Bull. Tohoku Univ. Museum. No. 18, pp. 5‒9, 2019© by The Tohoku University Museum

Late Triassic ammonoid Arcestes from the Saragai Groupin the South Kitakami Belt, Northeast Japan
Masayuki Ehiro
The Tohoku University Museum, Sendai 980-8578, Japan
Abstract: Late Triassic ammonoid Arcestes sp. is described from the Chonomori Formation (SaragaiGroup), crop out at Haizaka, Minamisanriku Town in the South Kitakami Belt, Northeast Japan. It
was collected from the Monotis scutiformis Zone, which occupies the lower part of the formation.
Although it is ill-preserved, based on the general shell morphologies, it can be compared withsome Arcestes species described from the lower Norian of Austria, being the Norian age of theChonomori Formation became more probable.

Introduction

On the geological age of the Saragai Group

Naumann (1881) reported the occurrence of Late Triassic
bivalve Monotis at Saragaizaka, Utatsu area, Minamisanriku
district in the South Kitakami Belt, Northeast Japan, which
was the first discovery of the Triassic fossils from Japan.
Subsequently, Mojsisovics (1888) described some Japaneseammonoids (Middle Triassic ammonoids) for the first timefrom Japan. Most of them were collected from Inai and its
environs, to the south of Utatsu. Yokoyama (1904) describedfirst Jurassic ammonoids of Japan from Hosoura, on thesouth of Utatsu. Through these works, the Minamisanriku
district came to attract attention as a representative localityof shallow marine Mesozoic strata, especially Triassic andJurassic ones.
The Monotis-bearing Saragai beds (Shimizu and Mabuti,1932, 1941) were regarded as the type strata of the Upper
Triassic Saragian Stage (Ichikawa, 1950). Onuki and Bando(1958) subdivided the beds (Saragai Group) into the lowerShindate and the upper Chonomori formations. Molluscan
fossils from the Saragai Group are all from the ChonomoriFormation. The Saragai Group has been considered to becorrelated with the Carnian to Norian stages, but opinion
is divided on the precise correlation of it. It is partly due toscarce number of age-diagnostic fossils, such as ammonoid.
Recently an ammonoid specimen collected from theChonomori Formation was brought to the present author.
This paper describes this ammonoid and discusses its
stratigraphic significance.

The Saragai Group is mainly distributed in the Motoyoshiand Utatsu areas in the southern part of the Southern
Kitakami Massif, Northeast Japan (Figure 1), overlying theLower–Middle Triassic (Olenekian–Ladinian) Inai Groupand while underlying the Lower Jurassic Shizugawa Groupboth with unconformity. Shimizu and Mabuti (1932, 1941)
correlated the Saragai beds (the Chonomori Formation)with the Norian Stage based on the occurrence of two
ammonoids, Arcestes aff. oligosarcus Mojsisovics andPlacites aff. oxyphyllus Mojsisovics. Ichikawa (1951) also
considered that his Saragian Stage is roughly correlatedwith the Norian. Onuki and Bando (1958) divided theChonomori Formation into four Monotis zones: C 1 to C 4in ascending order, and considered that two ammonoids
reported by Shimizu and Mabuti (1932, 1941) were comefrom the C2 zone (Entomonotis densistriata Zone = Monotis
ochotica densistriata Zone of Ando, 1983). They placed
the Carnian/Norian boundary between the C1 (Entomonotis
scutiformis Zone = Monotis scutiformis Zone of Ando, 1983)
and C2 zones, based on the Monotis fauna and the abovementioned ammonoids. These ammonoids, however, have
not yet been described and the whereabouts of them areunknown. Nakazawa (1964a) established six molluscan
zones in the Chonomori Formation: the Tosapecten Zone,
Dictyoconites Zone, Monotis typica Zone, M. ochotica
densistriata Zone, M. ochotica Zone and M. zabaikalica
Zone, in ascending order, and considered that the horizon of
the above-mentioned ammonoids to be the M. typica Zone
or Dictyoconites Zone. Ando (1983) renamed the M. typica

