ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 19: 89–104 (2020)© by the Fukui Prefectural Dinosaur Museum

REVISION OF THE NARIWA FLORA AND ITS IMPLICATIONS IN TERRESTRIAL
CLIMATE CHANGES DURING THE LATE TRIASSIC IN EAST ASIA
Hirokazu YUKAWA, Ge SUN , Shigeyuki SUZUKI and Takuya IMAI
1

2, 3

4

1, 5

1

Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, JapanKey Laboratory of Evolution of Past Life and Environmental Changes in Northeast Asia, Ministry of Education of China,
Jilin University, 2699 Qianjin St, Chaoyang District, Changchun, Jilin 130026, China
3
College of Paleontology, Shenyang Normal University, 253 Huanghe N St, Huanggu District, Shenyang, Liaoning 110034, China
4
Department of Earth Sciences, Faculty of Science, Okayama University, Okayama 700-8530, Japan
5
Insitute of Dinosaur Research, Fukui Prefectural University, 4-1-1, Matsuoka-kenjojima, Eiheiji, Fukui 910-1195, Japan
2

ABSTRACTThe Nariwa Group is well-known for the distinctive Late Triassic Nariwa flora and fossil marine bivalvescharacterized by the presence of Monotis ochotica. Based on the recently proposed stratigraphic scheme of the
Nariwa Group, localities of the previously known Nariwa flora are assigned to the lower and upper terrestrial
horizons that are intercalated by a marine deposit that yields Monotis ochotica and rich in plant fossils. In this study,
we re-examine the previously collected specimens of the Nariwa flora based on the new stratigraphic scheme anderect two distinct floristic assemblages within the Nariwa Group, namely Niga and Hina-Hinabata floristic
assemblages. The lower plant fossil assemblage, the Niga Floristic Assemblage, from the Niga Formation ischaracterized by a greater diversity of Dipteridaceae and cycadophytes, while the upper Hina-Hinabata Floristic
Assemblage from the Hina and Hinabata Formations is characterized by the increase of Ginkgoales and cycadophytesremains with the lower abundance of Dipteridaceae. The ages of the Niga and Hina-Hinabata floristic assemblages
are inferred to be the Norian and the Norian-Rhaetian?, respectively, based on the floristic correlation. The floral
changes recognized in the Nariwa Group may imply the changes in the climate conditions from subtropical to warm
temperate with increased seasonality in eastern Eurasia during the latest Triassic.
Key words: Nariwa Group, Late Triassic, Norian, Rhaetian, Floristic assemblage, Paleoclimate
湯川弘一・孫 革・鈴木茂之・今井拓哉（2020）成羽植物群の再検討と東アジアでの後期三畳紀の陸上気候変化における意義．福井県立恐竜博物館紀要 19：89‒104.
成羽層群は特徴的な後期三畳紀の成羽植物群と，モノチスに代表される海生二枚貝化石でよく知られている．近年の成羽層群の層序学的研究では，モノチスを産する海成層を挟んで，上下共に植物化石を多産する陸成層が存在することが確認された．そこで本研究では，新しい層序を基に，先行研究で報告されていた産地および植物化石を見直し，２つの植物化石群集（仁賀植物化石群集と日名‐日名畑植物化石群集）を認めた．仁賀植物化石群集は，地頭層の下位に位置する仁賀層から産出する植物化石からなり，ヤブレ
ガサウラボシ科のシダ植物やベネチテス類，ソテツ類などの裸子植物の多様性が高いことで特徴付けられる．一方，日名‐日名畑植物化石群集は，地頭層の上位に位置する日名層及び日名畑層から産出する植物
化石からなり，仁賀植物化石群集に比べてイチョウ類やベネチテス類，ソテツ類の多様性の増加，および
ヤブレガサウラボシ科のシダ植物の多様性の減少が示される．植物化石の対比により，仁賀植物化石群集と日名‐日名畑植物化石群集は，それぞれ Norian および Norian-Rhaetian? と示唆される．また，両植物化
石群集における構成の変化から，三畳紀末期にかけて東ユーラシアでは亜熱帯から暖温帯の気候へと変化
していたことが示唆される．

INTRODUCTION
Nariwa flora is a Late Triassic plant-fossil assemblage uniqueto the Nariwa Group distributed in western Okayama of Honshu
Island, Japan (Fig. 1). The group has been extensively studied
for its paleobotanical record since the preliminary report by
Yokoyama in 1905 (Oishi, 1932; 1940; Oishi and Huzioka,
1938; Huzioka, 1970; Kimura and Ohana, 2000).

Received September 24, 2019. Accepted February 18, 2020
Corresponding author―Hirokazu YUKAWA
E-mail: h-yukawa＊dinosaur.pref.fukui.jp89

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HIROKAZU YUKAWA, GE SUN, SHIGEYUKI SUZUKI AND TAKUYA IMAIN

TottoriAxial trace of foldTakahashi
Okayama

Fault

Kobe

Osaka

Unconformity

N
Nariwa Group30

Tokushima

10

20 km

21

A

64

63 90

93

Eda

33

87

16

96

1

er

a Riv

Nariw

Nariwa

0 km
V

iver

V
V

ke R

65

Ryo

88

Hina

62
46

89

47

50

40

55

2 km

V

V

54

Intrusive boundary

95

68
66
86

V

Jito

V

45
94

V

69

48

97
100

49

Granite44

Hinabata

Hayama Fm.

