Late Early Cretaceous (Albian) Sasayama Flora from the Sasayama Group in Hyogo Prefecture, Japan

Abstract

Plant fossils are reported on from the Lower Cretaceous Upper Formation of the Sasayama Group, including the new species Otozamites toshioensoi sp. nov. The Sasayama Flora proposed here is characterized by an abundance of microphyllous conifers such as Brachyphyllum spp. and Pseudofrenelopsis sp., as well as by the rare occurrence of pteridophytes. These floristic components suggest that an arid climate prevailed in the land of the Sasayama Group and that the Ryoseki-type flora, which generally represents the Outer Zone of Japan during the Late Jurassic to Early Cretaceous, flourished in the Inner Zone of Japan during the late Early Cretaceous. Albian or Cenomanian ages have previously been proposed for the Upper Formation of the Sasayama Group by radiometric datings, but an Albian age is preferred here in regards to the extremely rare occurrence of possible angiosperms.

Full text

Toshihiro Yamada et al.112
Paleontological Research, vol. 22, no. 2, pp. 112–128, April 1, 2018
© by the Palaeontological Society of Japan
doi:10.2517/2017PR014
Late Early Cretaceous (Albian) Sasayama Flora from the
Sasayama Group in Hyogo Prefecture, Japan
TOSHIHIRO YAMADA1, JULIEN LEGRAND2

HARUFUMI NISHIDA2,3
1Department of Biological Sciences, Faculty of Natural Systems, Institute of Science and Engineering, Kanazawa University, Kakuma-machi,
Kanazawa 920-1192, Japan (e-mail: ptilo@mb.infoweb.ne.jp)
2Department of Biological Sciences, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
3Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received April 17, 2017; Revised manuscript accepted July 27, 2017
Abstract. Plant fossils are reported on from the Lower Cretaceous Upper Formation of the Sasayama Group,
including the new species Otozamites toshioensoi sp. nov. The Sasayama Flora proposed here is characterized by
an abundance of microphyllous conifers such as Brachyphyllum spp. and Pseudofrenelopsis sp., as well as by the
rare occurrence of pteridophytes. These oristic components suggest that an arid climate prevailed in the land
of the Sasayama Group and that the Ryoseki-type ora, which generally represents the Outer Zone of Japan
during the Late Jurassic to Early Cretaceous, ourished in the Inner Zone of Japan during the late Early Cre-
taceous. Albian or Cenomanian ages have previously been proposed for the Upper Formation of the Sasayama
Group by radiometric datings, but an Albian age is preferred here in regards to the extremely rare occurrence
of possible angiosperms.
Key words: Albian, Early Cretaceous ora, Paleoclimate, Paleophytogeography, Sasayama Flora, Sasayama
Group
Introduction

have been recognized as consisting of two major types,
the Ryoseki- and Tetori-types (Kimura, 1987, 1988, 2000;

found in the Outer Zone of present-day Japan, the geo-


as indicated by the occurrence of various microphyllous
conifers, cycadophytes and pteridophytes (Kimura, 1987,
1988, 2000; Ohana and Kimura, 1995). Similar coeval

 
to those of the Eurosinian paleophytogeographic region
      
have been reported from the Inner Zone facing the Sea
of Japan. Their components, consisting of pteridophytes,
macrophyllous cycadophytes and ginkgoaleans, suggest
a warm temperate climate with moderate precipitation
(Kimura, 1987, 1988, 2000; Ohana and Kimura, 1995).
-
parable with that of the Siberian paleophytogeographic
region (Vakhrameev, 1991).
  
    -
ing the Late Jurassic to Early Cretaceous epochs (Kimura,
1987, 1988, 2000; Ohana and Kimura, 1995). How-
ever, this inference was based only on paleobotanical
records from the Tetori Group, which is distributed in
the Hokuriku and Hida regions of central Japan (Oishi,
1940; Kimura, 1958, 1975; Kimura and Sekido, 1976a, b;
Kimura et al.
-
tions may have changed with time even inside the Tetori
Group (Yabe et al., 2003; Yamada and Uemura, 2008;
Yamada, 2009a).
Enso (1958) reported some plant fossils from the Lower
Cretaceous Sasayama Group distributed in the Sasayama
basin of Hyogo Prefecture, Inner Zone of Japan. This pio-


results were not incorporated into later paleophytogeo-
graphic works. Specimens reported by Enso (1958) have
unfortunately been lost; therefore, we decided to revisit
his fossil localities in order to collect new specimens.
In this paper, we newly report on plant fossils from the


Early Cretaceous Sasayama Flora 113

 
-
ristic changes that occurred at the eastern margin of the
Eurasian continent.
Geological settings and fossil localities
The Lower Cretaceous Sasayama Group is distributed
in the Sasayama basin of Hyogo Prefecture, in the Inner
Zone of southwest Japan (Yoshikawa, 1993; Figure 1).
The Sasayama Group is lithologically divided into the
Lower and Upper formations (Yoshikawa, 1993). The
Lower Formation mainly consists of conglomerate, sand-
   -
calated between them. The Upper Formation is composed
     
mudstone. Clam shrimps (Enso and Nakazawa, 1956;
Hayashi et al., 2010) and vertebrates (Kusuhashi et al.,
2013; Saegusa and Ikeda, 2014) have been reported from
the Sasayama Group.
We collected plant fossils from the Upper Formation
of the Sasayama Group at Ojiyama and Sasayama Cas-
tle, Sasayama City, Hyogo Prefecture, from where Enso
(1958) previously reported a few plant fossils (Figure 1;
see details of the Ojiyama site given by Matsuura and
Yoshikawa (1992) and Hayashi et al. (2010)). All spec-
imens are stored in the Museum of Nature and Human
Activities, Hyogo and their specimen numbers are indi-
cated by those starting with “D1”. Sasayama Castle was
constructed on a natural mound and has several outcrops
in the moat. Sasayama City excavated a moat at the
southeast corner of the castle in 2012 for improvement of
drainage. We were able to collect plant fossils there from
abandoned slabs with the permission of Sasayama City.
Several fossils were also collected from the Sasayama
Castle site, but only one specimen (D1-32322) was taxo-

