mt.
Gionyama and occurs diverse fossils from limestone
conglomerate of the G2 Member. Locality 4 is a road side
outcrop of sandstone belonging to the G2 Member, fromwhich Geisonocerina? sp. was listed by Hamada (1961).
This species is neither figured nor described until now,furthermore I cannot detect an additional material at theidentical locality. Locality 5 is a river bed of the Gokase Rivernear Kyowa, where many float blocks of limestones derived

from the G3 Member are recognized.
Repository.―All examined specimens herein are housedin the Tohoku University Museum, Sendai (prefixed IGPS).

Cephalopod assemblage
The present Gionyama assemblage is composed byeight species of cephalopods, namely Michelinoceras(Mi chel i noceras ) al ti cameratum K obayash i, 1984,
Kopaninoceras kobayashii Niko, Hamada and Yasui,
1989, Orthocycloceras sp., Arionoceras densiseptum
Kobayashi, 1983, Geisonocerina? sp. (see Hamada, 1961;
not confirmed in this study), Haloites gionyamaensis sp.
nov., Gordonoceras? sp., and Subdoloceras sp. The latter
three species are placed within the Order Pseudorthocerida
that was not recorded previously in the Kurosegawa
Belt. Not only Kopaninoceras but also Michelinoceras,
Orthocycloceras, Arionoceras and Subdoloceras are thetypical genera of the Kopaninoceras Fauna (Niko et al.,2017) of which the main territory was the northern marginof Gondwana (peri-Gondwanan) including North Africa,
Variscan Europe, southern Afghanistan, western Tibet, andMalaysia. This assemblage is one of the representatives ofthe fauna same as the Yokokurayama (Kobayashi, 1983,
1984, 1988; Kobayashi and Hamada, 1985; Niko et al.,1989) and Suberidani (Niko, 2021) assemblages in the
Kurosegawa Belt.

Shuji Niko

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Figure 1. Map showing locations of the Kurosegawa Belt (KB), Miyazaki Prefecture, Gionyama, Yokokurayama andSuberidani in Southwest Japan (A), and cephalopod localities, 1, 4, 5 in the Kuraoka area (B). Used topographic map is“Digital Japan Basic Map” published by Geospatial Information Authority of Japan.
Systematic paleontology

Figures 2.A, 2.B

Subclass Orthoceratoidea Teichert, 1967
Order Orthocerida Kuhn, 1940
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 sp. α, sp. nov., Kobayashi, 1983, p. 293,
fig. 1.
Michelinoceras alticameratum Kobayashi, 1984, p. 245,
pl. 3, fig. 1; 1988, p. 1.
Michelinoceras (Michelinoceras) alticameratum; Niko,2021, p. 2, 4, figs. 2.A–2.D, 2.G.

Michelinoceras (Michelinoceras) alticameratum
Kobayashi, 1984

Description.―Conchs orthoconic with very gradualexpansion and circular transverse sections; the largest
specimen of imperfect phragmocone (IGPS coll. cat. no.
112840) attains 3 mm in approximate diameter; conch→
Figure 2. A, B. Michelinoceras (Michelinoceras) alticameratum Kobayashi, 1984, thin sections. A, IGPS coll. cat.
no. 112838, longitudinal (slightly oblique) section: B, IGPS coll. cat. no. 112839, transverse section. C–I. Arionoceras
densiseptum Kobayashi, 1983, thin sections. C, IGPS coll. cat. no. 112824, longitudinal (off siphuncle) section, note welldeveloped cameral deposits; D, IGPS coll. cat. no. 112827, transverse (slightly oblique) section: E, G–I, IGPS coll. cat. no.
112826: E, partial enlargement of H to show details of apical phragmocone; G, partial enlargement of H to show details of
siphuncle; H, longitudinal (slightly oblique) section; I, partial enlargement of H to show details of septal neck (arrow): F, IGPS
coll. cat. no. 112828, longitudinal section. Scale bar is 3 mm in A-C, E; 6 mm in D, F, H; 2.5 mm in G; 0.6 mm in I.

