Diversity of Fagaceae on Hainan Island of South China During the Middle Eocene: Implications for Phytogeography and Paleoecology

The Fagaceae family is currently widespread throughout tropical and temperate regions of South America and the Northern Hemisphere, especially East Asia, and has likely been so since the Eocene, according to fossil records. In China, Fagaceae fossils are rare in the lowest latitudes of South China. Here, we describe 12 species in 5 genera of Fagaceae (i.e., Berryophyllum, Castaneophyllum, Quercus, Castanopsis, and Lithocarpus) based on leaf morphology and trichomes. These fossils are recovered from the Changchang Formation of Changchang Basin, Hainan Island, South China, indicating that Fagaceae has been distributed in the tropical low latitudes since the Eocene. Given that our fossils are closely related to the tropical and subtropical extant species, we speculate that Fagaceae lineages have likely diverged since the Eocene and that each extant lineage, such as Quercus sect. Cyclobalanopsis, became highly differentiated no later than middle Eocene. Based on the current living conditions of the extant species, we further speculate that the climate of Hainan Island was warm and wet during the middle Eocene, suitable for the growth and differentiation of the family.

In China, previously reported Fagaceae fossils were mainly recovered from the Eocene to Pleistocene strata in the northern and southwestern regions, including Liaoning, Shandong, Tibet, Sichuan, and Yunnan (Writing Group of Cenozoic Plants of China [WGCPC], 1978;Liu et al., 1995;Tao et al., 2000;Xiao et al., 2006;Xing et al., 2013;He et al., 2014;Wu et al., 2014;Li et al., 2015). Reports on Fagaceae fossils from the tropical low latitudes of South China are rare. The fossil history and its biogeographic implications of Fagaceae have been previously discussed (Zhou, 1999;Chen, 2007) given the continuous increase of this family's fossil record in recent years (Mindall et al., 2007(Mindall et al., , 2009Vikulin, 2011;Xing et al., 2013;He et al., 2014;Wu et al., 2014;Jia et al., 2015;Li et al., 2015;Xu et al., 2016;Wilf et al., 2019), the diversity, and phytogeographic history of the family need to be further studied.
In this study, we studied 41 leaf fossils of Fagaceae recovered from the middle Eocene of Changchang Formation, Changchang Basin, Hainan Island, South China. Twelve species within 5 genera (Berryophyllum Jones et Dilcher, Castaneophyllum Jones et Dilcher, Castanopsis, Lithocarpus, and Quercus) have been described based on the leaf morphology and trichomes via Scanning Electron Microscopy (SEM). The present discovery documents a tropical low latitude distribution of Fagaceae in the middle Eocene. Moreover, our fossils are closely related to the extant tropical and subtropical elements, providing an important contribution to the understanding of the historical biogeography of this family and the paleoecology of Hainan Island during the middle Eocene.

MATERIALS AND METHODS
The present fossils were collected from the Changchang Formation of Changchang Basin (Figure 2, 19 • 38 03 N, 110 • 27 04 E), located near Jiazi Town, Qiongshan County, in the northeastern part of Hainan Island, South China and housed in the Museum of Biology, Sun Yat-sen University, Guangzhou, China.
The Changchang Formation is composed of two parts: the lower coal-bearing part is ca. 52-54 m thick and consists of clastic terrigenous and coal deposits with mudstone, coaly shale, oil-bearing shale, muddy siltstone, and sandstone, and coal; the upper part is ca. 37-40 m thick and consists predominantly of lacustrine and fluvial mudstones, siltstones, and sandstones (Spicer et al., 2014). Numerous well-preserved plant macrofossils,  including the fagaceous leaf fossils investigated here, were mainly collected from the lower part of the Changchang Formation. These deposits also contain diverse pollen and spore assemblages (Zhang, 1980;Lei et al., 1992;Yao et al., 2009;Hofmann et al., 2019), as well as bivalve and gastropod remains, and fish bones and scales. The age of the Changchang Formation was originally considered to be the late Palaeocene to early/middle Eocene based on floral composition (Guo, 1979). Later, the Changchang Formation is dated on the basis of palynological data as the middle Oligocene (Zhang, 1980), the late early Eocene to early late Eocene (Lei et al., 1992), and early middle Eocene (Yao et al., 2009). Recently, Spicer et al. (2014) dates the Changchang Formation as middle Eocene (Lutetian-Bartonian, ca.48-38 Ma) based on comprehensive analysis on the macrofossil flora, its similarity with the adjacent deposit Youganwo Formation in Maoming Basin, Guangdong Province, South China, and previously published palynological data. Here we followed the age assessment by Spicer et al. (2014).
