Leaf, specimen no. MMJ3-008-1a (part) and MMJ3-008-1b (counterpart), designated here (Figures 3A,B,D, 4A,B, 5H, 6A,D,F,H).

The Museum of Biology of the Sun Yat-sen University, Guangzhou, China.

Maoming Basin, Guangdong Province, China.

The Huangniuling Formation, upper Eocene.

From Eocene and Sina (lat.) – China.

Leaves are simple, petiolate, laminar size mesophyll, length to width ratio about 3:1. Lamina is obovate, slightly asymmetrical at the base. Laminar apex angle acute with straight or acuminate apex shape, base angle acute with cuneate shape. Primary venation pinnate. The surface of the primary vein is pubescent. Major secondary veins alternate or occasionally opposite, semicraspedodromous or brochidodromous with spacing decreasing proximally. In the axils of primary and secondary veins leaf domatia developed. Leaf margin untoothed undulate or irregularly serrate in the upper half of lamina. Tooth spacing is irregular with one order of teeth. Sinus shape rounded, tooth shapes typically concave/convex or concave/flexuous. Leaves hypostomatic. Abaxial epidermis is composed of polygonal ordinary cells, anomocytic stomata, and trichomes of two types: simple elongate unicellular and glandular with uniseriate 2–3-celled stalk and single-celled head. Adaxial epidermis is composed of isodiametric ordinary cells with glandular trichomes.

The leaves are simple, petiolate, with marginal petiole attachment. The incompletely preserved petiole is up to 0.8 cm long and 0.15 cm wide. Laminar size is mesophyll. The biggest specimen is 17.5 cm long and up to 6.1 cm wide, the length-to-width (L/W) ratio is approximately 3:1. The laminar shape is obovate. The leaf apex is acute, straight, or acuminate in shape. The leaf base is slightly asymmetrical, with an acute angle and cuneate shape.

Primary venation is pinnate. The primary vein is straight or slightly curved at the base, thick in the basal half and tapering upward, terminating at the apex. The surface of the primary vein is pubescent. Major secondary veins (14–17 pairs) are alternate or occasionally opposite, mixed semicraspedodromous or brochidodromous (mostly in the lower part of the lamina and near the leaf apex) with spacing that decreases proximally. The secondary vein angle is smoothly increasing proximally (from 45 to 50° in the upper part of the lamina to 70° in the lower part). The leaves possess numerous leaf domatia in the axils between primary and secondary veins (Figures 3H, 4E). Interior secondaries are absent. Minor secondary veins, if present, are semicraspedodromous. Intersecondary veins are occasionally present, usually, their length is less than 50% of the subjacent secondary vein, the intersecondary proximal course is almost perpendicular to midvein, while the intersecondary distal course is perpendicular to subjacent secondary vein. Intercostal tertiary veins are mixed percurrent with an obtuse angle to midvein and consistent vein angle variability. Epimedial tertiary veins are mixed percurrent with proximal course perpendicular to the midvein and distal course parallel to intercostal tertiaries. Exterior tertiaries are looped. Quaternary vein fabric is an alternate percurrent. Quintarnary vein fabric is irregular reticulate. Areolation is well developed. Freely ending veinlets are unbranched. Marginal ultimate venation is looped.

The leaf margin is untoothed undulate or irregularly serrate in the upper half of leaf lamina. Tooth spacing is irregular with one order of teeth and usually less than one tooth per 1 cm. Teeth are small and 1–1.5 mm high. Sinus shape is rounded, tooth shapes are concave/convex or concave/flexuous. The principal vein terminates at the tooth apex.

The leaf epidermal characters were obtained from the cuticle fragmentary preserved on the midvein and near domatia. The cuticle is very thin. Ordinary epidermal cells of a costal area are rectangular in shape, with straight anticlinal walls, 40–70 μm long and 9–15 μm wide, bearing numerous trichome bases (Figures 6A,D,F,H).

The epidermis of the leaf abaxial surface is composed of polygonal ordinary cells, randomly oriented stomata, and trichomes of two types: simple elongate unicellular and glandular (Figures 7A,C, 8A,B). The length of the unicellular non-glandular trichomes is up to 600 μm, their diameter at the base is approximately 20–30 μm (Figures 7D,E, 8A, 9C–E). Glandular trichomes have uniseriate stalks that usually consist of 2–3 cells and a single-celled head, the length of trichomes is 45–60 μm, their diameter at the base is about 20 μm (Figure 8A). Stomata are anomocytic with four to six subsidiary cells (Figures 8B, 9B,C,H), stomatal size ranges between 28 and 33 μm in length and between 25 and 28 μm in width.