91

V

Hinabata Fm.

92

Oga
98

V

V
V

V

V

Mogamiyama

Mogamiyama Fm.

V
V

Jito Fm.

Niga

Niga Fm.
VV

B

N a riw a G ro u p

Hina Fm.

58

Acidic tuff
Paleozoic
successions

FIGURE 1. A, Location of the Nariwa Group; B, geological map of the Nariwa Group, modified from Suzuki and Asiedu (1995) and localities of the plantfossils of the group. See Table 1 for the references of each locality.

Based on the compositional similarity to the Rhaetic floras of
Sweden (Nathorst, 1878), Greenland (Harris, 1931) and
Vietnam (Zeiller, 1903), the age of the Nariwa flora has been
assigned to the Rhaetian (Oishi, 1932; 1940). On the other hand,
Kobayashi et al. (1937) argues for its Norian age based on theoccurrence of fossil bivalve Monotis ochotica (Keyserling,1848) in the uppermost horizon of the Nariwa Group. Teraoka(1959) defines the formations of the Nariwa Group as the
Mogamiyama, Hinabata and Jito Formations in ascending order.
Based on the occurrence of M. ochotica in the Jito Formation,
Teraoka (1959) assigns the Nariwa Group to the Norian. This

assignment has been widely accepted, and the Nariwa flora hasserved as a representative flora for the Norian in East Asia
(Kimura, 1987; Sun, 1993; Dobruskina, 1994; Sun et al., 1995;
Shorokhova et al., 2009; Kustatscher et al., 2018).
Suzuki and Asiedu (1995) challenges the traditional
stratigraphic scheme of the Nariwa Group and re-defines it as
the Niga, Jito, Mogamiyama, Hina, and Hinabata formations inascending order (Fig. 2). Suzuki and Asiedu (1995) further
suggests that the Nariwa flora can be stratigraphically divided
into the lower and the upper units. Notably, it has beensuggested that the floristic assemblage of the Nariwa Group

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REVISION OF THE NARIWA FLORA OF EASTERN EURASIASuzuki and Asiedu, 1995

Hinabata Fm.

-Rhaetian?

Nariwa Group

Hina Fm.

Nariwa Group

NorianLate

Triassic

NorianTeraoka, 1959

Jito Fm.
Hinabata Fm.
Mogamiyama Fm. (upper part).

Mogamiyama Fm.
Jito Fm.

Niga Fm.

Mogamiyama Fm. (lower part).
Monotis ochotica

plant fossils

FIGURE 2. Stratigraphy of the Nariwa Group after Suzuki and Asiedu (1995).

differs from those typical for the Norian (Oishi, 1932; 1940),
and re-evaluation of the Nariwa flora and its age is in order.
Here, we re-examine the stratigraphic positions for previously
collected specimens of the Nariwa flora in light of the newstratigraphic scheme (Suzuki and Asiedu, 1995). Clarifying theiroccurrences within the group reveals that the Nariwa flora is infact composed of two distinct floristic assemblages. Thesefloristic assemblages allow constraining the depositional age ofthe formations within the Nariwa Group and help paleoclimaticreconstruction of eastern Eurasia during the Late Triassic.
GEOLOGICAL SETTING
The Nariwa Group represents a shallow-marine to terrestrial
sedimentary sequence that crops out in western Okayama,
Honshu Island, Japan (Fig. 1). The Paleozoic strata, which arecomposed of the complexly folded Carboniferous to Permian
sediments, underly the Nariwa Group unconformably to the east
and west. The Nariwa Group is unconformably overlain by theLower Cretaceous Hayama Formation characterized by thereddish mudstone and conglomerates. The intrusion of the
granitic rocks is present in the northern outcrops of the Nariwa
Group, which likely occurred during the Late Cretaceous basedon the thermal metamorphism observed in the Early Cretaceous
Hayama Formation. Since the preliminary report of the Nariwa
Group by Yokoyama in 1905, its fossil assemblages andstratigraphy have been extensively studied for wealth of
paleoenvironmental and paleobotanical data for the Triassic ofEast Asia. (Yokoyama, 1905; Oishi, 1932; 1940; Oishi and
Huzioka, 1938; Huzioka, 1970; Kimura and Ohana, 2000). Here,
we provide a brief overview of the study history and previous
understanding about the stratigraphy and depositional
environment of the Nariwa Group.