Leaf fossils were photographed using a Canon EOS 500D digital camera (Canon, Tokyo, Japan). Small cuticular fragments of some species were recovered from the leaf fossils. Remnant rock particles adhering to the leaf fossils were removed using 40% HF for 24 h. The specimens were then rinsed in distilled water and mounted on stubs, coated with gold, and then examined and photographed using a JSM-6330F SEM (JSM, Tokyo, Japan).
We examined the extant specimens of Quercus, Lithocarpus, and Castanopsis represented in herbaria of the South China Botanical Garden, Chinese Academy of Sciences (IBSC, Guangzhou), Sun Yat-sen University (SYS, Guangzhou), Harvard University (HUH, Boston) and the University of Florida (FLAS, Gainesville). Leaf terminology follows Ellis et al. (2009). The following states and abbreviations are used for interpreting tooth types: convex (CV), straight (ST), concave (CC), retroflexed (RT; tooth flank is basally concave and apically convex). Tooth shape is described in terms of the distal and proximal flank curvatures relative to the midline of the tooth. The distal flank shape is always given first, e.g.: CV-ST indicates that the tooth is concave on the distal flank and straight on the proximal flank.
Comparison The present specimens are assigned to Berryophyllum because they have lanceolate leaves, cuneate bases, serrate margins with CC-ST teeth and craspedodromous secondary veins (Figures 3A-C). They are similar to Dryophyllum puryearensis Berry, D. anomalum Berry, and B. tennesseensis (Berry) Jones et Dilcher from the early Eocene of southeastern North America on the leaf shape (Berry, 1916;Jones and Dilcher, 1988), but they are different on venations. The present fossils are distinguished with B. dewalquei (Sap. et Mar.) Zhou (1996) and B. yunnanense (Colani) Zhou (1996) by the leaf shape, the trend and angle of secondary veins and the teeth (Writing Group of Cenozoic Plants of China [WGCPC], 1978;Zhou, 1996). Our specimens conform to the diagnosis of B. relongtanense (Colani) Z. K. Zhou previously recognized from Writing Group of Cenozoic Plants of China [WGCPC] (1978) and Zhou (1996).
Diagnosis Leaf narrowly lanceolate, apex elongate, acuminate or caudate bending to the right. Margin serrate up to the apex, teeth CC-ST to CC-CV with rounded sinus. Midvein straight to slightly bend in apex; secondary veins pinnate, nearly opposite, craspedodromous, bend inward in margin, with angles 40-50 • between the midvein and secondary veins; Tertiary veins mixed percurrent; Quaternaries regular, rectangular to polygonal reticulate. Leaf surface rugose with verrucae.
Holotype CC-1244 (a, b) Paratypes CC-1103, CC-1118 (a, b) Etymology The specific epithet "hainanensis" refers to the Hainan Island from which the specimens were collected.
Comparison The present specimens are distinguished from B. relongtanense which is also known from the same site, because they are narrowly lanceolate in shape while B. relongtanense is lanceolate. The present fossils are also different from the linear or extremely narrowly lanceolate leaves of Berryophyllum tenuifolia Jones and Dilcher (1988) and the lanceolate leaves of B. dewalquei, B. yunnanense, and B. relongtanense (Writing Group of Cenozoic Plants of China [WGCPC], 1978;Zhou, 1996). Our fossils are similar to Dryophyllum berendtianum (Goepp.) Kirchh. from the Eocene of Ukraine and Kaliningrad, Russia (Takhtajan, 1982) on having elongate acuminate or caudate apex with clear teeth. However, the teeth of our fossils have more irregular spaced teeth and narrower leaves than D. berendtianum. The features of narrowly lanceolate leaf shape with elongate acuminate or caudate apex, irregularly spaced CC-ST to CC-CV teeth and pinnate secondary veins without forming a loop convinced us to assign these fossils to a new species B. hainanensis sp. nov (Figure 4). Etymology The specific epithet "hainanensis" refers to the Hainan Island from which the specimens were collected.
