Introduction

Two types of floras were flourishing in Japan during the Late Jurassic to Early Cretaceous, i.e., the Tetori- and Ryoseki-type floras (Kimura, 1979, 1987). The Tetori-type flora is only reported from the Tetori Group which is distributed in the Hokushinetsu (Fukui, Ishikawa, Toyama, Niigata and Nagano Prefectures) and Hida (Gifu Prefecture) districts of the Inner Zone of Japan (Maeda, 1961; Fujita, 2002; Sano, 2015; Sano and Yabe, 2017; Yamada, 2017; Yamada and Sano, 2018). It is characterized by the presence of macrophyllous ginkgoalean and bennettitalean gymnosperms that are thought to have flourished under a warm and humid climate (Kimura, 1979, 1987). On the other hand, the Ryoseki-type flora is reported from the Outer Zone of Japan. It contains matoniaceous ferns, microphyllous conifers of the Cheirolepidaceae and Araucariaceae, and the bennettitalean genera Zamites and Ptilophyllum (Kimura, 1979, 1987). It is suggested that Ryoseki-type plants favored drier climatic conditions than those from the Tetori Group (Kimura, 1979, 1987). The Tetori- and Ryoseki-type floras were compared to the Siberian and Eurosinian floras, respectively (Kimura, 1979, 1987), which are known for Eurasian continental strata of the same age (Vakhrameev, 1991).

The stratigraphic range of the Tetori-type flora has long been thought to extend from the Bathonian until the Albian (Kimura, 1979, 1987), but recent studies showed that this flora appeared no earlier than the Tithonian (Yamada and Uemura, 2008; Yamada, 2018). It was also pointed out that some Ryoseki-type plants were found along with Tetori-type plants in the early Aptian Nochino Formation (Sakai et al., 2020) and middle Aptian Kitadani Formation (Yabe and Kubota, 2004; Yabe and Shibata, 2011; Terada and Yabe, 2011; Sano and Yabe, 2017) of the Tetori Group. These paleobotanical data imply that composition of the vegetation in the Tetori Basin gradually changed around the Aptian (Yamada et al., 2018), but more information is required to infer when this floristic transition exactly started.

A palynological analysis would provide powerful data to test the hypothesis about floristic provincialism in Japan, as well as to grasp when the above floristic change began. We already published robust palynological data on the Barremian Ashikajima and Kimigahama formations of the Choshi Group distributed in the Choshi Peninsula of Chiba Prefecture (Legrand et al., 2011) and the late Barremian Nishihiro Formation in Wakayama Prefecture (Legrand et al., 2014), where Ryoseki-type macrofloras have been reported (Kimura and Kansha, 1978a, b; Asama et al., 1979; Kimura et al., 1985, 1991). Some palynomorphs (anemiaceous spores of Cicatricosisporites spp., conifer pollen of Taxodiaceaepollenites spp. and unidentified bisaccate pollen, one fungal spore and the acritarchs Baltisphaeridium spp.) were also reported from the Hauterivian-Barremian Arida Formation in Wakayama Prefecture (Matsumoto, 2009). However, until this study, no palynological record was available for the Barremian part of the Tetori Group, thus preventing any comparison between coeval palynofloras from Ryoseki- and Tetori-type localities. In addition, the lack of Barremian palynological data from the Tetori Group also prevented tracing floristic changes through the Barremian to Aptian, although palynological data is available for the Aptian Nochino (Umetsu and Matsuoka, 2003; Umetsu and Sato, 2007) and Kitadani (Legrand et al., 2013) formations.

Figure 1.

Distribution of the Tetori Group with location of the Itsuki, Kuwajima, Nochino and Kitadani localities (redrawn from Yamada and Sano, 2018).

We report here palynofloras newly obtained in the Tetori Group from the Barremian Itsuki Formation of the Itoshiro area, Fukui Prefecture and the Barremian Kuwajima Formation of the Shiramine area, Ishikawa Prefecture (Figure 1). We compare these assemblages to coeval assemblages of Ryoseki-type localities to elucidate the floristic provinces in terms of palynofloras. Moreover, we also compare them with Aptian palynological data available for the Tetori Group to test hypotheses on floristic changes within the Tetori Basin. Finally, we discuss floral provincialism by comparing our assemblages to coeval palynofloras of eastern Asia.