Silurian cephalopods from Miyazaki

3Shuji Niko4

surface probably smooth; apex, adoral phragmocone andbody chamber are missing. Camerae long to relatively longwith form ratios (maximum width/length) of 1.1–1.6 anddeeply concaved septa; sutures are possibly transverse.
Siphuncle relatively large for the genus with ratios ofsiphuncular diameter per corresponding conch diameterattaining 0.15 and subcentral in position; connecting rings
cylindrical; septal necks orthochoanitic and long. Cameral
deposits are not observable.
Material examined.―IGPS coll. cat. nos. 112833, 112836,
112838–112840.
Occurrence.―All specimens examined herein occurred inlimestone boulders at locality 5. Lithologically, these fossilbearing limestones obviously belong to the Ludlow (lower
upper Silurian) G3 Member of the Gionyama Formation.
Discussion.―Michelinoceras (Michelinoceras)
alticameratum is a common constituent of the late Silurian
faunas in the Kurosegawa Belt, Southwest Japan. It was
first described by Kobayashi (1984) from the upper Fukada
Formation of the Yokokurayama Group in Kochi Prefecture.
Subsequently, this species discovered from the SuberidaniGroup in Tokushima Prefecture (Niko, 2021). There is a
few tentative in the present specific identification of theGionyama specimens because they are fragments of theapical phragmocones.
Genus Kopaninoceras Kiselev, 1969
Type species.―Orthoceras jucundum Barrande, 1870.
Kopaninoceras kobayashii Niko, Hamada and Yasui,
1989
Figures 3.A, 3.B
Kopaninoceras kobayashii Niko, Hamada and Yasui,
1989, p. 61, 63, figs. 2.A, 2.B; Niko, 2021, p. 4, figs. 2.E, 2.F,
2.H.
Description.―Conchs orthoconic with circular transverse
sections; expansion of conch is relatively rapid for theorder, approximately 9°; the largest specimen of imperfectphragmocone (IGPS coll. cat. no. 113837) attains 5.5 mm
in approximate diameter; conch surface probably smooth;
apex and body chamber are missing. Camerae relatively
long to moderate with form ratios (maximum width/length)of 1.4–1.9; septa relatively deep; sutures faintly oblique topossibly transverse. Siphuncle narrow and composed bycylindrical connecting rings and long orthochoanitic septal
necks; position of siphuncle is subcentral. Cameral deposits
are episeptal-mural and hyposeptal.
Material examined.―IGPS coll. cat. nos. 112823, 112830,
112832, 112834, 112837.
Occurrence.―All examined specimens were collectedfrom float blocks of limestones derived from the Ludlow G3Member at locality 5.
Discussion.―Kopaninoceras kobayashii is also a typical
species in the Kurosegawa late Silurian cephalopod
assemblages. It commonly occurs in the Yokokurayama andSuberidani groups in Southwest Japan (Niko et al., 1989;
Niko, 2021).
Subfamily Leurocycloceratinae Sweet, 1964
Genus Orthocycloceras Barskov, 1972
Type species.―Orthocycloceras alayense Barskov, 1972.
Orthocycloceras sp.
Figures 3.C, 3.D
Description.―Longiconic and annulated orthocones withcircular transverse sections; conch expansion gradual.
Camerae short with form ratios (maximum width/length) of
3.1–5.2; septa shallow. Siphuncle central with cylindricalconnecting rings and short orthochoanitic septal necks.
Cameral deposits episeptal-mural.
Material examined.―IGPS coll. cat. nos. 112819, 112820,
112831, 112835.
Occurrence.―All examined specimens were collectedfrom float blocks of limestones derived from the Ludlow G3Member at locality 5.
Discussion.―There is a possibility that the present
Gionyama specimens are conspecific with Orthocyclocerassp. (Niko, 2021, p. 4, figs. 2.I–2.K) described from theSuberidani Group. Unfortunately, they are inadequate todeterminate, because their well-oriented section through thecentral axis of the siphuncle was not available.
Family Arionoceratidae Dzik, 1984
Genus Arionoceras Barskov, 1966
Type species.―Orthoceras arion Barrande, 1866.

→
Figure 3. A, B. Kopaninoceras kobayashii Niko, Hamada and Yasui, 1989, thin sections. A. IGPS coll. cat. no. 112830,
longitudinal (slightly oblique) section: B. IGPS coll. cat. no. 112837, transverse section. C, D. Orthocycloceras sp., IGPS
coll. cat. no. 112820, thin sections. C, longitudinal (slightly oblique) section: D, partial enlargement of C to show details of
siphuncle. E–G. Gordonoceras? sp., IGPS coll. cat. no. 112822, thin sections. E, F, partial enlargements of G to show detailsof siphuncle: G, longitudinal (slightly oblique) section. Scale bar is 3 mm in A, B, E, F; 6 mm in C, G; 2.5 mm in D.