Systematic paleobotany
Class Polypodiopsida

leptosporangiate ferns, but the species could not be iden-
 
      Cladophlebis
based on their ultimate segments having pecopterid-type
venations, i.e., those consisting of a distinct midvein and
dichotomously bifurcating lateral veins (Seward, 1910).
By contrast, the other three specimens were assigned to
genus Sphenopteris in that the ultimate segments have a
single vein or sympodially branching veins (Brongniart,
1822).
Family uncertain
Genus Cladophlebis Brongniart, 1849
Type species.—Cladophlebis albertsii (Dunker)
Brongniart, 1849.
Cladophlebis sp.
Figure 2A, B
Material examined.—D1-32301, 32411 (Figure 2A,
B).
Description.—Pinna fragment 10 mm long by 8.9 mm
wide. Pinnules elongate oblong, 4.0 to 6.6 mm long by
1.4 to 1.8 mm wide, alternately arising at ca. 40 degrees
to pinna rachis. Pinnule margins entire. Pinnule bases
slightly expanded on acroscopic side but decurrent on
basiscopic side. Pinnule apices obtuse. Midvein distinct.
Sixteen lateral veins alternately derived from midvein.
Each lateral vein usually bifurcates once or twice.
Genus Sphenopteris Sternberg, 1825
Type species.—Sphenopteris elegans (Brongniart)
Sternberg, 1825.
Sphenopteris sp. A
Figure 2C, D
Material examined.—D1-32279.
Description.—Frond at least bipinnate. Rachis 0.4 mm
wide. Pinnae lanceolate, 5.7 mm long, 0.5 mm wide at
the base, 1.7 mm at the widest part, departing at 25 to
30 degrees with ca. 1.7 mm intervals. Pinnules wedge-
shaped, weakly lobed, 1.8 to 2.5 mm long by 0.3 to
0.6 mm wide, arising at 10 to 20 degrees to pinna rachis.
Single vein entering pinnules. Vein branching twice at
most.
Remarks.—We could not estimate the exact thickness
of the lamina, but it might be thinner than the veins, judg-

Sphenopteris sp. B
Figure 2E, F
Material examined.—D1-32281, 32410 (Figure 2E, F).
Description.—Frond at least bipinnate. Rachis 0.3 mm
broad, alternately dividing pinnae at 15 to 18 degrees.
Pinnae wedge-shaped, 2.3 to 3.7 mm long, 0.4 mm wide
at the base, 1.1 to 1.5 mm at the broadest part, crowdedly
bearing two to three pinnules. Pinnules wedge-shaped,
1.5 to 2.0 mm long by 0.2 to 0.4 mm wide at the broadest

Toshihiro Yamada et al.114
part, with toothed or slightly lobed apical margins. Single
vein entering each pinna. Vein sympodially branching to
form six veins at maximum.
Remarks.—We probably have two Sphenopteris spe-

pinnule densities.
Class Pteridospermopsida
Order and Family uncertain
Genus Pachypteris Brongniart, 1828a emend. Harris,
1964
Type species.—Pachypteris lanceolata Brongniart,
1828a.
Pachypteris? sp.
Figure 2G, H
Material examined.—D1-32277 (Figure 2G, H), 32413.
Description.
than 23 mm long by 9 mm wide. Pinnae lanceolate, crowd-
edly arising at 10 to 15 degrees. Basal pinnae 18 mm long
by 3 to 6 mm wide. Apical three pinnae 10 to 13 mm long
by 1.8 to 2.1 mm wide. Pinna rachis 0.5 to 0.7 mm wide,
with distinct median groove. Lateral pinnules lanceolate,
alternately diverging at 15 to 35 degrees with 1.5 to 1.8
mm intervals. Terminal pinnules lanceolate, sometimes
lobed, larger than lateral pinnules. Pinnule apices acute.
Basiscopic bases of lateral pinnule decurrent. Acroscopic
base of lateral pinnule almost straight. At least one vein
entering pinnules. Vein concealed, ending on margin. No
lateral vein visible in lateral pinnules, but a few in termi-
nal pinnules.
Remarks.—Laminae of the obtained specimens are
      -
tion, rachises are completely concealed by continuous
laminae and pinnules are not isolated. These features are
rarely found in pteridophytes. Therefore, these specimens
likely represent pteridosperm leaves, although the exact

epidermal characters.
Our specimens resemble both Stenopteris and Pachyp-
Figure 1. Geological map of the Sasayama basin showing the distribution of the Lower and Upper formations of the Sasayama Group
   