The epidermis of the adaxial surface consists of isodiametric cells, usually pentagonal with undulate anticlinal cell walls, 37–60 μm long, 23–36 μm wide (Figure 8C). There are only glandular trichomes of the same morphology as on the abaxial surface (Figures 7B, 9A,G).

MMJ3-008-1a, b; MMJ3-321; MMJ3-549-1; MMJ3-837-3a, b; MMJ3-898; MMJ3-2175-1a, b; MMJ3-2205-1; MMJ3-2206-2; MMJ3-2207-1; MMJ3-2207-2.

Leaves of the new species are most similar to the fossil leaves of Meliosma sp. cf. M. simplicifolia from the Eocene Clarno Formation of Oregon, United States (Manchester, 1981) in the size and shape of the lamina, the number of secondary veins, as well as the tertiary and higher order venation patterns. Meliosma eosinica sp. nov. differs from this species mostly in terms of the leaf margin characters: fossil leaves from the Maoming Basin have irregular small teeth only in the upper half of the lamina or untoothed leaf margin. By contrast, leaves from the Clarno Formation are characterized by more prominent teeth arranged almost along the whole leaf margin. However, fossil leaves from the Clarno Formation have not been described in detail.

The leaves of M. eosinica are also very similar in general morphology to the fossil leaves M. shanwangensis Hu et Chaney originally described from the lower-middle Miocene of Shandong, eastern China (Hu and Chaney, 1940). Both species are characterized by an obovate shape of the leaf lamina with an acute or acuminate apex and cuneate base, a similar number of secondary veins (12–16 pairs in M. shanwangensis and 14–17 pairs in M. eosinica) arising at angles of 50–70° from the midvein and distally curving up to the leaf margin, and a slightly asymmetrical base. Leaves of M. shanwangensis differ in having remotely serrate margin and serration starting in some cases from the leaf base.

Leaves of M. eosinica resemble leaves of M. obtusifolia Tao described also from the lower-middle Miocene of Shandong, eastern China (WGCPC (Writing Group of Cenozoic Plants of China), 1978), in respect of their obovate lamina shape and irregularly serrate margin, but differ by having an acute apex. Tao compared these fossil leaves with the leaves of extant M. parviflora Lecomte, which also possess an obovate lamina with an obtuse rounded apex. In addition, leaves of M. eosinica differ from those of M. obtusifolia by their larger lamina size and their greater number of secondary veins (14–17 pairs compared to 10–13 in Meliosma obtusifolia).

The leaves of M. eosinica are similar to those of M. longifolia (Heer) Hickey from the Golden Valley Formation (lower Eocene) of Western Dakota, United States (Hickey, 1977), in the number of secondary veins and their angle of divergence. However, M. eosinica leaves can be distinguished by their obovate shape and irregularly serrate margin.

Unlike the leaves of two species, M. duktotensis Wolfe and M. kushtakensis Wolfe, described from the Eocene Kulthieth and Kushtaka formations of the Gulf of Alaska (Wolfe, 1977), M. eosinica leaves are characterized by a larger lamina size, a greater number of secondary veins, and an irregularly serrate or undulate leaf margin.

The leaves of M. eosinica differ significantly from the leaves of two Paleocene species from North Dakota, United States, M. thriviensis Peppe et Hickey from the lower Paleocene Ludlow Member of the Fort Union Formation and M. vandaelium Peppe et Hickey from the middle Paleocene Tongue River Member of the Fort Union Formation (Peppe and Hickey, 2014). Both American species are characterized by a small number of secondary veins, 9–10 pairs in M. vandaelium and 8–14 (average 10) in M. thriviensis, compared with 14–17 pairs in M. eosinica. The secondary vein angle in M. eosinica is smoothly increasing proximally (from 45 to 50° in the upper part to 70° in the lower part of the lamina) whereas in M. vandaelium the secondary vein angle decreases basally, and in M. thriviensis secondary vein spacing and angle are uniform. These species also differ in their leaf margin characteristics. In M. eosinica, the teeth are sparse, developed mainly in the upper part of the leaf or the leaf margin is untoothed, while in M. vandaelium the margin is serrate to dentate throughout the entire leaf, the teeth are smaller and more frequent (three teeth per 1 cm). In M. thriviensis the teeth are uniformly spaced along the entire lamina (one to two teeth per 1 cm).

The only fossil leaves of Meliosma with studied epidermal characters were reported from the upper Miocene of Tiantai, Zhejiang, China in a conference abstract (Jia and Sun, 2011). Leaves of M. eosinica differ from those of M. cf. flexuosa Pamp. from Zhejiang in having a larger lamina size (6.2–9.8 cm long and 2.2–4.2 cm wide in M. cf. flexuosa), an acute apex (obtuse or narrowly acuminate in M. cf. flexuosa), and smaller irregular teeth (compared with sharp coarse marginal teeth in M. cf. flexuosa). Meliosma cf. flexuosa also differs in having only sparse multicellular hairs composed of 6–7 cells and smaller stomata (15–20 μm long, 12–15 μm wide). However, these Zhejiang fossils were not subsequently published in full.

