Bull. Tohoku Univ. Museum, No. 15, pp. 1–4, 2016
© by The Tohoku University Museum

Trematoceras hikichii sp. nov., an Early Triassic
orthocerid cephalopod from the Osawa Formation, Miyagi
Prefecture, Northeast Japan
Shuji Niko*, Masayuki Ehiro** and Yukihiro Takaizumi****Department of Environmental Studies, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashihiroshima 7398521, Japan, **The Tohoku University Museum, Sendai 980-8578, Japan, ***1-9-1 Shoryo, Izumi-ku, Sendai 981-3108, Japan
Abstract: A new Olenekian (late Early Triassic) species of orthocerid cephalopod, Trematoceras
hikichii, is described from the Osawa Formation in the Utatsu area, Miyagi Prefecture, NortheastJapan. The new species more or less resembles T. mangishlakense Schastlivtceva, 1981, fromKazakhstan and T. insperatum Schastlivtceva, 1988, from Caucasus. Diagnostic characters ofthe species from these similar species are following; 4° in angle of conch expansion and veryfine transverse lirae on the shell surface. This discovery represents the first record of the genus
Trematoceras in Japan.

Introduction
So far only limited orthocerid cephalopods from the Mesozoic
have been reported, namely Triassic Paratrematoceras
Schastlivtceva, 1981, Pseudotemperoceras Schastlivtceva,
1986 and Trematoceras Eichwald, 1851, and Cretaceous
Zhuravlevia Doguzhaeva, 1994. Among them Trematocerasis the most common and best-known genus whose records
are recognized in the Olenekian (late Early Triassic) toRhaetian (late Late Triassic) marine faunas of the Tethys,
Boreal and Panthalassa regions. The present paper
describes a new Olenekian species of Trematoceras fromthe Osawa Formation in Northeast Japan.
Abbreviations used for repositories are IGPS (Instituteof Geology and Paleontology, Faculty of Science, Tohoku
University, Sendai; kept in the Tohoku University Museum)and UIM (Utatsu Ichthyosaur Museum, Minamisanriku Town).

Systematic Paleontology
Order Orthocerida Kuhn, 1940
Superfamily Orthoceratoidea M’Coy, 1844
Family Orthoceratidae M’Coy, 1844
Subfamily Michelinoceratinae Flower, 1945
Genus Trematoceras Eichwald, 1851
Type species.—Orthocera [sic] elegans Münster, 1841;
Carnian (lower Upper Triassic) in Carnic Alps.
Trematoceras hikichii sp. nov.
Figure 1

Diagnosis.—Species of Trematoceras with approximately4° in angle of conch expansion and relatively short camerae
with 1.6‒2.3 in from ratios (maximum width per length);transverse lirae on shell surface are very fine.
Description.—Longiconic orthocones with circular crosssections; conch expansion is gradual for the family andmoderate for the genus, indicating approximately 4° inangle; holotype is incomplete phragmocone with 68 mm inlength; the largest specimen (paratype, IGPS coll. cat. no.
111571) attains 99 mm in length and 7 mm in approximate
diameter (reconstructed), adoral 51 mm of which specimenrepresents body chamber; no apical part of conch andperistome preserved; shell surface ornamented by very finetransverse lirae; there are 12‒16 ridges in 1 mm of conch
length. Sutures are essentially straight and transverse;
septa indicate moderate curvature; camerae are relatively
short for the genus indicating from ratios (maximum widthper length) of 1.6‒2.3; siphuncle is nearly central (faintlyventral from center); position ratios of siphuncle (distance ofcentral axis of septal foremen from ventral shell surface percorresponding conch diameter) are 0.45‒0.47; siphuncularwall consists of orthochoanitic (to suborthochoanitic)
septal necks and cylindrical connecting rings; length ofnecks is short, 0.19‒0.23 mm. Endosiphuncular deposits
absent; cameral deposits are well-developed, episeptalmural, and thicker in venter than dorsum; in later stages,
cameral deposits exhibit mammillary growth and extend tohyposeptal area.
Material examined.—Holotype, IGPS coll. cat. no. 111572.
Paratypes, IGPS coll. cat. nos. 111569‒111571 and UIM

