A NEW RECORD of Agathoxylon from the OLIGOCENE-MIOCENE of SOUTH CHINA
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338 IAWAIAWA Journal Journal 36 (3), 36 2015: (3), 2015 338–344 A NEW RECORD OF AGATHOXYLON FROM THE OLIGOCENE-MIOCENE OF SOUTH CHINA Xinxin Feng1, 2, 3, Alexei A. Oskolski 4, 5, Xiaoyan Liu1, Wenbo Liao1 and Jianhua Jin1, 2, * 1State Key Laboratory of Biocontrol and Guangdong Provincial Key Laboratory of Plant Resources, School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China 2State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China 3Key Laboratory of Southern Subtropical Plant Diversity, Shenzhen Paleontological Museum and Shenzhen Fairylake Botanical Garden, Shenzhen 518004, China 4Komarov Botanical Institute of the Russian Academy of Sciences, St. Petersburg 197376, Russia 5Department of Botany and Plant Biotechnology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa *Corresponding author; e-mail: [email protected] ABSTRACT A gymnosperm wood is described from the Oligocene-Miocene of Hainan Island, South China. It is characterized by circular, thin-walled tracheids with resin plugs, 1–3-seriate alternate or opposite intertracheary pits in radial walls, 1–2-seriate rays, cross fields with 3–14 araucarioid cross-field pits. These are features found in the Araucariaceae and the fossil is designated as Agathoxylon sp. Fossil woods with anatomical characteristics seen in the Araucariaceae are extremely rare in the North Hemisphere after the K/T boundary. Thus, this Agathoxylon from the Oligocene-Miocene of South China has significance for biogeographic studies. Keywords: Araucariaceae, Araucarioxylon-type of woods, Agathoxylon, Oli- gocene-Miocene, Hainan Island, South China. INTRODUCTION Fossil woods that are anatomically similar to modern Araucariaceae have been called Araucarioxylon. Worldwide, over 400 Araucarioxylon species have been described from the Carboniferous to the Cenozoic (Philippe 2011), particularly widespread during the gymnospermous optimum of the Mesozoic (Stockey 1980; 1982). In China, 11 Arau- carioxylon morphospecies have been described, most of which were collected from the Permian, Jurassic, and Cretaceous of North and Central China (Zheng et al. 2008). Nomenclature of Araucariaceae-like fossil woods has been debated over the years. At least 16 morphogeneric names have been used to describe fossil woods with features similar to Araucariaceae (Philippe 2011). Philippe (1993) conducted a nomenclatural review of these generic names and argued that Agathoxylon Hartig is the most appropri- ate name to accommodate Araucariaceae-like woods (Philippe 2011). The frequently © International Association of Wood Anatomists, 2015 DOI 10.1163/22941932-20150104 Published by Koninklijke Brill NV, Leiden Downloaded from Brill.com09/26/2021 05:16:46AM via free access Xinxin Feng et al. – Oligocene-Miocene Agathoxylon 339 used generic names Araucarioxylon and Dadoxylon are synonyms of Agathoxylon and therefore are invalid (Philippe 1993; Philippe & Bamford 2008). Although still controversial, this proposition has been supported and accepted by most colleagues (Philippe 2011; Rößler et al. 2014). In this article, we report a new record of Agathoxylon from the Oligocene-Miocene Qiutangling Formation of Ledong, Hainan Island. MATERIALS AND METHODS Ledong (18° 85' N, 109° 13' E; Fig. 1) is located in the south-western portion of tropical Hainan Island, South China. The Geologic Survey Team (764#) (1958–1964) reported on the Lower Cretaceous Lumuwan Formation and Upper Cretaceous Baowan Forma- tion in Ledong (Bureau of Geology Mineral Resources of Guangdong Province, 1988). The Panxi Geologic Survey Team (1988) restudied the outcrop in Qiutangling Hill and re-designated the stratum as Eocene Wayao Formation. The Yichang Institute of Geology and Mineral Resource (2004) collected 17 taxa of fossil plants, and reassigned the stratum in Qiutangling Hill to Oligocene-Miocene Changpo Formation based on plant assemblages and geologic properties. The Geologic Survey Institute of Hainan Province (2010) restudied the stratum from the lithostratigraphic point of view. Both rock and paleontological assemblage of the stratum is distinctively different from the Wayao Formation and Changpo Formation and therefore Li et al. (2011) proposed a new lithostratigraphic unit, Qiutangling Formation. Exposed in the Qiutangling Hill (Fig. 1), the Qiutangling Formation is composed of coarse siliceous clastics whose lithology is grey, yellowish-grey quartzose fine conglomerate, sandy conglomerate, and uneven sandstone (Li et al. 2011). The studied Agathoxylon wood and another gymnosperm wood were collected in situ from the outcrop in the Qiutangling Hill. Besides silicified woods, Qiutangling