Early Cretaceous Sasayama Flora 115
teris       
pinnules (Harris, 1964). Especially, S. cyclostoma Saiki,
Kimura et Horiuchi (1991) from the Lower Cretaceous
Choshi Group of Japan is closely similar to our speci-
mens in acute pinnule apices. However, our specimens
are distinguished from Stenopteris species in that Stenop-
teris pinnules lack lateral veins (Harris, 1964; Saiki et al.,
1991).
Our specimens are similar to some leaves of Pachyp-
teris lanceolata Brongniart reported from the Lower
Jurassic of Yorkshire (Brongniart, 1828; Harris, 1964)
and P. indica (Oldham et Morris) Bose et Roy from the
Lower Cretaceous of Kachchh, India (Bose and Roy,
1968; Bose and Banerji, 1984) in the lanceolate pinnules
with acute apex. Leaves of both species exhibit consider-
able variations in pinnule shape, which overlap between
P. lan-
ceolata from P. indica without leaf epidermal characters
(Bose and Roy, 1968; Bose and Banerji, 1984).
There are only two Pachypteris records from Japan:
Pachypteris? sp. from the Aptian Ryoseki Formation
(Oishi, 1940) and Pachypteris sp. from the Lower Cre-
taceous Kiyosue Formation (Kimura and Ohana, 1987c).
However, both specimens are too fragmental to compare
Figure 2. Cladophlebis, Sphenopteris, Pachypteris? and Otozamites from the Upper Formation of the Sasayama Group. A, B, Clado-
phlebis sp.; A, D1-32411; B, line drawing of A; C, D, Sphenopteris sp. A; C, D1-32279; D, line drawing of C; E, F, Sphenopteris sp. B;
E, D1-32410; F, line drawing of E; G, H, Pachypteris? sp.; G, D1-32277, H, line drawing of G; I, J, Otozamites toshioensoi T. Yamada,
Legrand et H. Nishida sp. nov.; I, D1-32303 (holotype); J, line drawing of I. Shaded part indicates rim-like thickening. Scale bars = 2.5 mm
(A–D), 1 mm (E, F), 5 mm (G–J).

Toshihiro Yamada et al.116
with our specimens.
Class Bennettitopsida
Order Bennettitales Engler, 1892
Family uncertain
Genus Otozamites Braun, 1843 emend. Watson et
Sincock, 1992
Type species.—Otozamites (Zamites) brevifolius Braun,
1843.
Otozamites toshioensoi T. Yamada, Legrand et H.
Nishida sp. nov.
Figures 2I, J, 3A–C
Otozamites sp. Enso, 1958, p. 117, middle photo of the top row in a
plate.
Holotype.—D1-32303 (Figure 2I, J).
Paratype.—D1-32328 (Figure 3A).
Other material examined.—D1-2292, 32297, 32300
(Figure 3C), 32306, 32308, 32309, 32315, 32326 (Figure
3B).
Etymology.—Commemorating the late Toshio Enso

Diagnosis.    
alternately at 30 to 60 degrees to rachis. Rachis almost
      

      
       
      
-
ment. Veins bifurcate more than once. Vein density ca. 3
to 4 per mm at maximum.
Description.—Leaf compound, linear, at least 36 mm
        
at 30 to 60 degrees. Rachis 1 to 2 mm wide, almost com-
      

falcate with concave acroscopic and almost straight
basiscopic margins, 5.8 to 10 mm long, 2.7 to 5 mm wide
at the base, 1.25 to 3.75 mm wide at the middle. Ratio
      -
    
expanded to form prominent auricle crossing rachis.
-
tinctly thickened to make ca. 0.3 mm-wide rim. Six to

center of lamina bifurcate several times, but those on the
marginal area bifurcate only once. Vein density ca. 3 to 4
per mm. Cuticle not preserved.
Comparison     
        
obtained (e.g. Figure 3B, C). This feature suggests that
our specimens are “cycadophyte” leaves because pinna
abscissions are rare in pteridophytes (Yamada, 2009b). In

quite unique among “cycadophyte”, thus our specimens
Otozamites based on this character.
Otozamites toshioensoi sp. nov is best characterized

rounded to obtuse apex, and the prominent auricle on the
-
  
characters were consistently observed in all specimens of
O. toshioensoi examined, although O. toshioensoi shows

2I, J, 3A).
Otozamites toshioensoi shares many similarities in
leaf shape with O. titaniae Watson et Sincock (1992)
reported from the late Berriasian Ashdown Beds Forma-
tion of southern England and O. kerae Ohana et Kimura
(1991) reported from the Coniacian to Santonian Yezo
Group of Japan (Table 1). All three species have falcate
       -
sity of ca. 4 per mm and a well developed auricle on the
   
of the rachis. However, O. titaniae and O. kerae do not
have any obvious thickenings along the leaf margin. Oto-
zamites kerae  O. toshioensoi in its

The following species have a thickened margin at least
  Otozamites beani (Lindley et
Hutton) Brongniart, O. simpsoni Harris and O. tenuatus
(Leckenby) Phillips from the Lower Jurassic of Yorkshire
(Harris, 1964), O. linearis Halle and O. latior Saporta from
the Lower Jurassic of Antarctica (Rees and Cleal, 2004),
O. tenellus Zhou from the Lower Jurassic of Guangxi,
China (Zhou, 1983b), and O. walkamotaensis Bose et
Zeba-Bano from the Lower Cretaceous of Kachchh, India
(Bose and Zeba-Bano, 1981; Bose and Banerji, 1984).
Otozamites beani is similar to O. toshioensoi in the den-
 O. beani  
O. toshioensoi. In the other species, there are clear over-
 
the rachis.
Some Asian Otozamites species, Otozamites anglica
(Seward) Harris from the Lower Cretaceous Jianshangou
Formation of China (Sun et al., 2001) and O. kachchhensis
Bose et Banerji from the Lower Cretaceous of Kachchh,
India (Bose and Banerji, 1984), are somewhat similar to
O. toshioensoi
but the auricle of these species is less developed than that
of O. toshioensoi (Table 1). Marginal thickenings are not


