2026 年 30 巻 p. 149-163
A new species of mountain avens, Dryas nukabiraensis sp. nov., is described based on well-preserved leaf fossils from the Late Miocene lacustrine mudstone of the Tokachihoroka Formation distributed in Lake Nukabira area, eastern central Hokkaido, Japan. Based on the preservation of the new leaf fossils, which is consistent with predopositional transport, and the mixed physiognomic signals of the co-occurring leaf fossils, as well as the modern climate range of extant Dryas, D. nukabiraensis may have inhabited in an open, boreal to cold-temperate highland environment. The present discovery is important as this species is the earliest fossil record of the genus Dryas, extending its range to the Late Miocene. It also represents the southernmost fossil record of the genus in the Northern Hemisphere. We hypothesize that the genus first appeared at mid-latitude areas of East Asia under boreal to cold-temperate climatic conditions, and subsequently spread to Arctic/Subarctic habitats during the Pliocene and Pleistocene.
Modern mountain avens (Chōnosukesō; Dryas L.) is one of the most famous alpine shrubs in northern hemisphere and is well-loved by many plant enthusiasts and mountaineers. Pollen fossils of Dryas are widely used as indicators of cold environments during the Quaternary, and the Older and Younger Dryas stadials that followed the Last Glacial Maximum are named after this genus, making it relatively familiar to geologists and paleoclimatologists as well.
Today, the genus Dryas has a circum-Arctic distribution that extends south to alpine and subalpine regions in temperate latitudes, such as the Rocky Mountains in western North America, the Alps in Europe, the Japanese Alps in Chubu, Japan, and the Daisetsu Mountains (i.e., Daisetsuzan Volcanic Group) in Hokkaido, Japan (e.g. Hultén, 1959; Marcysiak, 2014; Hirao et al., 2017; Flora of North America Editorial Committee, 2024). Dryas exhibits considerable morphological variation, and many taxa were formerly treated as subspecies or varieties of D. octopetala L. According to Plants of the World Online (2024), the genus currently comprises 11 accepted species worldwide. Among these species, only Dryas ajanensis Juz. is native to the Japanese Archipelago, including Hokkaido.
The frequent recovery of Dryas pollen from numerous Late Pleistocene and Holocene sediment cores—especially across Europe and the Arctic—demonstrates that this genus had an wider range throughout the Northern Hemisphere during Quaternary glacial periods than it does today (e.g. Pons et al., 1987; Fyles et al., 1998; Birks and Birks, 2008). Compared to pollen records, macrofossil records of Dryas are scarce. But Dryas leaf fossils resembling D. octopetala or D. integrifolia Vahl have been recovered from Pliocene to Pleistocene deposits in highland Alaska and lowland arctic or subarctic Canada (Fletcher et al., 2021). It is thought that, during the last glacial period, the genus extended its distribution to lower latitudes, including the Japanese archipelago, before retreating to alpine zones in response to post-glacial warming estimated from the recent distribution of this genus, fossil records, and molecular phylogeographic studies (e.g. Murray, 1995; Hirao et al., 2017; Ikeda, 2022). However, the exact origins of this genus and many other “circumpolar plants” are still uncertain. Some theories propose that such “circumpolar plants” may have appeared in the mountainous regions of Asia or North America in the late Neogene (Murray, 1995), but such hypotheses remain to be tested.
Here, we describe an assemblage of new Dryas leaf fossils from lacustrine strata of the Upper Miocene Tokachihoroka Formation in the Lake Nukabira area of eastern central Hokkaido and discuss their chronological and palaeogeographical implications.
The study area is near Lake Nukabira, which is located at an elevation of 520 m on the eastern side of the Daisetsu Mountains in eastern central Hokkaido (detailed in Geological Setting). In this area, the mean annual temperature is approximately 3.5°C, and annual precipitation reaches 1,297.7 mm. Monthly mean temperatures range from a maximum of 22.4°C to a minimum of −17.3°C (a difference of 39.7°C), and snow cover persists for 8 months from October to May (Ministry of the Environment, Hokkaido Regional Environmental Office, 2011).
