Bulletin of the Geological Survey of Japan, vol. 70 (1/2), p. 43–89, 2019

ArticleConodont-based age calibration of the Middle Triassic Anisian radiolarian biozones
in pelagic deep-sea bedded chert
Shun Muto1,*, Satoshi Takahashi1, Satoshi Yamakita2, Katsuhito Soda3 and Tetsuji Onoue3
Shun Muto, Satoshi Takahashi, Satoshi Yamakita, Katsuhito Soda and Tetsuji Onoue (2019) Conodontbased age calibration of the Middle Triassic Anisian radiolarian biozones in pelagic deep-sea bedded
chert. Bull. Geol. Surv. Japan, vol. 70 (1/2), p. 43–89, 14 figs, 2 tables, 10 plates.
Abstract: The age of pelagic Panthalassic deep-sea bedded chert has been assigned based on radiolarianbiostratigraphy. However, Triassic radiolarian biostratigraphy is in many cases not precisely correlated
to the conodont zones and the standard geological timescale. In this study, we investigated the conodont
biostratigraphy of two radiolarian-controlled bedded chert sections of Anisian age: the Ajiro Island section
in Oita Prefecture and the Kurusu section in Aichi Prefecture. We recognised six conodont biozones
in the studied sections: the upper Olenekian Novispathodus brevissimus-Icriospathodus collinsoni andTriassospathodus homeri Zones, the lower Anisian Chiosella timorensis Zone, the middle AnisianParagondolella bulgarica Zone, the upper Anisian Paragondolella excelsa Zone and the uppermost
Anisian to lowermost Ladinian Paragondolella trammeri Zone. These conodont zones were successfully
correlated to the standard Triassic radiolarian zonation proposed by Sugiyama (1997, Bull. Mizunami
Foss. Mus., vol. 24, p. 79–193). Sugiyama’s radiolarian TR 1 Zone, previously considered to be of
Olenekian age, extends to the middle Anisian. The TR 2A Zone, the TR 2B Zone and the lower part of the
TR 2C Zone are correlated to the middle Anisian, while the upper part of the TR 2C Zone and the lower
part of the TR 3A Zone are correlated to the upper Anisian. The upper part of the TR 3A Zone and thelower part of the TR 3B Zone are probably correlative to the uppermost Anisian, but the possibility thatthey are correlative to the lowermost Ladinian cannot be ruled out.
Keywords: biostratigraphy, conodont, radiolarian biozone, Panthalassa, Middle Triassic

1. Introduction
Carboniferous to Jurassic pelagic deep-sea bedded chert
preserved in the accretionary complex of Japan provides
precious sedimentary records of the Panthalassa sea ﬂoor
(Kojima et al., 2016; Matsuda and Isozaki, 1991; Wakita
and Metcalfe, 2005) (Fig. 1A). For instance, evidenceof oceanic anoxia (Fujisaki et al., 2016; Takahashi
et al., 2009), bolide impact (Sato et al., 2013, 2016)
and climate change (Nakada et al., 2014) have been
documented from Triassic bedded chert in Japan. The
prerequisite for such studies is the recognition of reliablegeochronological controls for the bedded chert sequence,
which was attempted by biostratigraphic correlations. Theage assignment of bedded chert is commonly based onradiolarian biostratigraphy, which has been successfully
established in many regions due to the abundant occurrenceof radiolarians from bedded chert (e.g. Matsuoka andYao, 1986; Sugiyama, 1997). Currently, the radiolarianbiozones of Sugiyama (1997) established in the Inuyama
area in central Japan are regarded as the most complete
radiolarian zonation of Triassic deep-sea bedded chertdeposited in the pelagic Panthalassa (e.g., O’
Dogherty etal., 2010). However, the Triassic radiolarian biozones of
Sugiyama (1997) is not precisely correlated to the global
standard geological timescale (Ogg, 2012). Hence, it isnecessary to calibrate the radiolarian biozones based ongeochronological controls that can be robustly correlated
to the global standard geological timescale. Calibrationby conodont biostratigraphy as in Sato et al. (2013, 2016)
and Nakada et al. (2014) is a useful method, because
although much rarer than radiolarians, conodont fossils
occur reasonably frequently from deep-sea bedded
chert (Hayashi, 1968; Isozaki and Matsuda, 1982, 1983;

1

Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo, 113-0033, Japan
Faculty of Education, University of Miyazaki, 1-1, Gakuen Kibanadainishi, Miyazaki, 889-2192, Japan
3
Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan*
Corresponding author: S. Muto, Email: sm.s.u-tokyo.ac.jp
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Fig. 1 (A) Palaeogeographic map of the Anisian (Ziegler et al.,1998) showing the location of the depositional site of thepelagic deep-sea sediments in the Jurassic accretionary
complex of Japan. (B) Location of the studied sections.
The distribution of Jurassic accretionary complexes isafter Isozaki et al. (2010).

Nishikane et al., 2011). In this article, we focused on theradiolarian biozones previously assigned to the AnisianAge (the TR 2A to TR 3A Zones of Sugiyama, 1997), andcalibrated their ages based on conodont biostratigraphy.
Previous reports of Anisian conodonts from bedded
chert have been sporadic (Ando et al., 2001; Matsuda
and Isozaki, 1982; Mizutani and Koike, 1982; Sugiyama,
1997; Tanaka, 1980). The conodont Chiosella timorensis,
which has been proposed as a marker for the base of theAnisian (Ogg, 2012; Orchard, 2010) has been found frompelagic deep-sea sediments including bedded chert in the
Kuzuu area in east Japan (Muto et al., 2018), the Nichihara
area in southwest Japan (Tanaka, 1980) and the Kamiaso
area in central Japan (Matsuda and Isozaki, 1982). In
particular, Muto et al. (2018) indicated that the radiolarian
based Olenekian-Anisian boundary, which is placed at

the base of the TR 2A Zone of Sugiyama (1997) is within
the middle Anisian. Therefore, the age of the“lower
Anisian”radiolarian biozones of Sugiyama (1997) needs
to be reinvestigated. On the other hand, the finding of
middle Anisian conodonts Paragondolella bulgarica andNicoraella kockeli from the TR 2B and TR 2C Zones in
the Inuyama area (Ando et al., 2001; Mizutani and Koike,
1982; Sugiyama, 1997) ensures that a part of these zones
is correlative to the Anisian. However, the full range ofthese conodonts with regard to the radiolarian biozones
remains unknown. The occurrence of the late Anisianconodont Paragondolella excelsa followed by that of
the Ladinian conodont Budurovignathus hungaricus
from a continuous bedded chert section in the Nichihara
area (Tanaka, 1980) likely indicates the presence of theAnisian-Ladinian boundary. Unfortunately, radiolarians
were not reported from this section, and the boundary
cannot be correlated with radiolarian biozones. A more
intensive investigation on conodont biostratigraphy andits correlation with radiolarian biozones based on highresolution sampling are required to calibrate the age ofthe radiolarian biozones.
Two sections were selected for investigation of
conodont biostratigraphy in this study. The first is the
Ajiro Island section in the Tsukumi area, southwest Japan(Onoue et al., 2011; Soda et al., 2015; Uno et al., 2012)(Figs. 1B, 2). This section was chosen because the TR
2A to TR 3A Zones can be collectively observed in one
continuous section (Onoue et al., 2011). Although thestratotype of the radiolarian biozones of Sugiyama (1997)
was defined in the Inuyama area, the stratotype sectionsof the TR 2A to TR 3A Zones were defined in separate
outcrops, which is a large disadvantage to the Ajiro Island
section. In addition, the Ajiro Island section was logged
bed-by-bed in Onoue et al. (2011) and Soda et al. (2015).
Radiolarian biostratigraphy was based on their logs, whichmeant that we could compare conodont and radiolarianbiostratigraphy at bed-by-bed resolutions.
The second section is the Kurusu section in the Inuyama
area in central Japan (Yao and Kuwahara, 1997) (Figs. 1B,
3). Although this is not one of the stratotype sections of
Sugiyama (1997), it provides crucial evidence on the age of
the TR 2A Zone, as detailed below. The stratotype sectionof the TR 2A Zone is Section M in the Inuyama area, but
the base of this zone is not observed there (Sugiyama,1997). In the Ajiro Island section, the base of the TR 2A
section was determined, but the underlying TR 1 Zone did
not yield the radiolarian Parentactinia nakatsugawaensis,
which is the species that most characterises the TR
1 Zone. Therefore, the TR 1 Zone in the Ajiro Island
section may not be directly correlative to its type sectionin the Inuyama area. In contrast, in the Kurusu section,
the Pa. nakatsugawaensis Assemblage Zone is overlain
by the Hozmadia gifuensis Assemblage Zone (Yao andKuwahara, 1997) and the boundary of the two zones canbe approximated as the base of the TR 2A Zone. The
boundary of the Pa. nakatsugawaensis Assemblage Zone

− 44 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)and the H. gifuensis Assemblage Zone was also reported
from the Momotaro-jinja section in the Inuyama area (Fig.
3; Yao and Kuwahara, 1997). However, the H. gifuensis
Assemblage Zone in this section did not yield the nominal
species H. gifuensis. Furthermore, according to Sugiyama(1997) who investigated the lateral extension of the
Momotaro-jinja section (Section T of Sugiyama (1997)),
the interval assigned to the H. gifuensis Assemblage Zone
by Yao and Kuwahara (1997) is within the TR 1 Zone. Insum, the Kurusu section is the only well-studied sectionthat includes the base of the TR 2A Zone.
In this article, we present the lithostratigraphy andconodont biostratigraphy of the Ajiro Island and Kurusu
sections. The conodont biostratigraphy is illustrated
parallel to previous results on radiolarian biostratigraphy.
Based on correlation with conodont biostratigraphy
established in the Tethys region and North America,
including the global boundary stratotype section and point(GSSP) candidate for the base of the Anisian and the GSSPof the base of the Ladinian, the radiolarian biozones were
calibrated to the global standard geological timescale.