Early Cretaceous Sasayama Flora 117
Class Coniferopsida
Order Pinales Gorozhankin, 1904
Family Cheirolepidiaceae Takhtajan, 1963
Genus Pseudofrenelopsis Nathorst, 1893 emend.
Watson, 1977
Type species.—Pseudofrenelopsis varians (Fontaine)
Watson, 1977.
Figure 3. Otozamites, Pseudofrenelopsis, Classostrobus and Brachyphyllum from the Upper Formation of the Sasayama Group. A–C,
Otozamites toshioensoi T. Yamada, Legrand et H. Nishida sp. nov.; A, D1-32328 (paratype); B, D1-32326; C, D1-32300; D–J, Pseudofre-
nelopsis sp.; D, D1-32270; E, close-up of D; F, D1-32340; G, D1-32396, showing leaves by arrowheads; H, D1-32369; I, D1-32341; J,
D1-32347; K, L, Classostrobus sp.; K, D1-32303; L, close-up of K; M–O, Brachyphyllum sp. A; M, D1-32304; N, line drawing of M; O,
close-up of a leaf marked with arrowhead in M (epoxy replica). Scale bars = 5 mm (A, D, F–H), 2.5 mm (B, C, I, J), 1 mm (E, L), 5 mm
(K, M, N), 0.5 mm (O).

Toshihiro Yamada et al.118
Table 1. Comparison of leaflet characters in selected Otozamites species.
Characters/Taxa O. toshioensoi O. anglica2) O. beani O. kachchhensis O. kerae O. latior
Length (mm) 5.8–10 17 30 3.0–5.0 12 6.5–23
Width (mm)1) 1.25–3.75 3.0–4.0 15 2.0–3.0 4.5 1.0–3.0
Length/Width 2.7–4.6 4.3–5.6 2 1.5–1.7 2.6 2.2–10
Shape slightly falcate slightly falcate round to
ovate-rhomboid
slightly falcate slightly falcate linear to triangular
Acroscopic auricle prominent moderate prominent moderate prominent weak
Basiscopic base contracted contracted contracted slightly contracted contracted straight
Apex broadly rounded
to obtuse
obtuse subacute to round round to obtuse round to obtuse acute to obtuse
Vein density per mm 3–4 ? 4–5 4–6 4 3–7
Thickening along
margin
present absent present absent absent rarely found
Angle to rachis
(degrees)
30–60 70–80 70 70–80 40 40–90
Overlap between

absent absent absent present present present
Distribution Japan China England India Japan Antarctica, Australia
Stratigraphic range Aptian to Albian Lower Cretaceous Bajocian Lower Cretaceous Coniacian to
Santonian
Lower Jurassic
Reference this study Sun et al. (2001) Harris (1969) Bose and Banerji
(1984)
Ohana and Kimura
(1991)
Rees and Cleal
(2004)
Characters/Taxa O. linearis O. simpsoni O. tenellus O. tenuatus O. titaniae O. walkamotaensis
Length (mm) 5.5–24 5–10 5 5 13 3–10
Width (mm)1) 1.5–3.5 3–4 4–4.5 5 5 2.5–6.0
Length/Width 2.0–10 1.7–2.5 1.1–1.25 1 2.6 1.2–1.7
Shape linear to triangular triangular widely ovate nearly circular slightly falcate round to triangular
Acroscopic auricle moderate weak moderate moderate prominent weak
Basiscopic base straight to slightly
contracted
contracted contracted contracted contracted contracted
Apex acute to obtuse round to subacute obtusely pointed obtusely pointed broadly-round subacute to obtuse
Vein density per mm 4–9 4–5 2–3 5–7 4–5 2–3
Thickening along
margin
rarely found present present present absent present
Angle to rachis
(degrees)
40–90 70–85 45 45 50 40–80
Overlap between

present present present present absent present
Distribution Antarctica, Australia England China England England India
Stratigraphic range Lower Jurassic Bajocian Lower Jurassic Bajocian late Berriasian Lower Cretaceous
Reference Rees and Cleal
(2004)
Harris (1969) Zhou (1983b) Harris (1969) Watson and Sincock
(1992)
Bose and Banerji
(1984)

2) Note that this description is not based on the type specimen.