Description Leaf lanceolate ( Figure 5A), preserved part 3.2-10.1 cm long, 1.2-2.0 cm wide, base cuneate, symmetric ( Figure 5A). Margin entire near base, serrate from > 1/3 of the leaf to the apex ( Figure 5A), teeth ST-CC to CC-CC with rounded sinus; Tip slightly bend inward ( Figure 5B). Midvein thick, straight; Comparison The present specimens are attributed to the Castaneophyllum rather than Castanea because their lanceolate leaf shape, craspedodromous and bend inward secondary veins and mixed percurrent tertiary veins are consistent with the Castaneophyllum (Figures 5A,B). Our specimens differ to Castanea on the secondary and tertiary veins. The secondary veins of Castanea are decurved near the midribs with two adjacent secondary veins near the midribs closer than those near the margin. The tertiary veins of Castanea are opposite percurrent. This new species is similar to Castaneophyllum tennesseense (Berry) Jones et Dilcher (1988) from the Eocene of Tennessee, North America on the lanceolate leaf shape, but it is different on the teeth characters and arrangement of the secondary veins. Our specimens are distinguished from C. moorii (Lesq.) Jones et Dilcher (1988) which is elliptic to narrowly elliptic, 23 cm long and secondary veins closely spaced from the Eocene of Tennessee, by the leaf shape, size and venations. Our fossils greatly differ from C. fushunense (Chen et Wang) Z.K. Zhou from the Eocene of Fushun, Liaoning Province in teeth type and angles between midvein and secondary veins (Writing Group of Cenozoic Plants of China [WGCPC], 1978;Zhou, 1996).
Species Castaneophyllum lanceolata X-Y Liu et J-H Jin sp. nov. (Figure 6) Diagnosis Leaf lanceolate, apex elongate acuminate, base cuneate. Margin entire near base, serrate from > 1/3 of the leaf to the apex, teeth ST-CV to CC-CV with rounded sinus. Midvein thick, straight; secondary veins 15 pairs, opposite from base to middle, pinnate from middle to apex, craspedodromous, bend in ward near the margin; Tertiary veins opposite percurrent; Quaternaries unclear. Leaf surface rugose with solitary trichomes.
Holotype CC-1106 (a, b) Etymology The epithet "lanceolata" refers to the specimen has elongate lanceolate leaf.
Comparison The present fossil is attributed to Castaneophyllum because its leaves lanceolate with elongate acuminate apex, cuneate base, and serrate margin, secondary and tertiary veins (Figures 6A-F). This new species differs from C. hainanensis, described above, by the venation and tooth type. The present fossil is very similar to C. tennesseense (Jones and Dilcher, 1988), for both having lanceolate leaf and variable teeth type, but the present fossil has more elongate acuminate apex and smaller leaf than C. tennesseense. Our specimen with the length of 12.6 cm is much smaller than C. moorii and C. fushunense (Writing Group of Cenozoic Plants of China [WGCPC], 1978;Zhou, 1996).  (Figure 7B). Midvein thick, straight; secondary veins more than 9 pairs irregularly spaced, nearly opposite, craspedodromous, with stable angles 55 • ( Figure 7A); Tertiary and quaternaries veins unclear. Leaf surface rugose with stellate trichomes with 9 solitary branches; Branches 22.7-32.7 µm (mean = 27.7 µm) long, 1.8-2.3 µm (mean = 2.1 µm) wide (Figures 7C-E).

Species
Comparison The present fossil is confirmed to Castaneophyllum because its lanceolate leaf shape, serrate margin, secondary and tertiary veins (Figures 7A,B). It is closest to C. moorii by having consistent characteristics of teeth type, similar angles between the midvein and secondary veins and the arrangement of secondary veins, but they are different in leaf shape and size. The present specimens are similar to Q. relongtanense Colani and Quercus cf. relongtanense Colani from the Miocene-Pliocene of To-tang, Yunnan Province, Southwest China (Colani, 1920) in venation, but our specimens have ST-ST teeth with rounded sinus and stellate trichomes, while the To-Tang species are lacking the details of leaf margin and surface.