Geological setting

The Tetori Group outcrops in the Hokushinetsu and Hida districts of the Inner Zone of Japan (Maeda, 1961; Fujita, 2002; Sano, 2015; Sano and Yabe, 2017; Yamada, 2017; Yamada and Sano, 2018) and consists, in the Itoshiro area (Ono city, Fukui Prefecture; Figure 2), of the Yambara, Ashidani, Obuchi, Itsuki, Nochino, and Chinaboradani formations in ascending order (Oishi, 1933; Maeda, 1961; Fujita, 2002). The Itsuki Formation mainly consists of brackish mudstones and sandstones and was deposited in a fan-delta plain (Masuda et al., 1991; Kusuhashi et al., 2002). A rich freshwater to marine molluscan assemblage (Maeda, 1961; Tamura, 1990), dinosaur remains and footprints (Azuma et al., 1988; Manabe, 1999) and plant remains (Maeda, 1961; Kimura, 1975a; Sakai et al., 2020) were reported from the formation, which is assigned to the Barremian based on U-Pb ages (127.2 Ma) of detrital zircons (Kawagoe et al., 2012), as well as on an ammonoid record (Goto, 2007). The Itsuki Formation is conformably overlain by the lower Aptian Nochino Formation, from which the Tamodani flora (Kimura, 1975a) and a palynological assemblage (Umetsu and Matsuoka, 2003) were reported.

Figure 2.

Age correlation between strata of the Inner (Itoshiro and Shiramine areas) and Outer (Choshi and Kii peninsulas) Zones of Japan considered in this study, with mention of reports on sporo-pollen assemblages (Umetsu and Matsuoka, 2003; Umetsu and Sato, 2007; Matsumoto, 2009; Legrand et al., 2011, 2013, 2014).

In the Shiramine area, Hakusan City, Ishikawa Prefecture (Figure 2), the Tetori Group is divided into the Gomijima, Kuwajima, Akaiwa and Kitadani formations. The Kuwajima Formation mainly consists of mudstones and sandstones and is considered as having been deposited in a floodplain environment (Isaji et al., 2006). Freshwater, brackish and marine mollusks, a diverse vertebrate assemblage (Hasegawa et al., 1995; Hirayama, 2000; Matsuoka, 2000) with footprints (Azuma and Takeyama, 1991; Azuma et al., 1992; Matsuoka et al., 2001), and plant fossils corresponding to the Oguchi Flora (Yokoyama, 1889; Oishi, 1940; Kimura and Sekido, 1961, 1965, 1966, 1967, 1972a, 1972b, 1974, 1975, 1976; Kimura, 1961, 1991; Matsuo and Omura, 1968; Kimura et al., 1978; Matsuo and Sekido, 2000) were reported. It has been assigned to a Barremian age based on the U-Pb ages (130.7 Ma) of zircons in tuff beds (Matsumoto et al., 2006) and the freshwater (Tatsukawa-type) bivalve fauna (Maeda, 1958, 1962; Tamura, 1990; Kozai et al., 2001, 2002; Okazaki and Isaji, 2008; Kusuhashi, 2008).

Material and methods

The material from the Itsuki Formation was collected in 2006 as a part of a project conducted by the National Museum of Nature and Science and in 2007 with the permission of the Ono City Board of Education. We collected samples from two horizons of the Itsuki Formation (called “lower” and “upper” samples hereafter) along the Itoshiro River, Ono City, Fukui Prefecture. The lower samples were collected from mudstones in the type locality of the formation (horizon 248 of Sakai et al., 2020), while upper ones were collected from muddy sandstones in the uppermost part of the formation near Nochino (horizon 62 of Sakai et al., 2020; Figure 1). Palynomorphs of the Kuwajima Formation were extracted from blocks of mudstone and sandstone collected in 2007 with the permission of the Hakusan City Board of Education and during an excursion of the 13th International Palynological Congress / 9th International Organization of Palaeobotany Conference 2012 (IPC XIII / IOPC IX 2012) from the “Kaseki-kabe (fossil bluff)” locality, Shiramine area of Hakusan City, Ishikawa Prefecture (Figure 1). The “Kaseki-kabe” locality is historically important as it is the place where fossil plants were first reported from the Tetori Group as well as from Japan (Geyler, 1877). The locality represents the uppermost part of the formation and consists of freshwater siltstones, mudstones, and coarse-grained sandstones (Ishikawa Prefecture Board of Education, 1978). Although we collected blocks that had fallen onto the road along the Kaseki-Kabe, these blocks presumably came from the upper bed bearing silicified woods shown in Figure 1 of Isaji et al. (2005), judging from lithological features of the block.