Relatively low elevations near Lake Nukabira are characterized by lower mixed conifer–broadleaf forests and other montane vegetation, including conifers such as Picea jezoensis and Abies sachalinensis and broadleaf species such as Acer pictum, Quercus crispula and Tilia japonica. In the surrounding area, subalpine vegetation is dominant, such as Picea jezoensis–Abies sachalinensis and Betula ermannii forests (Hokkaido Government, 1981). The Daisetsu Mountains reach higher elevations to the west and north, and alpine windblown rocky gravel dwarf scrub and herbaceous communities (Dicentro-Violetum crassae) inhabit rocky terrain at around 2,000 m elevation, including the Dryas octopetala (= D. ajanensis) subassociation (Hokkaido Government, 1981). In limestone areas of Hokkaido such as Kirigishi Mountain to the west of the Daisetsu Mountains, Dryas octopetala lives within Juniperus communis var. nipponica communities at elevations of 785–990 m, and Potentilla hyparctica communities dominated by Dryas octopetala (= D. ajanensis; Chounosuke-sō vegetation units) are found at elevations of 820–1,050 m (Sato, 2007).
Late Miocene lake deposits are widely distributed in the Nukabira area (Narita and Oppata, 2023). This area extends from Kuroishidaira in Kamishihoro Town, located southeast of Lake Nukabira, through Nukabira Gensenkyo, to the Horoka region to the north of Lake Nukabira, encompassing the Otofuke River basin (Figure 1). The detailed geology of this area was described by Yamagishi and Matsunami (1976) and Yamagishi (1976). Narita and Oppata (2023) reported on the plant macrofossil assemblages from the Tokachihoroka (the Tokachihoroka flora) and the Taushubetsu formations (the Taushubetsu flora) distributed around the Lake Nukabira area, together with the stratigraphy of the Cenozoic formations in the area to discuss the ages of each assemblage indicated by these fossils. Accordingly, the geology of the Lake Nukabira area consists of the Upper Cretaceous–Paleocene Hidaka Group as the basement on the western side (Watanabe and Iwata, 1987; Kiminami et al., 1990). This is overlain, in acending stratigraphic order, by the Middle Miocene Shikaribetsugawa (Watanabe and Iwata, 1985) and Tokachihoroka formations, the Upepesanke Welded Tuff and hydrothermally altered andesite (“propylite”), Lower Pleistocene lava flows (such as Onsenyama Lava, Setayama Lava, and Fujigawa Lava), and the contemporaneous Taushubetsu Formation, Eboshiyama Volcanic Ejecta, and the Tokachimitsumata Formation (Figures 1, 2; Narita and Oppata, 2023).
Among these formations, the Tokachihoroka Formation is widely distributed around the Lake Nukabira area and is subdivided into three informal members: the lower Alternating Beds of Conglomerate, Sandstone and Tuff Member; the middle Green Tuff Breccia Member; and the upper Rhyolite Welded Tuff Member (Figures 1, 2; Yamagishi, 1976; Yamagishi and Matsunami, 1976; Narita and Oppata, 2023). The lower member is intercalated by brecciated hydrothermally altered andesite (so-called “propylite”) at the base.
Plant macrofossil assemblages (the Tokachihoroka flora) are found in the lower Alternating Beds of Conglomerate, Sandstone and Tuff Member, particularly at the Kuroishidaira and the Maruyamabashi sites (Loc. Kd, My; Figure 1; Narita and Oppata, 2023). The Dryas fossil specimens reported in this study were all collected from the member, with twenty-five fossil leaves from the Kuroishidaira site (Loc. Kd; Figure 1) and one from the Maruyamabashi site (Loc. My; Figure 1).
Although the plant fossil-bearing lower member is not well dated, its age can be inferred from the underlying and overlying strata. The Fission Track (FT) age of 6.8 ± 0.6 Ma was reported for the middle Green Tuff Breccia Member, part of which is interfingered and is the overlying bed of the plant fossil-bearing lower member (Koshimizu and Kim, 1986; Figure 2). FT ages of 6.6 ± 0.5 and 6.7 ± 0.6 Ma were reported for upper part of the Horokabiribetsugawa Formation (Koshimizu and Kim, 1986; Figure 2). A FT age of 14.0 ± 1.2 Ma was reported for Green Tuff Member, the lower part of the Horokabiribetsugawa Formation (Koshimizu and Kim, 1986; Figure 2). The Shikaribetsugawa Formation, which is covered by the Tokachihoroka Formation, yields Middle Miocene benthic foraminifera like Ammodiscus sp. (Watanabe and Iwata, 1985). On the other hand, the Upepesanke Welded Tuff, which overlies the Tokachihoroka Formation, has been dated to 4.6 ± 0.2 Ma using the K–Ar method (Watanabe and Iwata, 1987; Figure 2). Considering the FT ages obtained from the Green Tuff Breccia Member, which overlies the Alternating Bed of Conglomerate, Sandstone and Tuff Member, and from the Horokabiribetsugawa Formation, which is temporally correlative with the Dryas fossil-bearing strata, the Alternating Bed of Conglomerate, Sandstone and Tuff Member can be reliably constrained between 7–6 Ma. If the age of the middle Green Tuff Breccia Member is taken into account, the Tokachihoroka flora containing Dryas fossils can be further constrained to ca. 6.8 Ma (i.e., Late Miocene).
