New coryphoid fossil palm leaves (Arecaceae: Coryphoideae) from ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

September 2013 Vol.56 No.9: 1493–1501 doi: 10.1007/s11430-013-4681-7

New coryphoid fossil palm leaves (Arecaceae: Coryphoideae) from the Eocene Changchang Basin of Hainan Island, South China ZHOU WenJun1,2, LIU XiaoYan1,2, XU QingQing1,2, HUANG KangYou3* & JIN JianHua1,2† 1

State Key Laboratory of Biocontrol and Guangdong Key Laboratory of Plant Resources, School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China; 2 State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS), Nanjing 210008, China; 3 Department of Earth Sciences, Sun Yat-sen University, Guangzhou 510275, China Received November 30, 2012; accepted March 27, 2013

Modern palms are diverse and widespread in the tropics and subtropics, especially the tropical rainforests. However, most fossil palm records have been reported from regions of relatively higher latitude. In this paper seven species of coryphoid palm from the Eocene Changchang Basin of Hainan Island of South China are reported. These are Sabalites asymmetricus sp. nov., S. robustus sp. nov., S. tenufolius sp. nov., S. szei Guo, S. changchagnensis Guo, Livistona sp., and Amesoneuron sp. According to leaf morphological characteristics, they can be classified as members of the subfamily Coryphoideae. In extant flora of Hainan Island, only three species of Livistona have costapalmate leaves. These palm fossils indicate that, during the Eocene, flora in Hainan Island included a diversity of coryphoid palms, which have some bearings on the reconstruction of the palaeoclimate of South China. coryphoid palm, Eocene, Changchang Basin, Hainan Island, South China Citation:

Zhou W J, Liu X Y, Xu Q Q, et al. New coryphoid fossil palm leaves (Arecaceae: Coryphoideae) from the Eocene Changchang Basin of Hainan Island, South China. Science China: Earth Sciences, 2013, 56: 1493–1501, doi: 10.1007/s11430-013-4681-7

Palms comprise the largest proportion of all fossil monocotyledons and are represented by a great number of fossil species, with almost all parts of the plants having been recorded [1]. These fossil records ranged from modern tropical and subtropical areas to warm temperate zones [2, 3]. According to molecular data, the evolutionary history of palms is earlier than those evidenced by the present fossil record [4]. The earliest palm fossil records are represented by fruits and stems. Hyphaeneocarpum aegyptiacum Vaudois-Midja et Lejal-Nicol, the earliest palm fruit, is recorded from the Aptian of Egypt [5]. Palmoxylon andegavense Crié and P. ligerinum Crié, the earliest records of palm stems, are de-

*Corresponding author (email: [email protected]) †Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013

scribed from the Turonian of France [1, 6]. The earliest unequivocal leaf fossils are Sabalites carolinensis Berry from the upper Coniacian-lower Santonian of South Carolina [2] and S. magothiensis Berry from the Santonian Magothy Formation of New Jersey and Maryland [7, 8]. Spinizonocolpites Muller is the earliest pollen record, with a wide distribution throughout the Maastrichtian. Numerous fossil records show that palms flourished at the CretaceousTertiary boundary and reached a peak of geographic distribution in the middle Eocene [9]. Some studies of fossils from Late Cretaceous suggest that an ecological relationship between animals and palms, such as dinosaur herbivores, may have played an important role in palm evolution [10]. Only a few fossil palm leaves have been recorded so far from China. Sabalites chinensis Endo has been described earth.scichina.com

