Bull. Tohoku Univ. Museum. No. 22, pp. 1‒6, 2023
© by The Tohoku University Museum

Middle Permian orthoconic nautiloids from theTakakurayama Formation in the Yaguki area,Fukushima Prefecture, Northeast Japan
Shuji Niko* and Masayuki Ehiro***Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University,Higashihiroshima 739-8521, Japan, **The Tohoku University Museum, Sendai 980-8578, Japan
Abstract: Four species of orthoconic nautiloids belonging to the orders Orthocerida andPseudorthocerida are described from the Wordian (middle Permian) Kashiwadaira Member of theTakakurayama Formation, Northeast Japan. They are Geisonoceras? sp., Iwakiella ichiroi Hatai,
Kotaka and Noda, 1972, Pseudorthoceras? sp., and Bitaunioceras sp. Our observations of the newly
acquired two specimens of I. ichiroi have resulted in addition to the generic diagnosis, especially innatures of the annulations and surface ornamentation of the adult shells and the septal characters.

Introduction
A thick sequence of the Permian clastic rocks in the Yaguki
area, Fukushima Prefecture, Northeast Japan is assignedto the Takakurayama Formation (Iwao and Matsui, 1961;Onuki, 1966), in which three stratigraphic units, namely
the Iriishikura, Motomura and Kashiwadaira Members inascending order, are recognized (Yanagisawa and Nemoto,1961; Yanagisawa, 1967). Following Ehiro (2008, 2022),
that were described ammonoids and coiled nautiloids, thepresent study focuses on orthoconic nautiloids belonging
to the orders Orthocerida and Pseudorthocerida occur in
the Wordian (middle Permian) Kashiwadaira Member as
the third fascicle. Detailed geologic setting and locality
information of the examined specimens herein are referable
in the preceding papers.
Repository.― All specimens described in this study andthe holotype of Iwakiella ichiroi Hatai, Kotaka and Noda,
1972, are reposited in the Tohoku University Museum,Sendai (prefixed IGPS).
Systematic paleontology
Subclass Nautiloidea Agassiz, 1847
Order Orthocerida Kuhn, 1940
Family Geisonoceratidae Zhuravleva, 1959
Genus Geisonoceras Hyatt, 1884
Type species.― Orthoceras rivale Barrande, 1866.
Geisonoceras ? sp.
Figures 1.A, 1.B

Description.― A single external mold of fragmentary andstrongly deformed specimen was available for study; it islongiconic orthocone with 38 mm in length and very gradualconch expansion; reconstructed conch diameter as circularcross section is 7 mm. Surface ornamentation consists oftransverse narrow bands; adjacent bands divided by striae;
subtriangular salients developed; no internal structure
preserved.
Material examined.― IGPS coll. cat. no. 112740.
Occurrence.― Lower part of the Kashiwadaira Member at
B-valley (T1 locality).
Discussion.― The poorly preserved specimen isquestionably placed in Geisonoceras on the basis of its
gross conch shape and characteristic ornamentationconsisting of the narrow bands and striae.
Family Sphaerorthoceratidae Ristedt, 1968
Genus Iwakiella Hatai, Kotaka and Noda, 1972
Type species.― Iwakiella ichiroi Hatai, Kotaka and Noda,
1972.
Emended diagnosis.― Annulated orthocone with gradualconch expansion; initial chamber spherical; annulations
oblique and closely spaced in juvenile shell; spacings ofannulations become slightly wider in adult shell including
body chamber; interspaces of annulations nearly flat;
longitudinal surface lirae restrictedly occur in juvenile shell;
oblique lirae developed in interspaces of annulations;camerae relatively short.
Discussion.― Because the type and only included
species, Iwakiella ichiroi, of the genus was monotypic, itsgeneric diagnosis was previously based only on the holotype

2Shuji Niko and Masayuki Ehiro

Figure 1. A, B. Geisonoceras? sp. IGPS coll. cat. no. 112740. A, silicone rubber cast, side view: B, partial enlargementof A to show details of surface ornamentation. C–H. Iwakiella ichiroi Hatai, Kotaka and Noda, 1972. C–E, H, IGPS coll. cat.
no. 112737; C, partial enlargement of D to show details of surface ornamentation at adoral shell; D, side view; E, silicone
rubber cast prepared from apical shell, showing details of annulations and surface ornamentation; H, partial enlargement of
D to show details of septa: F, G. IGPS coll. cat. no. 86628; F, side view; G, silicone rubber cast prepared from apical shell,showing details of annulations and surface ornamentation. Scale bar is 12 mm in A, D, F; 3 mm in B, E, H; 7.5 mm in C;
6 mm in G.