The Dryas leaf fossils reported here occurred at the same localities as the Tokachihoroka flora (Narita and Oppata, 2023). The materials used in this study were collected from two localities of the Tokachihoroka Formation (Locs. Kd and My; Figure 1). The fossil-bearing horizon consisted of parallel-laminated mudstone. Plant macrofossils from both the Kuroishidaira (Loc. Kd; Figure 1) and Maruyamabashi (Loc. My; Figure 1) mostly comprise leaves and other organs such as samaras and winged seeds that were scattered parallel to the bedding plane. According to Narita and Oppata (2023), the Tokachihoroka flora comprises 47 taxa in 18 families and 27 genera, and the dominant species is Betula miomaximowicziana, associated with other deciduous Betulaceae species such as B. protojaponica, B. protoglobispica, B. onbaraensis, Alnus protohirsuta, A. protomaximowiczii as well as Acer rotundatum, Sorbus uzenensis, Fagus palaeojaponica and evergreen conifers such as Picea sp., Pinus sp. cf. P. palaeopentaphylla and Tsuga sp. (Narita and Oppata, 2023). The flora represents a combination of lakeside, slope and subalpine vegetation types and indicates humid and cool temperate climate condition (Narita and Oppata, 2023).
Dryas leaf fossils are sparsely distributed along parallel lamination planes of mudstone within the Tokachihoroka Formation along with other plant macrofossils of the Tokachihoroka flora, including Betula leaves.
Most of the plant fossils are preserved as impressions, yielding no structurally preserved material. The specimens were photographed using an OM System TG-7 digital camera with a LG-1 LED light guide. Some specimens were also photographed using a Dino-Lite Edge M Fluorescence GFBW under both LED and UV illumination.
The descriptive terminology for dicotyledonous leaves follows the protocols of Ellis et al. (2009). The taxonomic arrangement of angiosperms is based on Angiosperm Phylogeny Group (2016). The specimens used in this study are stored at the Hokkaido Museum (HOKM) and Higashitaisetsu Nature Center (HTMNH).
Order Rosales Bercht. and J.Presl
Family Rosaceae Juss., 1789
Genus Dryas L. 1753
Dryas nukabiraensis Narita and Oppata sp. nov.
[New Japanese name: Nukabira-Chōnosukesō]
Diagnosis.—Leaf laminae small in size, less than 2.2 cm in length, oblong to elliptic in shape; apex obtuse, rounded; base reflex, cordate; petiole marginal, long, thick, more than 1.0 cm long, visibly covered with fine, whitish hairs; abaxial surface sparsely covered with short, thin hairs, presumably tomentose; adaxial surface with long, sparsely distributed villous hairs; abaxial hairs short, matted, distributed broadly across the lamina and secondary veins; adaxial filamentous hairs scattered, arising predominantly from secondary and primary veins; adaxial filamentous hairs fine in particular near the marginal teeth and intercostal areas; margin crenate-serrate with regularly-spaced teeth; tooth apex rounded, terminal apex retuse; sinuses angular, wide acute; venation pinnate; secondary veins craspedodromous, thin, arranged in 6–8 subopposite to alternate pairs, diverging from primary vein at 60–80°, with basal few pairs decurrent at 80–100°, ending in marginal teeth; tertiary veins thin, indistinct, irregular reticulate; higher order veins thin, irregularly reticulate; areoles developed.
Material.—Holotype, HOKM-190942-1 (Figures 3A, B, G; 4G, I; 5E, F), HOKM-190942-2 (Figure 3C); Paratypes, HOKM-190944 (Figure 3D, I), HOKM-190943-1 (Figure 3E), HOKM-190943-2 (Figure 3F), HOKM-190941 (Figure 3J, K), HTMNH-FO-1126 (Figures 3H; 4E; 5G, H), HTMNH-FO-481 (Figure 4D), HTMNH-FO-298 (Figure 4A, B, C), HTMNH-FO-1425b (Figure 5A), HTMNH-FO-1425a (Figure 5B), HTMNH-FO-1119a (Figure 5C), HTMNH-FO-1119b (Figure 5D).
