Introduction

Ptilophyllum pachyrachis was established as a new species in the genus Ptilophyllum (Bennettitales) by Oishi (1940) based on four syntypes collected from Mochiana, Kamianama Village (presently Ohno City), Fukui Prefecture, Central Japan. The exact location of the type locality was not provided, but is considered to be included in the Upper Jurassic (Tithonian) to Lower Cretaceous (Berriasian) Ashidani Formation of the Tetori Group (sensu Yamada and Sano, 2018) (Figure 1; Fujita, 2002; Sato and Yamada, 2005; Yamada and Uemura, 2008; Yamada, 2017). Kimura and Ohana (1987) later proposed a reclassification of this species to Pterophyllum pachyrachis (Oishi) Kimura and Ohana without designating a lectotype; although they cited Oishi's 1940 monograph, they did not specify the page or plate where the basionym appeared. Therefore, their proposal is invalid under Article 41.5 of the International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code) (Turland et al., 2018).

We recently reexamined syntypes of Ptilophyllum pachyrachis Oishi (1940) deposited at the Hokkaido University Museum and found that they included both Pterophyllum and Ptilophyllum species. In this paper, we formally propose a transfer of Ptilophyllum pachyrachis Oishi to the genus Pterophyllum, with lectotypification and some changes to specific diagnostic characteristics and further re-describe one syntype as an uncertain species of the genus Ptilophyllum. We also reevaluated the paleophytogeographical significance of Oishi's (1940) findings, which had been previously overlooked.

Material and methods

Plant-bearing beds in Mochiana (Figure 1) had been assigned to the Ochiai Formation (Maeda, 1961), but these were reassigned to the lower part of the Ashidani Formation in later studies (Fujita, 2002; Sato and Yamada, 2005; Yamada and Uemura, 2008). The Ashidani Formation is composed of conglomerates, sandstones and mudstones of non-marine origin, while the upper part consists of marine facies (the Kamihambara Formation) in the East of Izumi area where Mochiana is located (Fujita, 2002; Figure 2). The Late Jurassic (Tithonian) to Early Cretaceous (Berriasian) age is inferred for the Ashidani Formation based on ammonoids collected from the Kamihambara Formation (Sato and Yamada, 2005; Yamada, 2017; Figure 2).

Syntypes of Ptilophyllum pachyrachis are leaf impressions preserved on mudstone in which cuticular layers had been lost through diagenetic processes (Figure 3). We photographed them by D200 digital camera equipped with AI AF Micro-Nikkor 60 mm f/2.8D lens (Nikon, Tokyo, Japan) under incandescent light.

Figure 1.

Distribution of the Tetori Group [based on Maeda (1961) and Yamada and Sano (2018)] around the type locality of “Ptilophyllum” pachyrachis. The inset map shows a geological map [based on Fujita (2002)] of the boxed area shown in the small-scale map. The type locality, Mochiana (indicated with a star in the inset map), is now submerged below Lake Kuzuryu. The locations of Kuwajima, Kitadani, and Nochino are also indicated.

Figure 2.

Stratigraphies of the Tetori Group showing localities of Eurosinian-type plant fossils compiled from Yamada and Uemura (2008), Sano (2015), and Yamada (2017). Plant macrofossils of the Nochino and Kitadani formations are based on Sakai et al. (2020) and Yabe and Shibata (2011), respectively. The number of vascular plant palynospecies were compiled based on Legrand et al. (2021). Shading indicates periods of marine transgression (Sato and Yamada, 2005; Sano, 2015; Yamada, 2017). Fm., Formation; Km. Fm., Kamihanbara Formation.

Figure 3.

Pterophyllum pachyrachis (A, B) and Ptilophyllum sp. (C). A, lectotype (UHR8485B); B, UHR8485C; C, UHR8485D. Scale bars = 1 cm.

Systematic palaeobotany

Figure 4.

Line drawings of Pterophyllum pachyrachis (A) and Ptilophyllum sp. (B). A, lectotype (UHR8485B); B, UHR8485D. Scale bars = 1 cm.

Discussion

The Siberian and Eurosinian floristic provinces existed in Eastern Asia from the Middle Jurassic to Early Cretaceous, with the former located north of the latter (Vakhrameev, 1987, 1991). Siberian-type vegetation was characterized by a variety of pteridophytes and macrophyllous gymnosperms, which favored a warm temperate climate (Vakhrameev, 1987, 1991). By contrast, microphyllous gymnosperms flourished in the Eurosinian province due to a seasonally dry climate (Vakhrameev, 1987, 1991). It has been suggested that typical Siberian-type vegetation dominated in the land of the Tetori Group during its period of deposition, i.e., from the Late Jurassic to the Early Cretaceous, whereas Ryoseki-type floras of the Eurosinian-type were coevally distributed in the Outer Zone of Japan along the Pacific coast (Kimura, 1987, 2000). However, Eurosinian-type plants were recently found in the Aptian portion of the Tetori Group, as well as with Siberian-type ones (Yabe et al., 2003; Yabe and Kubota, 2004; Yabe and Shibata, 2011; Legrand et al., 2013; Sakai et al., 2020). Furthermore, Eurosinian-type coniferous pollen (e.g. Araucariaceae and Cheirolepidiaceae) has been found in the Barremian Kuwajima and Itsuki formations and palynoassemblages of these formations exhibit high similarities to those of the Ryoseki-type localities (Figure 2; Legrand et al., 2021).

Oishi (1940) reported the occurrence of “Ptilophyllum” pachyrachis from a horizon that is now included in the Tithonian to Berriasian Ashidani Formation (Fujita, 2002; Sato and Yamada, 2005; Yamada and Uemura, 2008; Sano, 2015; Yamada, 2017; Figures 1, 2). Given that Ptilophyllum is a characteristic genus of Eurosinian-type vegetation (Vakhrameev, 1987, 1991; Kimura, 2000), this record suggests that the vegetation of the Tetori Basin included some Eurosinian-type plants from the earliest stage. The importance of this record was overlooked when this species was invalidly transferred to the genus Pterophyllum (Kimura and Ohana, 1987). Here, we highlight the importance of Oishi's (1940) finding by confirming that one of the collected specimens (UHR8485D) belongs to a Ptilophyllum species. The Tetori Basin was located adjacent to the southern border of the Siberian province (Yamada et al., 2018), therefore, the vegetation of the Tetori Group may have been influenced by Eurosinian-type vegetation to some extent.

It has been argued that global warming from the Aptian onward triggered a northward migration of Eurosinian-type plants into the area of the Tetori Group (Sakai et al., 2018, 2020), but this argument is inconsistent with a climatic reconstruction that a humid zone prevailed at the latitude of the Tetori Basin (Hasegawa et al., 2012). Therefore, local topographic factors should also be taken into account to explain vegetation changes in this area, such as a rain shadow effect caused by high mountains (Yamada et al., 2018). The “rediscovery” of Ptilophyllum provides another example wherein local topography is shown to have affected the vegetation of the Tetori Group. In the Tetori Group, marine transgressions occurred at least twice, i.e., in the Tithonian to Berriasian and the Hauterivian to Barremian (Sato and Yamada, 2005; Sano, 2015; Yamada, 2017). These transgressions coincided with the appearance of Eurosinian-type plants (Figure 2), which implies that marine transgression may have provided drying coastal environments suitable for Eurosinian-type plants.