Comparison we decided to assign the present specimen to Castanopsis based on the venation and ST-RT teeth with rounded sinus (Figures 8A-C). Our fossils resemble the extant C. sclerophylla on the tooth type and secondary veins, but they are different in the arrangement of the teeth. Our specimens have teeth from base to apex, while only the top 1/3 part of C. sclerophylla has teeth. This new species is similar with the extant C. sclerophylla (Lindl. et Paxton) Schottky in the characteristic of leaf shape and teeth type, but they are obviously different in secondary veins and trichomes. Our specimen has stellate trichomes ( Figures 8D,E), while C. sclerophylla has thin-walled peltate trichomes. Our fossil has similar stellate trichomes with C. mekongensis A. Camus, but their leaf shape, size and angles between midvein and secondary veins are quite different. The present specimen is also distinctive from the previously fossil records of Castanopsis in the Cenozoic of China and North America (Wolfe, 1968;Tao et al., 2000;Wu et al., 2014;Li et al., 2015) by the teeth arrangement and small angle between midvein and secondary veins. Although Castaneophyllum cf. moorii also has stellate trichomes, these specimens are assigned to Castanopsis rather than Castaneophyllum for their ST-RT teeth and regularly spaced secondary veins.
Comparison Appressed parallel tufts (APT) trichome is unique trichome type in the genus Lithocarpus. The present fossils are confirmed to be Lithocarpus mainly based on leaf shape, venation and appressed parallel tufts (APT) trichomes with 2 thick-walled, unicellular elements (Figure 9). The tooth type and the secondary veins of the present fossils are similar to the extant L. fordianus (Hamsl.) Chun, but our fossils with the length of 4.1-6.0 cm are much smaller than L. fordianus with the length of 10-25 cm. The present fossils are different from all reported fossil records Lithocarpus leaves from the Cenozoic of China, Europe and North America. Therefore, our fossils are assigned to a new species Lithocarpus changchangensis sp. nov. secondary veins thin, 15-23 pairs regularly spaced, pinnate, craspedodromous, straight or slightly curved. Tertiary veins mixed percurrent; Quaternary veins regular, rectangular to polygonal reticulate. Leaf surface rugose with stellate trichomes with 4-8 solitary branches.
Comparison Our specimens are attributed to Quercus sect. Cyclobalanopsis by leaf shape, regularly spaced teeth and secondary veins as well as mixed percurrent tertiary veins (Figures 10A-D). The new species closest to Q. hypargyrea (Seemen ex Diels) C.C. Huang et Y.T. Chang, but they are significantly different: firstly, our fossils have cuneate base, while Q. hypargyrea is cuneate to subrounded; secondly, our specimens have15-23 pairs of secondary veins which is more than Q. hypargyrea (10-15); thirdly, the present fossils are longer and thinner than Q. hypargyrea (Huang et al., 1999). Quercus paleohypargyrea is distinctive by elliptic leaf shape, cuneate base, serrate margin with regularly spaced CC-CV to CC-CC teeth, multiple regularly spaced, pinnate, straight or slightly curved secondary veins, which is significantly different from the previously reported Cenozoic Q. sect. Cyclobalanopsis from China and North Amercia (MacGinitie, 1953;Axelrod, 1956Axelrod, , 1966bAxelrod, , 1992Axelrod, , 1995Axelrod, , 1998aAxelrod, ,b, 2000; Writing Group of Cenozoic Plants of China [WGCPC], 1978;Tao et al., 2000). Quercus paleohypargyrea differs to the aforementioned Castaneophyllum cf. moorii and Castanopsis sp. which also have stellate trichomes by CC-CV to CC-CC teeth and pinnate secondary venation.
Comparison The present fossils are assigned to Quercus sect. Cyclobalanopsis base on the leaf morphological characteristics such as oval long elliptic leaf, regularly spaced teeth and secondary veins and the mixed percurrent tertiary veins (Figures 11A-D). The new species is most similar to the extant Q. lamellosa Smith on the elliptic leaf shape and stellate trichomes. However, the secondary veins of our fossils are curved close to the leaf margin, while those of Q. lamellosa are straight. In addition, the teeth of the present fossils are CC-CV in a uniform size, sometimes curved inward like a hook, while the teeth of Q. lamellosa are thin and long, sometimes spiny. Our specimens are distinguished from the previously described fossils of Quercus sect. Cyclobalanopsis from the Cenozoic of China and North American by the leaf characteristics of large size, ovate, obovate or oblong shape, and the number of secondary veins (Writing Group of Cenozoic Plants of China [WGCPC], 1978;Meyer and Manchester, 1997). This new species is distinct from Q. paleohypargyrea, known from the same site, by the shape and size of teeth and secondary veins.
