Bull. Tohoku Univ. Museum. No. 19, pp. 1‒6, 2020© by The Tohoku University Museum

Trematoceras watanabei, a new orthoconic nautiloidspecies from the Middle Triassic Fukkoshi Formation,
Miyagi 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: A new species of an orthoconic nautiloid cephalopod, Trematoceras watanabei, isdescribed from laminated sandy mudstones in the Ozashi area, Jusanhama, Ishinomaki City,Miyagi Prefecture, Northeast Japan. Its type stratum is the Anisian (lower Middle Triassic) Fukkoshi
Formation of the South Kitakami Belt. Endosiphuncular deposits remaining in two well-preserved
specimens of the species support the decision that Trematoceras should be removed from theorder Orthocerida, and instead be placed in the Pseudorthocerida. The principal character thatdifferentiates T. watanabei from a comparable species (i.e., T. hikichii ) is the development ofdistinctly longer camerae.

Introduction
Little has been published on the taxonomy of Triassic
orthoconic nautiloid cephalopods in Japan. Only Nikoet al. (2016) described and named a single species ofTrematoceras (T. hikichii) from Olenekian (upper LowerTriassic) mudstones of the Osawa Formation, Miyagi
Prefecture. However, a considerable number of furtherspecimens comparable to their discoveries is found inpublications of faunal lists of various Japanese sites.
The first report was probably made by Yehara (1927),
who documented the occurrence of Orthoceras sp. fromthe Upper Triassic Kochigatani Group, Kochi Prefecture(Kobay as hi and Ic h i k a w a , 1 9 5 1 ). T h i s i s the onl yrepresentative of a Triassic orthocerid in the Kurosegawa
Belt up to now. Besides the abovementioned species, T.
hikichii Niko, Ehiro and Takaizumi, 2016, whose type stratum
is a constituent of the South Kitakami Belt, all further recordsof Triassic orthoconic nautiloid cephalopods derive from
the Maizuru Belt. Reports of these occurrences are listed
as follows: Nakazawa et al. (1954; Orthoceras sp. fromthe Anisian (lower Middle Triassic) Miyanooku Formation,
Okayama Prefecture; correspond Michelinoceras sp. inNakazawa, 1958), Nakazawa et al. (1957; Orthoceras sp.
from the Anisian Waruishi Formation, Kyoto Prefecture;correspond Michelinoceras sp. in Nakazawa, 1958),Nakazawa (1958; Michelinoceras sp. from the Olenekian
Hirobatake and Narawara Formations, Kyoto Prefecture),

Nakazawa and Nogami (1958; Michelinoceras sp. from theOlenekian part of the Oro Formation, Kyoto Prefecture),
and Ishibashi (2005; Michelinoceras? sp. from the Waruishi
Formation). The present paper describes Trematoceras
watanabei sp. nov. as the second species occurrence of anorthoconic nautiloid from the Triassic of the South Kitakami
Belt.
The cephalopod specimens described herein werecollected from a coastal outcrop (approximately latitude N38°36’ 18’’, longitude E 141°30’ 59’’) in the Ozashi area,Jusanhama, Ishinomaki City, Miyagi Prefecture, NortheastJapan (Figure 1). According to Kamada and Takizawa(1992) the Triassic rocks in this area are divided into theHiraiso, Osawa, Fukkoshi and Isatomae Formations inascending order. The newly described orthoconic nautiloid
species derives from the Fukkoshi Formation, that mainly
consists of sandstone and alternating beds of sandstone
and mudstone. The deposits intercalate with thick laminatedsandy mudstone beds, about 30 meters in total thickness, inits middle part. The two cephalopod-bearing beds, i.e., Fk1a and about 3 meters above Fk-1b, are in the topmost partof the mudstone beds (Figure 2). The Fukkoshi Formation
represents the submarine fan complex deposited in a deepwater environment (Kamada, 1984). Onuki and Bando(1959) assigned the formation to the Anisian, based on
the presence of age determining ammonoids, such as
Hollandites sp., Balatonites cf. kitakamicus (Diener), andGymnites cf. watanabei (Mojsisovics).

2Shuji Niko and Masayuki Ehiro

Figure 1. Index‌map (A) and geologic map (simplified from Kamada and Takizawa, 1992) (B) of the Ozashi area, showing
the fossil locality.
Repository. All examined specimens are reposited in theTohoku University Museum (Institution abbreviation: IGPS
= Institute of Geology and Paleontology, Faculty of Science,
Tohoku University, Sendai).

Systematic Paleontology
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;
Carnian (lower Upper Triassic) in Carnic Alps.
Discussion.─Two well-preserved specimens fromthe Fukkoshi Formation reveal that the new species ofTrematoceras has endosiphuncular deposits even in
the adoral shell. Although Jeletzky and Zapfe (1967) did
not describe these in their publication, similar deposits
are visible in their figure (pl. 4, fig. 3) of T. cf. triadicum
Mojsisovics, 1873, collected from the Rhaetian (upper UpperTriassic) Zlambach Marl, Austria. Judging, and based on thenew Japanese finding, the deposits in the Austrian specimen
seem organic. There are two opposing views about thehigher taxonomic position of the genus: (1) Trematoceras
belongs into the order Orthocerida (e.g., Sweet, 1964;
Shigeta and Nguyen, 2014; Niko et al., 2016), while (2)Schastlivtceva (1986, 1988) and Zakharov (1996) assigned
Trematoceras to the order Pseudorthocerida. These present
knowledges derived from the Japanese and Austrian species
support the latter decision advocating a pseudorthoceridaffinity of Trematoceras.
Trematoceras watanabei sp. nov.
Figure 3
Diagnosis.─Species of Trematoceras with adoralconch expansion of approximately 3° in angle andornamentation of fine transverse lirae; camerae moderate
to long, with 0.4−1.4 in maximum width/length ratio; septal
necks suborthochoanitic; connecting rings bear distinct
constrictions at siphuncular foramina; cameral deposits
episeptal-mural and hyposeptal; endosiphuncular deposits
unfused.