Figure 3.

List of palynomorphs obtained from the Itsuki and Kuwajima formations with their botanical affinities, and comparison with previously reported coeval palynofloras from the Outer Zone of Japan (Choshi Group and Nishihiro Formation) and Aptian palynoflora from the Inner Zone of Japan (Kitadani Formation). Species in dark grey cells not observed in the Itsuki and Kuwajima formations; species in light grey cells observed in the Tetori Group only; ◦, present; +, abundant.

Continued.

Continued.

Methods for pollen extraction followed Legrand et al. (2013). Palynomorphs were observed under a Leica DMRD Differential Interference Contrast Microscope equipped with a Leica MC170HD Camera. Position on the slide was recorded using an England Finder graticule, Pyser Optics.

Results

Palynological assemblage of the Kuwajima Formation.—Palynomorphs from the Kuwajima Formation (Figure 3) did not show any sign of metamorphism, but were often fragmented, most probably by oxidation and/or nature of the sediment. The assemblage is dominated by fern spores, mainly of Cyatheales (Cyathidites australis, Cy. minor, Verrucosisporites sp.) and Schizaeales (Cicatricosisporites minor, Ci. sp., Ischyosporites sp.), and a few spores with affinities to the lycophytes (Lycopodiumsporites sp., Echinatisporis sp.). Gymnosperm pollen is only represented by conifers, dominated by Classopollis sp. (Cheirolepidiaceae). We could also observe many wood and cuticle fragments, some showing stomata and most probably belonging to Cheirolepidiaceae and Cycadales. The freshwater green alga Chomotriletes minor is also present.

Figure 4.

Fossil spores (bryophytes, lycophytes and monilophytes: A–L), gymnosperm pollen (M–O), cyst of freshwater green alga (P), and epiphyllous fungus (Q) from the Upper Itsuki Formation. A, Cyathidites australis (2d L37); B, Cyathidites minor (2d U51-3); C, Biretisporites potoniaei (1b N30-2); D, Echinatisporis varispinosus (1b M34-3); E, Osmundacidites wellmanii (2a J22-3); F, Aequitriradites spinulosus (2d K64-2); G, Lycopodiumsporites sp. (1b W43-2); H, I, Reticulatisporites sp. (2d O24-1; H, proximal face; I, distal face); J, K, Polycingulatisporites reduncus (2d B39; J, proximal face; K, distal face); L, Gleicheniidites nilssonii (2a Y46); M, Classopollis torosus (1a R66-1); N, Araucariacites australis (2a F45); O, Pityosporites sp. (2f G43-2); P, Chomotriletes minor (2a Y37-2); Q, Phragmothyrites sp. (1b H60). Scar bar, 10 µm.

Palynological assemblage of the Itsuki Formation.—Among the three horizons examined in this paper, the best state of preservation was observed in the upper sample of the Itsuki Formation, which includes almost all palynomorph types observed in the Kuwajima Formation. The lower sample of the Itsuki Formation only yielded poor palynomorphs, and all types were also recovered from the upper sample (Figure 3).

The assemblage is composed of 32 species of spores and gymnosperm pollen, one algal cyst and two fungi (Figure 4A–Q). It is dominated by fern spores. Among them, cyathealean spores of Cyathidites minor (Figure 4B), Cy. australis (Figure 4A), Deltoidospora hallii and Verrucosisporites sp. are most abundant, followed by spores with affinities to the marchantiopsids (Aequitriradites [Figure 4F]) and lycophytes (Lycopodiumsporites [Figure 4G], Retitriletes, Echinatisporis [Figure 4D]). A few spores with affinities to the Pteridaceae (Contignisporites), Anemiaceae (Cicatricosisporites), Gleicheniaceae (Gleicheniidites [Figure 4L]), Lygodiaceae (Ischyosporites, Reticulatisporites [Figure 4H, I]) and Osmundaceae (Baculatisporites, Biretisporites [Figure 4C], Osmundacidites [Figure 4E]) are present. A number of spores of unknown affinity were also observed (Cingulatisporites, Granulatisporites, Polycingulatisporites [Figure 4J, K], Punctatisporites). The pollen assemblage is dominated by the bennettitalean/cycadalean Cycadopites and coniferalean Classopollis spp. (Cheirolepidiaceae; Figure 4M), Araucariacites (Araucariaceae; Figure 4N), Inaperturopollenites, and some small bisaccate pollen (Pityosporites [Figure 4O]). The freshwater green alga Chomotriletes minor (Figure 4P) and some fungi (Phragmothyrites [Figure 4Q], Pluricellaesporites) are also present.