History of the geological study of the Nariwa GroupThe first geological study of the Nariwa Group was conductedin Fukatawa, Nariwa, in 1888, in which fossil marine bivalve
Monotis ochotica was reported (Hiramatsu, 1992) and the
Nariwa Group was assigned to the Triassic (Otsuka, 1896).
Following the earliest study, Akagi (1925) and Kobayashi et al.
(1937) argue that the horizons yielding plant remains
corresponded to the lower part, and the horizon yielding M.
ochotica to the upper part of the group. Teraoka (1959) proceeds
the stratigraphic study of the Nariwa Group and defines theterrestrial Mogamiyama and Hinabata formations, and themarine Jito Formation in ascending order within the Group. On
the contrary, Otoh (1985) suggests that the Jito Formation infact underlies the Mogamiyama and Hinabata Formations. In
support of Otoh (1985), Suzuki and Asiedu (1995) proposes a
stratigraphic subdivision of the Nariwa Group: the Niga, Jito,
Mogamiyama, Hina, and Hinabata formations in ascendingorder (Figs. 1, 2). The present study follows the stratigraphic
definition of Suzuki and Asiedu (1995).
Stratigraphy
Otoh (1985) proposes a set of acidic tuff horizons in the Jito
Formation as the key bed for the Nariwa Group. Suzuki andAsiedu (1995) clarifies the stratigraphic relationship of the Jito
Formation and other formations within the group, and therevised stratigraphy places the marine Jito Formation overlying
the non-marine Niga Formation and underlying the non-marine
Mogamiyama, Hina and Hinabata formations (Fig. 2). The
stratigraphy of the Nariwa Group was most recently reviewed by
Suzuki (2009) which generally follows Suzuki and Asiedu

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HIROKAZU YUKAWA, GE SUN, SHIGEYUKI SUZUKI AND TAKUYA IMAITABLE 1. List of the localities of plant fossils from the Nariwa Group with references. The stratigraphic definitions follow Suzuki and Asiedu (1995).

(1995). We adopt Suzuki (2009) and herein provide a brief
description of the formations comprising the group.
The Niga Formation is composed of fluvial deposits andrepresented by fining-upward successions of coarse-grainedsandstone, alternating sandstone and mudstone, and coaly
mudstone. Each succession measures about 10 m thick. The
coaly mudstone produces well-preserved plant remains.
The Jito Formation represents as shallow to offshore marine
deposits. It includes weakly stratified, well-sorted
coarse-grained sandstone, muddy fine-grained sandstone, andturbiditic sandstone and mudstone, with intermittent horizons ofacidic tuff measuring up to 20 cm thick. The mudstone yields
abundant remains of Monotis ochotica, and plant remains areoccasionally present throughout the formation. This formation isequivalent to the Jito Formation in Teraoka (1959).
The fluvial Mogamiyama Formation is characterized byfining-upward successions of conglomerate, sandstone,
alternation of sandstone and mudstone, and coaly mudstone. The
thickness of successions is approximately 10 m. Plant remains
are present in the coaly mudstone layers. This formation isequivalent to the lower part of the Mogamiyama Formation in
Teraoka (1959).
The Hina Formation is composed of conglomerate dominatedfluvial deposits, which is characterized by fining-upward
successions of conglomerate and sandstone with a thin
mudstone horizon at the top. The formation contains
fragmentary plant fossils. It is equivalent to the upper part of theMogamiyama Formation in Teraoka (1959).
The Hinabata Formation is a mudstone-dominated floodplain
deposit. It is represented by the fining-upward successions ofcoarse-grained sandstone, alternation of sandstone andmudstone, and mudstone. The formation yields well-preserved,
numerous plant remains. It is equivalent to the Hinabata
Formation in Teraoka (1959).

Depositional environment
The channel deposits observed in the Niga Formation containmedium- to coarse-grained sandstone, exhibit a distinct lower
contact and cut into the underlying mudstone. The floodplain
deposits of the Niga Formation are composed of the overbank
mudstone horizons and a single sandstone horizon likely
representing a crevasse splay deposit (Suzuki, 2009) within ameandering fluvial system. The floodplain was probably
extensive, as indicated by the coal and coaly mudstone horizons
at the top of the fining-upward sequence occasionally present in
the formation. The depositional environment of the Jito
Formation transitioned from the shallow marine to offshore, andback to shallow marine through the time (Suzuki et al., 2013;
Masaoka and Suzuki, 2015). The Mogamiyama Formation
exhibits the deposits resembling those of the Niga Formation,
indicating that these two formations were deposited under asimilar sedimentary environment. The Mogamiyama Formationdiffers from the Niga Formation in presence of conglomerates
within the channel deposits and in lesser abundance of mudstone
horizons with an upward-fining sequence (Suzuki, 2009). The
Hina Formation exhibits conglomerates and sandstone
predominantly without associated mudstone, suggestingwell-developed meandering channels. The Hinabata Formationis composed largely of mudstone with upward-fining sequencethat sometimes lacks channel deposits (Suzuki, 2009), indicating
the predominance of the floodplain in a meandering fluvial
system.
RESULTS
Stratigraphic assignments of the fossil localities