2. Geologic setting of the study sections
The studied sections belong to the Jurassic accretionary
complex in southwest Japan. The Jurassic accretionary
complex is mainly composed of Carboniferous to Middle
Jurassic oceanic rocks (ocean ﬂoor basalt, deep-sea chert,
hemipelagic siliceous mudstone, seamount basalt andseamount-capping carbonate), and Middle Jurassic tolowermost Cretaceous trench-fill terrigenous clastic rocks(mudstone and sandstone) (Isozaki et al., 2010; Wakita andMetcalfe, 2005). Of the Carboniferous to Middle Jurassic
oceanic rocks, the deep-sea sedimentary rocks were
deposited in the pelagic Panthalassa superocean several
thousand kilometres away from the nearest continent (Fig.
1A), after which they traveled to a subduction zone along
the paleo-continent and formed a part of accretionary
complexes by the tectonic movement of the oceanic plates(Isozaki et al., 2010; Matsuda and Isozaki, 1991; Wakita
and Metcalfe, 2005).
2. 1 Ajiro Island section
The Ajiro Island section is located in Ajiro Island (Ajiro
Jima in Japanese) on the northern coast of the Youra
Peninsula, Tsukumi City, Oita Prefecture, Kyushu District(33°
4’10”N 131°
55’
10”E). The studied section belongs
to the Southern Chichibu Belt (Fujii, 1954; Hoshizumi
et al., 2015; Kambe and Teraoka, 1968: Matsuoka et al.,1998), which is mostly composed of a Jurassic accretionary
complex (Isozaki et al. 2010; Matsuoka and Yao, 1990;
Matsuoka et al., 1998). The studied section is a part of the
Shakumasan Group, which preserves the upper part of theoriginal sequence on the oceanic plate (Nishi, 1994). The
Shakumasan Group is further subdivided into the lower
Enoura Formation and the upper Youra Formation (Nishi,
1994). The Enoura Formation is composed of Lower

Triassic to Middle Jurassic pelagic deep-sea sedimentaryrocks (mostly chert) (Matsuoka, 1986; Nishi, 1994), andis overlain conformably by trench-fill clastic rocks of theYoura Formation (Fig. 2; Nishi, 1994). The successionof these two formations occurs repeatedly due to theimbricate structure formed by thrusts (Fig. 2; Nishi, 1994).
The section investigated in this study belongs to thelowermost part of the Enoura Formation (Fig. 2B). The
Ajiro Island section is located in the southernmost part of
the Ajiro Island on the northern coast of the Tsukumi area.
The studied outcrop corresponds to the“alternation bedsof siliceous claystone and chert”and“red chert”in Fig.
2 of Uno et al. (2012). According to the age assignment
based on radiolarian biostratigraphy, the bedded chert of
the Ajiro Island has been dated as latest Spathian to early
Ladinian, and has a northward younging trend (Onoue etal., 2011; Takahashi et al., 1998). The palaeolatitude ofthe depositional grounds have been estimated to be 2.1 ±5.2°
S based on rock magnetic records (Uno et al., 2012).
2. 2 Kurusu section
The Kurusu section (Kr-2 section of Yao and Kuwahara,
1997) is located on the eastern bank of the Kiso Riverin Inuyama City, Aichi Prefecture, Chubu District(35°
25’
8”N 136°57’50”E). The Kurusu section belongs
to the Mino-Tamba-Ashio Belt, which is mostly composedof a Jurassic accretionary complex (Isozaki et al., 2010;
Yamakita and Otoh, 2000; Kojima et al., 2016). The
Kurusu section is a part of the Kamiaso Unit (Wakita,1988), which is a coherent tectonostratigraphic unit
composed of Lower Triassic siliceous claystone, Middle
Triassic to Lower Jurassic pelagic deep-sea bedded chert,
Middle Jurassic hemipelagic siliceous shale and Upper
Jurassic turbidite and massive sandstone in ascendingorder (Yao et al., 1980). Thus, the upper part of theoriginal sequence on the oceanic plate is preserved in
the Kamiaso Unit. The Kamiaso Unit in the Inuyama
area has been studied in detail by Kimura and Hori
(1993). They clarified that the coherent sequence frompelagic siliceous claystone and chert to trench-fill clastic(
rocks “Inuyama
Sequence”in their paper) repeated six
times as thrust sheets, which together form a synclinal
structure with a W-plunging, WNW-ESE trending axis
(the Sakahogi syncline; Mizutani, 1964). The Kurusu
section corresponds to the lowermost strata of“sheet 4”
in Kimura and Hori (1993), and is located on the northernwing of the Sakahogi syncline, close to the axis (Fig. 3).
Yao and Kuwahara (1997) studied the radiolarianbiostratigraphy of the Kurusu section and indicatedthat the section is correlated to the Parentactinia
nakatsugawaensis Zone and the Hozmadia gifuensis Zone.
The radiolarian biostratigraphy indicates a southward
younging trend for this section. This younging trend isconsistent with the fact that the south of the section faces
the axis of the Sakahogi syncline. The palaeolatitude ofthe depositional site of the Anisian bedded chert in the
Inuyama area has been estimated to be 12.3 ± 15.6°
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Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Fig. 2 (A) Distribution of the Chichibu Belt in Kyushu District (after Isozaki et al., 2010). (B) Geological map of
the Tsukumi area (Nishi, 1994).

or S (Oda and Suzuki, 2000) or 5.6 ± 2.2°
S by (Andoet al., 2001) based on palaeomagnetic records. Ikeda
et al. (2010) investigated the“Kr-2 section of Yao andKuwahara (1997)”for cyclostratigraphy, but their section
was observed in a different part of the outcrop (compare
Fig. 4 of Yao and Kuwahara (1997) and Fig. 1S in the
supplementary materials of Ikeda et al. (2010)). In thisstudy, we investigated the studied section of Ikeda et al.
(2010) in addition to the section of Yao and Kuwahara(1997).

3. Methods
3. 1 Field mapping
For both the Ajiro Island and Kurusu sections, geologicalmaps of the outcrop were made based on examination of
the geological structure and lithology at the sub-metre
scale. For the major part of the Ajiro Island section,
detailed lithostratigraphy has been reported by Onoueet al. (2011), Soda et al. (2015) and Uno et al. (2012).
We conducted a biostratigraphic investigation buildingon the lithostratigraphic section constructed by thesestudies. For the lower part of the Ajiro Island section,
lithostratigraphy has not been investigated thoroughly
by previous studies. Therefore, we reconstructed thelithostratigraphy of this part of the outcrop based on
detailed mapping with careful attention to deformationand continuity. Lithostratigraphy of the Kurusu section

has been reported by Yao and Kuwahara (1997) and Ikeda
et al. (2010). However, both studies did not investigate theentire outcrop. In this study, we present an original outcrop
sketch map and lithostratigraphic columns that include,
but are not restricted to, stratigraphic intervals that were
investigated by these studies.
3. 2 Conodont biostratigraphy
Conodonts were found by observing the surfacesof sample rocks cleaved parallel to the bedding into
small“chips”(the“chip method”; Muto et al., 2018;
Takemura et al., 2001). This method allows us to findconodonts that are cracked or preserved as moulds andimpossible to extract by dissolving the host rock. A
preliminary search for conodonts by the“chip method”
was conducted at the outcrop using a hand lens (20magnifications), and horizons that yielded conodonts
were selected for further investigation. Following Mutoet al. (2018), 2–5 kg blocks of the selected horizons werebrought back to the laboratory, and then cleaved into thinplanar chips. These chips were moistened with tap waterand then examined under a stereoscopic microscope of 20
to 60 magnifications. Early attempts of the“chip method”
resulted in larger numbers of specimens from greenish
grey parts of the bedded chert compared to red or purple
parts. This is probably because conodont fossils have morechances of being found in the more transparent greenish− 46 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Fig. 3 Geological map of the Inuyama area (Kimura and Hori, 1993).

grey chert compared to purple or red chert. Therefore,
the parts of strata that have been altered to greenish grey
were targeted in the later attempts to find conodonts fromdominantly purple or red intervals.
For specific identification, the chips were observed
under a stereoscopic microscope of 100 magnifications.
The surface of the chips were dried, soaked with water orcoated with oil depending on the type of host rock and thestate of preservation of the specimens. Moulds in siliceous
claystone without grey fillings were best observed on dry
surfaces, while moulds in siliceous claystone with grey
fillings were better observed when soaked with water.
Specimens in chert were best observed when the surfaceof the chert was coated with oil.

4. Results
4. 1 Ajiro Island section
4. 1. 1 Field mapping
The Ajiro Island section is exposed on the southerncoast of Ajiro Island, Tsukumi City, Oita Prefecture (Fig.
2). The outcrop of the Ajiro Island section consists of twoparts that have different geological structure and lithology.
The main part of the section (the AJR section), whichis exposed along the south to southeast coast of Ajiro
Island, comprises bedded chert (Fig. 4). The AJR section− 47 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Fig. 4 Geological map of the southeast coast of Ajiro Island. The stratigraphic way up of the bedded chert is to thenorth (Onoue et al., 2011; Takahashi et al., 1998; Uno et al., 2012).

has already been studied in detail by previous studies(Onoue et al., 2011; Soda et al., 2015; Uno et al., 2012),
and therefore we will only give a brief description of theoutcrop. The other part is exposed at the southern tip of
Ajiro Island, and comprises alternating beds of siliceous
claystone and chert that are more intensely folded andfaulted compared to strata of the AJR section (Figs. 4, 5).
This part has been mapped by Onoue et al. (2011) andUno et al. (2012), but lithostratigraphy was not examined
in detail. Here, we provide a more detailed geological
map and lithostratigraphic columns for this part of thesection (the AJ-1, AJ-2, AJ-3-1 and AJ-3-2 sections).
In the following text, we will explain the geologicalstructures and lithofacies of the Ajiro Island section in
stratigraphically ascending order (stratigraphic order is
explained in Section 4.1.2.).
The lower part of the Ajiro Island section corresponds
to the AJ-1, AJ-2, AJ-3-1 and AJ-3-2 sections exposed at
the southern tip of Ajiro Island (Fig. 5). The strata here
generally strike ENE-WSW and dip steeply to the north,apart from the northwestern part where the strata strikeE-W to WNW-ESE. Isoclinal folds with wavelengths up
to 2 m are present throughout the outcrop, and the strata
are locally overturned. The axial planes of these folds
are almost parallel to the bedding. The faults in this partof the outcrop can be distinguished into the following− 48 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Fig. 5 Geological map of the southern tip of Ajiro Island. The lower part of the Ajiro Island section is exposed inthis area. Sections (a) to (p) are labelled in stratigraphically ascending order.

two types. Major faults with displacements of at least
several metres were identified in the northern part. Thesefaults are characterised by sheared zones about 50 cm
wide on either side of the fault plane. Minor faults withdisplacements of up to 2 m are present throughout theoutcrop. These faults are not accompanied by widesheared zones. Recognition of three key beds (Figs. 5,6) revealed that strata in the western half of the outcrop
are relatively weakly deformed. Therefore, we observed
the lithostratigraphy in the western half of the outcrop,
avoiding areas where strata are deformed by faultsand folds. The reconstructed lithostratigraphic column
comprises four continuous sequences: AJ-1 section, AJ-2

section, AJ-3-1 section and AJ-3-2 section in ascendingorder (Figs. 5, 9). On the other hand, in the eastern partof the outcrop, strata in the lower part of the AJ-1 section
are repeated by faults and folds, while the upper part of
the AJ-1 section, the AJ-2 section, the AJ-3-1 section andmost of the AJ-3-2 section are missing.
The AJ-1 section is approximately 8.4 m thick, and iscomposed mainly of alternating beds of grey siliceous
claystone and dark grey chert (Fig. 7A). An interval
composed of black siliceous claystone and black chert
occurs in the lower part of this sequence. The AJ-2 section
is approximately 4.8 m thick, and is composed mainly ofalternating beds of grey siliceous claystone and dark grey

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Fig. 6 Photographs of key beds in the lower part of the Ajiro Island section. White lines indicate the correlation
between the same key bed in different places (see Fig. 5 for location). (A, B) Key bed A. (C, D) Key bed C.

or black chert (Fig. 7B). Thin (~10 mm thick) red siliceous
claystone beds occur characteristically in this section.
The AJ-3-1 section is approximately 2.0 m thick, and iscomposed mainly of alternating beds of grey siliceous
claystone and grey or dark grey chert (Fig. 7C). The AJ3-2 section lies directly above the AJ-3-1 section, but aslip plane exists between the two sections, and therefore
they are not confirmed to be continuous (Fig. 7C, D).
The AJ-3-2 section is approximately 3 m thick, and iscomposed of dark grey bedded chert with intercalationsof grey siliceous claystone (Fig. 7D). This section is infault contact with greenish grey bedded chert to the north.
In the outcrop of the AJR section, the strata strike ENEWSW and dip steeply to the north (Fig. 4). A major fault
with an apparent displacement of ~10 m occurs in thenorthern part. Minor faults with apparent displacementsof up to 2 m occur throughout the outcrop. Deformationof the strata is negligible, apart from drags and shears near
faults. The colour of bedded chert changes from greenish
grey to purple, then to red in ascending order (Figs. 8A–D,