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from the Eocene of Fushun, Liaoning and S. cf. taishuensis Takahashi, S. szei Guo, S. changchangensis Guo, and S. sp. have been described from the Paleogene Guangdong, Guangxi, and Hainan Island [11, 12]. Livistona tibetica Tao was found in the Paleogene flora of Tibet [13]. Amesoneuron sp. from the Cretaceous of Pingzhou (Ping Chau) Island of Hong Kong is the oldest palm fossil record known from China [14]. The genus Sabalites was first described by Saporta in 1865 and revised in 1972 by Read and Hickey as a type of costapalmate leaf. Costapalmate leaves have a definite costa, or extension of the petiole, into the blade [15, 16]. Costapalmate leaves represented by Sabalites are common type of leaves in the subfamily Coryphoideae and this type of leaf can be found in more than twenty living palm genera, including Sabal Adanson, Livistona R. Brown, Borassus L., Corypha L., and others [9]. Most reports of Sabalites are from North America, especially in the United States. Livistona is an extant coryphoid genus that is widely distributed in the Horn of Africa, Arabia, the Himalayas, and Australia [9]. Numerous fossil seeds, pollen, leaves, and stems have been reported [9, 17]. Leaf fossils of L. tibetica Tao have been described from Tibet [13]. Fossil seeds of L. minima from the Eocene London Clay, L. atlantica from the middle Eocene of Germany, and L. australis from the lower Miocene of the Czech Republic are described [18, 19]. Although there are some stems and pollen assignable to Livistona, the cuticular characters of Livistona have not yet been reported. The genus Amesoneuron, revised in 1972 by Read and Hickey, is represented by unarmed palm leaf fragments lacking clear evidence of their original shape (palmate or pinnate, compound or simple, etc.) [16]. Seven species belonging to three genera are described in this paper. These are Sabalites asymmetricus sp. nov., S. robustus sp. nov., S. tenufolius sp. nov., S. szei Guo, S. changchagnensis Guo, Livistona sp., and Amesoneuron sp. from the Changchang Basin of Hainan Island, South China. Together with the palms previously described by Guo (1965) from same locality [12], our materials show a rich diversity of coryphoid palms occurring in the Eocene of Hainan Island. Modern palms are abundant and diverse in tropical areas, but most palm fossils have been discovered at relatively higher latitudes. The fossils are less reported in current tropical regions, so palm fossils are little known in tropical regions [20, 21]. The diversity of palm fossils from the tropical Changchang Basin may provide new insight into the phytogeography of the Arecaceae, especially the Coryphoideae, and will be helpful for reconstructing the palaeoclimate in South China.

1 Materials and methods Fossil palm leaves examined in this study were collected

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from the Changchang Basin, Hainan Island, South China. Most collections are from the upper part of the Changchang Formation in the Changchang Basin (19°38′03″N, 110°27′04″E), which is a Paleogene basin covering an area of 34 km2 located in the northeastern part of Hainan Island (Figure 1) [22, 23]. The Paleogene sediments of Changchang Basin are divided into the Paleocene Changtou Formation, and the Eocene Changchang and Wayao formations. The Changchang Formation consists of a lower multicolored lacustrine facies and an upper dark-colored lakeswamp facies; the age of the formation is dated palynologically as the early Eocene to early late Eocene [22, 24]. All specimens are housed in the Museum of Biology, Sun Yat-sen University, Guangzhou, China. The leaf specimens were photographed using a Canon EOS 500D digital camera, and then numbered. The fossil cuticles were macerated in Schulze’s solution (one part KClO3 to two parts HNO3), followed by a treatment of 5% KOH [25], then mounted on glass slides and photographed using a Zeiss Axioscope A1 microscope.

2

Systematics

Family: Arecaceae Subfamily: Coryphoideae Genus: Sabalites Saporta Species: Sabalites asymmetricus Jin et Zhou, sp. nov. (Figure 2, (a)–(c), (e), (f)) Specimens. CCM017, CCM063a, CCM063b, CCM064, CCM065, CC626. Holotype. CCM063a. Paratype. CCM063b. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Type locality. Changchang Basin, Hainan Island, South China. Stratigraphic horizon. Changchang Formation, Eocene. Etymology. Species named for the asymmetrical form of the leaf blade. Diagnosis. Leaves costapalmate, fan-shaped, costa long and strong; petiole wide, unarmed. Leaf segments narrow. Blade asymmetrically attached to costa; mid-veins clear, paralleled by numerous faint lateral veins. Lamina isolateral and amphstomatic; epidermal cells elongated to polygonal, arranged in rows. Stomata surrounded by 4–6 subsidiary cells (2 polar and 2–4 lateral). Guard cells, with a pair of contact subsidiary cells, sunken below the level of the epidermis. Description. The holotype is a positive compression showing the basal part of the fan-shaped leaf blade with a long straight costa (Figure 2(b)). The paratype is a counterpart showing the adaxial blade without a costa (Figure 2(a)). The petiole is 3–4.5 cm wide, smooth, spineless, tapering upward to form a straight costa on the abaxial surface

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Figure 1

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Map showing the location of the Changchang Basin, Hainan Island, South China [23].