Early Cretaceous Sasayama Flora 119
Pseudofrenelopsis sp.
Figure 3D–J
Equisetites? sp. Enso, 1958, p. 117, left photo of the top row in a plate.
Material examined.—D1-32270 (Figure 3D, E), 32303
(Figure 3K), 32340 (Figure 3F), 32341 (Figure 3I), 32347
(Figure 3J), 32369 (Figure 3H), 32374, 32375, 32377–
32379, 32393, 32394, 32396 (Figure 3G), 32401–32403.
Description.—Shoots sparsely branched at about 60
degrees, bearing one leaf per node in a spiral (Figure
3D–J). Internodes 1.6 to 2.5 mm long by 2.2 to 2.5 mm
wide in young shoots (Figure 3H, J), but usually 5.2 to
7.8 mm long by 3.9 to 5.3 mm wide (Figure 3D, F, G, I).
Surface of internodes and leaves striated alternately with
longitudinal ridges and grooves of about 80 μm width
(Figure 3E, I). Free part of leaf triangular, appressed and
1.7 to 2.0 mm long. Leaf tip acute (Figure 3D, F, H, J).
Remarks.—Specimens compressed in various direc-
tions were examined, but there were no axes in which a
node with more than two leaves is present. This character
distinguishes Pseudofrenelopsis from Frenelopsis, which
has two or three leaves per node (Watson, 1977).
Our specimens are identical to Pseudofrenelopsis cf.
parceramosa (Fontain) Watson reported from the Lower
Cretaceous Youngdong Group in having leaves less than
2 mm long and in striated internodes of the shoots (Oishi,
1940; Kim et al., 2012). Associated microsporangiate
cones, which are still putative for our specimens, are sim-
ilar to each other in the Youngdong and Sasayama speci-
mens (Kim et al., 2012; see below). Kim et al. (2012)
referred their specimens to P. parceramosa because epi-
dermal characters were not available. In gross morphol-
ogy, the Sasayama and Youngdong specimens also share
shoot characters with P. dalatzensis (Chow et Tsao) Cao
et Zhou (Zhou, 1995; Yang, 2008; Yang et al., 2009) from
the Aptian to Albian Dalazi Formation of northeast China,
P. heishanensis Zhou, 1995 from the Lower Cretaceous
(probably lower than Aptian) Lingxiang Group of Central
China, and P. guixiensis Bainian Sun et Dai (Sun et al.,
2011) from the Lower Cretaceous Zhoujiadian Formation
of southern China. Unfortunately, we could not compare
our specimens with these four species, which are diag-
nosed only by epidermal characters. In addition, P. parce-
ramosa probably consists of two natural species, because
two distinct Classostrobus species are reported as being
attached to this vegetative morphospecies (Axsmith et al.
2004). Thus, we tentatively assigned the Sasayama and
Youngdong specimens to an uncertain species of Pseudo-
frenelopsis. However, our specimens could be designated
     P. dalatzensis, because the
attached male cones of P. dalatzensis are elongate ovoid
in shape (Yang, 2008; see below for the male cone char-
acters of the Sasayama specimen).
Pseudofrenelopis glabra Saiki, 1999 from the Albian of
Hokkaido is the sole species of the genus ever reported in
Japan. It has smaller leaves than those of our specimens.
Genus Classostrobus Alvin, Spicer et Watson, 1978
Type species.—Classostrobus comptonensis Alvin,
Watson et Spicer, 1978.
Classostrobus sp.
Figure 3K, L
Material examined.—D1-32303.
Description.—Putative male cone born laterally to the
shoot of Pseudofrenelopsis sp. (Figure 3K) described
above. Cone spherical, about 10 mm in diameter. Sporo-
phylls arranged in a spiral, rhomboid and 1.1 to 1.6 mm
long by 1.0 to 1.4 mm wide. Sporophyll apices acute. No
in situ pollen obtained.
Remarks.—Although no in situ pollen was obtained,
we assigned the male cone to the genus Classostrobus
based on the biological connection to Pseudofrenelopsis-
type shoots. Three Classostrobus species among six spe-
cies described to date have a spherical cone shape (Yang,
2008) similar to our specimen: C. arkansensis Axsmith,
Krings et Waselkov (2004) from the Albian Potomac
Group of U.S.A., C. cathayanus Zhou (1983a) from the
Lower Cretaceous of Central China and C. comptonensis
Alvin, Spicer et Watson (1978) from the English Wealden.
Our specimen closely resembles Classostrobus comp-
tonensis described from the Lower Cretaceous Young-
dong Group of Korea (Kim et al., 2012) in its gross
morphology and cone size. Kim et al. (2012) assigned
their specimens to C. comptonensis  
distinguish C. comptonensis from C. arkansensis without
epidermal characters (Axsmith et al., 2004).
Family uncertain
Genus Brachyphyllum Brongniart, 1828a ex Lindley et
Hutton, 1836 emend. Harris, 1979
Type species.—Brachyphyllum marillare Brongniart ex
Lindley et Hutton, 1836.
 
distinguish leaf cushions exactly from the rests of leaves.
However, free parts of leaves should be limited, judging
from the appressed nature of leaves. Therefore, the fol-
lowing specimens are assigned to uncertain species of
genus Brachyphyllum after the emended diagnosis of the
genus by Harris (1979).
Brachyphyllum sp. A

Toshihiro Yamada et al.120
Figure 3M–O
Material examined.—D1-32288, 32296, 32304 (Figure
3M–O), 32321.
Description.—Shoot fragment about 1.5 mm wide
(Figure 3M, N). Leaves spirally arranged in 2 +3 phyllo-
taxy (inference based on Watson et al., 1987), rhomboidal
to lanceolate, 1.6 to 2.0 mm long by 1.0 to 1.2 mm wide
and appressed to axis. Leaf apices acute (Figure 3M, N).
Fine striations 10 to 15μm wide visible on the abaxial leaf
surface (Figure 3O).
Remarks.—Brachyphyllum     
Brachyphyllum sp. B in its phyllotaxis, but the phyllotaxis
of Brachyphyllum species may change within a shoot sys-
tem depending on the stem diameter (Harris, 1979). The
two types recognized in this study might belong to the
same species, but we tentatively separated them based on
their phyllotaxis.
Brachyphyllum sp. B
Figure 4A–C
Material examined.—D1-32278 (Figure 4A), 32311,
32316, 32322 (Figure 4B, C), 32371, 32400.
Description.—Shoot fragment about 5.8 to 6.9 mm
wide (Figure 4A–C). Leaves arranged probably in 3 +5
phyllotaxy (inference based on Watson et al., 1987),
adnate, triangular, 5.0 to 5.7 mm long by 4.6 to 5.6 mm
wide with length-to-width ratio of 1.0 to 1.2, broadest in
lower third of leaf length. Leaf apices obtuse. Leaf base
not contracted or slightly contracted. Leaf surface striated
with ridges and grooves of ca.160 μm wide (Figure 4C).
Remarks.—These specimens are comparable to
Brachyphyllum obesum Heer (1881) which was described
from the Lower Cretaceous of Portugal, based on leaves
with triangular shape and distinct striations on the abaxial
surface. However, without observations on the branching
patterns of shoots, B. obesum could not be discriminated
from B. rhombicum Wu (1999) from the Lower Creta-
ceous Yixian Formation of northeast China, B. squammo-
sum (Velenovský) Palibin from the Upper Cretaceous of
   B. vulgare (Stopes
et 
Group of Japan. In gross morphology, our specimens also
resemble both B. macrocarpum Newberry (Lesquereux,
    
the Upper Cretaceous Dakota Group of U.S.A. and B.
winklerprinsii van Waveren, van Konijnenburg-van Cit-
tert, van der Burgh et Dilcher (2002).
Yabe and Kubota (2004) described “Brachyphyllum
obesum” from the late Barremian (to early Aptian?)
Kitadani Formation of Japan. Their “B. obesum” could be
    
not conclude that this is so without epidermal features.
 