Within living species of Meliosma, leaves of M. rigida Siebold et Zucc., M. fordii Hemsl., and M. simplicifolia have the most similar leaf architecture to those of M. eosinica. Van Beusekom (1971) treated these species as subspecies of one highly variable species M. simplicifolia, but later they were considered as individual species (Guo and Brach, 2007). The leaves of these three extant species are very similar to each other; and like M. eosinica, they are obovate or elliptic in shape with an acute or acuminate apex and a cuneate base, pinnate semicraspedodromous or brochidodromous venation with distinctly ascending secondary veins, and untoothed or serrate margins usually toothed in the upper half of lamina (Figures 10A–G,I,L). Compared with the new fossil species, M. simplicifolia differs in having larger leaves (15–40 × 4–16 cm) and a greater number of secondary veins (15–28 pairs) (Guo and Brach, 2007). The leaves of M. rigida and M. fordii are usually less than 15 cm long and 5 cm wide, the number of secondary veins is often fewer than 15 pairs. In addition, despite the certain morphological similarity of leaves, extant taxa also differ from each other and fossil species in leaf epidermal features (Figures 6B,C,E,G,I, 11A,D–G, 12).

The epidermal characters of the new fossil species M. eosinica are most similar to those of M. simplicifolia (Figures 6C,G,I, 7F,G, 9F, 11A,D–G; Supplementary Table 2). The epidermis of the leaf abaxial surface in both extant and fossil species is composed of ordinary cells lacking papillae, with randomly orientated anomocytic stomata and trichomes of two types: non-glandular and glandular, which are especially numerous in the axils between primary and secondary veins forming domatia of hair-tuft type (e.g., O’Dowd and Willson, 1989). On the leaf adaxial surface of both species, mostly glandular trichomes of a similar morphology are presented. The stomata in the modern species are slightly larger, 38–45 μm long and 35–37 μm wide, compared with 28–33 μm long and 25–28 μm wide in M. eosinica. M. eosinica differs from M. simplicifolia in having unicellular non-glandular trichomes. The non-glandular trichomes of the modern species consist of 2–5 cells (Figures 6C,I), and the length of trichomes varies from 100 to 500 μm. The surface of non-glandular trichomes in the new species is smoother (Figures 9E, 11F). The glandular trichomes of M. eosinica are smaller than those of M. simplicifolia, in that they consist of 2–3 cells and are up to 60 μm long (compared to 4–5 cells in the trichomes of usually more than 100 μm long in M. simplicifolia).

Unlike M. eosinica, on the leaf abaxial surface of M. rigida, all epidermal cells have papillae, the stomata are smaller (10–14 μm long and 7–9 μm wide compared with 28–33 μm long and 25–28 μm wide in M. eosinica), sunken, and surrounded by subsidiary cells with large papillae which partly cover the stomatal pit (Figure 12G; Supplementary Table 2). The epidermal characters similar to those of M. rigida are observed also in the leaves of M. fordii (Figures 12A–E; Supplementary Table 2): all epidermal cells of the abaxial surface are papillate, the stomata are relatively small (17–23 μm long and 15–18 μm wide), and the ordinary cells of the adaxial surface are non-papillate, polygonal, with straight anticlinal walls, 40–50 μm long, and 26–30 μm wide (Figure 12F). Similar to the fossil leaves of M. eosinica, numerous non-glandular and glandular trichomes are present on the leaf abaxial surface of both modern species, particularly on the midvein (Figures 12A,C). As in M. eosinica, on the leaves of M. fordii and M. rigida, simple unicellular non-glandular trichomes are sometimes present (Figure 12C), although more often, they consist of several cells (Figure 12D). Non-glandular trichomes on M. rigida leaves are usually elongated with a short basal cell and a cylindrical terminal cell.

Leaves of the extant Japanese species M. tenuis Maxim. differ from those of M. eosinica in being of smaller leaf size, having a mainly elliptic (not obovate) lamina shape, straight secondary veins craspedodromously terminated at the tooth apex and a biserrate leaf margin toothed along the entire length of the lamina (Figures 10H,J,K). The leaf epidermal features of M. eosinica are similar to those of M. tenuis in stomatal size (26–37 μm long and 15–20 μm wide in M. tenuis compared to 28–33 μm long and 25–28 μm wide in M. eosinica), with non-papillate ordinary cells on abaxial and adaxial surfaces and numerous trichomes of two types (Figures 11B,C; Supplementary Table 2). However, in contrast to M. eosinica, the non-glandular trichomes of M. tenuis are much longer (up to 1,000–1,100 μm) and consist of several cells.