Middle Permian orthoconic nautiloids from Northeast Japan(IGPS coll. cat. no. 86669). Furthermore, internal structureof the holotype was mostly unknown. The study has resulted
in emendation for the diagnosis on the basis of knowledges
derived from the following two newly acquired specimens.
A specimen (IGPS coll. cat. no. 112737) reveals that theholotype is the juvenile shell and natures of the adult shelland the septa. Another specimen (IGPS coll. cat. no. 86628)
provides data of the body chamber.
Similar annulations with Iwakiella also appear rarely inother Late Paleozoic genera, such as Cycloceras M’Coy(1844; type species, C. laevigatum M’Coy, 1844; see Histon,1991), Lopingoceras Shimansky in Ruzhentsev et al. (1962;
type species, Orthoceras lopingense Stoyanow, 1910), andNeocycloceras Flower and Caster (1935; type species, N.
obliquum Flower and Caster, 1935). Although Cycloceras
clearly differs Iwakiella from in its strongly concave andnon-ornamented interspaces of the annulations, thegeneric distinctions between Iwakiella, Lopingoceras andNeocycloceras are undeterrable at present because of
their insufficient morphological informations. Solution of theproblems is beyond the purposes of this study.
Iwakiella ichiroi Hatai, Kotaka and Noda, 1972
Figures 1.C–1.H[?] Cycloceras sp., Yanagisawa and Nemoto, 1961, p. 282.
Michelinoceras? sp., Yanagisawa, 1967, p. 98, pl. 4, fig. 9.
Tentaculites sp., Yanagisawa, 1967, p. 104, pl. 3, fig. 7.
Iwakiella ichiroi Hatai, Kotaka and Noda, 1972, p. 2–5, textfig.; Niko, 2022, p. 229, 230, figs. 1.1–1.6.
Description.― Conchs are longiconic and annulatedorthocones with gradual conch expansion; in examined twospecimens herein, apex and adoral body chamber including
peristome are not preserved; a specimen of deformed
phragmocone (IGPS coll. cat. no. 112737) has 45 mm inlength, 1.7 mm in reconstructed diameter as circular conch
section near apical end, 4.5 mm in ditto near adoral end,
and approximately 4.5° in reconstructed expansion angle;
length of another specimen of distorted body chamber (IGPScoll. cat. no. 86628) is 33 mm. Annulations well-developed
throughout and oblique; in apical (= juvenile) shell, they areclosely spaced, 6 in conch length of 2 mm, and possessrounded crests; then, their spacings become slightly wider
with 2–3 annulations in conch length 5 mm and shapes ofcrests shift to rounded triangular in adult shell; approximate
angles of annulations per transverse plane of shell in thesedeformed specimens range from 20° to 30°; interspacesof annulations are nearly flat. Surface ornamentationconsists of very fine longitudinal lirae, that are restricted inapical shell, and oblique ones developed in interspeciesof annulations and parallel with these. Septal curvatures