still remains open.
Genus Cupressinocladus Seward, 1919
Type species.—Cupressinocladus massiliensis (Saporta)
Seward, 1919.
Decussate phyllotaxis is often found in species of the
Araucariaceae, Cheirolepidiaceae, Cupressaceae, and
Podocarpaceae (e.g. Watson, 1977), but detailed classi-
      -
acters. The following specimens lack these features, thus
are assigned to uncertain species of the morphogenus
Cupressinocladus which is diagnosed by having a decus-
sate phyllotaxis (Seward, 1919).
Cupressinocladus sp. A
Figure 4D, E
Material examined.—D1-32318.
Description.—Shoots up to 1.6 mm wide bearing
leaves decussately. Lateral shoots alternately branched at
about 30 degrees, closely spaced in apical region. Leaves
entirely attached to the axis at the base. In expanded
shoots, leaves lanceolate, 2.0 to 2.8 mm long by 0.7 to 1.0
mm wide, with a length-to-width ratio of ca. 2.8. Young
leaves rhomboidal, 0.5 to 0.7 mm long by 0.3 to 0.6 mm
wide, with a length-to-width ratio of 1.2 to 1.7. Leaf api-
ces acute to obtusely pointed and free from the axis.
Remarks.—Cupressinocladus sp. A is distinguished
from Cupressinocladus sp. B by having crowdedly
branched shoots in the proximal shoot system.
Cupressinocladus sp. B
Figure 4F–H
Material examined.—D1-32271 (Figure 4F, G), 32245
(Figure 4H).
Description.—Shoots up to 3.0 mm wide bearing
leaves decussately (Figure 4F–H). Lateral shoots alter-
nately branched at 16 to 21 degrees (Figure 4H). Leaves
entirely attached to the axis at the base, lanceolate, 1.0
to 5.1 mm long by 0.6 to 0.9 mm wide, with a length-
to-width ratio of 1.7 to 5.7 (Figure 4F–H). Leaf apices
acute to obtusely pointed, appressed in young leaves (Fig-
ure 4H), but slightly recurved abaxially in mature leaves
(Figure 4F, G).
Remarks.—Cupressinocladus sp. A and B have leaves
         
from all other Early Cretaceous Cupressinocladus spe-
cies reported from Japan, i.e., C. japonicus (Yokoyama)
Kimura et Matsukawa (1979), C. mimotoi Kimura et

Early Cretaceous Sasayama Flora 121
Ohana (1987a), C. obatae Okubo et Kimura (1991) and
Cupressinocladus sp. (Kimura et al., 1992).
Genus Elatocladus Halle, 1913 emend. Harris, 1979
Type species.—Elatocladus heterophylla Halle, 1913.
Elatocladus sp.
Figure 4I, J
Material examined.—D1-32276, 32295, 32299, 32305,
32319, 32320 (Figure 4J), 32380 (Figure 4I).
Description.—Shoots obtained without the apex, up to
20 mm long by 11.5 mm wide. Lateral shoots branched
at ca. 60 degrees. Leaves spirally attached to the stem at
60–90 degrees with a decurrent basal cushion. Free part
of lamina falcate, varying from 5.4 mm long by 1.4 mm
Figure 4. Brachyphyllum, Cupressinocladus, and Elatocladus from the Upper Formation of the Sasayama Group. A–C, Brachyphyl-
lum sp. B; A, D1-32278; B, D1-32322; C, close-up of B (epoxy replica); D, E, Cupressinocladus sp. A; D, D1-32318; E, line drawing of
D; F–H, Cupressinocladus sp. B; F, D1-32271; G, line drawing of F; H, D1-32245; I, J, Elatocladus sp.; I, D1-32380; J, D1-32320. Scale
bars = 10 mm (A, F, G), 2.5 mm (C), 5 mm (B, D, E, H–J).

Toshihiro Yamada et al.122
wide to 7.7 mm long by1.8 mm wide. Leaves obtusely
pointed at apices, decurrent at the base, and margins
entire. Single vein entering leaf.
Remarks.—Elatocladus is a morphogenus applied for
       
characterized by leaves which are spirally arranged, dor-
-
rent basal cushion (Harris, 1979).
Our specimens could not be distinguished from Ela-
tocladus manchuricus, originally described from the
Lower Cretaceous of Liaoning, China, in having leaves
with obtuse apex attached to the stem at 60–90 degrees
(Yokoyama, 1906; Yabe, 1922; Oishi, 1933; Li et al.,
1986; Yang, 2003). However, a similar external morphol-
ogy is also found in E. laxus (Phillips) Harris, 1979 and
E. confertus (Oldham et Morris) Halle (McLoughlin and
    
based only on the external morphology.
Order Magnoliopsida?
Family uncertain
Genus uncertain
Figure 5
Material examined.—D1–32324.
Description.—Putative reproductive shoot 5.4 mm
long by 5.3 mm wide, consisting of two outer bracts, two
inner bracts and three fusiform organs at least. Fusiform
organs independent from each other, inserted above two
whorls of bracts, 4.6 mm long by 1.2 mm wide, consist-
        
part 3.2 mm long by 1.2 mm wide. Filiform part 1.4 mm
long, 0.5 mm wide at the base, tapering to the apex. Outer

