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Bull. Tokushima Pref. Mus.
No. 21 : 1-10, 2011

【Article】

Discovery of Late Triassic ammonoid Arcestes in the Kochigatani Group,
Sakawa Town, Kochi Prefecture, Japan
1

2

Yasuyuki Tsujino , Yasunari Shigeta and Kenji Mimoto3

Abstract : A well preserved specimen of Triassic ammonoid Arcestes sp. is describedfrom the middle part of the Kochigatani Group in the Otogo area of Sakawa Town in
Kochi Prefecture, Japan. The shell, measuring 68.8 mm in diameter, shows some mature
modifications such as coiling change and contraction of the last quarter whorl,
development of an apertural constriction, and formation of a ventrolateral salient at theapertural margine. Arcestes is the cosmopolitan genus of the Late Triassic period. The
occurrence of the specimen and bivalves such as Halobia and Tosapecten leads to theconclusion that the middle part of the Kochigatani Group in the Otogo area is that ofCarnian-Norian interval.
Key words : Carnian-Norian, Halobia, Otogo area, Proarcestes aff. bicarinatus,
Tosapecten

Introduction
Mesozoic deposits are widely distributed in Sakawa Town, Kochi Prefecture, Japan and haveattracted the attention of many scientists since the pioneering work of Edmund Naumann (Naumann,
1885; Naumann and Neumayr, 1890; Yehara, 1927 etc.). The Kochigatani Group in the region yields
numerous marine Upper Triassic fossils, and many bivalves and ammonoids have been described(Shimizu, 1927, 1930, 1931; Kobayashi and Ichikawa, 1950a, 1950b, 1950c; Bando, 1964 etc.).
Although most fossils from the Kochigatani Group are fragments, steinkerns, or flattened ones bycompaction, coauthor Kenji Minato recently discovered an exceptionally well preserved Arcestes
specimen together with bivalves, gastropods and crinoids from an outcrop in the Otogo area of
Sakawa Town (Figs. 1, 2; Mimoto, 2001). We describe the specimen hereafter and discuss thesignificance of the finding.

Note on stratigraphy
Kobayashi et al. (1940) divided the Kochigatani Group into three lithologic units: lower, middle,
and upper parts, and recognized four biostratigraphic units based on bivalve fauna, in ascending order:
Oxytoma-Mytilus, Halobia-Tosapecten, Myoconcha and Entomonotis beds.
Katto et al. (1956) subdivided the group into two subgroups: the Lower and Upper Kochigatani
subgroups. The lower part of the Lower Kochigatani Subgroup is composed of lightly greenish,
Receive 28 February, 2011; Revised manuscript accepted 4 March, 2011.
1 Tokushima Prefectural Museum, Bunka-no-Mori Park, Hachiman-cho, Tokushima 770-8070, Japan.
2 Department of Geology and Paleontology, National Museum of Nature and Science, 3-23-1, Hyakunin-cho, Shinjukuku, Tokyo 169-0073, Japan.
3 Mizuki 1-310-8, Kochi, 780-0976, Japan.

―1―Yasuyuki Tsujino, Yasunari Shigeta and Kenji Mimoto

Fig. 1. Index map showing the fossil locality from which Arcestes sp. was collected. A specimen described
as Proarcestes aff. bicarinatus by Shimizu (1931) was collected from the Yamaguchi area. Note that
Kobayashi et al. (1940) plotted the position of the Yamaguchi area somewhat north from the actual
location.

massive, fine sandstone with gravel (Hata, 1991). The upper part consists of sandy mudstone, finesandstone and alternating beds of sandstone and mudstone (Hata, 1991). The Upper Kochigatani
Subgroup is composed mainly of sandy mudstone and massive muddy sandstone, which are frequently
intercalated with gravelly layers (Hata, 1991).
Toshimitsu et al. (2007) recently subdivided the group into three parts: lower part (= lower part of
the Lower Kochigatani Subgroup by Katto et al., 1956); middle part (= upper part of the Lower
Kochigatani Subgroup by Katto et al., 1956); upper part (= Upper Kochigatani Subgroup by Katto etal., 1956).
The middle part of the group is well exposed at a large outcrop along the National Highway 33 in
the Otogo area, Sakawa Town, Kochi Prefecture (Figs. 1, 2), in which a 56 m thick continuous
succession is visible. The strata strike E-W and dip northward at an angle of 70-80°, and the verticaland overturned beddings are partly exposed (Fig. 3). They are divided into three litho-facies, inascending order as follows: 1) black mudstone, 2) alternating beds of sandstone and mudstone, and 3)
sandy mudstone (Figs. 3, 4; Mimoto, 2001).
The black mudstone facies (greater than 30 m) consists mainly of slightly siliceous black mudstoneintercalated with abundant greenish-white acidic tuff layers, which are 1-100 cm thick. This facies
yields abundant fossils such as bivalve Otapiria dubia (Ichikawa) and Halobia sp., gastropod, andstalks of crinoid Balanocrinus sp. (Fig. 4; Mimoto, 2001).
The alternating beds of sandstone and mudstone facies (19 m) overlies the black mudstone facies
with erosional surfaces by channel (Mimoto, 2001). The lower part of this facies is composed mainlyof very fine to fine-grained sandstone intercalated with thin mudstone layers. The sandstone beds areseveral tens to 100 cm thick, and gradually become thinner toward the upper part (Fig. 4). Most