Locality and horizon.—Only known from the Late Miocene Tokachihoroka Formation (6.8 Ma: Koshimizu and Kim, 1986), located in Kuroishidaira (Loc. Kd) and Maruyamabashi (Loc. My) localities, Nukabira-gensenkyo, Kamishihoro-cho, Kato-gun, Hokkaido (Figure 1).
Etymology.—Species named after the type locality Lake Nukabira in eastern central Hokkaido, Japan.
Description.—Leaf laminae symmetrical, length 0.8–2.2 cm, width 0.5–1.2 cm, length:width ratio 1.25–2.2:1, oblong to elliptic in shape; apex angle obtuse, convex, rounded in shape; base angle reflex, cordate in shape; petiole marginal, long, thick, more than 1.0 cm long, ca. 0.5 mm wide at base visibly covered with fine, whitish hairs, forming a pale indumentum; abaxial surface sparsely covered with short, thin hairs, presumably tomentose; abaxial hairs short, matted, distributed broadly across the lamina and secondary veins; adaxial surface with long, sparsely distributed villous hairs; adaxial filamentous hairs scattered, arising predominantly from secondary and primary veins; adaxial filamentous hairs fine particularly near the marginal teeth and intercostal areas; margin unlobed, crenate-serrate; teeth regularly-spaced with one order of teeth and 4–5 teeth/5 mm; distal flank of tooth convex or retroflexed, proximal flank convex; tooth apex rounded, terminal apex retuse; sinuses angular, wide acute; teeth medially supplied by principal vein; venation pinnate; primary vein stout, nearly straight at the base of the laminae, tapering toward the apex; secondary veins craspedodromous, thin, arranged in 6–8 subopposite to alternate pairs, variation of secondary angle smoothly increasing proximally, diverging from primary vein deflected at 60–80°, with basal few pairs decurrent at 80–100°, ending in the marginal teeth, majority of secondaries attachment to midvein decurrent, partly excurrent at the base; intersecondary absent; tertiary veins thin, indistinct, irregular reticulate; quaternary veins thin, irregular reticulate, quinternary vein irregular reticulate; areoles moderately developed; veinlets thin, mostly unbranched.
Comparison.—The new specimens have clear affinities to Dryas species with petiolate leaves, oblong to elliptic shapes with cordate bases, convex to rounded apex shape, crenato-serrate margins, craspedodromous secondary veins and abaxial tomentose hairs. They are characterized by having lobe-like marginal teeth (Figure 4G) and remarkable tomentose hairs on the adaxial surface (Figures 4E; 5G, H). This combination of characteristics is unique to Dryas, and the new specimens are assigned to that genus. These specimens therefore represent the first reported occurences of Dryas from the Miocene.
Owing to their oblong to elliptic leaf shape, reflex base angle, number of secondary pairs, length–width ratio and crenate-serrate margin, the new fossil Dryas leaves can be clearly differentiated from the extant D. integrifolia, D. drummondii Richardson ex Hook, D. alaskensis Porsild, D. incisa (Malyschev) Juz. ex Jurtzev, D. hookeriana Juz., D. grandis Juz., D. sumneviczii Serg., D. viscosa Juz. and D. oxydonta Juz., all of which exhibit notably different leaf shape (Table 1). The new Dryas fossils can also be distinguished from the extant D. octopetala by their obtuse apex angle, convex and rounded apex shape, and a relatively lower number of secondary veins (Table 1). This combination of morphological characteristics closely resembles that of living D. ajanensis (syn. D. octopetala var. asiatica), which lives in the alpine regions of central Honshu to Hokkaido in Japan, East Siberia, Magadan, inner Mongolia, Kamchatka, western North America including Alaska, and the Korean Peninsula. However, the new specimens differ from D. ajanensis in terms of numbers of secondaries and scattered adaxial filamentous hairs (Table 1). Although the number of secondary veins in the extant D. octopetala is known to vary with precipitation (Marcysiak, 2014), our fossil specimens consistently exhibit fewer secondary veins than extant D. ajanensis, suggesting a stable morphological distinction. Compared with extant Dryas species, although the sparse hair density may be partly influenced by the preservation of the fossil, the new fossil leaves show much less dense hairs on the abaxial surface (Figures 4E; 5G, H). The new specimens are clearly distinguished by the presence of long villous hairs on the adaxial surface (Figures 3F, G; 4G, I; 5E, F). While extant D. ajanensis shows fine villous hairs densely spread between secondary veins (Figure 4H, J), the new fossil specimens are characterized by hairs that are prominent long or partly emerging from vein axes, particularly on the secondary and primary veins (Figures 3F, G; 4G, I). In contrast to the extant D. ajanensis, in which no hairs are observed near the leaf margins (Figure 4H), the new fossil specimens exhibit numerous long hairs on and around the crenate-serrate margins (Figures 3F, G; 4G; 5E, F). Thus, the tendency toward a reduced number of secondary veins, much less dense tomentose hair on the abaxial surface, the presence of long hairs arising from the secondary, and primary veins on the adaxial leaf surface and the consistent occurrence of long hairs around the marginal teeth distinguish the new fossil specimens morphologically from any of the extant species, but they may be closely related Dryas ajanensis (Table 1). Based on these differences, we propose our fossil specimens as a new fossil species, Dryas nukabiraensis.