6Masayuki Ehiro

Figure 2. Locality map of Hosoura–Niranohama area
showing the fossil locality (Haizaka). The base map is theDigital Map of Japan issued by Geospatial Information
Authority of Japan.
Figure 1. Index map (A) and generalized geologic mapof the Utatsu area and its environs in the South Kitakami
Belt, Northeast Japan (B).
Zone to the M. scutiformis Zone, but did not mention theprecise position of the Carnian/Norian boundary.
Nakazawa (1964a) described newly discovered
ammonoids, Rhacophyllites sp. (one specimen) andArcestes sp. (two specimens), from the M. typica Zone at
Haizaka, small pass between Hosoura and Niranohama(Figure 2). According to Nakazawa (1964a), Rhacophyllites
from Haizaka somewhat resembles some Carnian species
known from Kyushu, Japan and New Zealand, and hisspecimens of Arcestes are similar to. A. oligosarcus
Mojsisovics, 1875 from the Norian of Austria and A. trauthi
Diener, 1921 from Carnian-Norian mixed fauna of Austria.
He also discussed the age of the Dictyoconites Zone andconsidered that it is probably Carnian in age, because
the zonal fossil Dictyoconites nipponicus Shimizu and
Mabuti, 1941 was considered to be similar to Carnian–Norian D. multisulcatus Bulow from Timor and D. aff. haueriMojsisovics from the Carnian of Himalayas (Shimizu andMabuti, 1941). Based on these fossils, Nakazawa (1964a)
tentatively correlated the lower two zones (the Tosapectenand Dictyoconites zones) and upper four zones (above theM. typica zone) with the upper Carnian and lower Norian,
respectively, but later, he (Nakazawa, 1964b) considered theage of the lower two zones to be lower Norian rather thanlate Carnian, based on the correlation of the Monotis fauna.
Recently, new genus Miyagiteuthis was proposed (Niko andEhiro, 2018) for D. nipponicus, and, therefore, the Carnian
age of the Dictyoconites Zone needs to be reconsidered.
The present specimen of Arcestes was collectedfrom the Monotis scutiformis Zone (M. typica zone ofNakazawa, 1964a) at Haizaka (Figure 2). It is ill-preserved
and deformed by tectonic deformation, making it difficult
to identify at the specific level. However, as discussed in
the systematic section, based on the estimated shell form
and its characteristic rostrum, the present species can becompared with some Arcestes species, such as Arcestes
oxycephalus Mojsisovics and A. dicerus Mojsisovics bothfrom the lower Norian of Austria. Therefore, the early Norianage of M. scutiformis Zone became more probable.

Systematic description
Order Ceratitida Hyatt, 1884
Superfamily Arcestoidea Mojsisovics 1875
Family Arcestidae Mojsisovics 1875
Genus Arcestes Suess 1865
Arcestes sp.
Figures 3a–eMaterial: One specimen; IGPS coll. cat. no. 112167,
collected by Yudai Torihata, 2018.
Description: A small, partly squashed specimen isat hand. It is involute with completely closed umbilicus.
The shell shape is looks like extremely discoidal, but thisdiscoidal form is due in part to the tectonic deformation. ItLate Triassic ammonoid Arcestes from the Saragai Group in the South Kitakami Belt, Northeast Japan
is squashed, and the lateral sides are dented irregularly(Figure 3a) and the ventral part is collapsed in a narrow roof
shape (Figure 3b). The living chamber is long and occupies
about 1.25 volution. Near the apertural end the ventral partis projected forward as forming a prominent rostrum. The
cross section of the rostrum is sub-rectangular, the ventral
side of which is flattened and become concave near thenose, having sharp corners at both sides (Figures 3c, 3d).
The shell surface is smooth without any ornamentation. The
shell diameter (D) attains 42.3 mm, and its correspondingheight (H) is ca. 27 mm. The maximum width of shell in the
deformed state is near the peripheral end (frontal part ofthe rostrum) and measures 12.5 mm (WR/D = ca. 0.3; WR:width of the rostrum). Except for the rostrum, the apparent