Silurian cephalopods from Miyazaki

5Shuji Niko

6
Arionoceras densiseptum Kobayashi, 1983
Figures 2.C–2.I

Arionoceras densiseptum Kobayashi, 1983, p. 293, fig. 5;
1984, p. 246, 247, pl. 3, figs. 3–5; The Tokai Fossil Society,1995, p. 48, upper fig.
Description.―Conchs longiconic orthocones with slightly
elliptical (dorsoventrally depressed?) transverse sections;
expansion angles of conchs are relatively large for the order,
ranging from 5° to 8°; the largest specimen of imperfectphragmocone (IGPS coll. cat. no. 112826) attains 9 mm inapproximate diameter; conch surface probably smooth; apex
and body chamber are missing. Camerae moderate to shortwith form ratios (maximum width/length) of 2.0–3.2; septal
curvature moderate; sutures slightly oblique leaning toward
the dorsum. Siphuncle shifts to the venter and subcentral
in position; siphuncular walls composed by short orthoconic
septal necks and nearly cylindrical connecting rings.
Cameral deposits are episeptal-mural and hyposeptal, butdevelopments of the latter deposits are partial and restricted
to the ventral side.
Material examined.―IGPS coll. cat. nos. 112824–112829.
Occurrence.―All examined specimens were collectedfrom float blocks of limestones derived from the Ludlow G3Member at locality 5.
Discussion.―Characteristics of the examined six
specimens from the Gionyama Formation are quite identicalwith those of Arionoceras densiseptum, whose type locality
is the upper Fukada Formation. This discovery represents
the first record of the species except for the Yokokurayama
Group.
Order Pseudorthocerida Barskov, 1963
Family Pseudorthoceratidae Flower and Caster, 1935
? Subfamily Pseudorthoceratinae Flower and Caster,
1935
Genus Haloites Chen in Chen, Liu and Chen, 1981
Type species.―Haloites bellus Chen in Chen, Liu andChen, 1981.
Haloites gionyamaensis sp. nov.
Figures 4.A–4.F
Diagnosis.―Longiconic pseudorthoceratid with gently
exogastric cyrtocone in apical and orthocone in adoral parts;
conch expansion gradual; camerae short; siphuncle near
central with position ratio of 0.55; siphuncular segments
subglobose to pyriform; septal necks cyrtochoanitic;
cameral deposits mural and hyposeptal; annulosiphonate
endosiphuncular deposits indicate subtriangular protrusion.
Description.―An only available specimen that designatedherein the holotype is longicone with circular transverse
section and consists of imperfect phragmocone and apicalbody chamber; its apical part exhibits gentle exogastriccurvature and adoral one is orthoconic; measurementsof the holotype are 69 mm in preserved length, 15 mm in
maximum observable diameter, and 33 mm in preserved
length of body chamber; conch expansion gradual indicating
approximately 4° at phragmocone; body chamber may
exhibit weak constriction; detailed character of shell surface
unknown because it embedded in matrix, but distinct
ornamentation is not detected in longitudinal and transverse
sections; apex and peristome are missing. Camerae shortindicating 3.0–3.4 in form ratio (maximum width/length);septa shallow; in observations of longitudinal sections,sutures seem transverse. Siphuncle nearly central, but
faint shifts to dorsum in position with siphuncular position
ratio (minimum distance from ventral shell surface tocentral axis of siphuncle/corresponding conch diameter)of 0.55; siphuncular wall composed by short cyrtochoanitic
septal necks and strongly inflated connecting rings;
septal neck shapes asymmetrical indicating hook-like inventral side with approximately 0.6 mm in brim lengthand simple cyrtochoanitic in dorsal side with 0.3–0.4 mm
in ditto; longitudinal profiles of segments are subglobose
to pyriform with form rations (maximum width/length) of
0.8–1.0; connecting rings undifferentiated in structure, butslightly thickened attaining 0.08 mm. Cameral deposits welldeveloped and differentiated into mural and hyposeptal
types; endosiphuncular deposits annulosiphonate, slightly
thicker in ventral side than in dorsal one, and microgranularin structure; longitudinal section of well-preserved
endosiphuncular deposits indicates subtriangular protrusion
on adoral surface of septum.
Material examined.―Holotype, IGPS coll. cat. no. 112818.
Occurrence.―The holotype occurred in flat block oflimestone deriving from the G3 Member of the Gionyama
Formation. The fossil-bearing block was collected from theriverbed at locality 5.
Etymology.―The specific name is derived from the typestratum.
Discussion.―The present species from the Gionyama
Formation is tentatively assigned to an insufficiently
diagnosed genus Haloites, because its gross conch shapeand siphuncular structure are similar to those of the generictype, H. bellus Chen (in Chen et al., 1982, p. 44, 45, pl. 10,
figs. 14, 15), described from the Wenlock Xiushan Formationin Hubei, Central China, and as far as I know there is nocomparable Silurian genus with it. These two species canbe distinguished by their siphuncular position rations, i.e.,
0.55 in the Gionyama species versus 0.57–0.63 in H. bellus.
These numeral values indicate less eccentric siphuncular
position in the former than the latter. Therefore, the author

Silurian cephalopods from Miyazaki

7

Figure 4. Haloites gionyamaensis sp. nov., holotype, IGPS coll. cat. no. 112818, thin sections. A, longitudinal section,
venter on left. B, transverse section of body chamber (arrow). C, D, partial enlargements of A to show details of siphuncle.
E, partial enlargements to show details of ventral septal neck. F, partial enlargements to show details of dorsal septal neck.
Scale bar is 12 mm in A; 6 mm in B, C; 3 mm in D; 0.6 mm in E, F.

