1 Introduction

Eurya Thunb. is a large genus of the tea family Theaceae, comprising approximately 130 species (Min and Bartholomew, 2007). Its extant species are mainly distributed in tropical to subtropical Asia, and in the western and southern Pacific Islands (Min and Bartholomew, 2007), with a few species in tropical regions of the New World (Łańcucka-Środoniowa, 1966). They are evergreen shrubs, small trees, and rarely large trees, commonly found as dominant understory elements in tropical to subtropical broadleaved evergreen forests (WGVY, 1987; Min and Bartholomew, 2007).

Although Eurya is not distributed in Europe now, its fossil record is especially rich in this continent, where at least 12 extinct species from the Late Cretaceous to early Pleistocene have been documented (Mai, 1960, 1971; Knobloch, 1977; Łańcucka-Środoniowa, 1981; Friis, 1985; Knobloch and Mai, 1986; van der Burgh, 1987; Friis et al., 2011; Martinetto et al., 2015). Unlike Europe, the fossil occurrences of Eurya in East Asia are rather scarce (Tanai and Uemura, 1991; Momohara, 1992; Momohara and Saito, 2001; Yamakawa et al., 2015), even though this region shows the highest species richness of the genus today. Although molecular data can explain the emergence and diversification of Eurya (Wu et al., 2007), the lack of fossil evidence limits further understanding of the evolution and establishment of its high modern species diversity in East Asia.

In this study, we report a new fossil record of Eurya based on three-dimensionally preserved seeds from the late Pliocene of northwestern Yunnan, southwestern China. We perform detailed morphological and anatomical examinations of these fossil seeds, and discuss the paleobiogeographic implications of this new fossil finding.

2 Materials and methods 2.1 Fossil site and geological horizon

The present fossil seeds were collected from deposits at Nanbanbang Village, Heqing Basin, northwestern Yunnan Province (26°310 N, 100°100 E; 2200 m a.s.l.; Fig. 1). The Heqing Basin is located at the southeastern fringe of the Qinghai-Tibet Plateau within the renowned Hengduan Mountains. It is a tectonic default basin surrounded by mountains with a distinguishable vertical vegetation belt spectrum (Shen et al., 2007). The local climate is primarily influenced by the Indian Monsoon, which is characterized by warm, wet summers and cool, dry winters (Xiao et al., 2010; An et al., 2011). The section studied is unconformably overlain by Quaternary deposits and is composed of horizontally laminated carbonaceous layers imbedded by fine gray sandstones. These carbonaceous layers contain abundant plant remains, mainly fruits and seeds. The age of the carbonaceous layers was previously assigned to the early Pleistocene (Zhu et al., 2016). The latest geological survey shows that the underlying sedimentary layers beneath these carbonaceous layers contain abundant leaf fossils dominated by evergreen sclerophyllous oaks (Quercus sect. Heterobalanus). In northwestern Yunnan, the dominance of evergreen sclerophyllous oaks in a fossil flora is a common indicator of the Sanying Formation (Tao, 1986), which has been determined to be the late Pliocene based on stratigraphic correlations, palynological information, mammal fossils, and paleomagnetic data (Tao and Kong, 1973; Tao, 1986; Ge and Li, 1999; Su et al., 2011; Li et al., 2013). Therefore, the age of the present fossil seeds and fruits is assigned to the late Pliocene.

2.2 Fossil materials and examination

More than 1000 specimens of charcoalified fruits and seeds were collected from the fossil site. Among them, more than 80 seeds and seed fragments of Eurya were identified through observations under a binocular microscope (Leica, S8AP0). The fossil seeds were cleaned by an ultrasonic cleaner at 40 kHz (KO-50M) for 5-10 s. Air dried, they were then observed under a 3D Super Depth Digital Microscope (ZEISS Smartzoom 5) and images were taken. Five seed specimens were further studied under a scanning electron microscope (SEM, Zeiss EVO LS10) both morphologically and anatomically. For comparative analysis, extant seeds of Eurya obtained from herbarium specimens housed at the Herbarium of Kunming Institute of Botany (KUN) were also examined using the same procedure as the fossils. The descriptive terminology mainly follows Friis (1985). All studied fossil specimens are numbered and kept at the KUN.