―2―

Arcestes from the Kochigatani Group

Fig. 2. A: Satellite photograph of the Otogo area, Sakawa Town, Kochi Prefecture, Japan. B: Outcrop of
the Kochigatani Group from which Arcestes sp. was collected. The photograph was taken in May 2000.

sandstone beds in the upper part of the facies are 20-30 cm thick. Bivalves and gastropods were foundfrom a float block of thick sandstone, which probably came from this facies (Mimoto, 2001).
The sandy mudstone facies (greater than 7 m) consists mainly of sandy mudstone intercalated withacidic tuff layers, which includes a thick layer in the middle part. The acidic tuff layers are
significantly less than the black mudstone facies. This facies contains numerous plant remains andbivalves such as Tosapecten suzukii suzukii (Kobayashi) (Fig. 4).

―3―

Yasuyuki Tsujino, Yasunari Shigeta and Kenji Mimoto

Fig. 3. Plan view of the outcrop in
Fig. 2. Showing rock types andstructual attitude of the beds.
ss.: sandstone, ms.: mudstone.

Fig. 4. Columnar section of the outcrop shown on Figs. 2 and 3, showing stratigraphic occurrences ofArcestes sp. and other fossils. ss.: sandstone, ms.: mudstone.

―4―Arcestes from the Kochigatani GroupA specimen assignable to Arcestes sp. was obtained from the black mudstone facies from this large
outcrop (Fig. 4).

Paleontological description
Morphological terms are those used in the Treaties on Invertebrate Paleontology (Arkell et al.,1957). Quantifiers used to describe the shape of ammonoid shell are the same as those reported by
Matsumoto (1954, p. 246) and modified by Haggart (1989, table 8.1).
Institution abbreviation: TKPM = Tokushima Prefectural Museum.

Order Ceratitida Hyatt, 1884
Superfamily Arcestaceae Mojsisovics, 1875
Family Arcestidae Mojsisovics, 1875
Genus Arcestes Suess, 1865

Type species: Ammonites galeiformis Hauer, 1846.
Discussion: Arcestes is morphologically very similar to Proarcestes Mojisisovics, 1893, but
periodic constrictions are confined to the phragmocone, and the body chamber shows some mature
modifications (Spath, 1951; Arkell et al., 1957). In contrast, Proarcestes possesses constrictions andflared ribs on phragmocone and body chamber. Also, the body chamber shows no modification.
Occurrence: Carnian to Rhaetian in the Arctic, Pacific and Tethys regions (Arkell et al., 1957;
Tozer, 1981; Konstantinov, 2008; Charlton et al., 2009).

Arcestes sp.
(Figs. 5, 6)

Material examined: TKPM.GFI5387, measuring 68.8 mm in diameter, was extracted from acalcareous concretion embedded in black mudstone in the middle part of the Upper Triassic
Kochigatani Group at an outcrop exposed along the National Highway 33, the Otogo area, Sakawa
Town, Kochi Prefecture (133°15’ 42.1”E, 33°31’ 18.3”N).
Description: Very involute and depressed shell with semicircular whorl section. Maximum whorl
width occurs on umbilical shoulders. Coiling rate clearly decreases at three-quarters of outer whorl andlast quarter whorl is slightly contracted. Umbilicus probably very narrow. Shell surface smooth, but
exhibits very weak ribs, which increase in strength toward the apertural margine, on apertural part; astrong apertural constriction and a ventrolateral salient at apertural margine. Suture line is finely anddeeply incised, partly visible on inner whorl. Outer whorl is occupied by body chamber.
Remarks: The body chamber of TKPM.GFI5387 is characterized by having some changes incoiling on the last quarter whorl, slightly contracted last quarter whorl, development of an apertural
constriction and formation of a ventrolateral salient at apertural margine. Similar features have been―5―Fig. 5. Arcestes sp., TKPM.GFI5387, from the Otogo area, Sakawa Town, Kochi Prefectrure, Japan. 1: Apertural view. 2: Right lateral view. 3: Ventral view. vs: Ventrolateral salient. Arrow of 2
indicates position of decrease of coiling rate.