8Shuji Niko

advocates a new species as H. gionyamaensis even thoughsome unclear points concerning the generic placement still
remain.
The exogastric curvature of the apical shell andthe subglobular siphuncular segments of Haloites
gionyamaensis are also recognized in the Devonian to
Carboniferous pseudactinoceratine pseudorthocerids, suchas Pseudactinoceras Schindewolf, 1943, Campyloceras
M’Coy, 1844, and Macroloxoceras Flower, 1957. However,these siphuncles are distinctly ventral from the central axisof the conch. There is a possibility that Haloites and generaof the subfamily Pseudactinoceratinae Schindewolf, 1943,
are phylogenetically related.

Figure 5. Subdoloceras sp., thin sections. A, D, E, IGPS coll. cat. no. 112817: A, longitudinal (slightly oblique) section;
D, longitudinal (slightly oblique) section, showing details siphuncle, arrow indicates septal neck; E, partial enlargementof A to show details of septal neck. B, C, F, IGPS coll. cat. no. 112821: B, longitudinal (slightly oblique) section; C, partial
enlargement of B to show details of siphuncle; F, partial enlargement of B to show details of septal neck. Scale bar is 6 mm
in A, B; 3 mm in C, D; 0.6 mm in E, F.

Silurian cephalopods from Miyazaki
Subfamily Spyroceratinae Shimizu and Obata, 1935
Genus Gordonoceras Teichert and Glenister, 1953
Type species.―Gordonoceras bondi Teichert andGlenister, 1953.
Gordonoceras? sp.
Figures 3.E–3.G
Description.―A single fragment of longiconic cyrtocone
is available for this study; it consists of adoral phragmocone
with the last septum and apical body chamber; conch
diameter attains at least 16.5 mm. Camerae very short;
septal curvature deep. Siphuncle situates near midway
between conch axis and margin; septal neck achoanitic
in the last septum and cyrtochoanitic to suborthochoanitic
in the second one from the last; connecting rings more orless inflated. No endosiphuncular and cameral deposits
preserved.
Material examined.―IGPS coll. cat. no. 112822.
Occurrence.―This specimen was collected in flat block oflimestone deriving from the Ludlow G3 Member at locality 5.
Discussion.―In its general conch shape and siphuncular
position, the fragment is suggestive of Gordonoceras, whosetype species was described from the Silurian of Tasmania(Teichert and Glenister, 1953), but is not complete to enoughfor a confident identification.
Genus Subdoloceras Krōger, 2008
Type species.―Subdoloceras tafilaltense Krōger, 2008.
Subdoloceras sp.
Figures 5.A–5.F
Description.―Two fragmentary phragmocones areavailable in this study; they are longiconic orthocones withmoderate expansion for the order and circular transverse
sections; the larger specimen (IGPS coll. cat. no. 112821)
attains 12 mm in conch diameter. Camerae short, having
maximum width per length ratios of 3.1–4.8; septa deeply
concaved and form oblique sutures. Siphuncle subcentral in
position; septal necks suborthochoanitic, short; connecting
rings mostly cylindrical with abrupt constrictions at septal
foramina. No endosiphuncular and cameral deposits
preserved.
Material examined.―IGPS coll. cat. nos. 112817, 112821.
Occurrence.―A specimen (IGPS coll. cat. no. 112817)
was collected from limestone pebble in limestone
conglomerate belonging to the Wenlock (late early Silurian)G2 Member at locality 1. Another specimen (IGPS coll. cat.
no. 112821) occurs in flat block of limestone deriving fromthe Ludlow G3 Member at locality 5.
Discussion.―The Gionyama specimens display

9

similarities to Subdoloceras that was previously known
from the Pragian to upper Emsian (Lower Devonian) of
Morocco (Krōger, 2008), especially in their cameral lengthand siphuncular structure. However, the fragmentary natureof the present species prevents identification in the specificlevel.

Acknowledgements
The author is particularly grateful to the late Takashi
Hamada, who focused my attention on the Gionyama
fauna and provided the information of fossil locality andgeology in the Kuraoka area. I sincerely thank Tomio Adachi
and Yasuyoshi Hirata for their donations of part of the
cephalopod material described in this study. Special thanks
also go to Toshifumi Komatsu and Gengo Tanaka for theirsupports during the field work. This paper has benefited
from the criticisms of Masayuki Ehiro.

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