Remarks.—Fusiform organs consisting of conical and
        
bract-like structures. These organs may represent fruits,
         
them. Four bract-like structures and three possible fruits
are visible in the specimen obtained, but some organs
might still be buried inside the rock matrix.
The present fossil, however, shows certain similari-
ties to the seed-bearing organ of some Mesozoic gnetoid
plants, e.g. Liaoxia from the Early Cretaceous (early to
early late Aptian) Yixian Formation of China (Rydin et
al., 2006).
Discussion
Age and environment of the Sasayama Flora
We described 13 plant species from the Upper Forma-
tion of the Sasayama Group (Table 2) and propose here
the name Sasayama Flora for this plant assemblage. The
Sasayama Flora is characterized by the abundance of
microphyllous conifers such as Pseudofrenelopsis sp.
(Cheirolepidiaceae), Brachyphyllum sp. (family uncer-
tain) and Cupressinocladus sp. (family uncertain), which
are regarded to favor a warm climate with dry season(s)
(Alvin, 1982; Ziegler et al., 1993; Rees et al., 2004). The
cycadeoids and possible pteridosperms with thick and/or
coriaceous laminae also suggest prevalence of dry con-
   
     
might imply a drier climate than those that prevailed in

details). This climatic inference from plant fossils is har-
monious with sedimentological observations that red beds
containing caliches are well developed in many strati-
graphic levels throughout the Sasayama Group (Enso and
Nakazawa, 1956; Yoshikawa, 1993).
Several radiometric ages have been reported for a vol-
canic breccia intercalated just below the plant-bearing
horizon, i.e., 109 ±5 Ma by potassium–argon dating
(Matsuura and Yoshikawa, 1992), 100 ±5 Ma (Matsuura
and Yoshikawa, 1992), 100.9 ±    
dating (Hayashi et al., 2010), and 106.4 ±0.4 Ma by
Figure 5. -
ity, D1-32324. t1, putative outer tepal; t2, putative inner tepal; g,
putative gynoecium. Scale bar = 2.5 mm.

Early Cretaceous Sasayama Flora 123
uranium–lead dating (Kusuhashi et al., 2013). All these
age estimations are coherent if the breccia is slightly
older than 100 Ma. Therefore, a late Albian age can be
suggested for the plant-bearing horizon, according to the
latest International Chronostratigraphic Chart (Cohen et
al., 2013; updated). Hayashi et al. (2010) argued that the
Upper Formation of the Sasayama Group is Cenomanian
in age based on conchostracan biostratigraphy, but this
inference is not supported by paleobotany. We obtained
ca. 200 plant fossil specimens from the Sasayama Group,
     
an angiosperm (Figure 5). This extreme scarcity of angio-
       
  -
acterized by the abundance of broad-leaved angiosperms
such as platanoids (Matsumoto et al., 1982; Narita et al.,
2008; Yamada, 2009a; Nishida and Legrand, 2017).
Northward shift of the Ryoseki-type oras during the
late Early Cretaceous
Late Jurassic (Oxfordian) to Early Cretaceous (Albian)

(Siberian) or Ryoseki (Eurosinian) types (Figure 6A;
Kimura, 1987, 1988, 2000; Ohana and Kimura, 1995).
       

been reported (Kimura and Ohana, 1987b, c; Kimura et
al.
temperate climate as characterized by the abundance of
pteridophytes (e.g. Dicksoniaceae and Osmundaceae),
macrophyllous cycadeoids (e.g. Dictyozamites and Neo-
zamites) and conifers (e.g. Podozamites), czekanowski-
aleans, and ginkgoaleans (Kimura, 1987, 1988, 2000;
Vakhrameev, 1991; Ohana and Kimura, 1995). A cool
temperate climate (mean annual temperature of 10 ±4°C)
has been estimated for their distribution area based on
the oxygen isotope composition of apatite phosphate in
dinosaur teeth (Amiot et al., 2011), although this esti-
    -
      
consist of various microphyllous conifers (e.g. Brachy-
phyllum, Cupressinocladus and Frenelopsis), cycadeoids
(e.g. Otozamites, Ptilophyllum and Zamites), and diverse
pteridophytes including species of the tree fern family
Cyatheaceae (Kimura, 1987, 1988, 2000; Vakhrameev,
1991; Ohana and Kimura, 1995), suggesting that the

       
inferred from the presence of microphyllous conifers, the
simultaneous occurrence of diverse pteridophytes implies
the existence of both wet and dry periods within a year. In
accordance with this interpretation, spores of epiphyllous
fungi which favor wet conditions are abundantly found
et
al., 2011).
      
restricted to the Inner Zone of Japan, while the Ryoseki-

Outer Zone of Japan (Kimura, 1987, 1988, 2000; Ohana
and Kimura, 1995). It has been widely accepted that the
         
Japan throughout the Late Jurassic to Early Cretaceous
(Kimura, 1987, 1988, 2000; Ohana and Kimura, 1995;

has been explained by a scenario in which sediments con-
taining the Ryoseki-type plant fossils were shifted north-
eastwardly by sinistral strike-slip movements (Ohana and
Kimura, 1995; Golozoubov et al., 1999; Kimura, 2000).
However, there is controversy over whether such a trans-
form fault was present (Takahashi and Matsukawa, 2000)
and, even if present, whether the displacement of the sedi-
       -
cant as predicted (Haggart et al., 2006). In the case that
sediments were moved, it would have happened no later
Table 2. Plant fossils obtained from the Sasayama Group.
Species Locality
Class Polypodiopsida
Cladophlebis sp. Ojiyama
Sphenopteris sp. A Ojiyama
Sphenopteris sp. B Ojiyama
Class Pteridospermopsida
Pachypteris? sp. Ojiyama
Class Bennettiopsida
Otozamites toshioensoi T. Yamada,
Legrand et H. Nishida
Ojiyama
Class Coniferopsida
Pseudofrenelopsis sp. Ojiyama
Classostrobus sp. Ojiyama
Brachyphyllum sp. A Ojiyama
Brachyphyllum sp. B Ojiyama, Sasayama Castle
Cupressinocladus sp. A Ojiyama
Cupressinocladus sp. B Ojiyama
Elatocladus sp. Ojiyama
Order Magnoliopsida?
gen. et sp. indet. Ojiyama