Pollen grains recovered from the leaf surface of M. eosinica are small, prolate to subprolate, tricolpate (probably, tricolpate but this is not detectable on available SEM photos), and perforate (Figure 13). Perforations are distributed more or less regularly, at a distance 3–5 times greater than the perforation diameter. They become smaller on apocolpium regions and toward apertures; there is a narrow smooth margo along the aperture. The aperture membrane is granular. The colpus ends are pointed. The pollen outline is elongated in equatorial view and rounded in polar view (the polar outline is based on one pressed pollen grain and might be incorrect). The polar axis is 16.2–19.2 μm, the equatorial diameter is 10.5–12.5 μm, and the colpus length is 11.2–13.6 μm (based on three pollen grains).

We found no previous data on fossil Meliosma pollen. Studies on the pollen grains of extant Meliosma species are relatively few with the most detailed study SEM and transmission electron microscopy (TEM) by Furness et al. (2007), and short descriptions of SEM data by Li et al. (2011) and Miyochi et al. (2011). Earlier publications incorporating data from LM (see review in Mondal, 1983) are insufficient for a reliable determination of fossil Meliosma pollen. The pollen grains of extant Meliosma are small, spheroidal to subprolate, tricolporate, perforate to microreticulate; the exine sculpturing often becomes finer toward the colpi or forms a smooth margo along the colpus. Among the studied species, our fossil pollen grains show the most similarity to M. pinnata ssp. macrophylla (Merr.) Beusekom (Furness et al., 2007), M. rhoifolia Maxim. (Figures 14A,B, this study), and M. rigida (Figure 14C, this study). To a lesser extent, due to somewhat larger perforations, the studied fossil pollen grains also resemble M. buchananifolia Merr. (Li et al., 2011) and M. angustifolia Merr. (Figure 14D, this study). The pollen grains of M. fruticosa Blume (Figure 14E) and M. simplicifolia (Figure 14F) show larger perforations (microreticulation) and no smooth margo along the colpi that differentiate the here studied fossils. However, the pollen type and exine sculpturing of the found fossil pollen is rather common for eudicot angiosperms and indistinguishable from several other taxa (e.g., some Menispermaceae taxa, Harley and Ferguson, 1982).

Leaf domatia are tiny structures located on the underside of leaves, usually in the vein axils (Figure 3H). The main types of domatia of woody angiosperms are represented by cavities between veins (pockets), partial destruction of the external tissue near the vein bifurcation (pits), and hair tufts covering the area between veins. The combination of hair-tufts with pockets or pits is common for many plants (Nishida et al., 2005; Romero and Benson, 2005). Leaf domatia occur in woody angiosperms of over 80 families, including Sabiaceae, in both temperate and tropical regions but are most abundant in tropical and subtropical zones of Australia, Korea, China, Japan, New Zealand, Africa, and the United States, being limited mainly to humid areas (O’Dowd and Willson, 1989, 1991; Pemberton and Turner, 1989; O’Dowd et al., 1991; Ryu and Ehara, 1991; Willson, 1991; O’Dowd and Pemberton, 1994; Situngu and Barker, 2017). The function of leaf domatia has been debated for a long time (e.g., O’Dowd and Willson, 1989, 1991; Pemberton and Turner, 1989; Walter and O’Dowd, 1992a,b; O’Dowd and Pemberton, 1994; Agrawal, 1997; Norton et al., 2000, 2001; Romero and Benson, 2005; Ferreira et al., 2008; Rowles and O’Dowd, 2009; Schmidt, 2014; Situngu and Barker, 2017). Leaf domatia are often called acarodomatia because they are inhabited by mites (e.g., O’Dowd et al., 1991; O’Dowd and Pemberton, 1998; Norton et al., 2000; Romero and Benson, 2004, 2005; Situngu and Barker, 2017; Maccracken et al., 2019). The diversity and reproduction of mites are greater on leaves with domatia (e.g., Walter and O’Dowd, 1992a,b; O’Dowd and Pemberton, 1994; O’Connell et al., 2010). The majority of mites inhabiting leaf domatia are either predators or fungivores (e.g., O’Dowd and Willson, 1989, 1991; Pemberton and Turner, 1989; Walter and Denmark, 1991; Willson, 1991; Walter and O’Dowd, 1992a,b; Nishida et al., 2005), but saprophagous mites also utilize domatia (O’Dowd and Willson, 1989). Studies on leaves with several types of domatia revealed that different mite taxa dominate different domatia, even on the same leaf (Nishida et al., 2005). Since numerous tiny mites are characterized by a thin weakly chitinised integument, dry air, and high temperatures are very harmful to them. Domatia help to reduce moisture loss and maintain a relatively constant temperature, thereby preventing the desiccation of mite eggs and mite bodies during molting (Pemberton and Turner, 1989; Willson, 1991; Agrawal, 1997; Norton et al., 2000; Romero and Benson, 2005). Domatia also provide mites with refuges from predatory arthropods (e.g., O’Dowd and Willson, 1989; Agrawal, 1997; Norton et al., 2001; Romero and Benson, 2005; Schmidt, 2014; Situngu and Barker, 2017) and protect young predatory mites from cannibalism (Ferreira et al., 2008). Furthermore, domatia functions as pollen and fungal spore traps, providing some mites (e.g., some Phytoseiidae, Eupodidae) with additional food sources (e.g., Kreiter et al., 2002; Duso et al., 2004; Romero and Benson, 2005; Schmidt, 2014). In return, predaceous and fungivorous mites may protect leaves from epiphyllous pathogenic agents, including herbivorous arthropods and fungi (e.g., Pemberton and Turner, 1989; O’Dowd and Willson, 1991; Agrawal, 1997; Norton et al., 2000; Romero and Benson, 2004; Monks et al., 2007). Thus, leaf domatia facilitate a protective mutualism between mites and plants (Norton et al., 2000; O’Connell et al., 2010; Walter, 2017).

