New Record of Fossil Wood Xenoxylon from the Late Triassic in the Sichuan Basin, Southern China and Its Paleoclimatic Implications

PALAEO-07681; No of Pages 11 Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx

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New record of fossil wood Xenoxylon from the Late Triassic in the Sichuan Basin, southern China and its paleoclimatic implications

Ning Tian a,c,d, Yongdong Wang b,c,⁎,MarcPhilippee,LiqinLib,h, Xiaoping Xie f,ZikunJiangg a College of Paleontology, Shenyang Normal University, Shenyang 110034, China b Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China c State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS), Nanjing 210008, China d Key Laboratory for Evolution of Past Life in Northeast Asia, Ministry of Land and Resources, Shenyang 110034, China e Laboratoire de Géologie de Lyon, Université Claude Bernard Lyon 1 and CNRS, Campus de la DOUA, Bâtiment, Darwin A, 69622 Villeurbanne Cedex, France f College of Geography and Tourism, Qufu Normal University, Rizhao 276826, China g Chinese Academy of Geological Sciences, Beijing 100037, China h University of Chinese Academy of Sciences, Beijing 100049, China article info abstract

Article history: Fossil wood is one of the significant proxies for terrestrial paleoclimate and paleogeographical reconstruction in Received 6 October 2015 Earth history. Abundant and diversified Mesozoic fossil woods are well recorded in China; however, Triassic fossil Received in revised form 21 January 2016 wood is very scarce. Here, we report a new fossil wood from the Late Triassic Xujiahe Formation (Norian to Accepted 3 February 2016 Rhaetian) in Guangyuan of northern Sichuan Basin, southwestern China. The fossil wood material consists of Available online xxxx two well-preserved specimens yielding secondary xylem with distinct growth rings. Bordered pits on the radial tracheid walls are mostly contiguous, biseriate alternate, locally uniseriate and strongly flattened. Cross-fields Keywords: Xenoxylon show a large window-like pore. This anatomy is typical for the important fossil wood morphogenus Xenoxylon Late Triassic Gothan, and, based on a novel combination of radial and cross-field pitting type and ray height, a new species, Xujiahe Formation Xenoxylon guangyuanense sp. nov. is recognized. The finding of this new fossil wood taxon contributes to a better Paleoclimate understanding of the yet poorly documented Xenoxylon early radiation during the Late Triassic, as well as of the Sichuan Basin origin of X. meisteri group, a peculiar endemic group which diversified in Far-East Asia from the Triassic to the Southern China Early Cretaceous. Xenoxylon is a paleobiogeographically significant genus, being bound to cooler and/or wetter climates of Northern Hemisphere throughout its Late Triassic–Late Cretaceous range. The occurrence of Xenoxylon in the Sichuan Basin of southern China may indicate a short-term cooling event, sandwiched within a period during which warm and wet climate conditions largely prevailed over lower latitude regions of the Northern Hemisphere. Such a cooling event in the Sichuan Basin which was located at a low paleolatitude on the eastern rim of Tethys may be in accordance in time with the temperature decline event in Norian– Rhaetian boundary uncovered by oxygen isotope (δ18O) record in Italy of the western rim of Tethys. © 2016 Elsevier B.V. All rights reserved.

1. Introduction documented in both the northern and southern phytoprovinces of China (Zheng et al., 2008; Wang et al., 2009; Yang et al., 2013). Howev- Currently, the understanding of Mesozoic climates is mainly based er, Triassic fossil wood records are very scarce and limited in this coun- on marine paleoclimatic information; however, detailed information try. Here we report a new fossil conifer wood taxon recently collected on the climatic evolution of continental interiors is relatively rare, al- from the continental Triassic in the Sichuan Basin of southern China at though much wanted. Fossil wood is an important component of plant a low paleolatitude with implications for paleoclimate shift. remains, and plays a significant role in reconstructing the terrestrial Late Triassic floras of the Sichuan Basin have been well known in vegetation, paleoclimate and paleoenvironment in the geological past China for a long time (e.g. Schenk, 1883; Sze and Lee, 1952). In particu- (Creber and Francis, 1999; Uhl, 2006). Rich and diversified fossil wood lar, the floral composition and paleoenvironmental significance of the records are documented from China, ranging in age from the Late plant remains from the Upper Triassic Xujiahe (=formerly Hsüchiaho) Carboniferous to the Cenozoic (Zheng et al., 2008). For Mesozoic fossil Formation in Guangyuan region of the northern Sichuan Basin, have wood, the Jurassic and Early Cretaceous records are abundant and well been investigated in detail (Lee, 1964; Yang, 1978; Huang, 1995; Wu, 1999). Fossil plants of the Xujiahe flora are mostly represented by fo- liage impressions or compressions. In this flora, the cycadophytes are ⁎ Corresponding author at: Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. the dominant group, yielding diverse bennettitalean taxa (e.g. E-mail address: [email protected] (Y. Wang). Pterophyllum, Anomozamites and Otozamites). Ferns, including