Toshihiro Yamada et al.124
than the Hauterivian (Matsukawa et al., 1997; Haggart et
al., 2006). Therefore, the displacement scenario could not
account for the present distribution of sediments contain-

Other studies have considered the paleotopography of
eastern Asia during the Late Jurassic to Early Cretaceous

et al., 2011; Philippe et al., 2014). According to this pale-

existed on cordilleran slopes of the oceanic side with suf-

have been established in coastal lowland areas that sea-
sonally experienced a drought. However, although some
moderate topographic highs running parallel to the sub-
duction trench may have been present (Ito et al., 2006), it
should be noted that sedimentary basins of the Inner Zone
including the Tetori and Sasayama basins were located
behind the hypothesized “rain shade,” according to recon-
structed paleomaps (Maruyama et al., 1997; Matsukawa
et al., 1997).
Recent studies pointing out the occurrence of “Brachy-
phyllum obesum” Heer and Ptilophyllum sp. from the
Kitadani Formation of the Tetori Group permitted the
proposal of another scenario in which some Ryoseki-
type plants would have extended to the Inner Zone at
least during the late Barremian to early Aptian (Figure
6B; Yabe et al., 2003; Yabe and Kubota, 2004; Yabe and
Shibata, 2011; Legrand et al., 2013). The Kitadani plant
assemblage includes various pteridophytes characteristic

Ryoseki-type plants (Yabe and Shibata, 2011), indicat-
  
during this period (Yabe et al., 2003). In addition, the
microphyllous Cuppresinocladus sp. was reported from
the Barremian Wakino Formation of the Kwanmon Group
distributed in Yamaguchi Prefecture belonging to the
Inner Zone, although no other plants were reported from
this locality (Kimura et al.
that the distributional areas of the Tetori- and Ryoseki-
        
changed with time (Yamada, 2009a).
   
Inner Zone, which was probably established under a
drier and warmer climate than that of the late Barremian
to early Aptian Kitadani Formation, as suggested by the
dominance of microphyllous conifers and the rarity of

known from the Inner Zone, the continuation of drying
and warming trends during the Aptian is supported by the
development of thick red beds in the Lower Formation
Figure 6.      A,  
map based on Kimura (1987, 1988); B,     C,   
sites plotted on B and C were revised based on Sha (2007) for Chinese sites and Golozoubov et al. (1999) for Russian sites. See main text for
details on Japanese sites. Paleomaps (indicated by black lines) are based on Maruyama et al. (1997). Present position of Japanese Archipelago
is indicated by white lines. Bold line with triangles represents the subduction front.

Early Cretaceous Sasayama Flora 125

similar to the Sasayama Flora were reported from the
Youngdong Group of Youngdong, Korea (Seo and Kim,
2009; Kim et al., 2012), and the Dalazi Formation (Sha,
2007) of Longjing, northeast China (Zhou, 1995; Yang
and Deng, 2007; Yang, 2008). Therefore, the northern
 
the Eurasian continent would have continuously shifted
northward during the late Barremian to Albian, to reach
a paleolatitude of ca. 45 degrees (Eldridge et al., 2000)
before the end of the Albian (Figure 6C). Some Ryoseki-
type plants may possibly have reached further north to
lands around the Yezo forearc basin, which was located
north of the Sasayama basin in the late Albian (Figure
6C), as indicated by the occurrences of thermophilic chei-
rolepidiaceous conifers (Saiki, 1999) and ephedroid pol-
len grains (Takahashi, 1995).
Various evidence has already suggested that the cli-
matic system of the Earth was under a supergreenhouse
mode during the mid-Cretaceous period (Berner, 2006;
Fletcher et al., 2008; Price et al., 2013). The northward
        -
gered by this warming event. However, a humid zone
should have existed between the northern paleolatitudes
of 35 and 50 degrees during the mid-Cretaceous period
in response to the shrinkage of the Hadley circulation, as
represented by the coal-bearing deposits in the Gobi basin
of Mongolia (Hasegawa et al.   
between the global trend and the local climate inferred in
this study could be explained by either or both of the fol-
lowing scenarios: 1) the dry climate prevailing in the Inner
Zone and adjacent areas may have been caused locally by
a cold oceanic current from the boreal area (Haggart et
al., 2006); 2) alternatively, the “rain shade” model (Oh
et al., 2011; Philippe et al., 2014) can be applied to the
Inner Zone and adjacent areas at least until the late Bar-
remian to Albian. It would be desirable to further explore

Japan to test these two hypotheses.
Acknowledgements
We thank the landowner of the fossil site and Munici-
pal Board of Education, Sasayama City, for their permis-
sion to access the fossil sites. W. Adachi, K. Handa and H.
Saegusa kindly provided plant fossils they collected for
this study. We also thank the Ensos for their kind coopera-
tion on our fossil survey.
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Author contributions
T. Y. planned the research, collected fossils and con-
ducted taxonomic study. J. L. and H. H. collected fossils
and conducted taxonomic study. All authors were partici-
pated in writing this paper.