Bull. Tohoku Univ. Museum. No. 23, pp. 17‒21, 2024
© by The Tohoku University Museum

Two Olenekian (Early Triassic) species of longiconic
cephalopods from the Osawa Formation, Miyagi
Prefecture, Northeast Japan
Shuji Niko* and Masayuki Ehiro***Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashihiroshima 7398521, Japan,**The Tohoku University Museum, Sendai 980-8578, Japan
Abstract: Two species of longiconic cephalopods, Pseudotemperoceras? sp. and Trematoceras
osawaense sp. nov., are described from laminated mudstones of the Lower Triassic Osawa
Formation at Asadanuki, Tome City, Miyagi Prefecture, Northeast Japan. The new species isdifferentiated from other species assigned to Trematoceras by the morphological combinationof its very gradually expanding conchs with a 4°–5° apical angle, moderate to relatively longcameral lengths, nearly central siphuncular position, and well-developed cameral deposits.
Biostratigraphically, T. osawaense provides a regional index suggesting the latest Olenekian in theSouth Kitakami Belt.

Introduction
Following Bando and Ehiro (1982) and Ehiro (2022b),
the present study is the third fascicle of our publication
series concerning the latest Olenekian (late Early Triassic)
cephalopods from the uppermost part of the OsawaFormation, lower Inai Group, in the South Kitakami
Belt, Northeast Japan. We describe two longiconic
orthoceratoid species, namely Pseudotemperoceras? sp.
and Trematoceras osawaense sp. nov., based on material
collected from black laminated mudstones at the upper
reaches of Nameshi-zawa Valley in Asadanuki, Tome City,
Miyagi Prefecture. Their detailed geologic, stratigraphic,and geographic settings are given in the proceeding papers(Bando and Ehiro, 1982; Ehiro, 2022b).
Repository.–All specimens examined for this paper arereposited in the Toholu University Museum, Sendai (prefixedIGPS).
Systematic paleontology
Subclass Orthoceratoidea Teichert, 1967
Order Orthocerida Kuhn, 1940
Family Geisonoceratidae Zhuravleva, 1959
Genus Pseudotemperoceras Schastlivtceva, 1986
Type species.–Pseudotemperoceras pulchrum Schastlivtceva,
1986.

Pseudotemperoceras? sp.
Figures 1.A, 1.B
Description.–Two incomplete and flattened phragmocones
were examined; they are longiconic orthocones withmoderate expansion; the largest specimen (IGPS coll. cat.
no. 112845) attains 45 mm in length and approximately 12
mm in reconstructed diameter assuming it has a circularcross section; conch surface lacks distinct ornamentation.
Camerae short exhibiting approximately 3.6 in reconstructedwidth/length ratio; septal curvature relatively deep. Sutures
directly transverse. Siphuncle cannot observable.
Material examined.–IGPS coll. cat. nos. 112843, 112845.
Discussion.–Among the previously known Triassic generaof longiconic nautiloids, the preserved characters of thepresent species such as its moderately expanding conch andshort camerae, correspond to those of Pseudotemperoceras,
whose type species was described from the Olenekian of
Verkhoyansk, northeastern Siberia (Schastlivtceva, 1986).
We keep the genus assignment open for the Osawa species
as it is questionable because of the poor preservation until
better material becomes available.
Order Pseudorthocerida Barskov, 1963
Superfamily Pseudorthoceratoidea Flower and Caster, 1935
Family Trematoceratidae Zakharov, 1996
Genus Trematoceras Eichwald, 1851
Type species.–Orthocera [sic] elegans Münster, 1841.

18Shuji Niko and Masayuki Ehiro
Trematoceras osawaense sp. nov.
Figures 1.C–1.J

Diagnosis.–Species of Trematoceras with very small apicalangle between 4° to 5°, circular conch cross sections, andfine transverse lirae as surface ornamentation; camerae longto very long with approximately 1–2 in form ratio (maximumwidth per length); siphuncular position nearly central; ratio of
siphuncular diameter per corresponding conch diameter at
septal foramen attains 0.2; septal necks suborthochoanitic
and very short; cameral deposits well developed.
Description.–The type series consists of two incomplete
and more or less deformed phragmocones; an apical shell,
33 mm in length, is designated as the holotype (IGPS coll.
cat. no. 112844; Figures 1.E–J), that exhibits a slightly
cyrtoconic appearance and misalignment of camerae
probably by post mortem deformation; conch cross sectionsof an apical part of the holotype are circular; the paratype
is another deformed longiconic orthocone; it is a moreadoral part than the holotype with 46 mm in length andapproximately 4 mm in reconstructed conch diameter near
the adoral end; the angle of conch expansion is very small,approximately 4° in the holotype; reconstructed angle ofditto in the paratype is approximately 5°; conch surface
ornamented by fine transverse lirae; apex not preserved.
Sutures directly transverse; septal curvature shallow;
camerae long to very long for the superfamily; approximate
numbers of camerae per corresponding conch diameterare 1–1.5 in the holotype and 1.5–2 in the paratype (i.e.
form ratios (maximum width per length) approximately
1–2). Siphuncle nearly central in position; septal necks
suborthochoanitic and very short having 0.25 mm in theholotype; widths of septal foramina are large for the genus,
0.31–0.46 mm in the holotype, and ratio of their diameterper corresponding conch diameter attains 0.2; connecting
rings are not preserved. Cameral deposits well developed,
differentiated into episeptal-mural and hyposeptal types,
and partly indicate mamillary growth; no endosiphunculardeposits detected.
Material examined.–Holotype, IGPS coll. cat. no. 112844.
Paratype, IGPS coll.cat. no. 112842.
Etymology.–The specific name is derived from the Osawa
Formation.