Discussion

Overview of Barremian palynofloras of the Tetori Group.—The whole plant assemblage obtained from the Barremian Itsuki and Kuwajima formations is composed of 28 species of bryophytes, lycophytes and monilophytes (mainly ferns), and 8 species of gymnosperms. It greatly increases the palynological data presently available from stratigraphic horizons of the “canonical” Tetori-type flora, which are represented by only one report at the generic level from the lower Aptian Nochino Formation: five genera of fern spores (Appendicisporites, Cicatricosisporites, Cyathidites, Deltoidospora, Osmundacidites, Schizaeoisporites) and one genus of gymnosperm pollen, Classopollis (Umetsu and Matsuoka, 2003).

Cicatricosisporites, Cyathidites, Osmundacidites and Classopollis were also observed in the Itsuki and/or Kuwajima formations. Appendicisporites was not recovered in this study; the genus has been reported from the mid-Aptian Kitadani Formation (Legrand et al., 2013), thus it would have appeared after the early Aptian in the Tetori Basin. Schizaeoisporites is often reported from Lower Cretaceous strata of eastern Asia including the Nochino Formation, but most reports are misidentified gnetalean pollen of Ephedripites or Gnetaceaepollenites (Legrand, 2009; Legrand et al., 2013).

Differences in Barremian palynofloras between the Inner and Outer Zones of Japan.—From the Tithonian to Barremian period, two floras were proposed in Japan based on the macrofossil record, i.e, the Tetori-type flora for the Inner Zone and the Ryoseki-type flora for the Outer Zone (Kimura, 1979, 1987). The Tetori-type flora consists of an abundance of monilophytes (e.g., Dicksoniaceae, Osmundaceae), macrophyllous Bennettitales and Coniferales, Czekanowskiales, and Ginkgoales (Kimura, 1979, 1987; Vakhrameev, 1991; Ohana and Kimura, 1995), suggesting a warm temperate climate. On the other hand, the Ryoseki-type flora is characterized by diverse microphyllous Coniferales (e.g., Cheirolepidaceae, Araucariaceae), Bennettitales, and monilophytes including matoniaceous ferns and some cyathealean tree-ferns (; Vakhrameev, 1991; Ohana and Kimura, 1995), suggesting warmer and drier climatic conditions than those inferred from plants of the Tetori-type flora (Kimura, 1979, 1987). The juxtaposition of the Tetori-type and Ryoseki-type floristic provinces in Japan has been explained by a scenario in which sediments containing the Ryoseki-type plant fossils were located further to the south and shifted northeastward by sinistral strike-slip movements along the Median Tectonic Line (Ohana and Kimura, 1995; Golozoubov et al., 1999; Kimura, 2000) from the Late Jurassic until the close of the Hauterivian (Matsukawa et al., 1997). However, the existence of such a transform fault is controversial (Takahashi and Matsukawa, 2000), and, if correct, the displacement scenario could not be applied to post-Hauterivian rocks, from which most of the Ryoseki-type floras were reported (Ohana and Kimura, 1995). Therefore, the juxtaposition is presumably due to different factors, such as ocean currents (Haggart et al., 2006), local topography (Oh et al., 2011b) and global climatic fluctuations (Yamada et al., 2018; see below).