REVISION OF THE NARIWA FLORA OF EASTERN EURASIA

We verified the previously-reported plant fossil localities andassigned them to appropriate formations following thestratigraphic definition of Suzuki and Asiedu (1995) and thegeological map in this study (Fig. 1). These are summarized inTables 1 and 2.
The plant fossil localities of each formations are as follows(Table 1): The Niga Formation, locs. 1, 10, 16, 21, 30, 33, 54,
58, 63, 64, 85, 87, 90, 92, 93, 95, 96, 98, 99; the Jito Formation,
locs. 55, 89; the Mogamiyama Formation, locs. 62, 65, 88; theHina Formation, locs. 40, 46, 47, 50; the Hinabata Formation,
locs. 44, 45, 48, 49, 66, 68, 69, 86, 91, 94, 97, 100. Localities
1–96 were reported by Oishi (1931; 1932; 1940), Oishi andYamasita (1935), Oishi and Huzioka (1938), Kon’no (1962) andHuzioka (1970). Localities 97, 98, 99 and 100 were reported by
Numano and Tsuchiya (1990; 1991), Kimura and Ohana (2000),
Kobatake (1954) and Yukawa et al. (2012), respectively.
Floristic assemblages
Based on the revised stratigraphy of the Nariwa Group(Suzuki and Asiedu, 1995) and stratigraphic assignments of thelocalities in the present study, the Nariwa flora can be
stratigraphically divided into two parts, floristic assemblages ofwhich are distinct from one to the other: the Niga Floristic
Assemblage (Niga FA) of the Niga Formation and
Hina-Hinabata Floristic Assemblage (Hina-Hinabata FA) of theHina and Hinabata Formations.
Niga FA—The Niga FA consists of 54 species belonging to
27 genera, including Equisetales (ca. 9.3%), Filicopsida (ca.
35.2%), Caytonales (ca. 3.7%), Bennettitales (ca. 5.5%),
Cycadales (ca. 7.4%), cycadophytes incertae sedis (ca. 9.3%),
Czekanowskiales (ca. 3.7%), Ginkgoales (ca. 7.4%) and
Coniferales (ca. 18.5%) (Table 3). The taxa present only in thisassemblage include Annulariopsis inopinata? Zeiller, 1903,
Pseudolobatannularia densifolia Kobatake, 1954, Clathropteris
elegans Oishi, 1940, Dictyophyllum spectabile Nathorst, 1906,
Hausmannia (Protorhipis) crenata (Nathorst) Moeller, 1902, H.
(P.) dentata Oishi, 1932, H. (P.) nariwaensis Oishi, 1930,
Thaumatopteris nipponica Oishi, 1932, T. pusilla (Nathorst)
Oishi et Yamasita, 1936, Cladophlebis nariwaensis Oishi et
Huzioka, 1938, Sagenopteris nariwaensis Huzioka, 1970, S.
nilssoniana (Brongniart) Ward, 1900, Pterophyllum serratum
Oishi et Huzioka, 1938, Taeniopteris leclerei Zeiller, 1903, T.
minensis Oishi, 1932, T. richthofeni (Schenk) Sze, 1933, T.
stenophylla Kryshtofovich, 1910, Baiera guilhaumati Zeiller,
1903, Cycadocarpidium binerivium Kimura et Ohana, 2000,
Elatocladus plana (Feistmantel) Seward, 1919, Nageiopsis
rhaetica Oishi, 1932, Podozamites concinnus Oishi et Huzioka,
1938, P. schenki Heer, 1876 and Stenorachis elegans Oishi,
1932.
Hina-Hinabata FA—The Hina-Hinabata FA consists of 78
species belonging to 35 genera, including Equisetales (ca.
9.0%), Filicopsida (ca. 33.3%), Bennettitales (ca. 11.5%),
Cycadales (ca. 12.8%), cycadophytes incertae sedis (ca. 5.2%),