7maximum width (Wa) is near the middle part of the sides
and ca. 7.5 mm (Wa/D = 0.18).
The septa are partly destroyed and the suture lines are
only partly observable. The first lateral to the fourth lateralsaddles are preserved (Figure 3e). Although the second iscrashed, it is Arcestes-type, and lateral saddles are highwith sub-parallel, denticulate prongs.
Discussion: The specimen described is deformed bysevere tectonic deformation and lateral compaction, and its
shell width and ratio of Wa/D above are not original values.
It is estimated that the actual maximum width (W) is at theumbilicus and is equal or larger than that of the peripheralend (WR), and the original shell cross section is thought
to be thinly discoidal with slightly converging sides andFigure 3. Arcestes sp. collected from the Monotis scutiformis Zone of the Chonomori Formation at Haizaka (IGPS coll.
cat. no. 112167). a. lateral view; b. ventral view; c. ventral view including the peripheral end (rostrum); d. frontal view of therostrum, white dotted line shows the lower margin of the rostrum; e. external suture line of the last septum. Open allows
show view line directions of figure b, c and d.

8Masayuki Ehiro

narrowly rounded venter.
In having discoidal shell, the genus Arcestes somewhat
resembles the genus Proarcestes Mojsisovics, 1893. These
two genera, however, easily distinguished by there shellmorphology: The periodic constrictions of the genus Arcestes
are confined to the phragmocone and the living chambershows some mature modifications, while the living chamberof the genus Proarcestes possesses constrictions and shows
no modification (Diener, 1921; Spath, 1951). Because it
has a prominent rostrum at the peripheral end, the present
species is considered to belong to the genus Arcestes.
The shell widths (W) of already-known species ofArcestes, having flattened to concave venter at theperipheral end of the living chamber, described byMojsisovics (1875) and Diener (1921), are nearly the sameas those of flattened venter (WR) to ca. 1.3 times larger
than WR. Because the WR of the present specimen is very
narrow, with WR/D value of ca. 0.3, it original ratio of W/D
is estimated to be about or less than 0.4 (thinly discoidal).
Although the precise comparison with the known speciesof Arcestes is difficult, I attempt to compare the presentspecies with known ones based on the estimated shell form
and a characteristic rostrum with sub-rectangular crosssection and concave ventral side.
In the shell shape having extremely discoidal to discoidal
shell cross section, closed umbilicus and narrow (WR/D = 0.4or smaller) flattened venter at the apertural end with sharp
corners, it resembles Arcestes oxycephalus (Mojsisovics,1875, P. 141, pl. 38, figs. 2a–c) and A. dicerus (Mojsisovics,1875, p. 122, pl. 47, figs. 1a–c, pl. 53, fig. 20) from the lower
Norian of Austria. The apertural ends of latter two species,
however, do not form a rostrum and slightly wider than thepresent species (WR/D = 0.37–0.40).
A. oxystomus (Mojsisovics, 1875, p. 130, pl. 48, figs. 2a–c) from the lower Norian of Austria has a rostrum-shaped
peripheral end, but the rostrum has a tapered end. A.
piae (Diener, 1921, p. 352, pl. 1, figs. 4–6) from the mixed
Carnian-Norian fauna of Austria has a thinly discoidal shellwith prominent rostrum comparable to the present species,
but the living chamber of the former has a hook-shaped
bend. A. aff. frechi (Diener, 1921, p. 359, pl. 1, figs. 8a–c) from the Carnian of Austria has a discoidal shell witha strong rostrum, but differs from the present species byhaving a small but distinct umbilicus. Arcestes sp. describedfrom the same locality and horizon at Haizaka (Nakazawa,1964a) is distinguished from the present species by having
rather wide shell.
Occurrence: From the Monotis scutiformis Zone
(M. typica Zone) of the Chonomori Formation (SaragaiGroup), distributed at Haizaka, Minamisanriku-cho, Miyagi
Prefecture.

Acknowledgements
The author wishes to express his deep gratitude to Yudai
Torihata and Ryohei Hashimoto for providing the ammonoidspecimen. The manuscript was greatly improved by theconstructive review of Dr. Yasunari Shigeta.