Triassic orthoconic nautiloid from Miyagi

3Figure 2. Simplified‌
columnar section of the Lower-Middle Triassic Inai Group in the South Kitakami Belt, Northeast Japan,
showing the fossil horizon (Fk-1a and Fk-1b). The lowermost part of the Hiraiso Formation is not exposed in theOzashi area.
Description.─Longiconic orthocones with gradual conchexpansion; except for apical part of a paratype (IGPS coll.
cat. no. 112399), where cross sections of conch are circular,
most specimens are more or less deformed and exhibit
oval to flattened lenticular cross sections; the holotype
is imperfect phragmocone, which is 53 mm in length andapproximately 6 mm in reconstructed diameter near its
adoral end; body chamber attains approximately 9 mm inreconstructed diameter in a paratype (IGPS coll. cat. no.
112401); reconstructed angles of conch expansion are
4°−6° in apical decreasing to approximately 3° in adoralpart of the shell; apex and peristome not preserved. Conchsurface ornamented by fine transverse lirae. Sutures directlytransverse; septa moderately concave; camerae moderate
to long for the genus, having 0.4−1.4 in form ratio (maximumwidth per length). Siphuncle nearly central in position;

siphuncular wall consists of suborthochoanitic septal necks
and slightly thickened connecting rings; length of septal neckis very short (0.33 mm in the holotype); connecting rings
cylindrical with distinct constrictions at siphuncular foramina;
thickness of connecting rings maximum 0.04 mm in theholotype. Cameral deposits well developed, differentiatedinto episeptal-mural and hyposeptal; endosiphuncularparietal deposits unfused, developed mainly on ventral
siphuncular wall in apical shell of a paratype (IGPS coll.
cat. no. 112402) and adoral shell of the holotype; incipientparts of endosiphuncular deposits occur at apical partof connecting ring, then also grow adorally; siphuncularforamina lack deposits.
Material examined.─Holotype, IGPS coll. cat. no.
112409. Paratypes, IGPS coll. cat. nos. 112398, 112399,112401−112403, 112406−112408. In addition, three poorly

4Shuji Niko and Masayuki Ehiro

5

Triassic orthoconic nautiloid from Miyagi
preserved specimens, IGPS coll. cat. nos. 112400, 112404,112405, were also examined.
Occurrence.─Fk-1a (IGPS coll. cat. nos. 112398−112405)
and Fk-1b (IGPS coll. cat. nos. 112406−112409) in themiddle part of the Fukkoshi Formation, the Ozashi area,
Jusanhama, Ishinomaki City, Miyagi Prefecture, NortheastJapan.
Etymology.―The species name is to honor Mr. Yuta
Watanabe, who discovered important specimens including
the holotype and two of the paratypes (IGPS coll. cat. nos.
112407, 112408) and making them available for this study.
Discussion.─General conch shape and structure of
the siphuncular wall of the Fukkoshi specimens are
exclusive morphological characters restricted to the genus
Trematoceras. Among the previously known about 40
species of the genus, T. watanabei sp. nov. is similar tothe Olenekian species T. hikichii Niko, Ehiro and Takaizumi(2016, p. 1−3, fig. 1), from the Osawa Formation (typestratum) that is conformably overlain by the Fukkoshi
Formation. The principal difference between these species
is their cameral length. Form ratios (maximum width/length) of T. hikichii are 1.6−2.3, whereas T. watanabei has
much longer camerae and ratios of 0.4−1.4. In addition,stronger curvature of septal necks and deeper constrictionsof connecting rings of T. watanabei compared to thoseof T. hikichii are distinctive. Because endosiphunculardeposits are not preserved in T. hikichii, direct comparisonof T. watanabei and T. hikichii is impossible regarding thiscriterion. However, the pronounced differences of camerae,septal necks, and connecting rings support to separate thetaxa on the specific level.

Acknowledgements
We are especially grateful to Mr. Yuta Watanabe forallowing us to study some specimens from his personalcollection and donating them to the Tohoku UniversityMuseum. We also thank Mr. Yukihiro Takaizumi for his fieldassistance. Special thanks are also extended to Dr. Barbara
Seuss, who kindly reviewed the manuscript.

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←Figure 3. Trematoceras watanabei sp. nov. A−C: paratype, IGPS coll. cat. no. 112399, imperfect phragmocone; A, sideview; B, partial enlargement of A to show details of apical shell; C, partial enlargement of A to show details of surface
ornamentation. D, J−M: holotype, IGPS coll. cat. no. 112409, imperfect phragmocone; D, lateral view, venter on right; J,partial enlargement of L to show details of endosiphuncular deposits (arrow), septal neck and connecting rings; K, partial
enlargement of L to show details of cameral deposits; L longitudinal thin section, slightly oblique for dorsoventral plane,venter on left; M, transverse polished section near adoral end, venter down. E, G, I: paratype, IGPS coll. cat. no. 112402,imperfect phragmocone; E, lateral view, venter on right; G. longitudinal thin section, venter on right; I, partial enlargementof G to show details of endosiphuncular deposits (arrows). F: paratype, IGPS coll. cat. no. 112403, side view of imperfectphragmocone. H: paratype, IGPS coll. cat. no. 112401, side view of imperfect body chamber. Scale bar is 20 mm in A, D, E, F,H; 5 mm in B, C; 3 mm in G, K; 1 mm in I, J; 6 mm in L; 10 mm in M.

6Shuji Niko and Masayuki Ehiro

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