This provincialism has never been tested palynologically, mainly because no data had been published about Barremian palynofloras of the Inner Zone of Japan. Pre-Aptian palynofloras of the Outer Zone of Japan were reported from two areas, i.e., from the Barremian Ashikajima and Kimigahama formations of the Choshi Group, Choshi Peninsula (Legrand et al., 2011), and from the upper Barremian Nishihiro Formation of the Monobegawa Group, Kii Peninsula (Legrand et al., 2014). Forty-five species are found in both these areas, and 26 species of them are not recovered in the Barremian part of the Tetori Group (Figure 3). The unique occurrence of some palynotaxa is consistent with the macrofloral record in the Outer Zone of Japan, such as Matonisporites sp. (Matoniaceae), Balmeiopsis limbatus (Araucariaceae) and Callialasporites dampieri (Podocarpaceae), while palynofloras further include many groups unknown from the macrofloras, i.e., Foveosporites ryosekiensis and Lycopodiumsporites dentimuratus (Lycopodiaceae), Neoraistrickia sp. (Selaginellaceae), Manumia japonica (Pteridaceae), Nodosisporites choshiensis (Anemiaceae), Ischyosporites (Klukisporites) variegatus (Lygodiaceae), Gnetaceaepollenites sp. (Gnetales), etc. These taxa were suggested as characterizing palynofloras of the Ryoseki-type province (Legrand et al., 2011, 2013, 2014), and the present study confirms for the first time the argument by presenting palynological data from the Barremian part of the Tetori Group.

On the other hand, only Cicatricosisporites minor (Anemiaceae), Reticulatisporites sp. (Schizaeaceae), Gleicheniidites nilssonii and Gleichenidites sp. (Gleicheniaceae), and Punctatisporites sp. (Polypodiidae) are unique to the Barremian part of the Tetori Group, while the other 31 species are shared among the Tetori- and Ryoseki-type localities. Concerning gymnosperm taxa, the uniqueness of the Tetori-type palynoflora may be underestimated as both ginkgoalean and bennettitalean plants usually produce similar monosulcate pollen with simple morphology classified as Cycadopites minimus (Balme, 1995). However, the palynological commonality observed in both lycophytes and monilophytes must be noted. It has been hypothesized that different tree-fern families characterize the Tetori- and Ryoseki-type floras respectively, i.e., Dicksoniaceae in the Tetori-type versus Cyatheaceae in the Ryoseki-type (Kimura, 1979, 1987). Later, Cyathocaulis naktongensis Ogura (Cyatheaceae) was obtained as float from the Shokawa area of Gifu Prefecture, where the Tetori Group is distributed (Yabe et al., 2003). It remains to be clarified what stratigraphic horizon this cyatheaceous stem comes from, but this finding challenges the previous hypothesis. Our palynological data support the idea that there is no prominent difference in the tree-fern composition between the two floras since 5 of the 10 cyathealean species known from the Barremian of Japan are recovered from both the Tetori- and Ryoseki-type localities, representing all species obtained from the Tetori-type localities (Figure 3). Our data also suggest the commonality in ferns of the Osmundaceae between the Tetori- and Ryoseki-type floras, although osmundaceous ferns were once considered to characterize the Tetori-type flora (Kimura, 1979, 1987). A less diverse fern assemblage was suggested for the Ryoseki-type flora in comparison to the Tetori-type flora (Kimura, 1979, 1987). However, humid conditions should have partly and/or seasonally developed in the Ryoseki-type area, too, as indicated by the presence of epiphyllous fungi (Legrand et al., 2011, 2014), which would have allowed the growth of similar fern assemblages in both the Inner and Outer Zones of Japan.

We obtained two species with affinities to the Cheirolepidiaceae, Classopollis torosus from the Itsuki Formation and Classopollis sp. from the Itsuki and Kuwajima formations (Figure 3). We also observed cuticle fragments of probable Cheirolepidiaceae from the Kuwajima Formation. Classopollis torosus (Legrand et al., 2013) and cuticle fragments of Cheirolepidiaceae are reported from the Kitadani Formation of the Tetori Group (Legrand et al., 2013; see below for details) and Classopollis was found in the Nochino Formation (Umetsu and Matsuoka, 2003); in the macroflora, too, all previous cheirolepidiaceous reports concerned post-Aptian sediments (Yabe et al., 2003; Yabe and Shibata, 2011; Sano and Yabe, 2017; Sakai et al., 2020), and there is no record of Cheirolepidiaceae from sediments of the Tetori Group older than the Aptian. Therefore, it has been supposed that cheirolepidaceous plants were confined to the Ryoseki-type flora in pre-Aptian sediments of Japan.