Discussion.–Trematoceras osawaense sp. nov. mostclosely resembles T. clarum Schastlivtceva (1986, p.
79, 80, pl. 1, figs. 2, 3; 1988, p. 65, 66, pl. 2, figs. 8a,b, v) from the Olenekian of Verkhoyansk, northeastern
Siberia, especially their apical angles, cameral lengths andsiphuncular positions are nearly identical. The principal
differences between them are as follows: cameral deposits
are well developed in T. osawaense, whereas they are notobservable in T. clarum, and slightly larger diameter of thesiphuncle in T. osawaense than that of T. clarum, whose ratioof siphuncular diameter per corresponding conch diameteris 0.15. Trematoceras mangyshlakense Schastlivtceva(1981, p. 79, 80, pl. 1, figs. 3, 4a, b, v; T. mangishlakense[sic], 1988, p. 66, pl. 2, figs. 6a, b, 7a, b), described from theOlenekian of Mangystau, Kazakhstan, is also similar to T.
osawaense, but this species indicates slightly larger angles
of conch expansion, (6°–7°), than those of the new species
and a subcentral position of the siphuncle.
The new species is well different from the previously
known two Trematoceras species from the South Kitakami
Belt (T. hikichii Niko, Ehiro and Takaizumi, 2016, p. 1-3, figs.
1.1–1.7 and T. watanabei Niko and Ehiro, 2020, p. 2, 3, 5,figs. 2.A–2.M; see the following chapter for their geologicsettings) by its much finer transverse lirae than those of T.
hikichii and slightly larger apical angles (4°–5° versus 3° inT. watanabei) at the corresponding ontogenetic stages. Inaddition, the fused endosiphuncular deposits developed in T.
watanabei are not recognized in T. osawaense.
Biostratigraphic significanceExcept for the present Trematoceras osawaense sp.
nov., two species of the genus have been described fromthe Inai Group in the South Kitakami Belt as mentioned
above. Trematoceras hikichii and T. osawaense occur in the
middle and uppermost parts of the Osawa Formation. The
diverse ammonoid fauna comprises the taxa Hemilecanites
discus, Tardicolumbites aff. tardicolumbus, Yvesgalleticerassp., Hellenites elegans, Epiceltites sp. (described
originally as Columbites parisianus), Nordophiceratoidesbartolinae, etc. These taxa indicate that the former
horizon belongs the upper Olenekian (Ehiro et al., 2016).
By contrast, ammonoids associated with the present→Figure 1. A, B. Pseudotemperoceras? sp. A, side view of phragmocone, silicone rubber cast, IGPS coll. cat. no. 112843: B,side view of phragmocone, IGPS coll. cat. no. 112845, internal mold. C–J. Trematoceras osawaense sp. nov. C, D, paratype,IGPS coll. cat. no. 112842, adoral phragmocone: C, lateral view; D, partial enlargement of C to show details of surface
ornamentation and sutures: E–J, holotype, IGPS coll. cat. no. 112844, apical phragmocone: E, magnified detail of H to show
septal character; F, lateral view, silicone rubber cast; G, magnified detail of I to show siphuncular position and details ofcameral deposits; H, lateral view, internal mold; I, longitudinal thin section; J, magnified detail of I to show septal neck shape.
Scale bar is 12 mm in A, C, F, H; 15 mm in B; 6 mm in D, E, I; 3 mm in G; 0.6 mm in J.

Early Triassic cephalopods from Miyagi

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Shuji Niko and Masayuki Ehiro

new species in the latter horizon are Pseudosageceras
multilobatum, Procarnites kokeni, Japonites cf. meridianus
and Eodanubites aff. xinyuanensis, indicating the latest
Olenekian Eodanubites Zone (Ehiro, 2022b). Trematoceras
watanabei occurs in the early Anisian (early middle Triassic)
mudstone at the middle part of the Fukkoshi Formation
together with abundant and diverse ammonoids, such asJaponites cf. meridianus, Danubites floriani, Paradanubites
kansa, Procladiscites brancoi, Leiophyllites pitamaha, L.
suessi, and Ussuriphyllites amurensis (Ehiro, 2022a). Their
stratigraphic and chronologic distributions are given in
Figure 2. These three endemic orthoceratoid species can beconsidered as regional index fossils around the Olenekian/Anisian boundary in the South Kitakami Belt.