Fig. 9). Some white highly recrystallised chert beds occur
in the red bedded chert. Some parts of the purple and red
bedded chert are altered to greenish grey (Fig. 8D). Our
study section mostly corresponds to the section illustrated
in previous studies (Onoue et al., 2011; Soda et al., 2015;
Uno et al., 2012). However, we observed an additional~2 m of red bedded chert above the top of their section.
In addition, we identified a stratigraphic discontinuity atthe basal part of the lithologic column by Onoue et al.
(2011) (the two faults within the AJR section in Fig. 9).
Our reinvestigation of the geological structure showedthat the stratigraphic intervals corresponding to the TR
1 and TR 2A Zones in their section are separated bya fault. Furthermore, a block of greenish grey bedded
chert occurs structurally between these two intervals.
The lithostratigraphic column illustrated in this study
incorporates this new finding.
4. 1. 2 Conodont biostratigraphy
Conodont fossils were found from all of the investigated− 50 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Fig. 7 Outcrop photographs of the lower part of the Ajiro Island section. (A) Alternating beds of grey siliceous
claystone and dark grey chert in the AJ-1 section. (B) Alternating beds of grey siliceous claystone and darkgrey or black chert in the AJ-2 section. (C) Alternating beds of grey siliceous claystone and grey or dark grey
chert in the AJ-3-1 section. (D) Dark grey bedded chert in the AJ-3-2 section. Note the slip plane in (C) and(D). The stratigraphic way up is to the top in all photos. The black ruler in (A) to (C) is 1 m long. The blackboard in (D) is 20 cm × 30 cm.

samples (more than 2000 specimens in total). Conodonts
in siliceous claystone were preserved as moulds, while
those in chert were dark grey skeletons or white or orange
casts. Although most of the specimens were broken andunidentifiable, 194 specimens were identified at the
species level (Table 1; Plates 1–7). Conodont animals
are taxonomically classified based on their apparatuses

that comprise an assemblage of conodont elements (Clark
et al., 1981; Orchard, 2005; Sweet, 1988), but somemorphologically distinguishable elements are yet to be
assigned to any apparatus. Such elements are classified asform groups and are indicated in this paper with quotation
marks.
Conodonts from the siliceous claystone that alternates− 51 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Fig. 8 Outcrop photographs of the AJR section. (A) The upward lithofacies change from greenish grey bedded chert
to purple and red bedded chert. (B) Greenish grey bedded chert. (C) Purple and red bedded chert. (D) Red
bedded chert. Note that the colour is altered to greenish grey in some parts. The stratigraphic way up is to
the right in (A) and (C), and to the top in (B) and (D). The black ruler and board in the photos as scale are
the same as that in Fig. 7.

(p.53) Fig. 9
Lithostratigraphy, conodont biostratigraphy, radiolarian biostratigraphy andmagnetostratigraphy of the Ajiro Island section. The range chart for conodonts excludes
some specimens that were not identified at the species level. See Table 1 for full list andnumbers of identified conodonts. Radiolarian biostratigraphy is after Onoue et al. (2011)
and Uno et al. (2012). Magnetic polarity zones are after Uno et al. (2012). brevissimuscollinsoni Zone: Nv. brevissimus-I. collinsoni Zone; timorensis Z.: Ch. timorensis Zone;
excelsa Z.: Pg. excelsa Zone. The species names denoted in quotation marks are form
species defined based on discrete conodont elements.
− 52 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)

− 53 −

− 54 −AJR310

AJR377

AJR408

5

AJR517

1

AJR603-605

3
2
3

AJR630

AJR577

1

AJR669AJR722

AJR724+1.9m

AJR724+2.4m

1

3

3

1

1

1

1

(3)

(1)
1

3

2

1
1

1

1

2
2
1
1

2

1

1

1

3

1

3

6
(1)

3

2

1
(2)

3
(2)

1

3
(2)
(1)

1

7
(8)

1

3

3
(1)
(13)

6
(2)

3

Cornudina sp.

Table 1.	Occurrence list of conodonts from the Ajiro Island section. Underlined numbers indicate the number of conferred specimens. Bracketed numbers indicate
the number of questionable specimens. The species names denoted in quotation marks are form species defined based on discrete conodont elements.

Icriospathodus collinsoni (Solien)
Novispathodus brevissimus (Orchard)
Novispathodus sp.
Nv. abruptus (Orchard)
Nv. symmetricus (Orchard)
Triassospathodus homeri (Bender)
T. anhuinensis (Ding)
T. brochus (Orchard)
Triassospathodus sp.
Triassospathodus or Novispathodus sp.
Chiosella gondolelloides (Orchard)
Chiosella sp.
Chiosella timorensis (Nogami)"Neohindeodella benderi (Kozur & Mostler)""Kamuellerella gebzeensis Gedik"
Cornudina ? igoi Koike"Nh. aequiramosa Kozur & Mostler"
Paragondolella bulgarica Group
Nicoraella kockeli (Tatge)
Nicoraella sp.
Neogondolella sp.
Ng. hastata Golding & Orchard
Neogondolella regale Mosher
Neogondolella spp. or Paragondolella sp.
Ng. aff. hastata Golding & Orchard
post-Bithynian type Neogondolella sp.
Celsigondolella sp.
Pg. excelsa GroupPg. trammeri Group
Budurovignathus hungaricus (Kozur & Vegh)
Ng. aequidentata Kozur et al.
Ng. pridaensis (Nicora)
Cornudina breviramulis (Tatge)
Co. tortilis (Kozur & Mostler)
Paragondolella sp.

"Nh. triassica (Müller)"

2

3

1

1

5

"Neohindeodella sp."

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

1

1

1

8
1

1

2
1

1
2

1

1

1

1

− 55 −

2

1

AjL-C

150615-21

1(1)150614-06

150615-27

151005-03

1

3

2

1

1

1
1

2

1
1

150616-29

1

2

1
1

12

AjL-A

1

AjL-B

9

5

1

151005-13

5

2

1

1

AjL1-A

160819-03

2

AjL1-B

151015-08.5

170627-01

160818-02

Ajr-01

1

(1)

2
1

3

1

1

1

1

1

1
1

1

2
1

6
2

2

no identifiable elements

1

no identifiable elements

no identifiable elements

3

3

2

7

AJR 0

1
2
1

1

AJR101

AJR1443

170627-022

1170627-03

2

4

2

5

1

1

2
1

1
(1)
(1)

3

5

1

2

1

5

2

5

3

2

2

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019with chert in the AJ-1, AJ-2 and AJ3-1 sections include
many adult specimens, but conodonts from the bedded
chert in the AJR section are mostly juvenile forms.
Especially, species of Paragondolella, which occurredabundantly from the AJR section, are almost entirely
represented by juvenile stages with incomplete platform
development.
Species belonging to the genus Paragondolella are
difficult to distinguish in juvenile forms. Identificationof our specimens is further hindered by the fact that they
are partly immersed in the host rock. Hence, in the caseof specimens of Paragondolella, we referred to a groupof species that share some important morphologicalcharacters (see the palaeontological notes in Chapter
9 for details). Specifically, three“Groups”are used inthis paper: the Paragondolella bulgarica Group (Pg.
bulgarica, Paragondolella bifurcata and Paragondolella
hanbulogi), the Paragondolella excelsa Group (Pg.
excelsa, Paragondolella fueloepi and Paragondolellaliebermani), and the Paragondolella trammeri Group(Pg. trammeri and Paragondolella alpina). Note thatthese“Groups”do not necessarily represent taxonomic
lineages.
Based on the occurrence of conodonts, six conodont
biozones were recognised in the Ajiro Island section: theNovispathodus brevissimus-Icriospathodus collinsoni
Zone, the Triassospathodus homeri Zone, the Chiosellatimorensis Zone, the Pg. bulgarica Zone, the Pg. excelsa
Zone and the Pg. trammeri Zone in ascending order (Fig.
9).
The Nv. brevissimus-I. collinsoni Zone is characterisedby the occurrence of Nv. brevissimus and I. collinsoni.
Its base is not defined in the Ajiro Island section andits top is defined by the last occurrence (LO) of the two
species. This zone is recognised in the lowermost partof the AJ-1 section (Fig. 9). Other associated species are
Novispathodus abruptus, Novispathodus symmetricus andT. homeri.
The T. homeri Zone is characterised by the occurrenceof Nv. abruptus, Nv. symmetricus and T. homeri. Its baseis defined by the LO of Nv. brevissimus and I. collinsoni
and its top is defined by the first occurrence (FO) of Ch.
timorensis. This zone is recognised in the main part of the
AJ-1 section and the lower part of the AJ-2 section (Fig.
9). Other associated species are Chiosella gondolelloides,
Triassospathodus anhuinensis and “Neohindeodellabenderi”.
The Ch. timorensis Zone is characterised by the occurrenceof Ch. timorensis. Its base is defined by the FO of the nominal
species and its top is defined by the FO of the Pg. bulgarica
Group. This zone is recognised in the AJ-3-1 section (Fig.
9). T. homeri also occurred from this zone.
The Pg. bulgarica Zone is characterised by the occurrenceof the Pg. bulgarica Group. Its base is defined by the FOof the Pg. bulgarica Group and its top is defined by the
FO of the Pg. excelsa Group. This zone is recognisedin the AJ-3-2 section and the lower to middle part of