3 Systematics

Family: Theaceae Mirb., 1816

Genus: Eurya Thunb., 1783

Species: Eurya stigmosa (Ludwig) Mai, 1960

Fossil specimens: NBB 020 (Plate Ⅰ, 1), NBB 021 (Plate Ⅰ, 2), NBB 022 (Plate Ⅰ, 3), NBB 023 (Plate Ⅰ, 4), NBB 024 (Plate Ⅰ, 5), NBB 025 (Plate Ⅰ, 6), NBB 026 (Plate Ⅰ, 7), NBB 027 (Plate Ⅰ, 8), NBB 028 (Plate Ⅰ, 9), NBB 029 (Plate Ⅰ, 10), NBB 030 (Plate Ⅰ, 11), NBB 031 (Plate Ⅰ, 12), NBB 032 (Plate Ⅱ, 1, 8), NBB 033 (Plate Ⅱ, 2), NBB 034 (Plate Ⅱ, 3), NBB 035 (Plate Ⅱ, 4, 6, 7), NBB 036 (Plate Ⅱ, 5, 9, 10, 11, 12), and NBB 037-072.

Fossil locality: Nanbanbang Village, Heqing County, northwest Yunnan Province, southwest China (26°310 N, 100°100 E; 2200 m a.s.l.)

Stratigraphic horizon: The upper Pliocene Sanying Formation Description: Seeds are slightly campylotropous, laterally flattened, and subcircular to slightly angular in outline (Plate Ⅰ). They are 1.07-2.02 mm in length and 1.03-1.96 mm in width, with a lengthewidth ratio of 0.82-1.32. The seed surface is foveolate with funnel-shaped cells (Plate Ⅱ, 1-4, 6), which are concentrically arranged around the condyle-raphe region in 8-12 rows parallel to the seed margin (Plate Ⅱ, 1-4, 6-8). The cells are 0.03-0.15 mm (0.08 mm on average) in diameter, with thickened and finely pitted inner periclinal and anticlinal walls (Plate Ⅱ, 7), and with cell lumens that gradually narrow towards the base with polygonal facets (Plate Ⅱ, 6-7). The cavity of each seed valve is subdivided by a characteristic reversed and slightly curved Vshaped condyle into a reniform or horseshoe-shaped embryo cavity and raphe (Plate Ⅱ, 10). The condyle consists of two limbs with equal length but unequal width, which extend from the seed base to almost half of the seed width (Plate Ⅱ, 10). The raphe is marked by a subtriangular cavity, and is formed by the encompassment of the condyle and seed basal margin. The hilar scar is oblong and located between the raphe and micropyle. The seed internal surface is covered by elongated elliptic and slightly thickened cells, which are concentrically arranged around the condyle in rows parallel to the seed margin (Plate Ⅱ, 9-10). The seed wall is 0.06-0.16 mm thick, sclerotic, composed of a thick exotesta, with large cells, and a thin endotesta, with small cells (Plate Ⅱ, 5). The thin, membranous remains of the tegmen can be observed attached to the external surface of testa in several seeds (Plate Ⅰ, 12).

Morphological comparisons: Although seeds of Eurya are seemingly similar to those of its relatives, such as Adinandra W. Jack, Cleyera Thunberg, and Freziera Willd, they can be reliably distinguished from these genera by key structural details (Friis, 1985; Table 1). Generally, Eurya seeds are characterized by a reticulum of funnel-shaped cells with polygonal facets on the external seed surface (Friis, 1985). This diagnostic feature is consistent with our fossil seeds, which differs from the other three genera. Moreover, our fossils also share some other important anatomical features with extant Eurya seeds, such as the reniform or horseshoe-shaped embryo cavity, the prominent condyle in the seed valve, and the position of raphe and hilum (Friis, 1985). All of these traits suggest that our fossil seeds can be ascribed to Eurya. Among the examined modern seeds of Eurya, Eurya yunnanensis, a species presently found mainly in western and southeastern Yunnan, shows the greatest resemblance to the fossil seeds under study. However, the fossil seeds are distinct from this species by their slightly larger size, finely pitted cells on the seed external surface, and the difference in cell types on the condyle and seed internal surface (Plates Ⅱ and Ⅲ).