| Species | D. nukabiraensis sp. nov. | D. ajanensis Juz | D. octopetala L. | D. integrifolia Vahl | D. drummondii Richardson ex Hook | D. alaskensis Porsild |
|---|---|---|---|---|---|---|
| Age | Late Miocene | extant | extant | extant | extant | extant |
| Distribution | Japan (Hokkaido) | East Siberia, Magadan, inner Mongolia Kamchatka, Alaska, western North America, Korean peninsula, Japan (Hokkaido, central Honshu) | Europe, Siberia, Magadan, Mongolia, Tibet, Kamchatka, Alaska, Greenland | Siberia, Magadan, Alaska, northen and eastern Canada,western North America, Greenland | Alaska, Yukon, western America, western and eastern Canada | Alaska, Magadan,Yukon |
| Size (cm) | length 0.8–2.2 width 0.5–1.2 | length 0.8–1.6 width 0.5–1.2 | length 0.3–1.6 width 0.1–0.7 | length 0.2–3.8 width 0.5–1.1 | length 0.4–3.8 width 0.2–2.4 | length 0.5–3.9 width 0.2–1.9 |
| Length:width ratio | 1.3–2.2:1 | 1.3–1.6:1 | 1.5–2.2:1 | 2.0–3.5:1 | 1.6–2.0:1 | 2.1–2.5:1 |
| Shape | oblong to elliptic | oblong to elliptic | oblong or ovate to elliptic | oblong or lanceolate | oblong–elliptic to obovate | elliptic to narrow obovate |
| Apex angle | obtuse | obtuse to acute | acute | acute to obtuse or acuminate | obtuse | obtuse to reflex |
| Apex shape | convex, rounded | straight to convex, rounded, slightly emarginate (retuse) | convex, rounded | straight to convex, rounded, slightly emarginate (retuse) | convex, rounded, slightly emarginate (retuse) | convex, rounded |
| Base angle | reflex | reflex | reflex | obtuse to reflex | acute | obtuse to reflex |
| Base shape | cordate | truncate or cordate | truncate or cordate, or cuneate | truncate to slightly cordate, basal extension asymmetrical | cuneate, truncate or cordate | cuneate or cordate |
| Divergent angle of secondary from midvein | 60–80° ≥90° at basal part | 60–80° ≥90° at basal part | 60–80° ≥90° at basal part | 60–80° | 60–80° | 60–80° |
| Number of secondary pairs | 6–8 | 10 or >10 | 7–13 | >10 | 7–9 | 10–14 |
| Abaxial surface | tomentose or short, thin hairs | tomentose to wooly or lanate | tomentose to woolly | tomentose or long silky | woolly tomentose | tomentose or sparsely hairly |
| Adaxial surface | sparsely villous with long hairs from mid and second veins, many filamentous hairs on leaf margins | glabrous to sparsely hairy, proximally on midvein | glabrous to sparsely hairy, proximally on midvein | glabrous to sparsely hairy, proximally on midvein | glabrous to sparsely hairy | glabrous or sparsely hairy, proximally on midvein |
| Margin | crenate-serrate, partly erose | crenate-serrate, partly erose | coarsely dentate to serrate | entire or slightly crenate or revolute | crenate, dentate or coarsely once or twice serrate | coarsely dentate to serrate |
| Tooth features | tooth apex rounded | tooth apex rounded | tooth apex rounded | usually entire or dentate to crenate | tooth apex rounded | tooth apex rounded |
| Species | D. incisa (Malyschev) Juz. ex Jurtzev | D. hookeriana Juz. | D. grandis Juz. | D. sumneviczii Serg. | D. viscosa Juz. | D. oxydonta Juz. |
|---|---|---|---|---|---|---|
| Age | extant | extant | extant | extant | extant | extant |
| Distribution | Siberia, Magadan, Alaska, Yukon | Alaska, northern Canada, northwest North America | central and eastern Siberia, Magadan, Kamchatka | East Siberia | East Siberia | Altay, Kazakhstan, Mongolia, western and central Siberia |