While studies of the modern mite-plant associations are relatively abundant, the fossil record of leaf domatia considered to be inhabited by mites is sporadic. The oldest acrodomatia in the fossil record are known from the Upper Cretaceous of western North America (Maccracken et al., 2019). Fossil acrodomatia have also been described from the middle and upper Eocene of Victoria, Australia (O’Dowd et al., 1991), the middle Eocene of western North America (Liu et al., 2014), and the Miocene of New Zealand (Lee et al., 2010, 2012; Kaulfuss et al., 2015; Conran et al., 2016). However, the interaction between domatia-bearing plants and mites in the fossil record has not been proven by the presence of mites in domatia. In this paper, we report the first occurrence of fossil mites in a leaf domatium of M. eosinica (Figure 15). The recovered mite exoskeleton is considerably damaged, the covering was apparently soft, weakly sclerotized, without pronounced shields, and with numerous cuticular papillae and the thinnest of grooves (Figures 15C,D). Several characteristic densely pilose setae are preserved (Figures 15A–D), but a chaetome cannot be reconstructed due to incomplete preservation of the exoskeleton. Preserved legs are partly deformed, but presumably, they were longer than the body (Figures 15A,B). The gnathostome is not preserved, therefore the type of nutrition is not known with certainty. Based on all visible characters, we interpreted the studied mite as belonging to the superfamily Eupodoidea (Trombidiformes, Arachnida, Arthropoda), and probably the family Eupodidae, which comprises very small (up to 200 μm long) free-living mites with soft and weakly sclerotized bodies. Eupodids can be distinguished from other mites of the superfamily by chaetotaxy, setal and integument ornamentations, and the number of rhagidial solenidia (Strandtmann, 1971; Baker, 1990; Baker and Lindquist, 2002; Krantz and Walter, 2009; Khaustov, 2015a,b).

Eupodoidea has a global distribution, including both polar regions (e.g., Strandtmann, 1971; Olivier and Theron, 2000; Baker and Lindquist, 2002; Booth et al., 2009). The most described species of extant mites inhabit temperate to cold regions, whereas mites from tropical areas are much less understood (Baker and Lindquist, 2002; Booth et al., 2009). In China, extant Eupodidae mites were studied from the Shanghai City (Zha et al., 2003), Sichuan (He et al., 2012), Fujian (Zhang and Lin, 2012), and Jiangxi (Ding et al., 2008) provinces.

The greatest diversity and frequency of extant Eupodoidea mites are reported from the soil, leaf litter, mosses, lichens, decomposing wood, grasses, and shrubs where they may act as fungivores, predators, bryophytes, or phytophages (e.g., Baker, 1990; Krantz and Walter, 2009; Bizarro et al., 2020). Moreover, abundant Eupodoidea mites inhabit canopies from temperate to tropical forests living on the bark and under the bark of trees, as well as on leaves and stems (e.g., Walter and Behan-Pelletier, 1999; Romero and Benson, 2004; Situngu and Barker, 2017). Mites of the family Eupodidae are often very abundant in soil habitats. They are typically found in moist microhabitats, many of them are fungivores (Cowles, 2018). Most bark- and leaf-dweller eupodids are algivores, fungivores, and pollen-feeders (Walter and Behan-Pelletier, 1999).

We suppose that the fossil mites studied here lived in the forest canopy occupying the tomentous leaf surface and the domatia of the hair-tuft type.