the AJR section (Fig. 9).“Nh. benderi”, Cornudina?
igoi and “Kamuellerella gebzeensis” occurred fromthe lower part of this zone. The middle part of thiszone yielded Neogondolella cf. hastata, in addition to
Ng. aff. hastata and Neogondolella cf. regale, as well
as species of Neogondolella with a large cusp that is
fused to the posterior platform brim (post-Bithynian type
Neogondolella; detailed explanation in Section 5.3.).
Other associated species are Cornudina breviramulis,“Neohindeodella aequiramosa”and“Neohindeodellatriassica”.
The Pg. excelsa Zone is characterised by the occurrenceof the Pg. excelsa Group. Its base is defined by the FO of
the Pg. excelsa Group and its top is defined by the FO of
the Pg. trammeri Group. This zone is recognised in theupper part of the AJR section (Fig. 9). Other associatedconodonts are the Pg. bulgarica Group, Co. breviramulis,“Nh. aequiramosa”and“Nh. triassica”.
The Pg. trammeri Zone is characterised by the occurrenceof the Pg. trammeri Group. Its base is defined by the FOof the Pg. trammeri Group and its top is defined by the
FO of Budurovignathus cf. hungaricus. This zone isrecognised in the uppermost part of the AJR section (Fig.
9). Other associated conodonts are the Pg. excelsa Group,
Co. breviramulis, Cornudina tortilis and“Nh. triassica”.
At the top of this zone, Neogondolella aequidentata
and Neogondolella pridaensis co-occurred with B. cf.
hungaricus.
4. 2 Kurusu section
4. 2. 1 Field mapping
The Kurusu section is exposed on the east bank of theKiso River in Kurusu, Inuyama City, Aichi Prefecture,
Chubu District (Fig. 3). Strata in this outcrop are deformedby many faults and folds (Fig. 10). The outcrop is divided
into two parts that have different lithology by a fault(“dividingfault”in Fig. 10). The eastern part includes
the area mapped by Yao and Kuwahara (1997), and thesection observed in this part of the outcrop is named theKr-2-YK section (Fig. 10). The western part was studied
by Ikeda et al. (2010), and the section observed here isnamed the Kr-2-I section.
The outcrop of the Kr-2-YK section has a complicatedgeological structure (Fig. 10). The orientation of thebedding plane is variable due to folding. Tracing ofcharacteristic black claystone and black chert beds
revealed that the lower part of this section is repeatedby folds. In some parts, the bedding of the strata wasdifficult to identify due to strong deformation. The Kr-2YK section was observed in parts of the outcrop where
deformation was minimal, which include the area studied
by Yao and Kuwahara (1997). The Kr-2-YK section iscomposed mainly of grey bedded chert with intercalationsof grey siliceous claystone (Fig. 11A). Black claystone
and black chert beds also occur in several horizons. The
transition between grey chert layers and grey siliceous
claystone layers is gradual. Therefore, the contrast− 56 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)between the chert layers and the siliceous claystonelayers is not strongly expressed in the configuration of theoutcrop surface. This may have led to the description of
the strata as“siliceous claystone”by Yao and Kuwahara(1997). However, the rocks have moderate vitreous luster
and are not fissile, which indicate that its lithology isbetter described as chert. This is further supported by thepresence of numerous radiolarian tests and quartz matrix
confirmed in thin sections (Fig. 11D). Dolomite occurs as
beds or planar nodules in two horizons. The lithology of
the Kurusu section deserves further detailed observation,
because it likely represents a transitional lithofacies(
between the Lower Triassic siliceous claystone “Toishi”;)
Imoto, 1984 and Middle Triassic bedded chert. However,
it is beyond the scope of this paper, and we will tentatively
refer to the chert beds that grade into siliceous claystone
as“muddy”chert.
The section studied by Yao and Kuwahara (1997),
which corresponds directly to the upper 7 m of the Kr-2YK section, was regarded as continuous in the previous
studies. However, a fault was identified in the middle
part of this interval (Fig. 11C). This fault is accompanied
by a highly deformed sheared zone on both sides of thefault plane. Therefore, the displacement and consequent
stratigraphic gap of this fault could be considerable.
The Kr-2-I section is composed mainly of“muddy”
chert with intercalations of black claystone (Figs. 10,
11B). Intense folding such as that observed in the outcropof the Kr-2-YK section is not observed in this part of theoutcrop. However, the strata have experienced significant
deformation, represented by the distortion of the bedding
by numerous cm-scale faults and slip planes (Fig. 11B). The
Kr-2-I section corresponds almost exactly to the section
reported by Ikeda et al. (2010), but was not observed inexactly the same part of the outcrop, because some parts
that they observed had become covered by thick gravel.
Ikeda et al. (2010) assumed that the stratigraphic way up
in this section is to the northeast. However, the generalyounging of the strata is to the southwest in this part of
the Inuyama area (see Section 2.2.), which is consistent
with outcrop-scale biostratigraphy in the Kurusu section(Yao and Kuwahara, 1997). Therefore, it would be more
orthodox to assume that the stratigraphic way up isto the southwest, which will be adopted in this study.
Nonetheless, it should be noted that the way up in the
Kr-2-I section remains indeterminate, even considering
our biostratigraphic data presented in Subsection 4.2.2.
4. 2. 2 Conodont biostratigraphy
Conodonts were found from all of the investigatedsamples (more than 200 specimens in total). Conodonts
occurred as dark grey skeletons. Although most conodonts
were broken, 28 specimens were identified at the species
level (Table 2; Plates 8–10). Since conodont occurrence
from the Kurusu section is sporadic, independent zonation
for this section will not be defined. Instead, we will referto the conodont biozones recognised in the Ajiro Island

section (Fig. 12).
The Kr-2-YK section yielded Ch. timorensis and“Nh.
aequiramosa”. These conodonts indicate that the Kr-2-YKsection is mostly correlative to the Ch. timorensis Zone.
The Kr-2-I section yielded Nv. symmetricus, T. homeri,
Ch. gondolelloides and“Nh. benderi”. These conodonts
indicate that the Kr-2-I section is referable to the T. homeri
Zone. Co. breviramulis, Co.? igoi, Neostrachanognathus
tahoensis and“K. gebzeensis”also occurred from thissection. Since the Kr-2-YK section is referable to the Ch.
timorensis Zone, the Kr-2-I section is stratigraphicallybelow the Kr-2-YK section. Ikeda et al. (2010) andIkeda and Tada (2014) argued that the Kr-2-I section andSection M of Sugiyama (1997) in the Inuyama area canbe partly correlated by key beds to produce a continuous
lithostratigraphy, based on the assumption that the Kr-2-I
section is equivalent to the Kr-2-YK section. However,since the Kr-2-I section does not overlap with the Kr-2-YKsection, their correlation is not valid. Consequently, thelowermost part of the composite section of the Inuyama
bedded chert by Ikeda et al. (2010) and Ikeda and Tada
(2014) that concerns the Kr-2-I section is not continuous.

5. Age assignment of the conodont biozones
5. 1 The Lower-Middle Triassic (Olenekian-Anisian)boundary (OAB)
The global boundary stratotype section and point(GSSP) for the Olenekian-Anisian boundary (OAB)
is yet to be ratified. The Desli Caira section in north
Dobrogea, Romania has been proposed as a candidate(Gradinaru et al., 2007), while the Guandao section in
Guizhou Province, South China is a potential reference
section (Ogg, 2012). The OAB is classically defined by
the ammonoid Japonites welteri (Balini et al., 2010;
Bucher, 1989). This has been applied to the Desli Cairasection (Gradinaru et al., 2007), but the Guandao section
lacks a significant ammonoid fauna (Ogg, 2012). On
the other hand, Orchard et al. (2007a, b) and Orchard(2010) proposed that the first appearance datum (FAD) of
Chiosella timorensis, that occurs readily from both Desli
Caira and Guandao, can be used as a marker for the OAB.
Subsequently, Goudemand et al. (2012) concluded that the
FAD of Ch. timorensis is very close to, but is slightly below
the ammonoid-defined OAB and is within the uppermost
Spathian. Based on high-resolution investigation of
conodont biostratigraphy in the Desli Caira section and
the Guandao section, Goudemand et al. (2012) placed the
OAB above the FAD of Ch. timorensis and below the lastappearance datum (LAD) of Novispathodus symmetricus
and Triassospathodus homeri, so that the ammonoid based
age assignment (Bucher, 1989) is followed. Our definitionof the OAB will follow that by Goudemand et al. (2012).
In the Ajiro Island section, the first occurrence (FO) of
Ch. timorensis and the last occurrence (LO) of T. homeri
are both at the level of Sample AjL1-B, which is the baseof the Ch. timorensis Zone (Fig. 9). Therefore, the OAB− 57 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Fig. 10		Geological map of the outcrop of the Kurusu section. Note that the Kr-2 section of Yao and Kuwahara (1997)
and Ikeda et al. (2010) (Kr-2-YK and Kr-2-I sections, respectively) correspond to different parts of the outcrop.

is placed between the level of Sample AjL1-B and thelevel of Sample AjL1-A. The placement of the OAB at
the base of the Ch. timorensis Zone indicates that theunderlying Novispathodus brevissimus-Icriospathodus
collinsoni Zone and the T. homeri Zone are of Olenekian
age. The occurrence of Nv. brevissimus and I. collinsoni
is confined to the basal part of the Spathian (Chen et al.,2015; Koike, 1981, 2004; Orchard, 1995, 2007; Zhao et al.,2007). Therefore, the Nv. brevissimus-I. collinsoni Zone
is correlated to the lower Spathian, and the succeeding T.
homeri Zone to the upper Spathian.
In the Kurusu section, Nv. symmetricus and T. homeri did
not co-occur with Ch. timorensis. Therefore, the intervalreferred to the Ch. timorensis Zone in this section is likely
to be of Anisian age. Thus, the OAB is in the lower 1 m or
below the base of the Kr-2-YK section (Fig. 12).

The thickness of the Ch. timorensis Zone is different
between the Ajiro Island and Kurusu sections. An
explanation can be given by the slip plane between the
AJ-3-1 section and the AJ-3-2 section at Ajiro Island. Thatis, lowermost Anisian strata are missing at the slip plane
above the Ch. timorensis Zone in the Ajiro Island section.
If this is the case, the Ch. timorensis Zone in the Kurusu
section mostly corresponds in age to an interval that is
missing at the slip plane in the Ajiro Island section.
5. 2 The Anisian-Ladinian boundary (ALB)The Anisian-Ladinian boundary (ALB) was ratified at
the base of the Eoprotrachyceras curionii ammonoid zone
in the Bagolino section in Brescia, Northern Italy (Brack,2010; Brack et al., 2005). This horizon is above the FO of
species belonging to the Paragondolella trammeri Group− 58 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Fig. 11		(A–C) Outcrop photographs of the Kurusu section. (A) The Kr-2-YK section. (B) The Kr-2-I section.
(C) The fault in the Kr-2-YK section. The fault runs between the arrows. Broken lines represent bedding
planes. (D) Photo-micrograph of muddy chert from the Kr-2-YK section viewed in plane-polarised light.

− 59 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Chiosella sp.

2

1"Neohindeodella sp."

Ch. timorensis (Nogami)"Nh. aequiramosa Kozur & Mostler""Neohindeodella benderi (Kozur & Mostler)""Kamuellerella gebzeensis Gedik"

Chiosella gondolelloides (Orchard)

Triassospathodus homeri (Bender)

Novispathodus sp.

Novispathodus symmetricus (Orchard)

Neostrachanognathus tahoensis Koike

Cornudina ? igoi Koike

Cornudina breviramulis (Tatge)

Table 2		Occurrence list of conodonts from the Kurusu section. Underlined numbers indicate the number of conferred specimens.
The species names denoted in quotation marks are form species defined based on discrete conodont elements.