Comparison The present fossils can be assigned to Quercus sect. Cyclobalanopsis base on the leaf shape, regularly spaced teeth and secondary veins as well as the mixed percurrent tertiary veins (Figures 12A,B). These specimens are similar to the extant Q. myrsinifolia Blume in the characteristics of gradually stronger midvein, nearly parallelled secondary veins, slender cuneate teeth and trichomes. However, the present specimens are different from Q. myrsinifolia by the lanceolate leaf shape and elongate acuminate apex. Our fossils are different with Q. sinomiocenicum Hu et Chaney from the Miocene of Lintong, Shandong Province in the leaf shape, teeth type and venation (Nanjing Institute of Geology and Mineral Resources [NIGMR], 1982). This species is distinguished from Q. paleohypargyrea and Q. paleolamellosa from the same locality by the lanceolate leaf shape and tooth size.
Holotype CC-1287 (a, b) Paratype CC-961 Etymology The specific epithet "paleoargyrotricha" refers to its close affinity to the extant Quercus argyrotricha A. Camus.
Comparison The present specimens are assigned to Quercus sect. Cyclobalanopsis because the leaf shape, regularly spaced teeth and secondary veins, and mixed percurrent tertiary veins (Figures 12E-G). The new species is most similar to the extant Q. argyrotricha A. Camus in the characteristics of sparsely serrated margin, craspedodromous, slightly curved secondary veins, and stellate trichomes. However, the present specimens differ from Q. argyrotricha by the leaf shape, teeth details, and number of secondary veins. Our specimens have well-preserved trichomes (Figure 12H), while the previous reported fossil records from Cenozoic of east and west North America and southwest China lack trichome (Axelrod, 1956(Axelrod, , 1992Meyer and Manchester, 1997;Tao et al., 2000). The present specimens are different from Q. paleohypargyrea and Q. paleolamellosa, described above, in teeth shape and trichome branches.
Diagnosis Leaf lanceolate, apex elongate acuminate, base cuneate with petiole. Margin serrate from 1/3 of the leaf to apex, teeth regularly spaced, CC-RT with rounded sinus. Midvein thick to thin from base to apex, straight; secondary veins thin, 9 pairs regularly spaced, opposite to pinnate from base to apex, craspedodromous, with stable angles 50-60 • , slightly curved near the margin. Tertiary veins mixed percurrent; Quaternary veins regular, polygonal reticulate. Leaf surface rugose with simple uniseriate trichomes and air hole of stomata.
Holotype CC-960 Paratype CC-1102 Etymology The specific epithet "changchangensis" refers to the Changchang Formation from which the specimens were collected.
Comparison The present specimens are attributed to Quercus sect. Cyclobalanopsis because their leaf shape, regularly spaced teeth and secondary veins, and mixed percurrent tertiary veins (Figures 13A-C). This new species resembles the extant Q. schottkyana Rehd. et Wils. on the leaf shape and simple uniseriate trichomes (Figures 13A,D). This new species is distinct from the extant Q. schottkyana Rehd. et Wils. and Q. glauca Thunb. which have simple uniseriate trichomes with the length of 160 and 265 µm in length, respectively (Luo and Zhou, 2001), by having much smaller simple uniseriate trichomes. Most of the previously described fossils reported fossil leaves of Q. sect. Cyclobalanopsis from the Cenozoic of China have no trichomes, except for Q. praedelavayi Y.W. Xing et Z.K. Zhou described from late Miocene of XundianXianfeng Basin with typical stellate trichomes including 16 branches (Xing et al., 2013). The present fossils are easily distinguished from Q. praedelavayi and above 4 species of Quercus described herein by the simple uniseriate trichome and lanceolate leaf with CC-RT teeth.

Phytogeographic Implications
Previously published fossil records indicate that Berryophyllum was widely distributed in strata from the Paleocene to the Eocene in Asia, North America, and Europe ( Figure 14A; Writing Group of Cenozoic Plants of China [WGCPC], 1978;Takhtajan, 1982;Jones and Dilcher, 1988;Crepet and Nixon, 1989a;Mai, 1995;Zhou, 1996;Tao et al., 2000;Kvaćek and Walther, 2010). The distribution range decreased since the Oligocene and finally disappeared from North America (Figure 14A; Tao et al., 2000;Kvaćek and Walther, 2010). In China, this genus was present as early as Eocene in Fushun, Liaoning, and in Zhanhua, Shandong and become abundant in Yunnan after the Oligocene (Figure 14A; Tao et al., 2000). The present Berryophyllum fossils discovered on Hainan Island in South China indicates that the genus has been distributed in the low latitude tropical region at least since the Eocene.
