Bull. Tohoku Univ. Museum, No. 1 , pp. 1–4, 2015
© by The Tohoku University Museum

Early Permian (Asselian) orthoconic cephalopods fromthe Taishaku Limestone, Akiyoshi Belt, Southwest J apan
Shuji Niko*, Masayuki Ehiro**, Osamu Nishikawa*** and Isao Nishikawa*****Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashihiroshima 7398521, Japan, **The Tohoku University Museum, Sendai 980-8578, Japan, ***Mineral Industry Museum, Graduate Schoolof Technology and Resource Science, Akita University, Akita 010-8502, Japan, ****Otsu 1797-1, Yuki, Jinsekikogen-cho,Jinseki-gun, Hiroshima 720-1812, Japan
Abstract: Orthoconic cephalopods including two species of the orthocerid nautiloids,
Michelinoceras hasei sp. nov. and Bogoslovskya miharanoroensis Niko and Ozawa, 19 9 7, and abactritoid, Aktastioceras nishikawai Niko, Nishida and Hamada, 19 9 3, are described from the Lower
Permian (Asselian) limestone in the Miharanoro area of Hiroshima Prefecture, Southwest J apan.
Stratigraphically, the limestone belongs to the Uyamano Formation of the Taishaku Limestone.
Michelinoceras hasei differs from comparable taxa in the possession of the surface grooves.

Introduction
Following previous two papers (Niko et al., 1993; Ehiro etal., 2014) the present contribution deals subsequent resultsof detailed investigations of the cephalopod material thatcollected from the Taishaku Limestone over the ten years
in 1950’s to 1960’s and preliminary examined by Hayasaka
and Nishikawa (1962; faunal composition was introduced by
Koizumi, 1967a, b, 1975, as the personal communications
from the authors). Here, we additionally describe orthoconic
cephalopods. They are Michelinoceras hasei sp. nov.,
Bogoslovskya miharanoroensis Niko and Ozawa, 1997, andAktastioceras nishikawai Niko, Nishida and Hamada, 1993.

Systematic Paleontology
Class Cephalopoda Cuvier, 1797
Subclass Nautiloidea Agassiz, 1847
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.
Michelinoceras hasei sp. nov.
Figures 1.1 1.5, 1.14
Diagnosis.— Species of Michelinoceras with 4° in angleof conch expansion and transverse grooves as surface

ornamentation camerae relatively short indicating 1.3 2.0
in form ratios (maximum width/length); siphuncle central andnarrow.
Description.— An incomplete phragmocone is available
for study, that indicates longiconic orthocone with gradualconch expansion having approximately 4° in angle; its
approximate diameter and length are 9 mm near apicalend and 51 mm, respectively; cross sections of conch
are circular; shell surface ornamented by transverse finegrooves. Septa deeply concaved; sutures are not observablein the specimen, but apparent sinus and obliqueness ofsutures are not recognized in longitudinal sections; cameral
length relatively short for the genus; form ratios (maximumwidth length) of camera are 1.3 2.0 siphuncle is central
in position and consists of orthochoanitic septal necks
and cylindrical connecting rings; length of septal necks islong, 0.9 1.2 mm diameter of siphuncle is narrow, 0.9 1.0mm; ratios of siphuncular diameter per shell diameter areapproximately 0.1; endosiphuncular and cameral deposits
are not developed.
Material examined.—H olotype, IGPS coll. cat. no. 111503.
Occurrence and age.— Michelinoceras hasei sp. nov.
was discovered in massive limestone at locality 1 in Ehiroet al., (2014, fig. 1) in the Miharanoro area of Tojo-cho, on
the eastern end of Shobara City, Hiroshima Prefecture,
Southwest Japan. Stratigraphically, the fossil-bearing
limestone belongs to the Uyamano Formation of the central
facies in the Taishaku Limestone (Hase et al., 1974). The ageof the associated ammonoid fauna with M. hasei is highly
likely to be Asselian (early Early Permian; Ehiro et al., 2014).

2Shuji Niko, Masayuki Ehiro, Osamu Nishikawa and Isao Nishikawa

Etymology. The specific name honors the late r. kiraHase in recognition of his contributions for geology of theTaishaku Limestone.
Discussion.— Michelinoceras hasei sp. nov. somewhat
resembles M. magnum Shimansky (1968, p. 53, 54, pl.
1, figs. 1a, b, v, 2) from the upper ower Carboniferous ofsouthern Urals in its gross conch shape, relatively shortcamera for the genus, and narrow siphuncle. However,the Russian species lacks surface ornamentation.
Michelinoceras? sp. (Niko and Ozawa, 1997, p. 48, figs.
2.14 2.16) from the Triticites contractus Z one (late Gzhelian
to early Asselian; late Late Carboniferous to early EarlyPermian) of the Taishaku Limestone also differs from thenew species in having the smooth shell surface.

resembles B. akiyoshiensis Niko, Nishida and Kyuma (1995,p. 193, 195, figs. 1.1 1.14) from the Moscovian (UpperCarboniferous) strata of the Akiyoshi Limestone, but in shellsurface of the latter species is ornamented by fine transverse
lirae. Another Middle Carboniferous species of Bogoslovskya
from Japan, B. omiensis iko (2001, p. 115 118, figs.1.1 1.3,1.5 1.9), clearly differs from B. miharanoroensis by its
strongly eccentric siphuncular position.