Kr2-YK section
Kr1-02
Kr1-031

1
3
1

Kr1-04

3

1

no identifiable elements

Kr1-05Kr-2-I section
Kr2-03Kr2-012

1

1

3

and below the FO of Budurovignathus hungaricus (Bracket al., 2005). The same is true for the ALB (the base of the
curionii amonnoid zone) in the Balaton Highland, Hungary(Kovács, 1994). Therefore, the ALB can be placed within
the Pg. trammeri Zone in the Ajiro Island section (Fig. 9).
In the GSSP, the FO of the conodont Budurovignathus
praehungaricus is very close to the ALB (Brack et al.,2005), and provides a good correlation marker for theboundary (Chen et al., 2016b; Ogg, 2012; Orchard, 2010).
However, this species has not been found from the Ajiro
Island section. Another potentially useful marker species
is Neogondolella aequidentata, which has a short range
straddling the ALB (Chen et al., 2016b; Muttoni et al.,
2004; Orchard, 2010). In the Ajiro Island section, Ng.
aequidentata occurred from the top of the Pg. trammeri
Zone, suggesting that the ALB is near this horizon in theupper part of the zone.
5. 3 The Anisian substages
The Anisian Stage is divided into four substages:

3

1

1

2

1

1

1

1

1

3
1

Aegean, Bithynian, Pelsonian and Illyrian in ascendingorder. The Aegean is characterised by Ch. timorensis
all over the world (Chen et al., 2016b; Lehrmann et al.,2015; Nicora, 1977). In Europe and North America, theBithynian is characterised by Paragondolella bulgarica(Nicora, 1977), and the Pelsonian is characterised by
Nicoraella kockeli (Nicora, 1976; Pisa et al., 1980).
On the other hand, Ni. kockeli occurs from below theFO of Pg. bulgarica in South China (Lehrmann et al.,2015). Hence, the stratigraphic order of the FOs of these
species requires further investigation, and the recognitionof the Pelsonian based on the occurrence of Ni. kockeli
may not be valid on the global scale. In any case, theoccurrence of Ni. kockeli from our studied sections is
too sporadic to determine substage boundaries. Therefore,
we refrain from conclusively determining the BithynianPelsonian boundary. Based on the fact that species of theParagondolella excelsa Group characterises the Illyrian(Kozur, 1980), the Pg. bulgarica Zone is correlated to
the Bithynian to Pelsonian interval, and the Pg. excelsa− 60 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Fig. 12		Lithostratigraphy and conodont occurrence of the Kurusu section. The radiolarian biozones for the Kr-2-YKsection are after Yao and Kuwahara (1997). Pn Assemblage Zone: Parentactinia nakatsugawaensis Assemblage
Zone. Hg AZ: Hozmadia gifuensis Assemblage Zone. The species names denoted in quotation marks areform species defined based on discrete conodont elements.

Zone to the lower Illyrian (the Illyrian likely extends tothe lower part of the Pg. trammeri Zone; see section 5.2)
(Fig. 9).
It is notable that a type of Neogondolella characteristic
to the Pelsonian and younger age occurs from the upper
part of the Pg. bulgarica Zone. These are species of
Neogondolella that possess a large cusp that is fused tothe posterior platform margin and occupies the posterior
end of the element. Such forms can be referred to species
such as Neogondolella cornuta, Neogondolella constricta
and Neogondolella shoshonensis. In fact, some of our
specimens have characteristics that may be referable toNg. shoshonensis (Plate 3, Figs. 5, 6). The species of
Neogondolella mentioned above are known only fromthe Pelsonian and younger strata (Chen et al., 2016b).
Therefore, we refer to them as post-Bithynian type
Neogondolella. In the Ajiro Island section, the FO horizonof the post-Bithynian type Neogondolella is sample
170627-03 and the Bithynian-Pelsonian boundary may
be placed at this horizon (Fig. 9).

6. Age calibration of the radiolarian biozones
According to the radiolarian zonation proposed by
Sugiyama (1997), the OAB was regarded to be correlative
to the TR 1-TR 2A radiolarian biozone boundary, and the
ALB to the TR 3A-TR 3B radiolarian biozone boundary.
Our conodont-based age control proved that the age
assignment is erroneous for the TR 1-TR 2A boundary,and probably also for the TR 3A-TR 3B boundary (Fig.
13). In the following text, the age calibration of theradiolarian biozones TR 1 to TR 3B (Sugiyama, 1997) is
discussed in ascending order.
In the Ajiro Island section, the TR 1 Zone was− 61 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Fig. 13		Age calibration of the Anisian radiolarian biozones. Dashed lines represent uncertain boundaries. The Nv.
brevissimus-I. collinsoni Zone was not recognized in the sections of Sugiyama (1997), but is inferred to beencompassed in the TR 0 Zone, because conodonts from the Momotaro-jinja section indicate that the TR 0
Zone extends to the Smithian (Yamakita et al., 2010).

recognised in the Paragondolella bulgarica Zone, well
above the OAB (Onoue et al., 2011; Uno et al., 2012) (Fig.
9). In the Kurusu section, the TR 1-TR 2A boundary isapproximated by the boundary between the Parentactinia
nakatsugawaensis Asemblage Zone and the Hozmadia
gifuensis Assemblage Zone (Yao and Kuwahara, 1997),
which is well above the base of the Chiosella timorensis
Zone (Fig. 12). Thus, the TR 1 Zone includes part of thePg. bulgarica Zone, which is of Bithynian to Pelsonianage. This age assignment is consistent with the finding ofa radiolarian assemblage indicating the TR 1 Zone fromthe Bithynian to Pelsonian interval in the pelagic deepsea Ogama section in Kuzuu, Tochigi Prefecture, Japan(Muto et al., 2018). On the other hand, pelagic deep-sea
siliceous claystone correlative with the TR 1 Zone and theunderlying TR 0 Zone in the Momotaro-jinja section in the
Inuyama area (Sugiyama, 1997; Yao and Kuwahara, 1997)
yielded Spathian conodonts Novispathodus symmetricus
and“Neohindeodella benderi”(Takahashi et al., 2009).
Based on data of the Ajiro Island section, the Kurusu
section and the Momotaro-jinja section, the TR 1 Zone iscorrelated to the upper Spathian to the lower part of the
Bithynian to Pelsonian interval, and the OAB is within
this zone (Fig. 13).

The entire part of the TR 2A and TR 2B Zones and mostof the TR 2C Zone is encompassed in the Pg. bulgarica
Zone, and is correlated to the Bithynian to Pelsonian
interval (Figs. 9, 13). This age assignment is consistent
with the occurrence of Pg. bulgarica from the TR 2B
and TR 2C Zones reported from bedded chert sections in
the Inuyama area (Mizutani and Koike, 1982; Sugiyama,1997). If the placement of the Bithynian-Pelsonianboundary at the FO of post-Bithynian type conodonts in
the Ajiro Island section is correct, this boundary is within
the TR 2B Zone (Figs. 9, 13). The TR 3A Zone is mostly
correlated to the Illyrian Paragondolella excelsa Zone,
but its base may be within the uppermost part of the Pg.
bulgarica Zone, and its top is in the basal part of theParagondolella trammeri Zone (Figs. 9, 13). It is possiblethat the upper part of the TR 3A Zone extends into theLadinian (Fig. 13), but this is unlikely given that the ALB
is probably in the upper part of the Pg. trammeri Zone
(detailed in Section 5.4.).
The top of the TR 3B Zone was not recognised in the
Ajiro Island section (Onoue et al., 2011; Uno et al., 2012).
We assume that only the lower part of the TR 3B Zone
of Sugiyama (1997) is present in the Ajiro Island section,
because it lacks radiolarians such as Triassocampe− 62 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)postdewerveri that appear in the upper part of the TR 3B
Zone (Onoue et al., 2011; Uno et al., 2012). The lower
part of the TR 3B Zone is within the Pg. trammeri Zone,
and therefore falls within the upper Illyrian to the lower
Fassanian (lower substage of the Ladinian). Since the ALB
is probably in the upper part of the Pg. trammeri Zone, thelower part of the TR 3B Zone is likely to be correlative
to the upper Illyrian (uppermost Anisian). Unfortunately,
radiolarian biostratigraphy has not been investigated forthe upper part of the Pg. trammeri Zone. Therefore, wecannot determine whether the TR 3B Zone extends above
the top of the Pg. trammeri Zone. Further study is neededin order to clarify the age of the top of the TR 3B Zone.

7. Correlation with rock magnetic records
The Ajiro Island section has been studied for rockmagnetic records by Uno et al. (2012), who recognised
two magnetic polarity zones in the red bedded chert. A
reference record of Middle Triassic magnetic reversals
was compiled by Hounslow and Muttoni (2010) based
mainly on sedimentary records of the Tethys, many ofwhich have been studied also for conodont biostratigraphy.
We attempted to correlate the rock magnetic records of the
Ajiro Island section to the reference record by Hounslow
and Muttoni (2010) based on the framework of conodont
biostratigraphy (Fig. 14). However, a single solution was
not obtained and there are two possible options, as detailed
below.
In Option 1 (Fig. 14A), polarity zone β in the Ajiro
Island section is correlated to the reversed polarity inmagnetozone MT4 to MT5. The two stratigraphicallyisolated intervals of polarity zoneαare either correlated tothe two separate normal polarities in magnetozones MT6
and MT7, or together correlated to the normal polarity
in magnetozone MT7. In this case, the ALB in the Ajiro
Island section is above polarity zone α and below theFO of Budurovignathus cf. hungaricus, within the upper
1.5 m of the Pg. trammeri Zone. This option agrees with
Uno et al. (2012) in that polarity zoneαis correlated to anormal polarity and polarity zone βto a reversed polarity,
suggesting deposition in the southern hemisphere.
In Option 2 (Fig. 14B), polarity zoneβin the Ajiro Island
section is correlated to the normal polarity in magnetozone
MT6, whereas zone α of the Ajiro Island section iscorrelated to the reversed polarity in magnetozones MT6
to MT8. In this case, the ALB is placed in the lower partof the Paragondolella trammeri Zone, ~1 m above itsbase. This option assumes that polarity zonesαand β are
correlated to reversed and normal polarities, respectively,
contradictory to Uno et al. (2012). Hence, this option
suggests the deposition of the bedded chert of the Ajiro
Island section took place in the northern hemisphere.
The ambiguity in the correlation of mangnetic polarities
results mainly from two reasons. Firstly, the gaps in thepolarity zones in the Ajiro Island section due to thedifficulty in recovering primary magnetic records fromdeep-sea chert (Uno et al., 2012) allow a considerable
amount of speculation in the correlation. Another reason is
the lack of both biostratigraphic and magnetostratigraphic
data within the MT6 magnetozone of the reference
magnetostratigraphy from the Tethys sections (Hounslow
and Muttoni, 2010). Thus, further studies in both thePanthalassic deep-sea sections in Japan and the reference
Tethyan sections are required to establish a conclusive
correlation of integrated Anisian bio-magnetostratigraphy
between the two regions.

8. Conclusions
We investigated conodont biostratigraphy in the Ajiro
Island section in Oita Prefecture and the Kurusu section in
Aichi Prefecture. Six conodont biozones were recognised: thelower Spathian Novispathodus brevissimus-Icriospathodus
collinsoni Zone, the upper Spathian Triassospathodus
homeri Zone, the Aegean Chiosella timorensis Zone, theBithynian to Pelsonian Paragondolella bulgarica Zone, thelower Illyrian Paragondolella excelsa Zone, and the upper
Illyrian to basal Fassanian Paragondolella trammeri Zone,
in ascending order. The age of the radiolarian biozones
previously recognised in the studied sections were assigned
based on conodont biostratigraphy. The TR 1 Zone was
assigned to the upper Spathian to Bithynian (or Pelsonian)and thus the Olenekian-Anisian boundary is within thiszone. The TR 2A Zone, the TR 2B Zone and most of the
TR 2C Zone are correlated to the Bithynian to Pelsonian
interval. The uppermost part of the TR 2C Zone and thelower part of the TR 3A Zone are definitely correlated to theIllyrian. The upper part of the TR 3A Zone and the lower
part of the TR 3B Zone fall within the upper Illyrian to thebasal Fassanian, while the possibility that both intervals arecorrelative to the upper Illyrian is more likely. The AnisianLadinian boundary is within the TR 3B Zone or above it,
and further study is needed to clarify its position relative tothe radiolarian biozones. Our conodont data are helpful incorrelating the magnetic polarity zones detected from the
Ajiro Island section to the reference magnetostratigraphy of
the Triassic obtained in the Tethys realm. Further studies will
establish an integrated bio-magnetostratigraphic correlation
between the pelagic Panthalassic deep-sea sections and the
Tethyan sections.