Figure 2 Leaf morphological characteristics of Sabalites asymmetricus sp. nov. and S. changchangensis Guo, and leaf cuticle characteristics of S. asymmetricus. (a)–(c) Macrofossils of Sabalites asymmetricus sp. nov., the arrow points to the asymmetrical leaf base. (a) CCM063b; (b) CCM063a; (c) CC017; (d) macrofossil of Sabalites changchangensis, CC045a; (e) upper epidermal cells of S. asymmetricus from specimen CCM063b; (f) Lower epidermal cells of S. asymmetricus from specimen CCM063b. The scale bar is 1 cm for (a)–(d), and 50 μm for (e) and (f).

(Figure 2(a)–(c)). The costa is stout and straight, about 1.3 cm wide at the base and up to 12 cm long in some specimens (Figure 2(a)–(c)). Segments are partly preserved and emerge asymmetrically from the petiole apex and along both sides of the costa. Segments, about 26 pairs, are not more than 6 cm long and 0.6 cm wide; the mid-vein is clear with numerous faint lateral veins. Epidermal cells arranged in rows are elongated or polygonal (5.2–8.8 m (average 6.7 m) wide and 23.2–35.8 m (average 28.7 m) long) with straight anticlinal walls. Epidermal cells on the upper epidermis (Figure 2(e)) are usually shorter than those on the lower epidermis (Figure 2(f)). Stomata surrounded by 4–6 subsidiary cells, 2 polar and 2–4 lateral, present on both sides of the lamina. Guard cells are narrow, 3 m wide and 19–22 m long and are usually slightly sunken below the surface of the subsidiary cells. Comparison. This species is characterized by its asymmetrical leaf base, i.e., the blade is asymmetrically attached to the costa, which is uncommon among fossil palm leaves, although common in extant Sabal, such as S. bermudana Bailey and S. mexicana Mart. Asymmetrical blades also occur in Bismarckia, Latania, and Hyphaene. Bismarckia is a monotypic genus that was endemic to Madagascar. Originated in the Mascarene Islands in the Indian Ocean, Latania prefers areas that are seasonally dry and hot. It is most frequently found near the coastlines of these islands, located east of the Madagascar. Among three species of Latania, L. loddigesii sometimes has asymmetrical leaves, but its segments are much wider than that in the fossils. Hyphaene is distributed in Africa and from Arabia to western India. The extant genus Hyphaene species is usually armed with robust spines on the petiole, which is

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different from the fossils, so the affinity of Sabalites asymmetricus to Sabal, Bismarckia, or Latania is not yet clear. Sabalites sp. from the Eocene of the Changchang Basin has a petiole much narrower than that of S. asymmetricus and the costa of Sabalites sp. is straighter and not expanded at the base [12]. The sunken guard cells in the stomatal complexes of S. asymmetricus are similar to those of the living Sabal, but the longer and narrower epidermal cells and narrower outer lateral cells of the stomatal complexes are different. Genus: Sabalites Saporta Species: Sabalites robustus Jin et Zhou, sp. nov. (Figure 3, (a), (c), (e)) Specimen. Part CCM006a and counterpart CCM006b. Holotype. CCM006a. Paratype. CCM006b. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Type locality. Changchang Basin, Hainan Island, South

Figure 3 Leaf morphological and cuticle characteristics of Sabalites robustus sp. nov. and S. tenuifolius sp. nov. (a), (c) Macrofossils of Sabalites robustus sp. nov., CCM006a, CCM006b; (b) Adaxial surface of the leaf blade of Sabalites tenuifolius sp. nov., the arrow points to the hastula structure, CCM043; (d) Abaxial surface of the leaf blade of S. tenuifolius, CCM052; (e) Cuticle of S. robustus; (f) Lower epidermis of S. tenuifolius from CCM052, showing the trichome base cells and stomata; (g) Upper epidermis of S. tenuifolius from CCM052, showing the trichome base cells; (h), (i) Lower epidermal cells of S. tenuifolius from CCM043 and CCM052, respectively.; (j) Upper epidermal cells of S. tenuifolius from CCM052. The scale bar is 1 cm for (a)–(d), and 50 m for (e)–(j).