| Size (cm) | length 0.6–2.4 width 0.2–1.0 | length 0.4–2.5 width 0.1–1.0 | length 0.4–3.7 width 0.3–1.2 | length 0.8–2.2 width 0.4–0.8 | length 1.4–3.6 width 0.2–1.1 | length 0.4–1.4 width 0.1–0.6 |
| Length:width ratio | 2.4–3.0:1 | 2.5–4.0:1 | 1.7–3.1:1 | 2.0–2.8:1 | 3.3–7.0:1 | 2.3–4.0:1 |
| Shape | oblong to elliptic or lanceolate | oblong–elliptic to lanceolate | oblong to obovate | oblong to elliptic or obovate | elliptic to obovate | oblong to obovate |
| Apex angle | acute to obutuse | acute to obtuse | acute | acute to obtuse | acute | acute |
| Apex shape | straight or convex, rounded | convex, rounded | convex, rounded | convex, rounded | convex, rounded | straight or convex, rounded |
| Base angle | obtuse or reflex | acute to obtuse | acute | reflex | acute | acute |
| Base shape | cuneate, slightly cordate or convex, rounded | cuneate, truncate or cordate | cuneate | cordate | cuneate | convex, rounded or slightly cordate |
| Divergent angle of secondary from midvein | 50–60° 80–90° at basal part | 60–80° ca. 90° at basal part | 60–70° | 60–80° ≥90° at basal part | 60–80° ca. 90° at basal part | 70–90° |
| Number of secondary pairs | 9–11 | 8–12 | 9–11 | 9–11 | 8–10 | 6–9 |
| Abaxial surface | tomentose | tomentose to woolly (obscuring lateral veins) | tomentose | tomentose | tomentose | tomensose |
| Adaxial surface | glabrous or tomentose | glabrous or sparsely hairy, proximally on midvein | sparsely villous with long hairs from midvein | densely villous with long hairs, especially near midvein | glabrous | glabrous |
| Margin | strongly revolute to entire, or crenate, dentate to serrate | strongly revolute to flat, coarsely dentate or serrate | strongly revolute to entire, or coarsely dentate to serrate | crenate-serrate, partly erose | crenate-serrate, partly erose | crenate-serrate, partly erose |
| Tooth features | tooth apex rounded | tooth apex rounded | tooth apex acuminate to mucronate | tooth apex rounded | tooth apex rounded | tooth apex rounded |
Extant Dryas species are generally adapted to open alpine or arctic environments. The native range of the extant D. ajanensis (syn. D. octopetala var. asiatica), which closely resembles D. nukabiraensis, is East Siberia to the Russian Far East, the Korean Peninsula, and Japan. The extant D. ajanensis is a subshrub that grows primarily in subalpine or subarctic habitats (Plants of the World Online, 2024). Regional records indicate that it typically lives on open rocky and meadow sites at higher elevations such as windswept alpine areas (Wada et al., 2003; Wada, 2008). In the Daisetsu Mountains (including the Lake Nukabira area), populations of D. ajanensis also typically occur in open, well-drained alpine habitats such as wind-exposed fellfields and rocky slopes (Hokkaido Government, 1981). Dryas ajanensis can also occur at relatively lower elevations at Hokkaido, reflecting local differences in habitat conditions (Sato, 2007). Although the habitat of a fossil plant species may not always correspond to those of its closest extant relatives, if fossil species of Dryas occupied ecological niches broadly similar to those of extant species, D. nukabiraensis grew in an open boreal to cold-temperate highland environment.