9. Palaeontological notes(Form species defined by discrete elements rather thanmulti-element apparatuses are shown in quotation marks.)
Budurovignathus cf. hungaricus (Kozur & Vegh)(Plate 1, Figs. 1, 2.)Epigondolella hungarica Kozur & Vegh in Kozur &Mock, 1972. p. 8–9, Plate 2, Figs. 3–7.
Remarks: This species is distinguished by a short free
blade, a sinuous but unornamented platform and a posterior
process. The platform margins are unornamented unlike− 63 −

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Fig. 14		Correlation of magnetic polarity zones in the Ajiro Island section (Uno et al., 2012) to the reference
magnetostratigraphy by Hounslow and Muttoni (2010). The range of Ni. kockeli, Pg. bulgarica Group and Pg.
excelsa is not well understood in magnetozone MT6, but they are known to overlap to some extent (Kovács,
2011; Kozur, 1980). See Fig. 9 for legends of lithofacies and fossil occurrence.

younger species of Budurovignathus.
The specimen illustrated in Plate 1, Fig. 1 has a shortfree blade and a sinuous but unornamented platform. The
posterior denticles are broken, and therefore the presenceof a posterior process is not confidently confirmed. The
specimen illustrated in Plate 1, Fig. 2 has a sinuous
platform and a posterior process extending behind thebasal pit, which are characteristics of Budurovignathus.
The posterior platform margin is not pointed, which is acharacteristic only seen in B. hungaricus. The anterior
part is broken, and therefore the presence of a free blade
cannot be confirmed.
Chiosella gondolelloides (Bender)(Plate 1, Figs. 3, 4, Plate 9, Figs. 1, 2.)Spathognathodus gondolleloides Bender, 1970. p. 529–530,

Plate 10, Figs. 17, 19, 20.
Neospathodus gondolleloides (Bender). Orchard, 1995.
p. 115, Figs. 2.4, 2.5.
Remarks: The specimens are pectiniform elementscharacterised by a faint mid-lateral rib. Chiosella timorensis
is similar to Ch. gondolelloides, but the former has a moredeveloped mid-lateral rib that rises sharply from theblade, a higher length: height ratio (greater than 2.5:1)
and a smaller basal cavity. Ch. gondolelloides is similar
to Triassospathodus homeri, Triassospathodus brochus
and Novispathodus symmetricus in lateral profile, but theformer can be distinguished from the latter three species
by the presence of a mid-lateral rib. Ch. gondolelloides
differs from T. brochus also in having more fused denticles.

− 64 −

Chiosella timorensis (Nogami)

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)(Plate 1, Fig. 5, Plate 8, Figs. 1–6.)
Gondolella timorensis Nogami, 1968. p. 127–128, Plate
10, Figs. 17–21.
Neogondolella timorensis (Nogami). Koike, 1981. Plate
2, Fig 7.
Chiosella timorensis (Nogami). Goudemand et al.,2012. Figs. 2.1–14, 3.1–8.
Remarks: The specimens are pectiniform elementscharacterised by a prominent mid-lateral rib, small basal
cavity and increasingly inclined denticles at the posterior
end. The specimen illustrated in Plate 1, Figure 5 is ajuvenile form, which is shorter and has a less prominent
mid-lateral rib compared with adult forms. The basal
cavity, which is terminally located, is relatively small. The
specimen is similar to juvenile forms of Ch. gondolelloides,
T. homeri, T. brochus and Nv. symmetricus, but can be
distinguished from the latter four species by the narrower
basal cavity.
Cornudina breviramulis (Tatge)(Plate 1, Figs. 6–8, Plate 9, Figs. 3, 4.)Ozarcodina breviramulis Tatge, 1956. p. 139, Plate 5,
Figs. 12.
Cornudina breviramulis (Tatge). Koike, 1996. p.
118–119, Figs. 3.1–3.21.
Cornudina breviramulis (Tatge). Koike, 2016. p.
168–170, Figs. 4.1–4.3.
Remarks: The specimens are angulate pectiniform
elements with a thick long cusp and short anterior andposterior processes. The anterior process bears up to
four small discrete denticles that increase in size andinclination to the posterior. The posterior process bears
up to three denticles that are smaller than those of theanterior process.
Cornudina tortilis Kozur & Mostler (Plate 2, Figs. 2, 3.)Cornudina tortilis Kozur & Mostler, 1970. p. 432–433,
Plate 1, Figs. 10, 16, 20, 24.
Cornudina tortilis Kozur & Mostler. Koike, 1981. Plate
1, Fig. 34.
Remarks: The specimens are characterised by a shortanterior process and a very long and curved cusp. The
posterior process bears small denticles of equal size.
Cornudina tortilis is considered to be the P2 element of
Co. breviramulis (Koike, 1996, 2016).
Cornudina? igoi Koike(Plate 1, Fig. 9, Plate 2, Fig. 1, Plate 9, Fig. 5.)
Cornudina igoi Koike, 1996. p. 119–120, Figs. 4.1–4.20.
Remarks: The specimens identified as this species isdistinguished by a long cusp and a very short anterior
process that bears only two denticles. Although thisspecies may not belong to the genus Cornudina, which
typically has a posterior process, it is tentatively includedin this genus following the original description.
Icriospathodus collinsoni (Solien) (Plate 2, Fig. 5.)Neospathodus collinsoni Solien, 1979. p. 302–303,
Plate 3, Figs. 10, 12–20.
Neospathodus? collinsoni Solien. Koike, 1981. Plate
1, Figs. 42–44.
Remarks: Icriospathodus collinsoni (Solien) is a species
characterised by a biserial row of denticles that form ridges
extending laterally across the element, that commonly
possess short lateral processes. Although the biserial
nature of the denticles cannot be observed in lateral view,the ridge-like form of the denticles and the lateral process
can be observed in the specimen illustrated in Plate 2,
Fig. 5. This specimen also shows affinity to the holotype
in that the element is relatively low and decreases heightgradually to the anterior. The specimen illustrated in Plate
2, Fig. 6 also shows affinity to the holotype in lateral view,
but the biserial nature and ridge-like form of the denticles
cannot be confirmed.
“Kamuellerella gebzeensis Gedik” (form species)(Plate 2, Fig. 4, Plate 9, Fig. 6.)
Kamuellerella gebzeensis Gedik, 1975. p. 124–125,
Plate 8, Figs. 1, 2, 4.
Remarks: The specimens are bipennate ramiform elementscharacterised by an anterior process that is bent downwards
and bears relatively high denticles, and a posterior processthat bears numerous thin erect denticles.
Neogondolella aequidentata Kozur et al.
(Plate 2, Fig. 7.)Neogondolella aequidentata Kozur et al., 1994a. p.
278–279, Plate 2, Figs. 5–9.
Remarks: This species is characterised by a curved elementwith a platform that runs its entire length and a high carina
of uniform height. The lateral platform margins are subparallel. Neogondolella regale also has a carina of uniform
height, but it is not arched and it has more fused denticles.
Neogondolella cf. hastata Golding & Orchard(Plate 2, Figs. 8–11.)Neogondolella hastata Golding & Orchard, 2016. p.
1205–1206, Fig. 9.
Remarks: The specimens are characterised by a platformthat extends from the anterior end to the anterior side of
the base of the large terminal cusp and a uniform carina
of discrete denticles. The platform is widest around midlength. It does not surround the cusp, and terminates before
the cusp in juvenile forms. The cusp is conspicuously
higher and thicker than the other denticles.
Neogondolella aff. hastata Golding & Orchard(Plate 3, Figs. 1, 2.)
Remarks: The specimens are similar to Neogondolella
hastata in having a platform that is widest aroundmid-length and terminates at the anterior base of theconspicuously large cusp, and a relatively low carina of
uniform height. However, their carina is composed of
fused denticles as opposed to discrete denticles in Ng.

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hastata. This species differs from Ng. regale in having athick and high cusp.
Neogondolella pridaensis (Nicora, Kozur & Mietto)(Plate 3, Fig. 3.)Neogondolella pridaensis Nicora et al., 1981. p.
762–763, Plate 89, Figs. 1–9.
Remarks: The specimen is characterised by a reduced
platform and a large terminal cusp. The platform terminatesin a rib surrounding the posterior side of the cusp. The
denticles are of even height and are fused. Species ofParagondolella differ from Neogondolella pridaensis in
having a well-developed platform, which is particularly
evident near the posterior end.
Neogondolella cf. regale Mosher (Plate 3, Fig. 4.)Neogondolella regale Mosher, 1970. p. 741–742, Plate
110, Figs. 1, 2, 4, 5.
Neogondolella regale Mosher. Nicora, 1977. Plate 5,
Figs 1–7.
Remarks: This species is distinguished by an almost
straight element with a well-developed platform that runs
almost its entire length, and a high carina of uniform heightcomposed of moderately fused denticles. The platform has
mostly uniform width throughout its length, and does notsurround the terminal denticle. The specimens from the
Ajiro Island sections show these characteristics, although
they are partly not visible.

1982. Plate 8, Figs. 35–37.
Remarks: The specimens are bipennate ramiform elementscharacterised by an anterior process that bears relatively
large, anteriorly reclined denticles. The basal margin isdeﬂected downwards below the cusp and forms a convex
down outline below the anterior process. The specimen
illustrated in Plate 8, Fig. 8 is identical to“Neohindeodellaaequiramosa”in the configuration of the basal margin andthe denticulation of the anterior process, but its cusp andmuch of the posterior process is broken.
“Neohindeodella benderi (Kozur & Mostler)”(form
species) (Plate 3, Figs. 12, 13, Pl. 10, Figs. 1, 2.)Hindeodella benderi Kozur & Mostler, 1970 p. 440–441,
Plate 2, Figs. 10, 11, 13.
Neohindeodella benderi (Kozur & Mostler). Koike,
1981. Plate 1, Fig. 14.
Remarks: The specimens are bipennate ramiform elements
marked by a short anterior process that is conspicuously
downturned and bears an enormous denticle at the anterior
end. Apart from this large denticle, there are no denticles
or only one or two minute denticles on the anterior process.
This form species is identical to the S elements of theapparatus of Neostrachanognathus sp. A in Agematsu etal. (2008), which is referred to as Neostrachanognathus
tahoensis in this paper.

Neogondolella shoshonensis NicoraNeogondolella shoshonensis Nicora, 1976. p. 640–642,
Plate 83, Figs. 1–15, Plate 84, Fig. 17.
Remarks: This species is distinguished by a well-developed
platform, a low carina that becomes extremely low aroundmid-length and an enormous, posteriorly reclined cusp
with a large basal area that dominates the posterior end.
Some of the specimens identified as“post-Bithynian type
Neogondolella”(Plate 3, Figs. 5, 6) display the latter two
features, but the platform is not well preserved. Therefore,
the specific identification of these specimens is pending.
Other specimens identified as “post-Bithynian type
Neogondolella”(Plate 3, Figs. 7–9) are similar to Ng.
shoshonensis, but some important characters are different
or unconfirmed. The specimens illustrated in Plate 3, Fig.
7 and Fig. 8 have a large cusp and a very low carina,
but the cusp does not recline posteriorly. The specimen
illustrated in Plate 3, Fig. 9 has a very low anterior carina
and an enormous cusp that reclines posteriorly, but the
middle part of the carina is not visible.