Fossil seeds of Eurya that have previously been described are largely from Europe (Chandler, 1963; Mai, 1971; Knobloch, 1977; Łańcucka-Środoniowa, 1981; Friis, 1985; Knobloch and Mai, 1986; van der Burgh, 1987; Martinetto et al., 2015), and to a limited extent from Japan in East Asia (Momohara, 1992; Momohara and Saito, 2001; Yamakawa et al., 2015) and Nepal in South Asia (Bhandari et al., 2009, 2010). They are assigned to 12 fossil species and one modern species (Table 3). We compared our fossil seeds with these 12 fossil species using four key characteristics: size of the cells on the seed external surface, seed size, seed shape, and testa thickness (Table 2). Our fossil seeds most resemble the seeds of E. stigmosa (Ludwig) Mai reported from Paleocene to early Pleistocene of Europe (Table 3), that have similarly large cells on the seed external surface. Our fossils also have similar seed size (1.07-2.02×1.03-1.96 mm) and testa thickness (0.06-0.16 mm) with those of E. stigmosa. Other important seed characteristics, such as the subcircular to slightly angular seed shape, are also shared by our fossils and this species. We have therefore treated our fossils as belonging to E. stigmosa.

4 Paleobiogeographic implications

Eurya has a rich fossil record, represented primarily by seed remains and a few leaf impressions (Tao and Du, 1982; Ozaki, 1991; Tanai and Uemura, 1991; Bozukov et al., 2008). The oldest known records of the genus are seed remains from the middle to Late Cretaceous in Austria (Knobloch and Mai, 1986, 1991) and from the Late Cretaceous to Paleocene in the Czech Republic, central Europe (Knobloch, 1977). This may imply a central European origin. Fossil record also indicates that Europe, where about 12 known fossil species of the genus have been reported, may have been a center for Eurya evolution and diversification in the past (Chandler, 1963; Mai, 1971; Knobloch, 1977; Łańcucka-Środoniowa, 1981; Gregor, 1982; Friis, 1985; Knobloch and Mai, 1986; van der Burgh, 1987; Martinetto et al., 2015) (Table 3; Fig. 2). This hypothesis may be consistent with the warm and humid climate throughout the European Cenozoic (Mosbrugger et al., 2005). Based on the lack of fossil occurrences in the continent after the early Pleistocene, the genus likely disappeared from Europe by that time (Martinetto et al., 2015). This may be largely attributed to the more severe effect of the Quaternary glaciations on Europe compared with relatively low latitude regions (Ehlers et al., 2011).

In contrast to Europe, East Asia has revealed a much lower diversity of Eurya in its geological past. Despite flourishing in the modern era, there are only six documented fossil occurrences of Eurya in East Asia (Tao and Du, 1982; Ozaki, 1991; Tanai and Uemura, 1991; Momohara, 1992; Momohara and Saito, 2001; Yamakawa et al., 2015). The oldest known fossil record is dated to the late Oligocene (Tanai and Uemura, 1991), implying that Eurya probably first appeared in East Asia no later than the late Oligocene. The newly described fossil occurrence represents the first seed fossil record of Eurya in China, and one of the few fossil records of the genus in East Asia. As E. stigmosa has been frequently documented from the Paleocene to early Pleistocene of Europe (Mai, 1971; Łańcucka-Środoniowa, 1981; Gregor, 1982; Friis, 1985; van der Burgh, 1987; Teodoridis and Kvaček, 2006; Martinetto et al., 2015), it may indicate a close affinity between East Asian and European species of the genus. It may suggest the ancient species of East Asian Eurya originated in Europe. The genus may have further dispersed to other regions of Asia, as some other fossil occurrences of Eurya are also reported from the continent, e.g., the late Miocene and late Pliocene of Japan (Ozaki, 1991; Yamakawa et al., 2015), and the late Pleistocene of Nepal (Bhandari et al., 2009, 2010). These ossils may represent partial histories of the ancient lineages of modern Eurya in Asia.

Acknowledgments

We thank Lin-Bo Jia from Kunming Institute of Botany, Chinese Academy of Sciences, for help with fossil collection; and the editor and two anonymous reviewers for improving the manuscript. Fossil examinations with the 3D Super Depth Digital Microscope and SEM were performed in the Central Lab of Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences; and examination of the extant seeds with the SEM was performed in the Key Laboratory for Plant Diversity and Biogeography of East Asia, Kunming Institute of Botany, Chinese Academy of Sciences. Extant seeds of Eurya were provided by the Herbarium of Kunming Institute of Botany, Chinese Academy of Sciences. This study was supported by the National Natural Science Foundation of China (No. U1502231, 31300187). This study is a contribution to NECLIME (Neogene Climate of Eurasia).