Formation also yields abundant dicotyledonous leaves including Magnolia, Cinnamomum, Lindera, Machilus, Phoebe, Litsea, Cyclobalanopsis, Quercus, Acer, etc. Based on the fossil leaves assemblage and lithology, Li et al. (2011) consider the geologic age of the Qiutangling Formation to be Oligocene-early Miocene. There is only one specimen of the Agathoxylon wood (LDW002). It is silicified with a compressed cylindrical shape, a 6 cm diameter and 10 cm length, and a coarse surface without indication of rolling. Wood anatomical characters are described and measured in accordance to recommendations of the IAWA list of microscopic features for softwood identification (IAWA Committee 2004). The fossil wood specimen and thin sections are housed in the Museum of Biology, Sun Yat-sen University, Guang- zhou, China. Systematic description: Family: AR A UC A RI A CE A E . Genus: Agathoxylon Hartig 1848. Species: Agathoxylon sp. Specimen: LDW002 (Fig. 2–7). Downloaded from Brill.com09/26/2021 05:16:46AM via free access 340 IAWA Journal IAWA 36 (3), 2015 Downloaded fromBrill.com09/26/2021 05:16:46AM Figure 1. Map showing the location of Ledong, Hainan Island, South China (partly from Yao et al. 2009; He et al. 2010) and the simplified geologic map of Ledong (Li et al. 2011). Legends: ★: Location of Ledong. – ▲: Location of Qiutangling Hill (outcrop). – 1: Ordovician Nanbigou Formation. – 2: Low Cretaceous Lumuwan Formation. – 3: Upper Cretaceous Baowan Formation. – 4: Oligocene-Miocene Qiutangling Formation. – 5: Granite. – 6: Holocene alluvial. – 7: Fault. – 8: Unconformable contact boundary. – 9: Parallel unconformable contact boundary. – 10: Geologic boundary. via freeaccess Xinxin Feng et al. – Oligocene-Miocene Agathoxylon 341 Locality: Ledong (18° 85' N, 109° 13' E; Fig. 1), Hainan Island, South China. Geologic horizon: Qiutangling Formation, Oligocene-Miocene. Repository: Fossil wood sample and microscopic slides are deposited in the Museum of Biology, Sun Yat-sen University, Guangzhou, China. Description: Growth ring boundaries are distinct (Fig. 2). Tracheids are thin-walled, circular to oval in cross-sectional outline and 28–52 µm (mean 39 µm) in tangential diameter (Fig. 2). Resin-like plugs are occasionally present in tracheids. Tracheid pits in radial walls are 1–3-seriate (Fig. 3, 4), predominately 2-seriate; 1-seriate radial pits are bordered, circular to oval in outline and 12.3–17.5 µm (mean 14.9 µm) in vertical diameter (Fig. 3); 2–3-seriate pits are alternate (Fig. 3, 4), oc- casionally opposite (Fig. 3), bordered, polygonal in outline and 9.6–21.9 µm (mean 15.3 µm) in vertical diameter; all of the 1–3-seriate pits are contiguous with neighbour- ing pits and often somewhat flattened (Fig. 3, 4). Helical thickenings are absent in tracheids. Axial parenchyma is absent. Rays are 3–6/mm (mean 5/mm), predominately 1-seriate (Fig. 5) and rarely partial- ly 2-seriate (Fig. 6), completely composed of parenchyma cells, 66–469 µm (mean 186 µm) high, 3–20 cells (mean 8 cells) high (Fig. 5, 6), ray cells are oval or elliptical in tangential section, both vertical and horizontal end walls of ray parenchyma cells are smooth (Fig. 6). Cross-field pits are araucarioid; individual pits are distinctly bordered, circular to oval in outline, pit apertures are elliptical, included, narrower than the border; with 3–14 pits (mean 6) per cross-field arranged in alternate 1–4 rows (mostly 2 rows) with a tendency for crowding (Fig. 7). Intercellular canals and crystals are absent. DISCUSSION This Oligocene-Miocene wood from Ledong is characterized by absence of axial parenchyma and resin canals, 1–3-seriate alternate and opposite intertracheary pits, predominately 1-seriate rays, and alternate and crowded araucarioid cross-field pits. According to the ‘Key to Morphogenera of Fossil Conifer Woods’ illustrated by Philippe and Bamford (2008), the combination of anatomical characters unambigu- ously enables us to attribute the Oligocene-Miocene Ledong wood to the fossil genus Agathoxylon Hartig. There are over 400 morphospecies of Araucariaceae-type woods described from the Carboniferous to the Cenozoic around the world. Although the morphospecies are widespread in time and space, the features that have been used to establish different species are of doubtful systematic value. Booi et al. (2014) conducted a statistical analysis of a large collection (c. 250 specimens) of araucarioid woods from the Permian of Sumatra, Indonesia. They found that, although anatomical characters vary greatly, this large collection seemed to be a homogeneously coherent group. Downloaded from Brill.com09/26/2021 05:16:46AM via free access 342 IAWA Journal 36 (3), 2015 Figure 2–7. Wood anatomy of Agathoxylon sp. (LDW002) from Ledong, Hainan Island,