“Neohindeodella triassica (Müller)”(form species)(Plate 4, Fig. 1–3.)
Hindeodella triassica Müller, 1956. p. 826, Plate 96,
Figs. 4, 5.
Neohindeodella triassica (Müller). Koike, 1981. Plate
1, Fig. 25.
Neohindeodella triassica (Müller). Koike, 1982. Plate
8, Figs. 21–30.
Remarks: The specimens are bipennate ramiform elements
distinguished by a large cusp and denticles that are uprightat the anterior and become reclined in the posterior part.
The basal margin is convex under the cusp, and is relatively
straight to the posterior. There is a short anterior processwith relatively large denticles near the anterior end.“Nh.
aequiramosa”differs from this species in the bendingof the element below the cusp and the configuration of
the basal margin, which is convex downwards below theanterior process and not below the cusp.
This form species is considered to be the S3/4 elementsof Cornudina breviramulis (Koike, 2016), and also
shows affinity to the S3 element of the apparatus ofTriassospathodus reconstructed by Orchard (2005).

“Neohindeodella aequiramosa Kozur & Mostler”(form
species) (Plate 3, Figs. 10, 11.)
“Neohindeodella cf. aequiramosa Kozur & Mostler”(Plate 8, Fig. 8.)Neohindeodella aequiramosa Kozur & Mostler, 1970.
p. 445–446, Plate 2, Figs. 1, 2, 4, 5, 7, 8.
Neohindeodella aequiramosa Kozur & Mostler. Koike,

Neostrachanognathus tahoensis Koike.
(Plate 10, Fig. 3.)Neostrachanognathus tahoensis Koike, 1998. p. 127–128,
Figs. 9.10, 9.14, 9.16–23.
Neostrachanognathus sp. A. Agematsu et al., 2008. Fig. 8.
Remarks: This specimen is a coniform element with aposteriorly elongated basal area. Agematsu et al. (2008)− 66 −

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)redefined the original definition of Neostrachanognathus
tahoensis by Koike (1998), and argued that the P1 elementof this species is a coniform element without a posterior
elongation of the basal area. However, the holotype of
Nc. tahoensis has a posterior elongation of the basal area.
Agematsu et al. (2008) also described another speciesof Neostrachanognathus (Neostrachanognathus sp.
A), which has a coniform P1 element with a posterior
elongation of the basal area, and is assigned here to Nc.
tahoensis. The S element of Nc. tahoensis in Agematsu etal. (2008) (their Neostrachanognathus sp. A) is identical
to“Nh. benderi”.
Nicoraella kockeli (Tatge) (Plate 4, Fig. 4.)Ozarkodina kockeli Tatge, 1956. p. 137, Plate 5, Figs.
13, 14.
Neospathodus kockeli (Tatge). Koike, 1982. Plate 6,
Figs. 2–11.
Remarks: The specimens are pectiniform elementscharacterised by an angular profile of the upper margin
due to the denticles decreasing in height in both anterior
and posterior directions from the conspicuously widecusp. The cusp is located posterior to the mid-length ofthe elements. The decrease in height of the denticles is
rapid to the posterior side of the cusp. The basal cavity is
not expanded laterally, and is positioned slightly anterior
from the posterior end. This species is distinguished from
Novispathodus abruptus by the forward-shifted position
and unﬂared configuration of the basal cavity, and fromT. homeri by the small basal cavity.
Novispathodus abruptus (Orchard) (Plate 4. Fig. 5.)Neospathodus abruptus Orchard, 1995. p. 118–119,
Figs. 3.16–3.19, 3.23–3.26.
Remarks: The specimens are pectiniform elementscharacterised by denticles that decrease height rapidly at
the posterior end, resulting in the arcuate outline of theupper margin. This species is distinguished from T. homeri
by the lack of a distinct posterior process.
Novispathodus brevissimus (Orchard) (Plate 4, Fig. 6.)Neospathodus triangularis (Bender). Koike, 1981. Pl.
1, Fig. 6.
Neospathodus brevissimus Orchard, 1995. p. 119, Figs.
3.14–3.15, 3.20–3.22.
Remarks: The specimen is characterised by a blade
composed of thin fused denticles and a basal cavity that
expands widely in lateral directions and truncates abruptly
at the posterior end. The height of the blade is equal foralmost its entire length, giving the element a quadrate
outline in lateral view.
Novispathodus symmetricus (Orchard)(Plate 4, Figs. 7–10, Plate 10, Figs. 4, 5.)Neospathodus symmetricus Orchard, 1995. p. 120–121,
Figs. 2.6, 2.10–2.13, 2.18.
Remarks: The specimens are pectiniform elementscharacterised by sub-equal denticles that are slightly
to moderately inclined to the posterior. This species isdistinguished from T. homeri by the lack of a posterior
process and abrupt truncation of the basal cavity, and fromT. brochus by the more fused denticles and the lack ofradiating denticles at the posterior end. Orchard (2007)
assigned Nv. symmetricus to the genus Triassospathodus,
which was distinguished from Novispathodus based on
differences in their multielement apparatus (Orchard,2005), but the apparatus of this particular species is yet
to be reconstructed. As far as the P1 element is concerned,
Nv. symmetricus is closer to Nv. abruptus than to T. homeri
in that it lacks a well-developed posterior process. Hence,
we include Nv. symmetricus in Novispathodus.
Novispathodus triangularis (Bender)
Spathognathodus triangularis Bender, 1970. p. 530,
Plate 5, Fig. 22.
Neospathodus triangularis (Bender). Orchard, 1995. p.
116, 118, Figs. 3.1–3.4.
Remarks: Novispathodus triangularisi (Bender) was
redefined by Orchard (1995) as a species with a conspicuous
fold on the upper surface of the basal cup, that extends
vertically to a sub-terminal cusp. The specimen illustrated
in Plate 4, Figure 11 has a vertical groove above the basal
cup, which may be the“fold”described by Orchard(1995). The specimen also has a length-height ratio similar
to the specimens illustrated by Orchard (1995). However,the poor preservation of the specimen makes it difficult tomake a conclusion.
Paragondolella bulgarica Group (Plate 5, Fig. 2–8.)
Paragondolella cf. bulgarica Group (Plate 5, Fig. 1.)
Paragondolella bulgarica Budurov & Stefanov, 1975.
p. 794, Plate 1, Figs. 1–23.
Neogondolella bulgarica (Budurov & Stefanov).
Nicora, 1977. Plate 5, Figs. 8–14.
Neogondolella bulgarica (Budurov & Stefanov). Koike,
1981. Plate 2, Figs. 1–3.
Neogondolella bulgarica (Budurov & Stefanov). Koike,
1982. Plate 4, Figs. 1–19, 22–24.
Paragondolella bifurcata Budurov & Stefanov. Chen etal., 2016a. p. 728, Fig. 2.
Neogondolella unilobata Gedik, 1975. p. 133–134,
Plate 1, Figs. 9–25.
Neogondolella hanbulogi Sudar & Budurov, 1979. p.
50–51, Plate 1, Figs. 9, 10, Plate 2, Figs. 1–9, Plate
3, Figs. 1–12.
Remarks: This group is distinguished by an arched element
and a high-carina of moderately fused denticles that ends
with a strong terminal cusp. A small denticle is often
present behind the cusp. The last denticle is sometimes
fused to the posterior brim of the platform. In juvenile
specimens, the platform is not developed (Plate 5, Fig.
6) or confined to the posterior part (Plate 5, Figs. 4, 5).
In later stages, the platform runs almost its entire length.
These characteristics match with those of Paragondolella− 67 −

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bulgarica.
In juvenile stages, Pg. bulgarica is very similar to
Paragondolella bifurcata, which is considered as adescendant of the former species (Chen et al., 2016a).
Since our specimens are almost entirely juveniles, it is
impossible to separate them into the two species. We also
follow Nicora (1977) and Chen et al., (2016a) in treating
Paragondolella unilobata as a junior synonym of Pg.
bulgarica. Therefore, we will include Pg. bifurcata andPg. unilobata in the Pg. bulgarica Group. In addition,
Kovács and Rálisch-Felgenhauer (2005) reported
transitional morphotypes between both these two species
and Paragondolella hanbulogi. Hence, Pg. hanbulogi isalso included in the Pg. bulgarica Group.
Paragondolella excelsa Group (Plate 5, Figs. 9–14.)Paragondolella excelsa Mosher, 1968a. p. 938–939,
Plate 118, Figs. 1–8.
Paragondolella excelsa Mosher. Mosher, 1968b. Plate
120, Figs. 1–7.
Gondolella liebermani Kovács & Krystyn in Kovács,
1994. p. 492–493, Plate 6, Figs. 1–3.
Gondolella fueloepi Kovács, 1994. p. 493.
Gondolella fueloepi fueloepi Kovács, 1994. Plate 7,
Figs. 2, 3, Pl. 8, Fig. 4.
Gondolella fueloepi pseudobifurcata Kovács, 1994.
Plate 8, Figs. 1, 3.
Remarks: The specimens are segminiplanate elementswith a high blade and a thick platform developed in theposterior part of the element that surrounds the terminal
cusp. The cusp is thick, reclined posteriorly and its tip islower than the anterior denticles. These characteristics
match with juvenile forms of Paragondolella excelsa. In
adult forms, this species is characterised by a carina thatis high at the anterior and decreases height posteriorly
ending in a node-like cusp, and a massive platform thatsurrounds the terminal denticle. The low, thick cusp andwell-developed platform in our specimens are regarded
as a precursor of these adult characters. Paragondolellaliebermani and Paragondolella fueloepi also has acarina that decreases to a low, node-like cusp and a thickplatform that surrounds the posterior end. Hence, we treat
Pg. excelsa, Pg. liebermani and Pg. fueloepi as the Pg.
excelsa Group, although juvenile characters of the lattertwo species have not been thoroughly investigated.
In juvenile stages, the Pg. excelsa Group somewhat
resembles the Pg. bulgarica Group, but the former hasa thicker platform and a lower and more reclined cusp.
Paragondolella trammeri Group (Plate 6, Figs. 1–7.)
Gondolella haslachensis trammeri Kozur and Mock,
1972. p. 13, Plate 1, Figs. 3–7.
Paragondolella trammeri (Kozur) Kozur et al., 1994b.
p. 176–178, Plate 2, Figs. 19, 23, 24. Plate 3, Figs.
10, 13, 16.
Gondolella alpina Kozur & Mostler, 1982. p. 292–293,
Plate 1, Fig. 1, Plate 2, Figs. 4, 5.