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China. Stratigraphic horizon. Changchang Formation, Eocene. Etymology. Species named for the robust costa of the leaf. Diagnosis. Leaf costapalmate, large, fan-shaped, with thick costa; leaf segments narrow, arising from both sides of the costa and petiole apex; mid-vein distinct, paralleled by faint secondary veins; petiole stout, unarmed, with pronounced longitudinal ribs; costa robust, slightly enlarged at the base and then tapering distally; epidermal cells elongated, with tapering ends; stomatal complexes with six subsidiary cells, two polar and four lateral ones. Description. The holotype is a specimen (Figure 3(a)) showing the basal part of the leaf, and its counterpart (Figure 3(c)) is chosen as the paratype showing only the basal part of the blade. The apical part of the leaf blade and segments are missing. This specimen, about 13 cm wide and 15 cm long, shows the abaxial surface of a leaf. The costa is much wider than any palms known thus far from this basin. It is about 2 cm or more in width, slightly enlarged at the base, and tapers distally to form a long shoot. Leaf segments arise from the costa and petiole apex at an acute angle. Segments of the preserved part, about 20–25 pairs in number, are linear, about 0.5 cm wide and less than 7 cm long. Mid-veins and faint secondary veins are visible in this specimen. The prominent petiole is stout, unarmed, about 3.5 cm wide and 9 cm long, with distinct longitudinal ribs. Cuticle prepared from the counterpart shows the presence of stomata (Figure 3(e)), but the costal-intercostal arrangement of epidermal cells is not visible. Epidermal cells arranged in rows are irregular or elongated with tapering ends, about 3.7–8 m (average 5.4 m) wide and 10.1–37.7 m (average 20.9 m) long. Anticlinal walls are straight or rounded. Epidermal cells in the rows without stomata have oblique end walls and are longer than those in rows with stomata. Stomata are tetracytic, with 2 polar and 4 lateral subsidiary cells. Guard cells are superficial, approximately 3.3–4 m wide and 12.5–17 m long. Comparison. This new species is characterized by its robust petiole and large costa. It can be estimated that the entire leaf blade was probably above 1 m long. It can be compared to Sabalites imperialis Dawson from the early Paleocene of the Rocky Mountains [26], but later species has a straighter and continuous costal margin from the petiole to the blade. The epidermal cells differ greatly in size and shape. The polar subsidiary cells are usually short and nearly round. The stomata complex can be compared to the genus Livistona, and the uneven epidermal cells and smooth margins of petiole are distinct. Genus: Sabalites Saporta Species: Sabalites tenuifolius Jin et Zhou, sp. nov. (Figure 3, (b), (d), (f)–(j)) Specimens. CCM043, CCM052. Holotype. CCM052.

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Paratype. CCM043. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Type locality. Changchang Basin, Hainan Island, South China. Stratigraphic horizon. Changchang Formation, Eocene. Etymology. Species named for its narrow segments. Diagnosis. Leaves costapalmate, fan-shaped, with thick costa and narrow segments; petiole wide, unarmed; costa wide, prominent on the abaxial side of the leaf, absent from the adaxial side. Leaf segments very narrow, folded, pendulous; mid-vein paralleled by numerous finer secondary veins. Epidermal cells arranged in rows, polygonal or elongate; stomata surrounded by 6–8 subsidiary cells, abundant on both sides of the lamina; trichome bases comprised of eight or more radial cells. Description. This species is represented by two specimens that show the adaxial and abaxial surface of a leaf respectively. The specimen with a definite costa (Figure 3(d)) was selected as the holotype, and the other (Figure 3(b)) with a triangular hastula as the paratype. The holotype, about 6 cm wide and 14 cm long, shows the abaxial surface of the leaf as a partly preserved compression fossil. Leaf segments are very narrow, about 0.2 cm wide, and pendulous, about 28–30 pairs in number. The petiole is wide, about 3 cm in width, and unarmed, tapering upward forming a strong costa about 1.5 cm wide at the base. The costa is above 8 cm long. Some faint striations can be seen on the costa. Epidermal cells are usually arranged in rows (Figure 3(h), (i)) and stomatal complexes are abundant on both sides of the lamina. The upper and lower epidermal cells are nearly the same (Figure 3(i), (j)), but the lower side has more rows of elongate cells (Figure 3(h), (i)). Most epidermal cells are irregular or sometimes elongate, about 5.5–8.8 m (average 6.6 m) wide and 17–28.7 m (average 22 m) long. Anticlinal walls are rounded in short irregular cells or slightly undulate with small ridges in elongated epidermal cells. The trichome basal cells consist of 8–9 radial cells and can be found on both sides of the lamina (Figure 3(f), (g)). Stomatal complexes are superficial with 6-8 subsidiary cells, 2–4 polar, and 4 lateral ones. Guard cells are long and narrow, about 3–4 m wide and nearly 19 m long. Polar subsidiary cells are wide and exceptionally short. The paratype is also a compression showing the adaxial surface of a leaf according to the presence of a triangular hastula. It is almost identical in cuticle characteristics to that of the holotype. Comparison. This new species with extremely narrow and relatively pendulous basal segments is a rare occurrence in the fossil records. It is possible that the preserved segments were highly folded. The mid-veins in this species are not clear as there is dense plication in the basal part of the blade. It is also the only species from the Changchang Basin that the trichome base can be found in the cuticle and the trichome basal cells on both sides of the epidermis are virtually the same. The trichome base is one of the telling distin-