93

Czekanowskiales (ca. 2.6%), Ginkgoales (ca. 12.8%),
Coniferales (ca. 12.8%) and gymnosperms incertae sedis (ca.
1.3%) (Table 3). The taxa present only in this assemblage
including Equisetites nariwensis Kon’no, 1962, Lobatannularia
nampoensis (Kawasaki) Kawasaki, 1927, Neocalamites
hoerensis (Schimper) Halle, 1908, Phyllotheca sp., Asterothecaokafujii Kimura et Ohana, 1980, Todites princeps (Presl)
Gothan, 1914, T. williamsoni (Brongniart) Seward, 1900,
Gleichenites? sp., Coniopteris? sp., Clathropteris obovata Oishi,
1932, Cladophlebidium? okayamaensis Oishi et Huzioka, 1938,
Cladophlebis bitchuensis Oishi, 1932, C. gigantea Oishi, 1932,
C. pseudodelicatula Oishi, 1932, C. (Osmundopsis?)
subplectrophora Oishi et Huzioka, 1938, C. tenue Oishi et
Huzioka, 1938, Sphenopteris gracilis Oishi, 1932, Otozamites
huzisawae Oishi et Huzioka, 1938, O. lancifolius Oishi, 1932,
O. molinianus Zigno, 1852, Pterophyllum aequale (Brongniart)
Nathorst, 1878, P. ctenoides Oishi, 1932, Yabeiella sp., Ctenis
takamiana Oishi et Huzioka, 1938, C. yabei Oishi, 1932,
Nilssonia japonica Kimura et Tsujii, 1983, N. splendens Sun,
1993, cfr. N. tenuicaulis (Phillips) Fox-Strangways, 1892,
Ptilozamites tenuis Oishi, 1932, Taeniopteris lanceolata Oishi,
1932, Baiera elegans Oishi, 1932, B. furcata Heer, 1877, B.
paucipartita Nathorst, 1886, B. taeniata Braun, 1843, Ginkgoites
digitata (Brongniart) Seward, 1919 var. huttoni Seward, 1919,
Glossophyllum? sp., Podozamites distans (Presl) Braun, 1843,
Stenorachis bitchuensis Oishi, 1932, S. (Ixostrobus?) konianus
Oishi et Huzioka, 1938, cfr. Storgaardia spectabilis Harris,
1935, Swedenborgia cryptomerioides Nathorst, 1876 and S.
major Harris, 1935.
The taxa common in both the Niga and Hina-Hinabata
floristic assemblages include: Equisetites multidentatus Oishi,
1932, Neocalamites carrerei (Zeiller) Halle, 1908, Todites
fukutomii Kimura et Ohana, 1980, T. goeppertianus (Muenster)
Krasser, 1922, Clathropteris meniscoides (Brongniart)
Brongniart, 1828, Dictyophyllum muensteri (Goeppert)
Nathorst, 1875, D. nilssoni (Brongniart) Goeppert, 1846,
Thaumatopteris elongata Oishi, 1932, Cladophlebis denticulata(Brongniart) Fontaine, 1889, C. haiburnensis (Lindley et
Hutton) Brongniart, 1849, Pterophyllum schenki (Zeiller)
Zeiller, 1903, Ctenis japonica Oishi, 1932, Nilssonia acuminata(Presl) Goeppert, 1844, N. muensteri (Presl) Schimper, 1880, N.
simplex Oishi, 1932, Taeniopteris nabaensis Oishi, 1932,
Czekanowskia rigida Heer, 1876, Phoenicopsis sp., Baiera
filiformis Oishi, 1932, Ginkgoites sibirica (Heer) Seward, 1919,
Elatocladus tenerrima (Feistmantel) Sahni, 1928, Pityophyllum
longifolium (Nathorst) Moeller, 1903 and Podozamites
lanceolatus (Lindley et Hutton) Braun, 1843.
Additionally, the Jito Formation contains six species of fivegenera. Among them, Pterophyllum angustum (Braun) Gothan,1914 is absent in the Niga and Hina-Hinabata floristic
assemblages (Table 2). The Mogamiyama Formation contains
15 species of 13 genera. The following taxa occur only in thisformation: Goeppertella varida Oishi et Huzioka, 1938, cfr.
Pterophyllum distans Morris, 1862 (in Oldham and Morris,
1862), Nillsonia brevis Brongniart, 1825, Ptilozamites nilssoni

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HIROKAZU YUKAWA, GE SUN, SHIGEYUKI SUZUKI AND TAKUYA IMAITABLE 2. List of the fossil plants from the Nariwa Group with the locality numbers.

REVISION OF THE NARIWA FLORA OF EASTERN EURASIATABLE 2 (continued).

95

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HIROKAZU YUKAWA, GE SUN, SHIGEYUKI SUZUKI AND TAKUYA IMAI

Nathorst, 1878 and Campylophyllum hoermanni? Gothan, 1914
(Table 2).
DISCUSSION
Features of the Niga and Hina-Hinabata floristic assemblages
The floristic composition of the Niga FA is characterized asfollowing:
1. Filicopsida (ca. 35.2%) is predominant. In particular,
Dipteridaceae is highly diverse (ca. 22.2%) with occurrenceof Clathropteris elegans, C. meniscioides, Dictyophyllum
muensteri, D. nilssoni, D. spectabile, Hausmannia(Protorhipis) crenata, H. (P.) dentata, H. (P.) nariwaensis,
Thaumatopteris elongata, T. kochibei (Yokoyama) Oishi etYamasita, 1936, T. nipponica, and T. pusilla.
2. Cycadophytes (Bennettitales and Cycadales) occupies ca.
22.2% in composition. Bennetitales exhibits a low diversity
with occurrence of only Pterophyllum schenki, P. serratum,and P. sp. a. On the other hand, Taeniopteris is common (ca.
9.2%) with Teniopteris leclerei, T. minensis, T. nabaensis, T.
richthofeni, and T. stenophylla.
3. Coniferales is the third-most abundant (ca. 18.5%) with
Cycadocarpidium, Elatocladus, Nageiopsis, Pityophyllum,
Podozamites, and Stenorachis present.
4. In addition to the above taxa, Caytonales (Sagenopteris), andCzekanowskiales (Czekanowskia and Phoenicopsis) arerecognized.
The floristic composition of the Hina-Hinabata FA ischaracterized as following:
1. Filicopsida (ca. 33.3%) is the most abundant; numerous
species of Cladophlebis (ca. 12.8%) are present. Dipteridaceae iscommon (ca. 6.4%) and composed of Clathropteris obovata,
C. meniscoides, Dictyophyllum muensteri, D. nilssoni, andThaumatopteris elongata, while lacking Hausmannia.
Marattiales and Osmundaceae are present in most of thelocalities (Table 2).
2. Cycadophytes is the second most abundant (ca. 29.5%).
Among them, Bennettitales is diverse (ca.11.5%) and includes
Otozamites. Similarly, Cycadales (ca.12.8%) includes various
species of Nilssonia (N. acuminata, N. densinerve (Fontaine)
Berry, 1911, N. japonica, N. muensteri, N. simplex, N.
splendens, and cfr. N. tenuicaulis) and Ctenis.
3. Ginkgoales (ca. 12.8%) contains diverse species of Baiera (B.
elegans, B. filiformis, B. furcata, B. minuta Nathorst, 1878, B.
paucipartita, B. taeniata) and Ginkgoites (G. sibirica, G.
digitata var. huttoni), with notable occurrence of Glossophyllum.
4. Czekanowskia rigida and Phoenicopsis sp. (Czekanowskiales)
are found in many localities (Table 2).
5. Cycadocarpidium is absent, while Swedenborgia and Storgaardia
are present.
The Niga and Hina-Hinabata floristic assemblages commonly
contain Filicopsida as the most abundant component of theassemblages, while Filicopisda compositions differ between
these assemblages in which Dipteridaceae is more dominant in