Gondolella alpina alpina Kozur & Mostler. Kovács,
1994. p. 488–489, Plate 4, Figs. 2, 4, Plate 5, Figs.
1–3.
Remarks: The specimens are segminiplanate elementscharacterised by a large posteriorly reclined cusp and aplatform that merges at the posterior end into a pointed
denticle behind the cusp. The platform is thicker in larger
specimens (Plate 6, Fig. 6) reﬂecting ontogenic growth.
These characters match with those of Paragondolella
trammeri. Paragondolella alpina is similar to Pg.
trammeri, but can be distinguished from the latter in
adult specimens by the presence of a free blade. Immature
specimens of Pg. alpina have not been well studied. We
withhold from conclusively differentiating the two species
in our specimens, and refer to them as the Pg. trammeri
Group.
In earliest growth stages, the Pg. trammeri Groupsomewhat resembles late juvenile stages of the Pg.
bulgarica Group in having a pointed posterior platform
end behind the cusp. However, the former differs fromthe latter in the lower and more discrete denticles, andthe higher cusp. In later juvenile stages, the Pg. trammeri
Group differs from the Pg. bulgarica Group in having athicker platform. Pg. trammeri Group is distinguishedfrom the Pg. excelsa Group in having a small terminal
denticle fused to the posterior brim of the platform.
Triassospathodus anhuinensis (Ding) (Plate 6, Fig. 8.)Neospathodus anhuinensis Ding, 1983. p. 44, Plate 5,
Figs. 9, 10.
Remarks: The specimen is characterised by a blade
composed of small and fused denticles and a welldeveloped posterior process that occupies around one thirdof the element. It differs from T. homeri in having a much
longer posterior process and smaller denticles.
Triassospathodus brochus (Orchard) (Plate 7, Fig. 1.)
Triassospathodus cf. brochus (Orchard) (Plate 7, Fig. 2.)Neospathodus brochus Orchard, 1995. p. 119, Figs.
3.27, 3.28, 3.35, 3.36.
Remarks: This species is distinguished by a pectiniform
element that possesses relatively discrete denticles that
become increasingly reclined near the posterior end, sothat the terminal denticle is orientated at a low angle to thebasal margin. This species is distinguished from T. homeri
by the more discrete denticles and radiating denticles atthe posterior end.
The specimen illustrated in Plate 7, Fig. 2 is partly
broken at the anterior side, but the observed features are
identical to those of T. brochus.

− 68 −

Triassospathodus homeri (Bender)(Plate 7, Figs. 3–6, Pl. 10, Figs. 6, 7.)Spathognathodus homeri Bender, 1970. p. 528–529,
Plate 5, Figs. 16, 18.
Neospathodus homeri (Bender). Orchard, 1995. p.
115–116, Figs. 2.1–2.3, 2.7–2.9, 2.14–2.17, 2.20–2.21.

Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Remarks: The specimens are pectiniform elements marked
by a short posterior process that bears up to five denticles.
The posterior process is commonly downturned and bears
denticles that are sometimes smaller than the anterior
process. This species is distinguished from Nv. abruptus
and Nv. symmetricus by the presence of the distinct
posterior process.
Acknowledgements
We are grateful for an anonymous reviewer who provided
many fruitful comments. Sincere gratitude is expressed to
H. Shirozu, H. Matsumoto, K. Yoshizawa and residents ofthe Enoura district in Tsukumi City for their kind support
during fieldwork. This study was partly supported by
Grant-in-aid for JSPS Research Fellow Number 16J04796(to S. Muto) and 16J09728 (to K. Soda).

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Recieved December 19, 2017
Accepted January 18, 2019

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Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)

コノドント生層序による中部三畳系アニシアン遠洋深海層状チャートにおける放散虫化石帯年代の再検討
武藤

俊・高橋

聡・山北

聡・曽田勝仁・尾上哲治

要 旨
超海洋パンサラッサの遠洋域深海で堆積した層状チャートは，放散虫化石層序により年代が決められてきた．一方で，三畳系の放散虫化石帯は国際基準の地質年代との厳密な対比には用いられない場合が多い．本研究は，放散虫化石帯が確立され，中部三畳系アニシアン階に対比されていた大分県津久見地域の網代島セクションと愛知県犬山地域の栗栖セクシ
ョンの珪質粘土岩・層状チャートにおいてコノドント化石層序を検討した．結果，オレネキアン階の上部のNovispathodus
brevissimus-Icriospathodus collinsoni 帯とTriassospathodus homeri 帯，アニシアン階の下部のChiosella timorensis 帯，アニシア
ン階の中部のParagondolella bulgarica 帯，アニシアン階の上部のParagondolella excelsa 帯，アニシアン階の最上部からラディニアン階の最下部のParagondolella trammeri 帯が認識された．上記のコノドント化石層序との比較により，Sugiyama(1997, Bull. Mizunami Foss. Mus., vol. 24, p. 79–193)による放散虫化石帯の年代を再検討した．従来オレネキアン階に対比されていたTR 1化石帯はオレネキアン階の上部からアニシアン階の中部に対比された． TR 2A化石帯，TR 2B化石帯，及びTR 2C化石帯の下部はアニシアン階の中部に対比され，TR 2C化石帯の上部とTR 3A化石帯の下部はアニシアン階の上部に対比された．TR 3A化石帯の上部とTR 3B化石帯の下部はアニシアン階の最上部に対比される可能性が高いが，ラディニ
アン階の最下部に対比される可能性も否めない．

難読・重要地名
Tsukumi：津久見，Youra：四浦，Sambosan：三宝山，Yukagi：床木，Tsui：津井，Shakumasan：尺間山，Enoura：江ノ浦，Ajiro：網代，Kurusu：栗栖，Inuyama：犬山，Kamiaso：上麻生− 73 −

Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Plate 1

Stereo-photographs and sketches of conodonts from the Ajiro Island section. Figs. 3–5 are reversely arranged sothat the moulds appear as casts. All other figures are normally arranged. Scale bars are 200 µm.
1, 2: Budurovignathus cf. hungaricus (Kozur & Vegh). Sample AJR 724+2.4m.
3, 4: Chiosella gondolelloides (Bender). 3. Sample AjL-B. 4. Sample 151005-13.
5: Chiosella timorensis (Nogami). Sample AjL-1-B.
6–8: Cornudina breviramulis (Tatge). 6. Sample 170627-02. 7. Sample 170627-03. 8. Sample AJR 724+1.9m.
9: Cornudina? igoi Koike. Sample AJR 0.

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Plate 2

Stereo-photographs and sketches of conodonts from the Ajiro Island section. Fig. 5 is reversely arranged so thatthe mould appears as a cast. All other figures are normally arranged. Scale bars are 200 µm.
1: Cornudina? igoi Koike. Sample AJR 144.
2, 3: Cornudina tortilis Kozur & Mostler. 2. Sample AJR 722. 3. Sample AJR 724+1.9m.
4: “Kamuellerella gebzeensis Gedik”. Sample 170627-02.
5: Icriospathodus collinsoni (Solien). Sample AjL-C.
6: Icriospathodus sp.? Sample AjL-C.
7: Neogondolella aequidentata Kozur. Sample AJR 724+2.4m.
8–11: Neogondolella cf. hastata Golding & Orchard. 8, 9. Sample 160627-02. 10, 11. Sample 160627-03.

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Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Plate 3

Stereo-photographs and sketches of conodonts from the Ajiro Island section. Fig. 12 is reversely arranged so that the mouldappears as a cast. All other figures are normally arranged. Scale bars are 200 µm.
1, 2: Neogondolella aff. hastata (Golding & Orchard). Sample 170627-03.
3: Neogondolella pridaensis (Nicora, Kozur & Mietto). Sample AJR 724+2.4m.
4: Neogondolella cf. regale Mosher. Sample 170627-03.
5–9: post Bithynian type Neogondolella. 5, 7. Sample 170627-03. 6. Sample AJR 577. 8. Sample AJR 310. 9. Sample AJR 517.
10, 11: “Neohindeodella aequiramosa Kozur & Mostler”. 10. Sample AJR 144. 11. Sample 170627-02.
12, 13: “Neohindeodella benderi (Kozur & Mostler)”. 12. Sample 151005-13. 13. Sample AJR 0.

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Bulletin of the Geological Survey of Japan, vol. 70 (1/2), 2019

Plate 4

Stereo-photographs and sketches of conodonts from the Ajiro Island section. Figs. 5, 7–11 are reversely arranged
so that the moulds appear as casts. All other figures are normally arranged. Scale bars are 200 µm.
1–3: “Neohindeodella triassica (Müller)”. 1. Sample AJR 144. 2. Sample AJR 517. 3. Sample AJR 724+1.9m.
4: Nicoraella kockeli (Tatge). Sample AJR 101.
5: Novispathodus abruptus (Orchard). Sample 150615-21.
6: Novispathodus brevissimus (Orchard). Sample AjL-C.
7–10: Novispathodus symmetricus (Orchard). 7. Sample 150615-21. 8. Sample AjL-B. 9. Sample 151005-13.
10. Sample 160819-03.
11: Novispathodus sp. indet. Sample AjL-B.

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Plate 5

Stereo-photographs and sketches of conodonts from the Ajiro Island section. All
figures are normally arranged. Scale bars are 200 µm.
1: Paragondolella sp. cf. Paragondolella bulgarica Group. Sample 151015-08.5.
2–8: P aragondolella bulgarica Group. 2. Sample AJR 0. 3, 4. Sample AJR 144.
5. Sample AJR 310. 6, 7. Sample AJR 517. 8. Sample AJR 577.
9–14: P aragondolella excelsa Group. 9. Sample AJR 517. 10–12. Sample AJR
577. 13. Sample AJR 722. 14. Sample AJR 724+2.4m.

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Plate 6

Stereo-photographs and sketches of conodonts from the Ajiro Island section. Fig. 8 is reversely arranged sothat the mould appears as a cast. All other figures are normally arranged. Scale bars are 200 µm.
1–7: P aragondolella trammeri Group. 1. Sample AJR 603–605. 2–4. Sample AJR 630. 5. Sample AJR 722.
6, 7. Sample AJR 724+1.9m.
8: Triassospathodus anhuinensis (Ding). Sample 150616-29.

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Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Plate 7

Stereo-photographs and sketches of conodonts from the Ajiro Island section. All figures are reversely
arranged so that the moulds appear as casts. Scale bars are 200 µm.
1: Triassospathodus brochus (Orchard). Sample 151005-13.
2: Triassospathodus cf. brochus (Orchard). Sample 151005-13.
3–6: Triassospathodus homeri (Bender). 3. Sample 150615-21. 4. Sample AjL-B. 5. Sample 151005-13.
6. Sample AjL1-A.

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Plate 8

Stereo-photographs and sketches of conodonts from the Kr-2-YK section. All figures are normallyarranged. Scale bars are 200 µm.
1–6: Chiosella timorensis (Nogami). 1–3. Sample Kr1-04. 4. Sample Kr1-03. 5, 6. Sample Kr1-02.
7: Chiosella sp. Sample Kr1-04.
8: “Neohindeodella cf. aequiramosa Kozur & Mostler”. Sample Kr1-03.

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Conodont-based age calibration of Anisian radiolarian biozones (Muto et al.)Plate 9

Stereo-photographs and sketches of conodonts from the Kr-2-I section. All figures are normallyarranged. Scale bars are 200 µm.
1, 2: Chiosella gondolelloides (Bender). 1. Sample Kr2-01. 2. Sample Kr2-03.
3, 4: Cornudina breviramulis (Tatge). Sample Kr2-01.
5: Cornudina? igoi Koike. Sample Kr2-01.
6: “Kamuellerella gebzeensis Gedik”. Sample Kr2-03.

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Plate 10 Stereo-photographs and sketches of conodonts from the Kr-2-I section. All figures arenormally arranged. Scale bars are 200 µm.
1, 2: “Neohindeodella benderi (Kozur & Mostler)”. Sample Kr2-01.
3: Neostrachanognathus tahoensis Koike. Sample Kr2-01.
4, 5: Novispathodus symmetricus (Orchard). Sample Kr2-01.
6, 7: Triassospathodus homeri (Bender). 6. Sample Kr2-01. 7. Sample Kr2-03.

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