Castaneophyllum has been in Asia and North America since the Paleocene (Figure 14B; Takhtajan, 1982;Jones and Dilcher, 1988). This genus not only appeared in Asia and North America but also spread to Europe in the Eocene (Figure 14B; Walther, 1989, 2010). However, fossil records of the genus only occurred in Europe and central Asia during the Oligocene (Figure 14B; Takhtajan, 1982;Walther, 1989, 2010). In China, the genus was only previously found in the Eocene in Fushun, Liaoning ( Figure 14B; Tao et al., 2000). The Castaneophyllum fossils recovered here from Hainan Island are the lowest latitudinal distribution of the record for the genus.
Castanopsis fossils were widely distributed in Asia, North America, South America, and Europe during the Eocene, with the richest reproductive fossil records located in North America (Figure 14C; Wolfe, 1968;Huzioka and Takahasi, 1970;Takhtajan, 1982;Crepet and Nixon, 1989a;Manchester, 1994;Wilf et al., 2019). In China, however, Castanopsis fossils were mainly recovered from Yunnan, Sichuan, Zhejiang, and Guangxi during the Miocene to the Pliocene (Figure 14C; Tao and Du, 1982;Tao and Chen, 1983;Liu, 1993;Tao et al., 2000;Xia et al., 2009;Guo, 2011;Wu et al., 2014;Li et al., 2015). The Castanopsis fossils recovered here from the Eocene stratum on Hainan Island are both the earliest fossil records of the genus in China and also the lowest latitudinal distribution of the record in the world.
Lithocarpus was extensively distributed in North America and Europe in the Eocene, then extended into Asia during the Oligocene-Miocene, and finally almost disappeared from North America and Europe after the Miocene (Figure 15A; Andreansky and Kovaca, 1966;Axelrod, 1966aAxelrod, , 1998aTakhtajan, 1982;Kvaćek and Walther, 1989;Vikulin, 2011). In China, Lithocarpus fossils were mainly found from the Oligocene and Miocene strata of Yunnan, and diversified in the Miocene (Figure 15A; Writing Group of Cenozoic Plants of China [WGCPC], 1978;Tao et al., 2000;Guo, 2011;Mu et al., 2015). Additionally, they also occurred in the Pleistocene stratum of Guangxi ( Figure 15A; Tao et al., 2000). The Lithocarpus fossils here from Hainan Island have similar implications compared to Castanopsis in that they have the earliest record in China and the lowest distribution latitudes in the world.
Quercus, including subgenus Quercus and subgenus Cerris (including Q. sect. Cyclobalanopsis, Denk et al., 2017), has the richest and widest distribution of both modern and fossil species. The fossil records suggest that Q. subg. Quercus has been widely distributed in East and South Asia, western North America, and southern Europe since the Eocene (Figure 15B; MacGinitie, 1941MacGinitie, , 1953MacGinitie, , 1969Bones, 1979;Takhtajan, 1982;Daghlian and Crepet, 1983;Manchester, 1983Manchester, , 1994Mai and Walther, 1985;Kvaćek and Walther, 1989;Vikulin, 2011;Tao et al., 2000). Quercus also occurred in the Eocene stratum in Fushun, Liaoning ( Figure 15B; Tao et al., 2000). The fossil records of Q. sect. Cyclobalanopsis can be also dated back to the Eocene in western North America and eastern Germany (Figure 15C; Kvaćek and Walther, 1989;Manchester, 1994). In China, the earliest reliable Q. sect. Cyclobalanopsis fossils were discovered from the Oligocene stratum in Yunnan and Guangdong provinces ( Figure 15C; Writing Group of Cenozoic Plants of China [WGCPC], 1978;Liu et al., 2019). The Quercus fossils here from Hainan Island have a wide variety of species, including 5 species of Q. sect. Cyclobalanopsis, which are the lowest latitudinal distribution of the genus in the fossil record. Among these, the fossils assigned to Q. sect. Cyclobalanopsis are the earliest fossil records in China, as well as the earliest fossil records of the section in China. The above fossils suggest that the intragenus differentiation and the diversity evolution of the Q. sect. Cyclobalanopsis already started as early as the Eocene in South China.