the Niga FA. The Niga and Hina-Hinabata floristic assemblages
are also similar in containing cycadophytes as the second mostabundant component; however, the Bennettitales are morediverse in the Hina-Hinabata FA than in the Niga FA.
Coniferales are notably abundant in the Niga FA. In contrast,
Ginkgoales are well represented and Czekanowskiales arecommon in the Hina-Hinabata FA.
Comparison and the age assignments of the Niga andHina-Hinabata floristic assemblages
The Late Triassic Niga FA is paleogeographically comparablewith the Sadgorod Floral Assemblage (late Carnian) and the
Amba Floral Assemblage (middle Norian) of the Mongugai
flora of Primorye in Russia, the Mine flora of Yamaguchi inJapan (early–middle Carnian and Carnian–Norian) and the
Tianqiaoling flora of Jilin in China (Norian). Similarly, theHina-Hinabata FA can be compared with the Daedong flora of
the Korean Peninsula and the Tonkin flora of Vietnam(Norian–Rhaetian).
Niga FA—The Sadgorod Floral Assemblage from the
Sadgorod Formation consists of 43 species belonging to 25
genera (Shorokhova et al., 2009), while the Amba Floral
Assemblage from the Amba Formation consists of 86 species
belonging to 37 genera (Shorokhova et al., 2009). The Mineflora is found in the Mine Group of Omine Region(lower–middle Carnian) (Maeda and Oyama, 2019) and of Asa
Region (Carnian–Norian) (Nishimura, 2012) of Yamaguchi, andcomposed of 40 genera and 92 species (Naito, 2000). The
Tianqiaoling flora consists of 79 species belonging to 31 genera,
which is inferred as the Norian based on the plant fossil
assemblage (Sun, 1993) (Tables 3, 4).
The Niga FA exhibits some degree of similarity with the
Sadgorod Floral Assemblage and Mine flora, in which
dipteridaceous ferns are predominant, Taeniopteris is common,and the genera Czekanowskia, Phoenicopsis andCycadocarpidium are present. However, the Niga FA differsfrom them in the lower diversity of Conifelaes (particularly
Podozamites and Cycadocarpidium). Rather, the Niga FA is
better comparable with the Amba Floral Assemblage and the
Tianqiaoling flora. They are commonly characterized by equallyabundant ferns, namely Dipteridaceae (Clathropteris andDictyophyllum) and cycadophytes (Pterophyllum, Ctenis,
Nilssonia and Taeniopteris), with Conifelaes (Podozamites,
Cycadocarpidium and Elatocladus) being next abundant.
However, the Niga FA differs from the Amba Floral
Assemblage and the Tianqiaoling flora in the lower diversity ofConifelaes (particularly Podozamites and Cycadocarpidium) andthe presence of Marattiales and the genus Thaumatopteris. The
Niga FA also contains Czekanowskia and Phoenicopsis, which
are absent in the Amba Floral Assemblage.
Hina-Hinabata FA—The Daedong flora consists of 70
species belonging to 39 genera (Kimura and Kim, 1984), which
is roughly dated as the Late Triassic (Kustatscher et al., 2018).
The Tonkin flora is composed of 38 genera and 70 species

REVISION OF THE NARIWA FLORA OF EASTERN EURASIA

97

TABLE 3. Comparison of floristic assemblages of the Mine flora, the Nariwa flora (the Niga and Hina-Hinabata floristic assemblages) and theKuruma-type flora.