Analysis of plant interactions with arthropods and plant pathogens (especially fungi) in the fossil record provides recent ecological and evolutionary studies with comparative long-term data on the influence of climate change on the ecosystem structure and food-chain relationships (Currano et al., 2008; Wilf, 2008; Wappler, 2010; Wappler et al., 2012; Wappler and Grímsson, 2016), recovery of ecosystems after environmental crises and mass extinctions (Labandeira et al., 2002; Wilf et al., 2006; Wilf, 2008; Donovan et al., 2014, 2016), and their evolutionary implications (Labandeira, 2013, 2021; Labandeira and Currano, 2013).

Damage traces on the leaves of M. eosinica are caused mainly by external feeders and, more rarely, by fungi. Insect damage types (DTs) from the external foliage feeding group can be assigned to hole feeding, margin feeding, skeletonization, and surface feeding. The most common DTs are hole feeding types. Medium-sized circular and polylobate holes (DT2, DT3) range in maximum diameter between 2 and 5 mm; with weakly developed reaction tissue present along the edges of polylobate holes (Figures 3C,D, 5F,H). Larger circular holes (DT4) reach 6.5 mm in diameter (Figure 5F). Large-sized polylobate perforations (DT5) showing a narrow reaction rim are confined to the axils between the midvein and secondary veins (Figures 3E, 5D). A cluster of three holes located at the divergence point of secondary veins from primary vein is assigned to DT57 (Figures 3D, 5H). Margin feeding damage (DT15) is present as incisions (up to 7 mm wide and 10 mm deep) extending toward the midvein and showing a distinct reaction rim (Figure 3E). One leaf is skeletonized (Figures 5A,B). Polygonal patches of skeletonized tissue (DT17) are bounded by tertiary veins, with one elongate skeletonized zone adjacent to a secondary vein. Reaction tissue is usually present along the edges of the patches, and high order venation is preserved within skeletonized areas. Surface feeding damage (DT31) recorded on only one leaf is characterized by a circular shape and a distinct reaction rim, and measures 5 mm in diameter (Figure 3E).

A probable fungal infection mark with a wide diffuse reaction front bounded on one side by a tertiary vein is evident in the axillary zone of the primary vein and a secondary vein where domatia are developed (Figures 5F,G).

Meliosma has a rather long and well-documented fossil history and was widespread both in North America and Eurasia during most of the Paleogene and Neogene (Supplementary Table 1; Supplementary Figure 1). The earliest records assigned to the genus Meliosma have been reported from the Upper Cretaceous (Maastrichtian) of Europe (Knobloch and Mai, 1986). Endocarps of M. praealba Knobloch et Mai were described from Walbeck, Germany; this species corresponds to the subgenus Kingsboroughia according to the classification of Van Beusekom (1971). In North America, the presence of Meliosma is confirmed by endocarps of M. rostellata (Lesquereux) Crane, Manchester et Dilcher from the uppermost Cretaceous or lowermost Paleocene Denver Formation near Golden, Colorado, and the Paleocene Sentinel Butte Member of Fort Union Formation near Almont, North Dakota (Crane et al., 1990). Paleocene endocarps are also known in Europe. Mai (1987) described two species, M. gonnensis Mai and M. sheppeyensis Reid et Chandler, from the lower and upper Paleocene of Gonna near Sangerhausen, Germany. According to Mai (1987), the endocarps of M. sheppeyensis are very similar to those from the Lower Eocene London Clay Formation of southern England (Reid and Chandler, 1933).

The oldest occurrence of fossil leaves assigned to the genus Meliosma is from the Paleocene of Williston Basin, North Dakota, United States (Supplementary Figure 1). The species M. thriviensis was recorded from the lower Paleocene Ludlow Member of the Fort Union Formation, while M. vandaelium comes from the middle Paleocene Tongue River Member of the Fort Union Formation (Peppe and Hickey, 2014). Unlike most other Meliosma species, these two species are characterized by a relatively small number of secondary veins, 9–10 pairs in M. vandaelium and 8–14 in M. thriviensis. Peppe and Hickey (2014) compare these species with M. longifolia from the lower Eocene Golden Valley Formation of North Dakota, United States, and note a trend toward an increasing number of secondary vein pairs. They suggest that M. thriviensis, M. vandaelium, and M. longifolia might represent a pattern of evolutionary change within the genus. M. thriviensis could represent an evolutionary precursor to M. vandaelium, and M. vandaelium, in turn, could be a precursor to M. longifolia (Peppe and Hickey, 2014).