Overall, as the earliest fossils of Fagaceae, such as Berryophyllum and Castaneophyllum were mainly distributed in high latitude regions, and the present occurrence of above five genera of Fagaceae from the middle Eocene Changchang Formation of Changchang Basin, Hainan Island of South China, a possible divergence pattern for the family is proposed that the family might be boreotropical origin and then migrated southward by the middle Eocene and highly differentiated at that time. The extinct genera, Berryophyllum and Castaneophyllum arrived at the low latitude of South China at least by the middle Eocene, and 7 fossils species in extant Castanopsis, Lithocarpus, and Quercus section Cyclobalanopsis show the diversity of the family in the middle Eocene of South China. Paleoecological Implications The five species described here are assigned to the extinct genera Berryophyllum and Castaneophyllum complexes. Both the Berryophyllum and Castaneophyllum complexes could likely thrive in these many environments due to their considerable variation in ecologically important parameters (e.g., leaf area and length to width ratios) and due to their interbreeding complex, similar to the extant Quercus (Jones and Dilcher, 1988). The interbreeding strategy, highly adaptive in the fluvial and near coastal environments (Jones and Dilcher, 1988), is supported by the reconstructed environment of our fossil locality; Changchang Basin is very close to the coastal areas.
In the Eocene Hainan Island, the presence of Berryophyllum and Castaneophyllum complexes, as well as a great number of aquatic ferns, Salvinia , Alseodaphne (Li et al., 2009), palms (Zhou et al., 2013), and other angiosperms from the same layer, indicates a wet environment in the basin during that time. The other 7 species presented in this study have been assigned to the extant evergreen genera Castanopsis, Lithocarpus, and Quercus section Cyclobalanopsis. They are the most diverse groups within the family Fagaceae, confined to East and Southeast Asia, and are important dominants in the evergreen broadleaved forests (EBLF) in tropical and subtropical Asia (Tang, 2015). Castanopsis is generally distributed at lower altitudes, whereas Lithocarpus and Quercus section Cyclobalanopsis at higher altitudes, but sometimes they have been found to coexist in the same altitudinal range and be co-dominant in the same EBLF (Tang, 2015). Indochina, Southwest China, and South China have the highest species diversity of these three genera (Tang, 2015). The floristic composition of Hainan Island shows a strong tropical characteristics, with mostly tropical genera, e.g., Lithocarpus and some subtropical genera, e.g., Castanopsis and Quercus (Jiang et al., 2002). Many tropical genera, including Sabalites (Zhou et al., 2013), Alseodaphne (Li et al., 2009), Palaeocarya (Jin, 2009) closely related to extant Engelhardtia (Juglandaceae), and Morinda (Rubiaceae; Shi et al., 2012), have been recovered from the middle Eocene Changchang Formation of Changchang Basin, South China. Castanopsis, Lithocarpus, and Q. section Cyclobalanopsis, collected from the same layer with the above taxa, are the most abundant and diverse taxa (except for Lauraceae). This indicates that these three genera of Fagaceae dominated the evergreen tropical and subtropical forests in South China by at least the middle Eocene.
Palynological and Climate Leaf Analysis Multivariate Program (CLAMP) studies show the mean annual temperatures of 14.2-19.8 • C and ∼22 ± 4.7 • C, respectively, the mean annual precipitations of 784.7-1,113.3 mm, and growing season precipitation (GSP, effectively the mean annual precipitation) of 2020 ± 1220 mm, respectively for the middle Eocene Changchang Formation of Changchang Basin, South China (Yao et al., 2009;Spicer et al., 2014). Based on the above analysis of the living environment for the nearest living relatives of these fossils, we speculate that the climate of Hainan Island was warm and wet during the middle Eocene, which was suitable for the growth and differentiation of Fagaceae, especially for Quercus sect. Cyclobalanopsis which was well-developed and highly differentiated during the middle Eocene.

DATA AVAILABILITY STATEMENT
All datasets presented in this study are included in the article/Supplementary Material.

AUTHOR CONTRIBUTIONS
JJ and XL conceived and designed the study, conducted taxonomic treatments, phytogeographic, and paleoecological interpretations. JJ, HS, and XL photographed specimens and arranged the figures. XL carried out the cuticle experiments and data analyses and wrote the manuscript. HS formatted the references and figure captions. All authors read and approved the final manuscript.