Figure 2. Simplified columnar section of the Lower-Middle
Triassic Inai Group in the South Kitakami Belt, Northeast
Japan, showing the stratigraphic horizons of three
Trematoceras species.

Acknowledgements
Our sincere appreciation is extended to Yuta Watanabe,who discovered and donated the holotype of Trematoceras
osawaense sp. nov. We also appreciate the helpful reviewsby Christian Klug.
References
Bando, Y. and Ehiro, M., 1982, On some Lower Triassic ammonites
from the Osawa Formation at Asadanuki, Towa-cho, Tomegun, Miyagi Prefecture, Northeast Japan. Transactions andProceedings of the Palaeontological Society of Japan, New
Series, no. 127, p. 375–385, pl. 60.
Barskov, I. S., 1963, System and phylogeny of pseudorthoceratids.
Biulleten Moskovskogo Obshchestva Ispytatelei Prirody, Otdel
Geologicheskii, vol. 38, p. 149–150. (in Russian)
Ehiro, M., 2022a, Early Anisian (Aegean) ammonoids from theFukkoshi Formation (Inai Group) with special reference tothe Olenekian/Anisian boundary in the South Kitakami Belt,Northeast Japan. Bulletin of the Tohoku University Museum,no. 21, p. 39–84.
Ehiro, M., 2022b, Latest Olenekian (Early Triassic) ammonoids
from the uppermost part of the Osawa Formation (Inai Group)in the South Kitakami Belt, Northeast Japan. PaleontologicalResearch, vol. 26, p. 137–157.
Ehiro, M., Sasaki, O. and Kano, H., 2016, Ammonoid fauna of thelate Olenekian Osawa Formation in the Utatsu area, SouthKitakami Belt, Northeast Japan. Paleontological Research, vol.
20, p. 90–104.
Eichwald, E. von, 1851, Naturhistorische Bemerkungen, als Beitragzur vergleichenden Geognosie, auf einer Reise durch dieEifel, Tyrol, IIalien, Sizilien und Algier. Nouveauk Mémirs de laSociété de Naturalistes d’Histoire de Moscou, vol. 9, p. 1–464.
Flower, R. H. and Caster, K. E., 1935, The stratigraphy andpaleontology of northwestern Pennsylvania. Part II:Paleontology. Section A: The cephalopod fauna of theConewango Series of the Upper Devonian in New York andPennsylvania. Bulletins of American Paleontology, vol. 22, p.
199–271.
Kuhn, O., 1940, Paläozoologie in Tabellen, 50 p. Fischer, Jena.
Münster, G. zu, 1841, II. Beschreibung und Abbildung derin den Kalkmergelschichten von St. Cassian gefunden
Versteinerungen. In, Wissmann, H. L., and Münster, G.
zu, Beiträge zur Geognosie und Petrefacten-Kunde desSüdöstlichen Tirol’s Vorzüglich der Schichten von St. Cassian,p. 25‒152, pls. 1‒16, Bayreuth.
Niko, S. and Ehiro, M., 2020, Trematoceras watanabei, a new
orthoconic nautiloid species from the Middle Triassic Fukkoshi
Formation, Miyagi Prefecture, Northeast Japan. Bulletin of theTohoku University Museum, no. 19, p. 1–6.
Niko, S., Ehiro, M. and Takaizumi, K., 2016, Trematoceras hikichii
sp. nov., an Early Triassic orthocerid cephalopod from theOsawa Formation, Miyagi Prefecture, Northeast Japan. Bulletinof the Tohoku University Museum, no. 15, p. 1–4.
Schastlivtceva, N. P., 1981, On systematic position of Triassic
orthoceratoids of the southern USSR. Biulleten Moskovskogo
Obschestva Ispytatelei Prirody, Otdel Geologicheskii, vol. 56, p.
76‒82. (in Russian)Schastlivtceva, N. P., 1986, Some Triassic orthoceratids andEarly Triassic cephalopods from Miyagi
nautilids from North-East USSR. Biulleten Moskovskogo
Obschestva Ispytatelei Prirody, Otdel Geologicheskii, vol. 61, p.
122–129. (in Russian)Schastlivtceva, N. P., 1988, Triassic orthoceratids and nautilids fromUSSR. Akademii Nauk SSSR, Trudy Paleontologicheskogo
Instituta, vol. 229, p. 1–104, pls. 1–8. (in Russian)Teichert, C., 1967, Major features of cephalopod evolution. In,Teichert, C. and Yochelson, E. L. eds., Essays in Paleontology& Stratigraphy. R. C. Moore Commemorative Volume,

21

Department of Geology, University of Kansas SpecialPublication 2, p. 162–210. The University Press of Kansas,
Lawrence and London.
Zakharov, Y. D., 1996, Orthocerid and ammonoid shell structure:Its bearing cephalopod classification. Bulletin of the National
Science Museum, Series C, vol. 22, p. 11–35.
Zhuravleva, F. A., 1959, On the family Michelinoceratidae Flower,1945. Materialy k Osnovam Paleontologii, vol. 3, p. 47–48 (inRussian)