Dictyophyllum and Clathropteris, are the second most diverse group County (Fig. 1). Fossil woods from Yangliu and Gongnong Towns (Wang et al., 2010; Wang et al., 2015). Sphenophytes, pteridosperms, are not structurally preserved. conifers and ginkgoales are also present in this flora (Lee, 1964; Wu, The fossil wood specimens were collected from the Upper Triassic 1999; Wang et al., 2010). Compared to leaf impressions or compres- Xujiahe Formation, a coal-bearing sequence consisting of alternating sions, structurally preserved fossil wood remains are rarely documented deposits of mudstones and gray-yellow sandstones (Fig. 2). This forma- in this flora. Although a wood fragment was briefly noted from the tion crops out well in the Sichuan Basin, i.e. in the Guangyuan, Hechuan, Upper Triassic in western Sichuan Basin (Qian et al., 2010), no detailed Xuanhan, Daxian and Kaixian regions (Liu, 1982; Wang et al., 2010). information was provided regarding its systematic identification and In the Guangyuan region, the Xujiahe Formation consists of four affinity. lithological members, representing deltaic estuary sandbars and delta During a recent field survey in Guangyuan region of northern Si- plain swamp sedimentary environments, yielding diverse and well- chuan Basin, several fossil wood specimens were collected from the preserved plant fossil remains (Lee, 1964; Wang et al., 2010, 2015). Upper Triassic Xujiahe Formation. Among them, a new conifer wood The specimens studied here occurred in the gray-yellowish sandstone taxon, Xenoxylon guangyuanense sp. nov. was recognized, representing of the Xujiahe Formation Member 2, in Gaoyang Town (Fig. 2). The the first identified Triassic fossil wood data from the Sichuan Basin, Xujiahe Formation is dated as Norian to Rhaetian according to the asso- thus providing welcome evidence about the early radiation of the ciated invertebrate faunal assemblages (e.g. bivalves, ostracodes, genus Xenoxylon in Far-East Asia. As the wood taxon Xenoxylon is an in- conchostraceans) (Chen et al., 1979; Wei, 1982; Gou, 1993, 1998)and dicator of cool and/or wet climatic conditions (Philippe and Thévenard, palynological evidence (Liu, 1982; Lu and Wang, 1987; Liu et al., 1996; Amiot et al., 2011), the occurrence of this taxon at the low 2015a, 2015b). It is tentatively proposed that the Norian–Rhaetian paleolatitudes of southern China may document a short cool climatic boundary occurred in Member 1 of the Xujiahe Formation (Lu et al., event under the generally warm and humid climate that otherwise 2013). Hence the age of the Member 2 may represent early Rhaetian prevailed. age. The fossil flora of the Xujiahe Formation is paleobiogeographically considered as a typical hygrophilous “Dictyophyllum–Clathropteris” southern flora, dominated by ferns, cycads and bennettitales, with a 2. Geological setting, material and methods high occurrence frequency of sphenophytes (e.g. Neocalamites), pointing to humid climate conditions (Wang et al., 2010). Totally six fossil wood specimens were collected from three lo- For anatomical studies, the fossil wood specimens were cut trans- calities in Guangyuan city, northern Sichuan Province, including versely, longitudinally/radially and tangentially in order to prepare Gaoyang Town of Wangcang County (32°16′57.2″N, 106°16′43.1″ thin sections by standard methods (Hass and Rowe, 1999). These thin E), Xiaotangzi Village of Gongnong Town (32°29′20″N, 105°50′38″ sections were observed under light microscope for anatomical charac- E) and Yangliu Village of Yangjiayan Town (32°28′47″N, 105°46′ ters. Photographs were taken with Scope Image 9.0 (H3D) Software 15″E) (Fig. 1). The three fossil localities all belong to the same adapted to a Yongxin BM2000 Microscope. The specimens and slides tectonic unit and were located to the northern edge of the South are housed in the Palaeontological Museum of Liaoning, Shenyang, China Block. These material is generally silicified, and in particular, China, with registration numbers PMOL-B01250 and PMOL-B01251. two specimens are anatomically well preserved, both from the Xylological vocabulary, fossil wood taxonomy and nomenclature are same profile in Gaoyang Town, about 5 km north of Wangcang taken from Philippe and Bamford (2008).