(Dobruskina, 1994) (Tables 3, 4). It has been originallysuggested the Rhaetian (Zeiller, 1903). However, subsequent
analyses by Akagi (1954) raise a possibly of its Norian-Rhaetianage, and Kustatscher et al. (2018), while assigning the flora to
the Rhaetian, mentions the difficulty in distinguishing between
the Norian and the Rhaetian ages by floral composition.
Considering these disputes and the lack of definitive evidence torefute the Norian age of the flora, we concur with Akagi (1954)
and consider that the Tonkin flora is the Norian-Rhaetian (Table
4).
The Hina-Hinabata FA and the Daedong flora arecharacterized by abundance of ferns, namely the genera Todites
and Cladophlebis, and cycadophytes (Otozamites, Pterophyllum,
Ctenis and Nilssonia), in addition to the occurrences of thegenera Czekanowskia and Phoenicopsis. On the other hand, inthe Hina-Hinabata FA, Taeniopteris is less abundant andCycadocarpidium is absent. The Hina-Hinabata FA and the
Tonkin flora are characterized by abundant ferns (Clathropteris,
Dictyophyllum, Cladophlebis, Asterotheca and Sphenopteris)and cycadophytes (Otozamites and Pterophyllum), low diversityof Podozamites, and the absence of Cycadocarpidium. The
Hina-Hinabata FA differs from the Tonkin flora in containing
Czekanowskiales and higher diversity of Ginkgoales.
Age of the Niga and Hina-Hinabata floristic assemblages—
By the presence of Monotis ochotica, an index fossil of the late
Norian (Gavrilova et al., 2006), the age of the Jito Formation issuggested the late Norian. The contacts of the Jito Formation
with the underlying Niga Formation is unclear, nor is theoverlying Hinabata Formation. Therefore, the Niga FA is likely
late Norian and/or older, while the Hina-Hinabata FA late
Norian and/or younger.
Floristic comparisons of the Niga FA and the Hina-Hinabata
FA with other floras of known ages allow age-constraint ofthese assemblages. In the Niga FA, cycadophytes andFilicopsida are predominant, while in the Sadgorod Floral
Assemblage (late Carnian) and Mine flora (early–middle
Carnian in Omine Region), Coniferales and Filicopsida are themost common. Furthermore, the diversity of Equisetales islower in Niga FA than in the Mine flora. These differences infloristic compositions among the Niga FA, the Sadgorod FA andthe Mine flora suggest that the Niga FA is not assignable to theCarnian. Additionally, the Niga Formation underlies the Jito
Formation conformably (Masaoka and Suzuki, 2015),
eliminating the possibility that there is a hiatus between these
Formations. These lines of evidence indicate that the Niga FA is
at least younger than the Carnian. On the other hand,
cycadophytes and ferns, the plants that became prominent in the
Norian-Rhaetian, are present in the Niga FA as in the Amba
Floral Assemblage and the Tianqiaoling flora. Because theupper Norian Jito Formation caps the Niga Formation, it isconcluded that the Niga FA is assignable to the Norian.
The Hina-Hinabata FA is comparable to the Norian–Rhaetian
Tonkin flora (Akagi, 1954; Kustatscher et al., 2018) in the low
diversity of Podozamites and absence of Cycadocapidium,
which is in contrast to the Norian Amba Floral Assemblage andthe Tianqiaoling flora with moderately-diverse Podozamites and98

HIROKAZU YUKAWA, GE SUN, SHIGEYUKI SUZUKI AND TAKUYA IMAITABLE 4. Comparison of floristic assemblages of the Sadgorod Floral Assemblage and the Amba Floral Assemblage of the Mongugai flora, theTianqiaoling flora, the Daedong flora, and the Tonkin flora.

the common occurrence of Cycadocarpidium. Therefore, theHina-Hinabata FA is at least assignable to the late Norian to
Rhaetian. Notably, the Hina-Hinabata FA exhibits a partial
similarity with the Early Jurassic Kuruma-type flora. The
Kuruma-type flora in the Kuruma Group, central Honshu Island
is composed of 32 genera and 74 species of fossil plants(Kimura et al., 1988), and is assigned to Pliensbachian–Toarcian(Takeuchi et al., 2017). Both the Kuruma-type flora and theHina-Hinabata FA are characterized by abundant cycadophytes
in which Otozamites is present and Nilssonia is diverse, and by
the occurrence of Czekanowskiales (Czekanowskia andPhoenicopsis) and Coniferales (Storgaardia and Swedenborgia),
leaving a possibility that the age of the Hina-Hinabata FA isyounger than the Rhaetian.
Paleoclimate inferred from the Niga and Hina-Hinabata
floristic assemblages
The Niga Formation that yields the Niga FA and Hina andHinabata Formations that yield the Hina-Hinabata FA were
presumably deposited in meandering fluvial systems. Based on
the suggested depositional environments of the Niga Formation,
the Niga FA likely represents the fossil plants that inhabitednear the channels and floodplain environment. Similarly, theHina-Hinabata FA is constituted by the fossil plants that
inhabited near the channels and floodplain environment during