References
Ando, H., 1983, Paleontological significance of late Triassic
Bivalve Monotis Part 1: A review. Fossils, no. 33, p.
13–27. (In Japanese with English abstract)
Hyatt, A., 1884, Genera of fossil cephalopods. Proceedingsof the Boston Society of Natural History, vol. 22, p. 253–338.
Ichikawa, K., 1950, Chronological classification of theTriassic Period in Japan. Journal of the GeologicalSociety of Japan, vol. 56, p. 17–22. (In Japanese withEnglish abstract)
Ichikawa, K., 1951, Chronological classification of theTriassic Period in Japan. In: Geology of the Triassic
System in Japan. Report Geological Survey of Japan,Spec. No., p. 1–4. (In Japanese)
Diener, C., 1921, Neue Ammonoidea leiostraca ausden
Hallatatter Kalken des Salzkammergutes. Denkschriften
der Kaiserlichen Akademie der Wissenschaften,
Mathematisch-Naturwissenschaftliche Classe, vol. 97, p.
341-389.
Mojsisovics, E. von, 1875, Das Gebirge um Hallstatt. Eine
geologisch-paläontologische Studie aus den Alpen. Teil
I, Die Mollusken-Faunen der Zlambach- und HallstätterSchichten. Abhandlungen der Kaiserlich-Königlichen
Geologischen Reichsanstalt, vol. 6, no. 1, p. 83–174,
pls. 33–70.
Mojsisovics, E. von., 1888, Ueber einige JapanischeTriasfossilien. Beiträge zur Paläontologie ÖsterreichUngarns und des Orients, Bd. 7, p. 163–178, pls. 1–4.
Mojsisovics, E. von., 1893, Das Gebirge um Hallstatt, Theil I,
Die Cephalopoden der Hallstätter Kalke. Abhandlungender Kaiserlich-Königlichen Geologischen Reichsanstalt,,
vol. 6, no. 2, p. 1-835, pls. 71–200.
Nakazawa, K., 1964a, On the Monotis typica Zone in Japan.
Memoirs of the College of Science, University of Kyoto,
Series B, vol. 30, no. 4, p. 21–39, pls. 3–5.
Nakazawa, K., 1964b, On the Upper Triassic Monotis Beds,
especially, on the Monotis typica Zone. Journal of theGeological Society of Japan, vol. 70, p. 523–535. (inJapanese with English abstract)
N a u m a n n , E . , 1 8 8 1 , U e b e r d a s Vo r k o m m e n v o n
Triasbiludungen im nördlichen Japan. Jahrbuch dergeologischen Reichsanstalt, Wien, Bd. 31, p. 519–528.
Niko, S. and Ehiro, M., 2018, Aulacocerid coleoids from theLate Triassic ammonoid Arcestes from the Saragai Group in the South Kitakami Belt, Northeast Japan
Triassic of the South Kitakami Belt, Northeast Japan.
Bulletin of the Tohoku University Museum, no. 17, p.
1–8.
Onuki, Y. and Bando, Y., 1958, On the Saragai Groupof the Upper Triassic System (Stratigraphical andpalaeontological studies of the Triassic System in theKitakami Massif, Northeastern Japan:-1). Journal of
the Geological Society of Japan, v. 64, p. 481–493. (inJapanese with English abstract)
Shimizu, S. and Mabuti, S., 1932, Upper Triassic System in
the Kitakami Mountainland. Journal of the GeologicalSociety of Japan, vol. 39, p. 313–317. (In Japanese)

9Shimizu, S. and Mabuti, S., 1941, First discovery of
Dictyoconites from the Upper Triassic of the Kitakami
Mountainland, Northeast Japan. Jubilee Publicationin the Commemoration of Professor H. Yabe, M.I.A.,
Sixtieth Birthday, vol. 2, p. 919–926, pls. 48 .
Suess, F. E., 1865, Über Ammoniten. Sitzungsberichte
der kaiserlichen Akademie der Wissenschaften,
mathematisch-naturwissenschaftliche Klasse, vol. 52, p.
71–89
Yokoyama, M., 1904, On some Jurassic fossils fromRikuzen. Journal of the College of Science, Imperial
University of Tokyo, vol. 18, no. 6, p. 1–13.