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guishing features from the other palms found in South China. Genus: Sabalites Saporta Species: Sabalites szei Guo (Figure 4 (a)–(d)) Synonym and reference. Sabalites szei Guo (Guo, 1965, p.599, Plate I: figs. 1, 2). Specimens. CCM005, CCM007, CCM042, CC250. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Locality. Changchang Basin, Hainan Island, South China. Stratigraphic horizon. Changchang Formation, Eocene. Emended diagnosis. Leaves costapalmate, moderate, fan-shaped, with thick costa; leaf segments linear, emerging straight from both sides of the costa; petiole stout, unarmed; epidermal cells arranged in rows, elongated, rectangular, in rows with stomata often shorter than other epidermal cells; stomatal complexes with 4 polar and 4 lateral subsidiary cells; guard cells superficial. Description. Four relatively complete specimens can be assigned to Sabalites szei Guo, the largest of which is about 20 cm long, with a 12 cm long costa. Leaf segments emerge straight from both sides of the costa at an acute angle, about 40–50° (Figure 4(a), (b)). Segments are narrow, about 0.5 cm wide, 25–30 pairs in number. Petioles are well

Figure 4 Leaf morphological and cuticle characteristics of Sabalites szei Guo, Livistona sp. and Amesoneuron sp., and leaf cuticle characteristic of living Livistona jenkinsiana Griffith. (a), (b) Macrofossil of Sabalites szei Guo, CCM007, CCM042; (c), (d) Cuticles of S. szei Guo from CCM007 and CCM042, respectively; (e), (f) Cuticle and macrofossil of Livistona sp., CCM049; (g) lower epidermis of Livistona jenkinsiana Griffith; (h), (i) Cuticle and macrofossil of Amesoneuron sp., CCM020. The scale bar is 1 cm for (a), (b), (f) and (i), and 50 m for (c), (d), (e), (g) and (h).

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preserved, unarmed, and 3–3.5 cm wide. A long straight costa forms the extension of the petiole into the lamina. The costa in well-preserved specimens can reach a length of 12 cm and a width of 1 cm at the base. Epidermal cells arranged in rows (Figure 4(c), (d)), and are usually rectangular, about 5.4–5.9 m (average 5.7 m) wide and 13.7–53.7 m (average 31.2 m) long. The cuticle characteristics suggest that this species had isobilateral lamina. Stomatal complexes are abundant on both surfaces. Epidermal cells show some degree of costal-intercostal band arrangement. Epidermal cells in rows with stomata are shorter and more irregular than other cells. Most epidermal cells in the costal band have square end walls. Anticlinal walls are straight or rounded. Stomatal complexes are characterized by 8 subsidiary cells, 4 polar and 4 lateral ones. Guard cells are more or less crescent-shaped, 4–5 m wide and 15–16 m long. Comparison. This species was first proposed by Guo (1965) based on the distinctly wider petiole than the other specimens collected [12]. In his study, he described only the morphological characteristics. Based on the new material, we add the features of cuticle and additional morphological details in this study. It must have been one of the most dominant palm species, as it occurs in great abundance. Genus: Sabalites Saporta. Species: Sabalites changchangensis Guo (Figure 2(d)) Synonym and reference. Sabalites changchangensis Guo (Guo, 1965, p.600, Plate I: fig. 3). Specimen. Part CCM045a and counterpart CCM045b. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Locality. Changchang Basin, Hainan Island, South China. Stratigraphic horizon. Changchang Formation, Eocene. Emended diagnosis. Leaf fan-shaped, costapalmate, showing abaxial surface, with long delicate costa; leaf segments fused at base, narrow, folded, orientation of segments unknown; basal segments strongly pendulous; petiole somewhat slender, unarmed. Description. The specimen is an impression with most of the apical portion of the segments missing. The preserved part is about 8 cm long and 3.5 cm wide. From the clear impression of a relatively long straight costa, this specimen represents the abaxial part of the leaf. The petiole is about 1.5 cm in width and unarmed, extending upward to form a long straight costa. Costa is narrow, 0.5 cm wide and about 6 cm long. The leaf segments of about 32 pairs are fused at the base and emerge from the apex of the petiole and bilateral sides of the costa at an acute angle. Proximally the basal segments are first directed downward and then curve upward. Due to the missing apex, the orientation of the segments is unknown. The primary veins are clearly visible, and the secondary veins are not.