In the Eocene, the genus Meliosma reaches a wide distribution and diversity, confirmed by numerous occurrences of fossil fruits, leaves, and wood, including leaves of the new species M. eosinica and associated pollen grains described in this paper. Three species of endocarps, M. sheppeyensis Reid et Chandler, M. cantiensis Reid et Chandler, and M. jenkinsii Reid et Chandler, have been described from the lower Eocene London Clay Formation of southern England (Reid and Chandler, 1933; Chandler, 1961; Collinson, 1983). M. jenkinsii also has been reported from the middle Eocene Geiseltal Flora of eastern Germany (Mai, 1976).

The middle Eocene Nut Beds assemblage from the Clarno Formation of Oregon, United States is one of the most interesting fossil floras, which includes five species of Meliosma based on endocarps (Manchester, 1994), three species of woods (Wheeler and Manchester, 2002), and leaf species (Manchester, 1981). Manchester (1994) described four species of fossil endocarps, M. beusekomii Manchester, M. bonesii Manchester, M. elongicarpa Manchester, and M. leptocarpa Manchester, which correspond to the subgenus Meliosma, section Meliosma, and one species, M. cf. jenkinsii Reid et Chandler, corresponds to the subgenus Kingsboroughia. The Clarno Formation fossil wood is the oldest known wood with anatomical characteristics similar to those of extant genus Meliosma. There are three types of Meliosma wood in the Clarno Formation assemblage, M. brehmii Wheeler et Manchester, M. deweyii Wheeler et Manchester, and M. dodsonii Wheeler et Manchester, distinguished by variation in porosity, ray size, and ray cellular composition. All Meliosma woods have characteristics of the subgenus Meliosma, section Meliosma (Wheeler and Manchester, 2002). Only one well-preserved fossil leaf of Meliosma from this locality has been illustrated but so far has not been described (Manchester, 1981). The leaf from the Clarno Formation is most similar in morphology to those of extant M. simplicifolia.

Fossil leaves more or less similar in morphology to those of extant Meliosma have been described from several Eocene localities in North America and Asia. Two species, M. duktotensis and M. kushtakensis, were reported from the middle-upper Eocene Kulthieth and Kushtaka formations of the Gulf of Alaska (Wolfe, 1977). M. longifolia comes from the lower Eocene Golden Valley Formation of North Dakota, United States (Hickey, 1977). Previously, parts of these same leaves were assigned to Quercus sullyi Newberry by Brown (1962). According to Crane et al. (1990) and Manchester (2014), the leaves illustrated by Hickey, and most of those treated as Q. sullyi by Brown, appear to represent the extinct genus Dyrana Golovneva (Golovneva, 1994; Budantsev and Golovneva, 2009). The glandular (rather than spinose) marginal teeth with rounded sinuses may indicate that these leaves represent Platanaceae, rather than Sabiaceae (Manchester, 2014).

A single leaf from the Chalk Bluffs assemblage, Sierra Nevada, California (United States) assigned to M. truncata MacGinitie (1941) is similar in morphology to the leaves of extant M. parviflora (Van Beusekom, 1971), but in fact, such an important diagnostic feature as the shape of the leaf apex is unknown, since the apex has not been preserved.

In China, within the distribution area of extant Meliosma, occurrences of Eocene Meliosma-like leaves have been also repeatedly mentioned (Supplementary Table 1), but fragmentary preservation does not afford confidence in determining their taxonomic affinity. Among them, M. rigidofolia Guo was described based on one small incomplete leaf from the Guchengzi Formation of Fushun coal mine near Fushun City, Liaoning, northeastern China (WGCPC (Writing Group of Cenozoic Plants of China), 1978). Two poorly preserved incomplete leaves from the Shinao Basin, Panxian County, Guizhou, southwest China (Zhang, 1983) and one fossil leaf from the Nadu Formation, Fengling Town, Nanning City, Guangxi, South China (Feng et al., 1977) were assigned to Meliosma sp. M. augusta auct. was listed in the assemblage of presumably early Eocene in age from the Yilan coal mine, Heilongjiang, northernmost China, but this plant fossil has not been described and figured (He and Tao, 1997).