The presence of Classopollis in the Itsuki and Kuwajima formations points out that the floristic provincialism hypothesized so far (Kimura, 1979, 1987) may have already been weakened during the Barremian. Alternatively, the presence of Classopollis could be explained by a marine influence on the Itsuki (Goto, 2007) and Kuwajima (Masuda et al., 1991) formations. It has been suggested that some cheirolepidiaceous plants were halophytic (Alvin, 1982). In addition, some palynomorphs could be indicators of coastal environments under a warm and humid climate, such as Exesipollenites tumulus (Tanrikulu et al., 2018) and Cicatricosisporites sinuosus (Al-Ameri et al., 2001; Mahmoud et al., 2007). Whichever is the case, however, palynofloras of the Itsuki and Kuwajima formations suggest in total that the Tetori Basin harbored a mixed flora intermediate between the Siberian and Eurosinian floras rather than a Siberian flora during the Barremian, contradicting the previously hypothesized stratigraphic range of the Tetori-type flora in the Tetori Basin (Yabe et al., 2003; Yabe and Shibata, 2011; Terada and Yabe, 2011; Sano and Yabe, 2017; Yamada et al., 2018; Sakai et al., 2020).

Palynofloristic changes in the Tetori Basin from the Barremian to Aptian.—In the Tetori Basin, microphyllous conifers of the Ryoseki-type such as Brachyphyllum, Geinitzia and Pagiophyllum coexisted with plants typical of the Tetori-type flora after the Aptian (Yabe et al., 2003; Yabe and Kubota, 2004; Yabe and Shibata, 2011; Sano and Yabe, 2017; Sakai et al., 2020). This floristic change could be related to the supergreenhouse trend that is thought to have started in the Aptian (Fletcher et al., 2008; Hasegawa et al., 2012). Such a trend probably affected the local climate that prevailed on the Eurasian continental margin (Yamada et al., 2018). In this section, we compare the palynoflora of the Kitadani Formation (Legrand et al., 2013) to those of the Ryoseki- and Tetori-type palynofloras (see above) in order to evaluate if the palynological data can support these floristic histories inferred from macro-remains.

Almost all palynotaxa specific to the Tetori-type, i.e., Gleicheniidites sp., Reticulatisporites sp. and Cicatricosporites minor, are present in the Kitadani Formation. Abundance of Echinatisporis and the scarcity of bisaccate pollen are also common features between the Itsuki and Kitadani formations. On the other hand, 10 spore and 4 pollen species of the Ryoseki-type were recovered from the Kitadani Formation, including Manumia japonica (Pteridaceae), Plicatella sp., Ruffordiaspora australiensis (Anemiaceae), Balmeiopsis limbatus (Araucariaceae), Callialasporites dampieri (Podocarpaceae) and Gnetaceaepollenites sp. (Gnetales). However, Matonisporites, Foveosporites and Nodosisporites were not observed in the Kitadani Formation (Figure 3). All these points support the inference from the macrofossils that the “Mixed-type” flora was prevailing in the land of the Kitadani Formation (Yabe and Kubota, 2004; Yabe and Shibata, 2011; Terada and Yabe, 2011; Sano and Yabe, 2017).

We did not observe any angiosperm pollen from the Kuwajima nor Itsuki formations, while Clavatipollenites hughesii was found in the Kitadani Formation (Legrand et al., 2019). This result confirms the hypothesis (Legrand et al., 2014, 2019) that the appearance of angiosperms in Japan started earlier in the Outer Zone (late Barremian) than in the Inner Zone (mid-Aptian), hence supporting that early angiosperms expanded northward from the low- to high-latitudinal areas during the Early Cretaceous (Axelrod, 1959; Brenner, 1976; Legrand et al., 2015, 2019; Coiro et al., 2019). This assumes that the Inner Zone was located further north than the Outer Zone at this time, which is not certain.