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Comparison. This species is characterized by a small specimen with a distinct, narrow costa less than 0.5 cm in width and is similar in costal character to some species of Licuala, but there are no fossil records for comparison. Based on the stretched segments, it is not likely an immature leaf. The small size, narrow petiole, and pendulous segments on the basal bilateral sides all compare well to the characters of Sabalites changchangensis described by Guo [12]. The specimen described by Guo [12] shows the adaxial part of a leaf, whereas in this study the new material shows the abaxial part. Based on its small size, it is probable that S. changchangensis Guo is a kind of low understory shrub. Genus: Livistona R. Brown. Species: Livistona sp. (Figure 4(e), (f)) Specimen. CCM049. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Locality. Changchang Basin, Hainan Island, South China. Stratigraphic horizon. Changchang Formation, Eocene. Description. This specimen is represented by only one compression showing only the leaf segments (Figure 4(f)). Leaf is costapalmate. These segments average about 1 cm wide and 10 cm long. Mid-vein with numerous parallel narrow secondary veins are clearly visible. Cuticle characteristics are well preserved in the specimen. The epidermal cells are regularly elongated and arranged in alternating costal and intercostal bands (Figure 4(e)). Epidermal cells in costal bands are more regular and longer than those in intercostal ones. Overall, the epidermal cells are about 14.5–48.5 m (averaging 33.3 m) long and 3.6–6.0 m (averaging 4.9 m) wide. The epidermal cells typically have oblique end walls. Stomatal complexes abundant, closely spaced and arranged in rows on both lower and upper epidermis, but more numerous on the lower surface. Stomatal complexes have 2–4 polar and 4 lateral subsidiary cells. Guard cells are narrow and long, about 2.6–4.6 μm wide and 16–18 μm long. Comparison. The Hainan specimen is referred to the living genus Livistona based on the similarity of epidermal cell arrangements and stomatal complexes. One of the notable characters of the cuticle is the high density of stomatal complexes. The anticlinal walls are straight, whereas in extant Livistona they are commonly sinuous, although L. jenkinsiana Griffith (Figure 4(g)) has straight anticlinal walls, showing some similarity to the Hainan specimen in epidermal cells and stomatal complexes. Since the Hainan specimen is only partly preserved, it is unreasonable to refer it to a particular species or a new species, so we describe it as Livistona sp. Genus: Amesoneuron Göppert. Species: Amesoneuron sp. (Figure 4(h), (i))

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Specimen. CCM020. Repository. The Museum of Biology, Sun Yat-sen University, Guangzhou, China. Locality. Changchang Basin, Hainan Island, South China. Stratigraphic horizon. Changchang Formation, Eocene. Description. The Hainan specimen is known only from segment impressions with small areas of cuticle preserved (Figure 4(i)). The complete shape is palmate. The segments are unarmed, about 1.2 cm wide and 5.8 cm long. The midveins are clear, while the parallel veins are dim. Epidermal cells arranged in rows and elongated, ranging from 22.4 to 68.2 m (averaging 42.3 m) long, and 4.7–6.3 m (averaging 5.6 m) wide although it was difficult to determine the dorsiventral orientation from the isolated small pieces of cuticle. Anticlinal walls are straight. Stomatal complexes are surrounded by 2 polar and 4 lateral subsidiary cells (Figure 4(h)). Guard cells are narrow, about 2.2–4.1 m wide and 21–25 m long. Overall, epidermal cells in rows with stomata are shorter and less regular in shape than other epidermal cells. Comparison. Genus Amesoneuron is an organ genus established for the unarmed isolated segments with obvious mid-veins, which are not attached to a main rachis. Based on the definition of this genus, the Hainan specimen certainly belongs to Amesoneuron. Although the cuticular characters are preserved, the specimen is only partly preserved, and it is unreasonable to refer it to a particular species or a new species. Therefore, we describe it as Amesoneuron sp.