3moderate; camerae relatively short for the family andindicate approximate form ratio (width/length) of 3; sutures,
siphuncle and deposits are not observable.
Material examined.― IGPS coll. cat. nos. 86628, 112737.
Occurrence.― Middle part of the Kashiwadaira Member at
G2-valley (T7 Locality).
Discussion.― Apical shell morphologies of a newspecimen (IGPS coll. cat. no. 112737) are identical withthe juvenile shell characters of the holotype, that waserroneously assigned to Tentaculites (see Niko, 2022).
In addition, its annulation shape in the adoral shell isreasonably well with that of another specimen (IGPScoll. cat. no. 86628) representing the body chamber. The
latter specimen was described by Yanagisawa (1967) as
Michelinoceras? sp., but the presence of annulations in thisspecimen clearly excludes it from the genus Michelinoceras
Foerste, 1932, whose type species, Orthoceras michelini
Barrande, 1866, has non-annulated conch.
Cycloceras sp. in Yanagisawa and Nemoto (1961) may beconspecific with this species, but it remains undescribed.
Order Pseudorthocerida Barskov, 1963
Superfamily Pseudorthoceratoidea Flower and Caster, 1935
Family Pseudorthoceratidae Flower and Caster, 1935
Subfamily Pseudorthoceratinae Flower and Caster, 1935
Genus Pseudorthoceras Girty, 1911
Type species.― Orthoceras knoxense McChesney, 1859.
Pseudorthoceras? sp.
Figures 2.A–2.C
Description.― Conchs are longiconic orthocones withmoderate conch expansion and relatively large; a specimenof imperfect phragmocone (IGPS coll. cat. no. 112735)
attains 87 mm in length and has 9 mm in reconstructed
diameter as circular cross section near apical end, 15mm in ditto near adoral end, and approximately 5.5° inreconstructed expansion angle. Conch surface lacks
distinct ornamentation. Very shallow septa form roughly
transverse sutures; cameral length is short having form
ratios (reconstructed width/length) of 2.4–4.7; no siphuncle
preserved.
Material examined.― IGPS coll. cat. nos. 112735, 112738.
Occurrence.― Middle part of the Kashiwadaira Member at
G2-valley (T7 Locality).
Discussion.― The smooth conch surface and shortcamerae of the Takakurayama species may suggest it to be
Pseudorthoceras. Furthermore, there is a possibility that thespecies is conspecific with P. ouchii Endo and Mori (1969,p. 112, 113, fig. 1) because the holotype of the latter species
was collected also from G2-valley in the Yaguki area. The
preservations of the examined two specimens, however,4

Shuji Niko and Masayuki Ehiro

Figure 2. A–C. Pseudorthoceras? sp. A, IGPS coll. cat. no. 112735, silicone rubber cast, side view: B, C, IGPS coll. cat.
no. 112738; B, partial enlargement of C to show details of sutures; C, silicone rubber cast, shell wall mostly peeling off, sideview. D–F. Bitaunioceras sp. D, F, IGPS coll. cat. no. 112739; D, silicone rubber cast, side view; F, partial enlargement of
D to show details of surface ornamentation: E, IGPS coll. cat. no. 112736, side view, arrow indicates constriction at peeledpart of shell wall. Scale bar is 20 mm in A, C; 12 mm in B, D, E; 3 mm in F.

are incomplete to enough for the generic and specificidentifications.
Mooreoceras sp. indet. described by Ouchi (1971, p. 135,
pl. 1, figs. 1a–h) from G2-valley is similar in its gross conchshape and cameral length to Pseudorthoceras? sp., fromwhich it is separated by the presence of cancellate surface

ornamentation.
Subfamily Spyroceratinae Shimizu and Obata, 1935
Genus Bitaunioceras Shimizu and Obata, 1936
Type species.― Orthoceras bitauniense Haniel, 1915.

Middle Permian orthoconic nautiloids from Northeast Japan
Bitaunioceras sp.
Figures 2.D–2.F
Description.― Conchs are longiconic orthocones with
gradual conch expansion; a fragmentary and deformed
specimen (IGPS coll. cat. no. 112736) has 51 mm inlength, 6 mm in reconstructed diameter as circular crosssection near apical end, 8 mm in ditto near adoral end, andapproximately 3° in reconstructed expansion angle. Surface
ornamentation consists of transverse and slightly distantlirae that frequently form shallow salients; internal mold of
conch marked by relatively deep, transverse and periodic
constrictions provided by thickening of shell walls. Septum
shallow; suture and siphuncle are not observable.
Material examined.― IGPS coll. cat. nos. 112736, 112739.
Occurrence.― Middle part of the Kashiwadaira Member at
G2-valley (T7 Locality).
Discussion.― The distinctive constrictions formed byinward shell wall thickening of this species are exclusively for
Bitaunioceras. Although 11 species of the genus have beenreported from Italy (Gemmellaro, 1890), East Timor (Haniel,1915), Mexico (Miller, 1944), the southern Urals in Russia(Shimansky, 1954), Texas (Miller and Youngquist, 1947) andOklahoma (Niko et al., 2018) in North America, and Japan(Niko and Nishida, 1987), the present B. sp. is not complete
to enough for comparisons with these previously knownspecies.

Acknowledgements
We are thankful to Mr. Chisato Suzuki, who donatedcephalopod specimens for this study and provided fieldassistance. We express our thanks to Mr. Yukihiro Takaizumi
for his help in the field. The manuscript benefited fromsuggestive comments by Dr. Masayuki Fujikawa.

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