Comparison among coeval palynofloras of Japan and eastern Asia.—It is recognized that Inner Japan was originally located to the east of the Korean Peninsula with a connection between the Gyeongsang (southeast Korea) and Kwanmon (southwest Japan) Basins, implying a transport of sediments between the Gyeongsang and Tetori Basins (Yaskawa, 1975; Hisada et al., 2001, 2008; Sano, 2015). Sediments of the Gyeongsang Basin are divided into the Sindong, Hayang and Yucheon groups. The Sindong Group is divided into the Nakdong, Hasandong, and Jinju formations in ascending order, and considered to have been deposited from the Barremian to Albian. Similitudes between macrofloras of the Sindong and Tetori groups have already been pointed out, and floras of the Sindong Group are recognized as reflecting characteristics of the Mixed-type province (Kimura, 2000; Kim, 2009; Oh et al., 2011a; Kim et al., 2016). Palynological reports on Lower Cretaceous sediments of Korea are very scarce because of the geological setting of the Peninsula, as for the Inner Zone of Japan, but palynomorphs from the Sindong Group were reported from the Nakdong Formation (Yi et al., 1996) and its equivalent the Myogog Formation (Yi and Chun, 1993), and the Jinju Formation (Choi, 1985; Choi and Park, 1987; Yi et al., 1994, 1996).

We compared these assemblages from the Gyeongsang Basin with the assemblage obtained from the Itsuki Formation, which is more complete than that of the Kuwajima Formation. As could be expected, the assemblage from the Itsuki Formation confirmed a composition close to those of the Nakdong and Myogog formations, where 12 of the 16 morphogenera (75%) reported (Yi and Chun, 1993) are shared with the Itsuki Formation. In Korea, poor preservation of palynomorphs did not allow any identification at species level, but spores of the genera Lycopodiumsporites (lycophytes) and Baculatisporites, Biretisporites, Cicatricosisporites, Cingulatisporites, Deltoidospora, Ischyosporites (Klukisporites), and Polycingulatisporites (monilophytes) are common, while pollen is only represented by the genera Cycadopites, Classopollis and probable Inaperturopollenites, all also observed in the Itsuki Formation. We further note that the genus Cibotiumspora, present in the Myogog Formation but not observed in the Itsuki Formation, is reported from Barremian palynofloras of the Ryoseki- and Mixed-type provinces (Legrand et al., 2011, 2013).

Similarities have also been pointed out between macrofloras of the Inner Zone of Japan and those of northeast China (Kimura, 1975b). Among them, the palynoflora from the Itsuki Formation is closest to that reported from the Barremian Shitouhezi Formation of the Hegang Basin (Shang, 1994) in its composition and relative abundance in groups of palynomorphs: the spore assemblage is dominated by Cyatheaceae and a variety of Anemiaceae, while Osmundaceae, Dicksoniaceae, Selaginellaceae and Lycopodiaceae are present and mostly represented by species shared with the Itsuki Formation; Classopollis and Cycadopites are present in the pollen assemblage, which is however dominated by bisaccate pollen of Coniferales (Pinaceae and Podocarpaceae). Moreover, it should be noted that the Itsuki Formation has a palynological composition clearly distinct from that of the Yixian (= Chengzihe) Formation (Shang, 1997), which represents the basal formation of the latest Barremian-Albian Jehol Group; on the other hand, palynological compositions of the Yixian and Kitadani formations share strong common features (Legrand et al., 2019).

Conclusion

We report palynofloras from the Itsuki and Kuwajima formations. These are the first palynological records from the Barremian of the Inner Zone of Japan. We could find some taxa characterizing palynofloras of either the Inner or Outer Zone, but similarities between them are more prominent, especially in composition of osmundaceous and tree ferns. Moreover, cheirolepidiaceous conifers were commonly found in both the Inner and Outer Zones. These palynological data contradict the canonical hypothesis from macro-remains that the Ryoseki- and Tetori-type floristic provinces were distinct in Japan throughout the Early Cretaceous. The similarities imply that a weakening of this provincialism started at least from the Barremian. This tendency became more profound in the Aptian as the palynoflora of the Kitadani Formation suggests. The assemblage from the Itsuki Formation is close to those of the coeval Nakdong and Myogog formations in southeast Korea and Shitouhezi Formation in northeast China, but distinct from that of the latest Barremian-Aptian Yixian Formation. Finally, the absence of any angiosperm pollen supports the hypothesis that flowering plants appeared earlier in the Outer Zone of Japan and then expanded northward during the Early Cretaceous.