3 Discussion Extant palms are emblematic of the tropics, having a wide distribution in tropical and subtropical regions. Arecaceae is a large monocotyledonous family with five subfamilies and about 28 tribes [9]. Palm leaves fall basically into two types, pinnate and palmate. The pinnate leaves are feather-like with a long stout rachis bounded on each side by a number of pinnae or leaflets. Pinnate leaves are found in each palm subfamily. Palmate leaves are divided into the truly palmate type with no costa and the costapalmate type that has an extension of the petiole into the blade, forming the costa. Palmate leaves are characteristic of the subfamily Coryphoideae found in all members except the tribes Caroteae and Phoeniceae. Palmate leaves are also found in the tribe Mauritiinae of the subfamily Calamoideae [9]. Most of the fossil palms from Hainan Island have costapalmate leaves with a definite costa. Only some isolated segments have unclear overall shape, which can be truly palmate or costapalmate leaves. Read and Hickey [16] revised palm leaf fossils into six genera representing six types of leaves, i.e., Amesoneuron (isolated segments unarmed), Bactrites (isolated segments armed on one margin), Saba-

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lites (costapalmate leaves), Palmacites (pure palmate leaves), Phoenicites (pinnate leaves with reduplicate pinnae), and Phoenix (pinnate leaves with induplicate pinnae and spine-like ones based on the rachis). Such a system simplifies identification and classification of most palm leaves that have an unknown affinity to extant genera. But assignment of diverse palm fossils into six genera in this system will offer little insight into the phylogenetic study of a specific genus. In addition to these taxa, Daghlian [27] described two new genera, Palustrapalma and Costapalma, from the lower and middle Eocene of southeastern North America. These new genera have not been widely adopted. Daghlian also used cuticular analysis and even recognized one of his specimens as a fossil species of the extant genus Sabal. Daghlian and Dilcher [28] re-examined palm leaves from the Eocene deposits of western Kentucky and Tennessee using cuticular characteristics and found a greater diversity of palms than was found in an earlier study by Berry [29]. Van der Burgh [30] and Schaarschmidt and Wilde [31] also employed cuticular analysis in their studies of palm fossils. Cuticle features play an important role in species identification among a large number of specimens. As there are great similarities in the forms of palm leaves, cuticular analysis provides additional characters in the study of fossil palm leaves. In our study, we examined over one hundred specimens of fossil palm leaves from the Eocene Changchang Basin, and recognized seven species in three genera. Almost all the specimens have only the middle and basal part of the blades preserved. Segments are narrow, no more than 1.2 cm in width, and preserved basal parts are heavily plicate in form, which can make it difficult to understand the induplication or reduplication of the segments due to the lack of apical parts. Mid-veins are distinct from faint secondary veins and the numbers of secondary veins are not always clear. Margins of all the petioles are unarmed and most of the recognized species have isobilateral lamina with stomata on both sides. Such symmetry often occurs in the tribe Borasseae and is thought to correlate to light exposure or arid habitat [26, 32]. These cuticles also show some diversity in the arrangement of epidermal cells and stomatal characteristics, as shown in Table 1. All the fossil palm leaves from the Changchang Basin are fan-palms, which belong to the subfamily Coryphoideae. This is in contrast to the palm flora on Hainan Island today, which is dominated by pinnate-leaved palms. At present, there are about 26 species of modern palms in the Hainan flora, among which 17 species have pinnate leaves and 7 species have truly palmate leaves [33]. Only Livistona jenkinsiana Griffith and L. chinensis (Jacquin) R. Brown ex Martius have costapalmate leaves with a stout costa [33]. L. jenkinsiana and L. chinensis have a similar leaf shape and costa to the fossil L. sp. from the Changchang Basin, and L. jenkinsiana is comparable to L. sp. in cuticle features. Until now, there is no pinnate palm leaf fossil recorded. Maybe