Yasuyuki Tsujino, Yasunari Shigeta and Kenji Mimoto―6―Arcestes from the Kochigatani Group

Fig. 6. Suture line of Arcestes sp.,
TKPM.GFI5387, from the
Otogo area, Sakawa Town,
Kochi Prefectrure, Japan.
Arrow indicates the positionof siphuncle.

known in adult shells of many Paleozoic and Mesozoic ammonoids as mature modifications (Davis,
1972; Davis et al., 1996). These facts suggest that TKPM.GFI5387 is a mature shell.
TKPM.GFI5387 possesses no constrictions in the body chamber except for an apertural constrictionin addition to some body chamber modifications. These features enable us to identify it as Arcestes,
although no definitive species assignment can be made.

Discussion
Age of the Kochigatani Group in the Otogo area
Arcestes is one of the cosmopolitan genera of the Late Triassic (Arkell et al., 1957; Tozer, 1981;
Konstantinov, 2008; Charlton et al., 2009). Occurrence of Arcestes sp. from the black mudstonefacies of the middle part of the Kochigatani Group in the Otogo area suggests that the portion is of
Late Triassic Epoch (Carnian-Rhaetian). Mimoto (2001) collected bivalves assigned to Halobia sp.
from the black mudstone facies and Tosapecten suzukii suzukii from the Sandy mudstone facies in thesame outcrop (Fig. 4). Halobia typically ranges from Carnian to Norian (McRoberts, 2010). The taxon
range of Tosapecten is Late Triassic Epoch (Cox et al., 1969; McRoberts, 2010). Judging fromstratigraphic ranges of each taxa, depositional age of the Kochigatani Group in the Otogo area isrestricted from Carnian to Nornian.

Locality of “Proarcestes aff. bicarinatus”
Shimizu (1931) described a tiny but well preserved specimen as Proarcestes aff. bicarinatus
Münster, 1841 from the siliceous shale in the Yamaguchi area, Sakawa Town (Fig. 1). However, exact
locality and horizon of the specimen were not described. Some authors attempted to determine thelocality, but they could not find any exposures of siliceous shale in the Yamaguchi area (Kobayashi etal., 1940; Kobayashi and Ichikawa, 1950a; Kobayashi, 1962). We also had the same results.
Exposures of the siliceous shale are distributed in the Otogo area, which is located on the same
creek approximately 250 m upriver from the Yamaguchi area (Fig. 1). We discovered a specimen ofArcestes sp. described herein from that area. Three dimensional preservation of the specimen is very―7―

Yasuyuki Tsujino, Yasunari Shigeta and Kenji Mimoto

similar to that of Shimizu’s specimen. These facts certainly suggest that the Shimizu’s specimen may
have been obtained from a float block derived from the black mudstone in the Otogo area.
It should be noted that assignment of Shimizu’s specimen to Proarcestes aff. bicarinatus is
uncertain because of its tiny shell. It may simply be a juvenile arcestid shell.

Acknowledments
We would like to thank Yasuo Kondo (Kochi University) for providing valuable geologic
information of the study area. We greatly appreciate Angus A. McDonald (University of Tokushima)
for improvement of the English text.

References
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Untersuchungenn uber das Chichibu-Terrain in Shikoku-VI. Chikyukagaku, (26, 27): 1-9. (In Japanese, German
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Publishing Co., Tokyo. (In Japanese)*
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361-366. (In Japanese, English abstracts)
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Tosa (Kochi Prefecture), Shikoku Island, Japan, and its pelecypod-faunas. Journal of the Faculty of Science,
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Wissmann, H. L. and G. v. Münster. 1841. Beiträge zur Geognosie und Petrefaktenkunde des Südöstlichen Tirol’s,vorzüglich der Schichten von St. Cassian, IV. 152p. Bayreuth.
*English translation from the original written in Japanese―9―Yasuyuki Tsujino, Yasunari Shigeta and Kenji Mimoto摘要

高知県佐川町の川内ヶ谷層群より発見された三畳紀後期アンモノイド Arcestes
12

辻野泰之 ・重田康成 ・三本健二

3高知県佐川町乙川地域に露出する川内ヶ谷層群中部から保存良好な三畳紀アンモノイドの Arcestes sp. が産出した．殻直径は 68.8 mm，住房後半部での螺環高の増大率の減少，殻口部での殻の引き締まり，殻口縁に発達する強いくびれや突起など成熟殻の特徴を示す．Arcestes 属は三畳紀後期に汎世界的に分布したグループであり，Arcestes および同地域から産出する二枚貝化石（Halobia，Tosapecten）から，乙川地域に露出する川内ヶ谷層群中部の堆積年代は三畳紀後期のカーニアン期からノーリアン期を示す．
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