Fig. 1. Sketch map of fossil locality for petriﬁed wood in Guangyuan of Sichuan Province, southern China.

Fig. 2. The lithostratigraphic column and fossil wood horizon of the Upper Triassic Xujiahe Formation in Guangyuan, Sichuan Province, China.

3. Systematics more than 25 μm, contiguous, mostly biseriate alternate, or locally uniseriate and strongly flattened (xenoxylean radial pitting). Cross- Genus Xenoxylon Gothan, 1905. field pits large, window-like, one pit per cross-field. Rays uniseriate, Species X. guangyuanense Tian, Wang et Philippe sp. nov. low (1–15 cells). Resin canals and axial parenchyma absent.

3.1. Species diagnosis 3.2. Etymology Secondary xylem with distinct growth rings. Transition from early wood to late wood abrupt; early wood wide, late wood with 3–7rows The speciﬁc epithet guangyuanense refers to the fossil locality of of cells only. Bordered pits on the radial walls of tracheids usually Guangyuan City.

3.3. Holotype 3.5. Locality

Specimens PMOL-B01250 and PMOL-B01251 (sampled from Gaoyang Town of Wangcang County, Guangyuan City, Sichuan the same wood log) and slides PMOL-B01250-a, b, c, and PMOL- Province, P.R. China. B01251-a,b,c.

3.4. Repository 3.6. Horizon and age

Paleontological Museum of Liaoning, Shenyang, P. R. China. Xujiahe Formation, Late Triassic (Norian–Rhaetian).

Fig. 3. Xenoxylon guangyuanense Tian, Wang et Philippe sp. nov. from the Upper Triassic Xujiahe Formation in Guangyuan, northern Sichuan Basin, China a. Cross view, showing typical growth rings, scale bar = 3 mm. b. Cross view, growth-rings with wide early wood (EW) and narrow late wood, scale bar = 0.5 mm. c. Cross view, details of early wood (EW) and late wood (LW), scale bar = 0.1 mm. d. Radial view, tracheids with contiguous uniseriate or alternate biseriate pits, scale bar = 0.1 mm. e. Radial view, tracheidswithcontiguousand strongly ﬂattened uniseriate pits, scale bar = 0.1 mm. f. Radial view, details of contiguous uniseriate pits on the radial tracheid wall, scale bar = 0.1 mm. g. Radial view, details of contiguous alternate biseriate pits on the radial tracheid wall, scale bar = 0.1 mm. h and i. Radial view, window-like cross-ﬁeld pits, scale bar = 0.1 mm. Specimen number: PMOL-B01250.

3.7. Description ring, an abrupt transition occurs from the early wood to the late wood, marked by structural changes in tracheid wall thickness and radial The specimens are secondary xylem fragments without other tis- diameter (Figs. 3b, 4a). The mean number of tracheids between two sues. The transverse section shows distinct and well developed growth adjacent rays is 5, with a range of 3–9rows(Figs. 3b, 4a). Resin canals rings (Figs. 3a–b, 4a). The early wood counts 34 rows of tracheids on av- are absent. erage (16 to 65 rows), with a mean width of 1.9 mm (0.9–3.6 mm) In longitudinal section, the tracheids are about 40–50 μm wide. (Figs. 3b, 4a). Early wood tracheids are large and thin-walled with a Tracheid radial walls have areolate bordered pits. These pits are mostly square to polygonal shape in transverse section (Figs. 3c, 4b). The late biseriate (Figs. 3d, g, 4e), locally uniseriate and strongly ﬂattened wood, with an average width of 120 μm, is formed of 3–7 rows of tra- (Figs. 3d–f, 4c–d). The biseriate tracheid pits are typically alternate cheids, slightly thicker walled than those of the early wood tracheids (Figs. 3d, g, 4e), although some rare opposite pairs also occur (Fig. 3f). (Figs. 3c, 4b). Tracheid cross-sections in the late wood are narrow, The pits are generally oval in shape with a width of about 25 μmanda with a ﬂat elliptical to rectangular shape (Figs. 3c, 4b). Within a growth height of about 20 μm(Figs. 3f, g, and 4d, e); locally they are much

Fig. 4. Xenoxylon guangyuanense Tian, Wang et Philippe sp. nov. from the Upper Triassic Xujiahe Formation in Guangyuan, northern Sichuan Basin, China a. Cross view, growth rings with a narrow late-wood, scale bar = 0.5 mm. b. Cross view, details of early wood (EW) and late wood (LW), scale bar = 0.1 mm. c. Radial view, tracheids with contiguous uniseriate pits, scale bar = 0.1 mm. d. Radial view, details of contiguous uniseriate pits on the radial tracheid wall, scale bar = 0.1 mm. e. Radial view, details of contiguous alternate biseriate pits on the radial tracheid wall, scale bar = 0.1 mm. f. Tangential view, low rays, scale bar = 0.1 mm; g and h. Radial view, window-like cross ﬁeld pitting, scale bar = 0.1 mm. i. Radial view, details of the cross ﬁeld pits, scale bar = 40 μm. Specimen number: PMOL-B01251.