Dryas nukabiraensis sp. nov. is based on 26 leaf fossil specimens obtained from parallel-laminated mudstone of the Alternating Beds of Conglomerate, Sandstone and Tuff Member of the Tokachihoroka Formation. Since the plant-bearing sediments are assumed to have been deposited in a lacustrine environment, the fossil assemblage (i.e., the Tokachihoroka flora) is clearly allochthonous and possibly derived from several different parts of the landscape, having been transported by river currents and/or wind or having fallen directly from the surrounding forests. The following discussion regarding the habitat of Dryas nukabiraensis sp. nov. is based on the occurrence of fossil leaves together with the ecology of their nearest living relatives.
The leaf fossils of D. nukabiraensis are sparsely distributed along parallel lamination planes of mudstone within the Tokachihoroka Formation, and most of these leaves are somewhat damaged (Figures 3, 4, 5). These observations may indicate that the D. nukabiraensis leaves were transported some distance prior to their final deposition. The Tokachihoroka flora comprises 47 taxa, and it is especially dominated by such species as Betula miomaximowicziana, B. protojaponica, B. protoglobispica, Sorbus uzenensis, Picea sp., Fagus palaeojaponica and Alnus protohirsuta, while the new species of Dryas is relatively rare element represented only by the 26 isolated leaf remains reported here. It is hypothesized that the Tokachihoroka flora originated from riparian, montane, and subalpine vegetation behind the lake basin, considering the ecology of living equivalents of the taxa that comprised the flora (Narita and Oppata, 2023). Taking into account both the occurrence of the leaf fossils and the ecology of their living equivalents, Dryas leaf remains may have originated from highland vegetation.
Narita and Oppata (2023) conducted a Climate Leaf Analysis Multivariate Program (CLAMP: Wolfe, 1993, 1995; Wolfe and Spicer, 1999) analysis based on leaf morphology of the Tokachihoroka flora, estimating the paleoclimate conditions of the Lake Nukabira area during the Late Miocene. The results show that the mean annual temperature (MAT) was 8.4°C, the warmest month’s mean temperature (WMMT) was 20.1°C, the coldest month’s mean temperature (CMMT) was −2.5°C, the number of months with a mean temperature exceeding 10°C (GROWSEA) was 5.5 months, the precipitation during those periods (GSP) was 459 mm, and the precipitation during the wettest three months (3-WET) and the driest three months (3-DRY) were 482 mm and 169 mm, respectively. Although not directly calculated from CLAMP, the mean annual range of temperature (MART) was 22.6°C, calculated from the difference between WMMT and CMMT (Narita and Oppata, 2023). Considering the above temperature and precipitation data, Narita and Oppata (2023) concluded that the Tokachihoroka flora existed under a humid cool temperate climate, characteristic of a Mixed Coniferous Forest. Estimated MAT, WMMT, and CMMT values were significantly higher than those of the present Lake Nukabira area, while the 3-WET and 3-DRY values were notably lower. However, the paleoclimate parameters estimated by CLAMP represent averaged conditions over a broad region surrounding the fossil-bearing site, and do not necessarily reflect the specific climate under which the Dryas nukabiraensis grew. Considering the possibility that the leaves of Dryas nukabiraensis were transported to the depositional site and that its closest extant relatives typically inhabit higher-elevation environments, it is possible that the leaves of D. nukabiraensis were transported from a higher-elevation area that was cooler than the MAT estimated by CLAMP—that is, from a boreal to cold-temperate climatic environment.
Therefore, D. nukabiraensis may have occupied open boreal to cold-temperate highland habitats broadly comparable to those of extant Dryas, including D. ajanensis.
Phytogeographic and historical implications of Dryas nukabiraensisExtant Dryas species have a wide distribution across high-latitude cold and subarctic tundra surrounding the Arctic, representing a typical circumarctic pattern (Figure 6A). At the same time, has a disjunct distribution in mid-latitude alpine regions, such as the Rocky Mountains in North America and the high mountains of central Japan (Hultén, 1959). Eleven extant species of the genus Dryas are currently accepted (Plants of the World Online, 2024). Considerable regional morphological variation has also been reported, particularly within D. octopetala, which exhibits a wide circumpolar distribution in the Arctic and was previously considered to comprise several infraspecific taxa, such as D. octopetala var. asiatica (e.g. Jalas and Suominen, 1989; Flora of North America Editorial Committee, 2024). According to classical taxonomy, Dryas octopetala var. asiatica is distributed across East Asia, including Japan, however, recent taxonomic treatments (e.g. Flora of North America Editorial Committee, 2024) recognize this taxon as a distinct species, which is also found in Alaska and northwestern Canada.