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Table 1

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Comparison of the seven species of fossil palms from Hainan Island based on morphological and cuticular characters

Species Shape Petiole armed or not Petiole width (cm) Base width of costa (cm) Costa length (cm) Segment numbers (pairs) Segment width (cm) Width of epidermal cells (average) (m) Length of epidermal cells(average) (m) Number of polar subsidiary cells Number of lateral subsidiary cells Width of guard cell (m) Length of guard cell (m)

Sabalites asymmetricus Costapalmate not 3–4.5

S. robustus

S. tenuifolius

S. szei

S. changchangensis

Livistona sp.

Amesoneuron sp.

Costapalmate not 3.5

Costapalmate not 3

Costapalmate not 3–3.5

Costapalmate not 1.5

Costapalmate

Palmate

1.3

2

1.5

1

0.5

12

> 10

>8

12

6 32

26

20–25

28–30

25–30

0.6 5.2–8.8 (6.7) 23.2–35.8 (28.7)

0.5 3.7–8 (5.4) 10.1–37.7 (20.9)

0.2 5.5–8.8 (6.6) 17–28.7 (22)

0.5 5.4–5.9 (5.7) 13.7–53.7 (31.2)

1 3.6–6.0 (4.9) 14.5–48.5 (33.3)

1.2 4.7–6.3 (5.6) 22.4–68.2 (42.3)

2

2

2-4

4

2-4

2

2–4

4

4

4

4

4

3

3.3–4

3–4

4–5

2.6–4.6

2.2–4.1

19–22

12.5–17

19

15–16

16–18

21–25

the differences of habitat preference between pinnate palms and palmate palms are the reason. In any event, we can infer that coryphoid palms in the Eocene on Hainan Island were much more abundant than at present. A palynological study using nearest living relatives (NLRs) analysis suggests a subtropical climate during the early-middle Eocene for the upper part of the Changchang Formation [23]. The results show that the mean annual temperature and mean annual precipitation is 14.2–19.8°C and 784.7–1113.3 mm, respectively, which are similar to the Hunchun City area of Jilin Province in the Eocene. Later, Li et al.’s [34] study of the Alseodaphne (Lauraceae) record in the Changchang Basin documented a warmer climate than results by Yao et al. [23], as the mean annual temperature of extant distribution of Alseodaphne ranges is 20–22.6°C. Modern palms, widely distributed in the tropics and subtropics, have the most diverse palm flora in tropical rainforests and the family has the highest number of species between 5°N and 5°S latitude [9, 27]. Palm diversity is usually thought to be a reflection of high temperature and humid climate. Palm fossils are one of the most dominant groups among all fossils collected from the Changchang Basin. Combined with the rich fossils of Lauraceae, Euphorbiaceae, and Musaceae, it is most likely that Hainan Island in the Eocene should have a warmer tropical climate than was found in previous analyses. Apart from the warmer climate, we must consider the geographic environment and varied habitats, which may also be factors in the local palm diversity.

cluding three new species belonging to three genera. These are Sabalites asymmetricus sp. nov., S. robustus sp. nov., S. tenufolius sp. nov., S. szei Guo, S. changchagnensis Guo, Livistona sp., and Amesoneuron sp. from the Changchang Basin of Hainan Island, South China. As fossil palms from tropical areas are little known, these fossils provide important evidence for exploring the evolutionary history of diverse palms in tropics, and also provide hard evidence for the reconstruction of South China’s palaeoclimate in the Eocene. This study was supported by National Natural Science Foundation of China (Grant Nos. 41210001, 31070200 and 40972011), National Basic Research Program of China (Grant No. 2012CB822003), State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (Grant No. 123110), Fundamental Research Funds for the Central Universities (Grant No. 12LGJC04), and Key Project of Sun Yat-sen University for inviting foreign teachers and the Scientific Research Fund, Hongda Zhang, Sun Yat-sen University. The graduate students majoring in Plant Science at Sun Yat-sen University participated in the field collection of the sample fossils. We also thank Ms. Margaret Joyner, University of Florida, for polishing the manuscript. 1 2 3

4 5

4 Conclusions Here seven species of coryphoid palms were described, in-

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