2Shuji Niko, Masayuki Ehiro and Yukihiro Takaizumi

Figure 1. Trematoceras hikichii sp. nov. from the Early Triassic Osawa Formation in the Utatsu area, Minamisanriku
Town, Miyagi Prefecture. 1, 2. Paratype, IGPS coll. cat. no. 111569: 1, side view of mold; 2, partial enlargement of Figure 1.1
to show details of surface ornamentation, external mold. 3. Paratype, IGPS coll. cat. no. 111571, side view of mold, arrow
indicates the last septum. 4‒7. Holotype, IGPS coll. cat. no. 111572: 4, partial enlargement of Figure 1.5, to show details ofcameral deposits; 5, dorsoventral thin section, venter on left; 6, 7, partial enlargements of Figure 1.5 to show details of septal
necks. Scale bar is 10 mm in Figure 1.1; 2 mm in Figure 1.2; 20 mm in Figure 1.3; 1.4 mm in Figure 1.4; 4 mm in Figure 1.5; 0.4
mm in Figures 1.6, 1.7.
30622. In addition, two poorly preserved specimens, IGPS
coll. cat. no. 111573 and UIM 30623, were also examined.
Etymology.—The specific name honors Mr. YasuhiroHikichi, who discovered the holotype of the new species.

Occurrence.—All specimens of Trematoceras hikichii sp.
nov. were collected from laminated mudstones of the Osawa
Formation at the Tatezaki locality (38°42’50’’N, 141°32’07”E)in the Utatsu area, Minamisanriku Town, Miyagi Prefecture,Triassic cephalopod from Miyagi
Northeast Japan (see figures 1, 2 in Ehiro et al., 2015 for its
geographic point). The specific components of the Osawa
ammonoid fauna were revealed by Bando and Shimoyama(1974), Bando and Ehiro (1982), Ehiro (1993), Ehiro (2016),and Ehiro et al. (2016 in press). These results suggestthat the formation is correlative with the upper Olenekian(= Spathian; upper Lower Triassic).
Discussion.—In examined specimens, only the apicalshell of the holotype is undistorted. Thus, some importantcharacters, such as profile of the conch, siphuncular
structure and biotic deposits, in the above description arebased on this specimen.
In its ratios of the cameral form and siphuncular position,
Trematoceras hikichii sp. nov. resembles T. mangishlakense
Schastlivtceva (1981, p. 79, 80, pl. 1, figs. 3a, b, 4a, b) fromthe Olenekian of Kazakhstan. The most important featuresto separate these two species are external morphologies.
Trematoceras mangishlakense possesses slightly larger
angle of conch expansion (6°‒7°) and lacks evident surface
ornamentation. Transverse surface lirae are developed
also in T. insperatum Schastlivtceva (1988, p. 67, 68, pl. 2,figs. 12a, b, v) from the Anisian (lower Middle Triassic) ofCaucasus, but its interspaces of the liare are much wider,2‒3 ridges in 1 mm, than those of the new species. The typespecies of the genus, T. elegans (Münster, 1841, p. 125, pl.
14, figs 2a‒c; Bizzarini and Gnoli, 1991, p. 112, pl. 1, figs.
1‒4, pl. 2, figs. 1, 2) clearly differs from the new species inhaving strongly oblique surface liare and longer camerae.
This discovery from the upper Olenekian Osawa
Formation is noteworthy because it represents the firstrecord of Trematoceras in Japan. In addition, previous Early
Triassic occurrences of the genus were restricted in Albania,
Kazakhstan, northeastern Siberia, Primorye, and Vietnam(Kiparisova, 1961; Schastlivtceva, 1988; Shigeta andNguyen, 2014).

AcknowledgementsWe are greatly indebted to Mr. Yasuhiro Hikichi. The
present research is not possible without his discovery anddonation of an exceptionally well-preserved specimen(holotype). We also thank Dr. Harumasa Kano for collectingsome specimens. Thanks extend to the Educational
Committee of Minamisanriku Town and Abei-Gumi Co. Ltd.
for permissions to excavate fossiliferous beds at the Tatezaki
locality. Reviewer Dr. Yasufumi Iryu kindly provided helpfulcomments on a previous version of this paper.

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