Genus Bogoslovskya Z huravleva, 1978
Type species.— Bogoslovskya perspicua Z huravleva, 1978.

Aktastioceras nishikawai Niko, Nishida and Hamada,
1993
Figures 1.8 1.13, 1.15

Bogoslovskya miharanoroensis Niko and Ozawa, 1997
Figures 1.6, 1.7
Bogoslovskya miharanoroensis Niko and Ozawa, 1997,
p. 48, 50, figs 2.1 2.9.
Description.— Longiconic orthocones with gradual conchexpansion and circular (to slightly compressed? ) crosssections; apical end of a relatively well-preserved specimen(IGPS coll. cat. no. 111499) has 11 mm in approximate
diameter; shell surface ornamented by distant ridges forming
very narrow ribs, whose direction is oblique and towards
apex on siphuncular side. Sutures straight and transverse;
siphuncle submarginal in position.
Material examined. IGPS coll. cat. nos. 111498 111500. In
addition, two poorly preserved specimens, IGPS coll. cat. nos.
111501 and 111502, are questionably assigned to this species.
Occurrence and age.—S ame as Michelinoceras hasei sp.
nov.
Discussion.— Morphologies of the present specimens
are consistent with those of Bogoslovskya miharanoroensis
described by Niko and Ozawa (1997) from the Triticites
contractus Z one of the Taishaku Limestone. This species

Subclass Bactritoidea Shimansky, 1951
Order Bactritida Shimansky, 1951
Family Parabactritidae Shimansky, 1951
Genus Aktastioceras Shimansky, 1948
Type species.— Aktastioceras kruglovi Shimansky, 1948.

Orthoceras cfr. adrivnense[ sic] ; Koizumi, 1967a, p. 135;
1967b, p. 208.
Psevdorthoceras[ sic] cfr. adrianense (Gemmellaro, 1890);
Koizumi, 1975, p. 22.
Aktastioceras nishikawai Niko, Nishida and Hamada,
1993, p. 314, 315, figs. 2.1 2.8.
Description.— Longiconic orthocones with approximately
9° in angle of conch expansion; cross sections of conchs
are circular at apical shell and laterally compressed inadoral one; shell surface is marked by very fine lirae thatrun directly transverse to conch axis. Septa indicate deep
concavities; except for dorsal sinus, sutures are straightand transverse; cameral length moderate for the genus;
form ratios (maximum width length) of camera are 1.8 2.3
siphuncle situates dorsal margin with orthochoanitic septalnecks; length of septal necks is short, approximately 1.0mm; diameter of septal foramen is approximately 0.8 mm;
no connecting ring preserved;
Material examined.— IGPS coll. cat. nos. 111504 and111505.
Occurrence and age.— Same as Michelinoceras hasei sp. nov.

Figure 1. Orthoconic cephalopods from the Taishaku Limestone in the Miharanoro area of Tojo-cho, on the eastern end of
Shobara City, Hiroshima Prefecture. 1‒5, 14. Michelinoceras hasei sp. nov., holotype, IGPS coll. cat. no. 111503: 1, side view,× 1.5; 2, cross sectional view of polished apical end, × 1.5; 3, thin longitudinal section, × 3; 4, 5, partial enlargements of Figure
1.3 to show details of siphuncular structures, × 10; 14, surface ornamentation of transverse grooves, × 5. 6, 7. Bogoslovskya
miharanoroensis Niko and Ozawa, 1997: 6, IGPS coll. cat. no. 111500, lateral view, siphuncular side on right, × 2; 7, IGPS
coll. cat. no. 111499, septal view, arrow indicates siphuncle, × 2. 8‒13, 15. Aktastioceras nishikawai Niko, Nishida andHamada, 1993: 8, 11, IGPS coll. cat. no. 111505; 8, side view, × 1.5; 11, partial enlargements of Figure 1.8 to show surface
ornamentation of very fine transverse lirae, 3 9, 10, 12, 13, 15, IGPS coll. cat. no. 111504; 9, lateral view, venter on right,
× 1.5; 10, cross sectional view of adoral end, venter down, × 1.5; 12, dorsal view, arrow indicates siphuncle, × 3; 13, partialenlargement of Figure 1.15 to show details of siphuncular structure, × 8; 15, dorsoventral polished section, venter on right,× 2.

Early Permian cephalopods from Taishaku

34Shuji Niko, Masayuki Ehiro, Osamu Nishikawa and Isao Nishikawa

Discussion.— Records of bactritoid cephalopods fromJapan are rare and restricted in allochthonous limestonebodies in the Akiyoshi Belt, including Akiyoshi (Niko et al.,1991), Omi (Niko, 2001), and Taishaku (Niko et al., 1993;
Niko and Ozawa, 1997; this report).
Based on an associated label, it seems that a specimen(IGPS coll. cat. no. 111505) of Aktastioceras nishikawai
was assigned tentatively to an orthocerid genus,
Pseudorthoceras Girty, 1911, by r. Ichiro ayasaka andMr. Isao Nishikawa. This unpublished result was introduced
by Koizumi (1967a, b, 1975). These erroneous assignments
are corrected herein.

Acknowledgements
The specimens described here had been kept by r.
Tamio Nishida after the examination of Hayasaka andNishikawa (1962). He kindly permits the authors to study
them. r. Yasunari Shigeta is thanked for loan of the typesof Aktastioceras nishikawai. We also thank r. Masayuki
Fujikawa for his helpful comments on the manuscript.

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