Oligocene occurrences of leaf and fruit fossils of Meliosma are less common. Three species of fossil leaves morphologically similar to extant Meliosma were reported from the Goshen flora (early Oligocene), Lane County, Oregon (United States): M. rostrata Chaney et Sanborn (Chaney and Sanborn, 1933, depicted in Van Beusekom, 1971, p. 403, Figures 14-C 1, 2, M. goshenensis Chaney et Sanborn (Chaney and Sanborn, 1933, depicted in Van Beusekom, 1971, p. 399, Figure 10-A), and M. aesculifolia Chaney et Sanborn (Chaney and Sanborn, 1933, depicted in Van Beusekom, 1971, p. 391, Figure 14-B). M. goshenensis, according to Van Beusekom (1971), is very similar in morphology to the extant species M. rigida and M. fordii. Small lanceolate leaves (or leaflets?) that serrate along the whole lamina margin were described as M. nevolinae Pavlyutkin from the lower Oligocene Fatashinskaya Formation of South Primorye, Russia (Pavlyutkin et al., 2014). Furthermore, a single small leaf lacking an apex, probably pubescent, with a serrate margin in the upper part of the lamina, was described as Meliosma sp. from the same deposits (Pavlyutkin et al., 2014). A single incomplete leaf of Meliosma sp. was described (but not pictured) from the Liuqu Formation of Lazi County, Xizang, southwestern China (Tao, 1988). The age of this formation, according to various reports, has been thought to be Oligocene (Wei et al., 2011), Oligocene–Miocene (Li et al., 2015), or late Paleocene (Ding et al., 2017), this most recent determination being based on radiometric analysis of contained volcanic ashes.

Several Meliosma species based on endocarps are known from the Oligocene of Europe. M. reticulata (C. et E.M. Reid) Chandler was originally described from Bovey Tracey, Devonshire, England, and subsequently also recovered from the Haselbach member, Saxony, Germany (Chandler, 1957; Mai and Walther, 1978). Endocarps of M. europaea C. et E.M. Reid have been reported from several Oligocene and Miocene localities in Western Siberia (Dorofeev, 1963). According to Mai (1964), the older epithet wetteraviensis has priority over europaea, and, consequently, M. europaea was placed by Mai in the synonymy of M. wetteraviensis (Ludw.) Mai. This species is also known from the Miocene and numerous Pliocene localities of Europe (Supplementary Table 1). Van Beusekom (1971) noted that the similarity of these endocarps (especially from Pliocene) to modern ones of M. veitchiorum Hemsl. is very close; they are probably best attributed to M. veitchiorum.

Several species of Meliosma endocarps have been also found both in the Miocene and Pliocene of Europe. M. miessleri Mai was described from the Miocene of Germany, the Czech Republic, and the Pliocene of Italy (Supplementary Table 1; Mai, 1964; Van Beusekom, 1971; Martinetto, 1998; Holý et al., 2012), and M. pliocaenica (Szafer) Gregor has been recorded from the upper Miocene Sandanski Formation and upper Miocene–lower Pliocene Noviiskar Formation in Bulgaria (Mai and Palamarev, 1997).

Silicified wood of Meliosma mio-oldhami Suzuki et Watari is known from the Lower Miocene Nawamata Formation of central Japan (Watari, 1949; Suzuki and Watari, 1994). This species is similar to the extant species M. oldhami Miq. ex Maxim. in wood anatomy.

The only species of Meliosma leaves in the upper Miocene of North America, described as M. predentata MacGinitie, was reported from the Kilgore flora of Nebraska, United States (Van Beusekom, 1971, p. 402, Figure 10-D).

Fossil leaves of M. obtusifolia and M. shanwangensis have been described from the lower-middle Miocene Shanwang Formation of Shandong, eastern China (Hu and Chaney, 1940; WGCPC (Writing Group of Cenozoic Plants of China), 1978). Species M. shanwangensis are also known from the Miocene of Kobe Group, Japan, and South Primorye, Russia (Hori, 1983; Pavlyutkin et al., 2012). Miocene–Pliocene leaves from southwestern China were assigned to M. lincangensis Tao et Chen from the Lincang Region, Yunnan (Tao and Chen, 1983; Tao et al., 2000), and extant species M. buchananifolia (younger synonym of M. thorelii Lecomte) from Tengchong County, Yunnan (Tao and Du, 1982; Tao et al., 2000; Li et al., 2004). Meliosma sp. based on a single incomplete leaf was reported from the Tatsumitoge flora (late Miocene), Tottori Prefecture, Southwest Honshu, Japan (Ozaki, 1980).

The only fossil leaves with known epidermal characters assigned to the extant species M. cf. flexuosa were reported from the upper Miocene Shengxian Formation of Zhejiang Province, eastern China (Jia and Sun, 2011).

Pliocene endocarps of M. canavesana Martinetto, originally described as Meliosma aff. reticulata (Martinetto, 1998), have been reported from several fossil sites in northwestern and Central Italy (Martinetto, 2001a). Two species of fossil leaves similar to the leaves of modern species M. myriantha Siebold et Zucc. and M. tenuis were identified from the Kabutoiwa Formation of Central Honshu, Japan (Ozaki, 1991). Fossil endocarps of Meliosma cf. rigida Siebold et Zucc. were reported from the lower Pliocene of Hatagoya, Central Honshu, Japan (Miki, 1941; Van Beusekom, 1971, p. 402, Figures 13-E 1, 2).