the time of the Hina and Hinabata Formations, respectively.
Therefore, the habitats of these floristic assemblages were
probably not largely different.
Dipteridaceae is more abundant in the Niga FA than in the
Hina-Hinabata FA. On the other hand, the diversity ofcycadophytes, particularly Bennettitales, increases from the
Niga FA to the Hina-Hinabata FA. This is also true for
Ginkgoales. Additionally, Czekanowskia rigida andPhoenicopsis sp. (Czekanowskiales) are less common in the
Niga FA than in the Hina-Hinabata FA. The predominance ofthermophilous and hygrophilous Dipteridaceae andcycadophytes (Sun, 1993; Volynets and Shorokhova, 2007) inboth the Niga and Hina-Hinabata floristic assemblages is
consistent with a warm and humid, tropical to subtropicalsetting, while the abundance of the Czekanowskiales in the
Hina-Hinabata FA indicates warm and humid, warm-temperate
climate (Vachrameev, 1991). This difference in the floristic
compositions between the Norian Niga FA and theNorian-Rhaetian? Hina-Hinabata FA indicates that the
paleoclimate of eastern Eurasia generally transitioned fromsubtropical to warm-temperate. Additionally, mixed presence ofthermophilous Dipteridaceae and cycadophytes andnon-thermophilous Czekanowskiales in the Hina-Hinabata FA
may suggest more pronounced seasonality during the latest
Triassic.

REVISION OF THE NARIWA FLORA OF EASTERN EURASIACONCLUSIONS
1. Based on the revised stratigraphic relationship of theformations within the Nariwa Group (Suzuki and Asiedu,
1995) and comparison with coeval floras in eastern Eurasia,
the Nariwa flora can be assigned to two distinct floristic
assemblages, the Norian Niga FA and the Norian-Rhaetian?
Hina-Hinabata FA.
2. The Niga FA is characterized by abundance of Dipteridaceous
ferns (Clathropteris, Dictyophyllum, Thaumatopteris andHausmannia) and cycadophytes (Pterophyllum and Taeniopteris),and the occurrence of genera Sagenopteris, Cycadocarpidium
and Nageiopsis. This floristic composition of the Niga FA
likely represents a warm and humid subtropical climaticcondition during the Norian.
3. The Hina-Hinabata FA is characterized by abundant ferns(Cladophlebis, Todites, Marattia, Asterotheca and Clathropteris),
cycadophytes (Otozamites, Pterophyllum, Ctenis, Nilssonia)
and Czekanowskiales (Czekanowskia and Phoenicopsis), andthe occurrence with genera Glossophyllum, Storgaardia andSwedenborgia. This floristic composition of the Hina-Hinabata
FA reflects a shift to a warm-temperate condition with more
pronounced seasonality toward the latest Triassic in eastern
Eurasia.
ACKNOWLEDGMENTS
We thank Shintaro Tsuchiya (Kounoshimasoto Junior HighSchool) for kindly permitting us to study his specimens. We also
thank Tetsuo Yamamura (Itabashi Science and Education Hall)for preparing part of the figures and Sota Niiyama (Kanazawa
University) for sharing literatures. David Dilcher (Indiana
University) provided suggestions to our study. We are grateful
to our reviewers, Yondong Wang (Nanjing Institute of Geology
and Paleontology) and Atsushi Yabe (National Museum of
Nature and Science), whose comments greatly improved themanuscript.
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flora fossile che li distingue. Rivista Periodica dei Lavoridella Imperiale Reale Accademia di Scienze, Lettere ed Artidi Padova 1: 1–14. **********
Zeiller, R. 1903. Études des Gîtes Minéraux de la France,
Colonies Françaises, Flore Fossile des Gîtes de Charbon du
Tonkin: Texte. Imprimerie Nationale, Paris, 328 pp. *********: in Japanese with English abstract**: in Japanese***: English translation from the original title written in Japanese****: in Russian and English*****: in Chinese and English******: in Russian*******: in Swedish******** : in French********* : in Germany**********: in Italian

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HIROKAZU YUKAWA, GE SUN, SHIGEYUKI SUZUKI AND TAKUYA IMAI＜Asa
Fukatawa
Hina Formation
Hinabata Formation

厚狭
深乢
日名層
日名畑層

地名・地層名

Jito Formation
Kuruma Group
Mine Group
Mogamiyama Formation

＞
地頭層
来馬層群
美祢層群
最上山層

Nariwa Group
Niga Formation
Omine

成羽層群
仁賀層
大嶺

APPENDIX. List of the fossil plants from the Nariwa Group based on published papers: Huzioka (1970), Kimura and Tsujii (1980), Kimura and Ohana(1980), Kimura and Tsujii (1983), Numano and Tsuchiya (1990), Numano and Tsuchiya (1991), Kimura and Ohana (2000) and Yukawa et al. (2012). The
classification of the plant taxa in the table follows Nishida (2017). While Huzioka (1970) reports three types of Pterophyllum sp., we follow Oishi and
Huzioka (1938) and recognize two types of Pterophyllum sp.

REVISION OF THE NARIWA FLORA OF EASTERN EURASIAAPPENDIX (continued).

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Nishida, H. 2017. The Natural History of Fossil Plants.
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Tama Shitagiri, Takahashi City, Okayama Prefecture,
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