Circumarctic plants, including Dryas, are thought to have originated in the late Neogene during global cooling (Bennike and Böcher, 1990; Matthews and Ovenden, 1990), with their ancestors likely inhabiting mountainous regions of Asia and North America (Murray, 1995; Abbott and Brochmann, 2003). During Pleistocene glaciations, the ranges of these plants expanded southward, with some populations persisting in refugia such as Beringia. Following post-glacial warming, many populations dispersed from these refugia (e.g. Abbott et al., 2000), while southern populations retreated into cold high-altitude environments, producing the present-day disjunct alpine distribution (Weber, 1965; Murray, 1995). While this general evolutionary scenario of Arctic–alpine plants is broadly accepted (e.g. Takahashi, 2000; Wada et al., 2003; Wada, 2008), the precise geographic and temporal origins of each lineage remain poorly understood.
Major fossil occurrences of Dryas from the Pliocene and Pleistocene are summarized below to clarify the spatiotemporal context of the genus. Dryas leaf fossils are known from some localities in the Arctic/Subarctic region (Figure 6B). Leaf fossils comparable to D. octopetala, which resemble D. ajanensis are, known from the Early Pliocene Beaver Pond (3.9 Ma; Fletcher et al., 2021) and Fyles Leaf Beds (3.8 Ma; Fletcher et al., 2021), the Early Pleistocene Prince Patrick Island–Green Bay Beds (2.5 Ma; Bennike, 1990; Fletcher et al., 2021), Kap København (2.5 Ma; Bennike, 1990), the Hvitland Beds (2.4 Ma; Fyles et al., 1994, 1998), and the Fosheim Dome (1.5 Ma; Fletcher et al., 2021). Leaf fossils similar to D. integrifolia were recognized from the Early Pliocene Beaver Pond Site near Stathcona Fiord (3.9 Ma; Fletcher et al., 2021), the Fyles Leaf Beds (3.8 Ma; Fletcher et al., 2021), and Meighen Island (>3 Ma; Barendregt et al., 2021; Fletcher et al., 2021). Dryas fossils are also known from some other localities (e.g. Begunov: Sher et al., 1979; Circle Gravels: Ager et al., 1994; Figure 6B), but some of them were not identified at the species level and the others were reported from pollen. The Dryas leaf fossils from the Lake Nukabira area, reported in this paper, show morphological resemblance to D. ajanensis (Figures 3, 4, 5). As a Late Miocene fossil species, D. nukabiraensis suggests that the genus Dryas appeared by at least the Late Miocene.
Previous macrofossil records of Dryas have primarily been from the Arctic regions, including Alaska, Canada, and Greenland (Fletcher et al., 2021). Dryas pollen has been recognized from numerous localities across Europe and the Arctic (e.g. Pons et al., 1987; Fyles et al., 1998; Birks and Birks, 2008; Fletcher et al., 2021). The age of each fossil record is either Pliocene or Quaternary. The earliest record of Dryas fossils in a previous study, dating to 5–4 Ma, were found in the Circle Gravels of Alaska (Ager et al., 1994; Figures 6B, 7). Dryas nukabiraensis sp. nov. from the Upper Miocene Tokachihoroka Formation (ca. 6.8 Ma) in the Lake Nukabira area represents both the earliest and geographically southernmost fossil record of this genus (Figures 6B, 7). Based on these findings, we hypothesize that Dryas may have migrated northward from Hokkaido and dispersed widely to subarctic and Arctic areas, including Siberia, Alaska, northern Canada, Greenland, and Europe. We note that the genus Dryas might have appeared at mid-latitude areas of East Asia under boreal to cold-temperate climatic conditions, and then spread to habitats in the Arctic/Subarctic area during the Pliocene to Pleistocene. The occurrence of D. nukabiraensis in the Late Miocene of Hokkaido provides crucial evidence for the early evolution of the genus in mid-latitude East Asia.
We are grateful to Toshihiro Yamada (Hokkaido University) and Hajime Ikeda (University of Tokyo) for their advice. Thanks are also due to Masayuki Furuto and Sachiko Furuto for their cooperation in fossil collection at Kuroishidaira. The content of this manuscript was significantly improved thanks to the valuable comments and suggestions provided by Atsushi Yabe (National Museum of Nature and Science) and Tamara Fletcher (University of Adelaide), who served as reviewers. Fieldwork and fossil collection were conducted with the permission of the Ministry of the Environment and the Forestry Agency.
Both authors collected fossils and contributed geological aspect of this study. First author conducted taxonomic study and wrote this paper.