Fig. 5. Xenoxylon guangyuanense Tian, Wang et Philippe sp. nov. from the Upper Triassic Xujiahe Formation in Guangyuan, northern Sichuan Basin, China a–c. Tangential views showing low rays. a. scale bar = 1.0 mm. b. scale bar = 0.5 mm. c. scale bar = 0.2 mm. Specimen number: PMOL-B01250.

5. Discussion Carnian Xenoxylon diversity is thus poorly known, although it can be hy- pothesized to have been relatively low. Triassic fossil wood records are very limited in China; only six spe- During the Norian to Rhaetian, at least five species of Xenoxylon cies belonging to five genera have been documented (Zheng et al., are reported worldwide, including X. latiporosum, X. nariwaense, 2008). Among them, X. ellipticum and Protophyllocladoxylon szei Wang X. phyllocladoides, X. junggarensis,andX. guangyuanense sp. nov. were reported from the Hongweikeng Formation in northern Guang- (Shilkina, 1967; Yamazaki et al., 1980; Khudaiberdyev, 1993). dong Province, southern China (Wang, 1991a, 1991b), Scalaroxylon These, coupled to the data left in open nomenclature (e.g. Yukawa Vogellehner emend. Zhang et Zheng was described from the Lower Tri- et al., 2012)suggestthatXenoxylon diversity increased much after assic Laolongtou Formation of Inner Mongolia (Zhang et al., 2006), X. the Carnian, during the end of the Triassic. Then in the Early Jurassic, junggarensis was documented from the Upper Triassic Huangshanjie the species diversity of Xenoxylon kept on increasing, up to eight FormationinXinjiang,whileTianoxylon Zhang et Zheng and species (such as X. fuxinense, X. hopeiense, X. huttonianum, X. latiporosum, Liaoningoxylon Zhang et Zheng were identified from the Upper Tri- X. meisteri, X. peidense, X. phyllocladoides and X. suljuctense)withahigher assic Hongla Formation in Liaoning Province, NE China (Zheng et al., xylological disparity and a wider distribution range than during the Late 2008). The new species X. guangyuanense reported here represents the Triassic. first systematic report of fossil wood from the Sichuan Basin, which con- Interestingly, however, during the Norian, while the peculiar X. tributes to further understanding of the Late Triassic fossil wood diversity parvipunctatum disappeared, the three known subgroups of Xenoxylon in China as well as to the paleoclimatology and paleobiogeography of this (namely the Latiporosum, Meisteri and Phyllocladoides subgroups, time interval. which lingered at least until the end of the Early Cretaceous) diversified. The Meisteri subgroup was represented by three species during the Norian, including the present species X. guangyuanense sp. nov. and X. 5.1. Late Triassic radiation of Xenoxylon junggarensis from China. Much of the Early Jurassic Xenoxylon diversity thus originated from the Norian, a period during which the genus The genus Xenoxylon,erectedbyGothan (1905) is an important Xenoxylon must have experienced a remarkable radiation. component of the Mesozoic wood flora. It is peculiar for having a unique The present new species from China brings new insights into the anatomy with no modern equivalents (Philippe and Thévenard, 1996), specific diversity of Xenoxylon during the Late Triassic. It provides new although it is considered most probably as a member of Coniferales clues to the radiation of Xenoxylon in general, and of the Meisteri sub- (Marynowski et al., 2008). From the Jurassic to the Early Cretaceous, group in particular. This group became dominant in terms of species di- Xenoxylon was widespread in the Northern Hemisphere, with the ex- versity from the Early Jurassic to the end of the Early Cretaceous, and ception of North America. It is noted that northern part of Far-East thus played a key role in Xenoxylon diversity. In our current knowledge, Asia is the only region in the world where Xenoxylon persisted from it is impossible to hypothesize a cradle for the genus. From the Laurasian the Late Triassic to the Late Cretaceous (Philippe et al., 2009). low-paleolatitudes it occupied during the Carnian, the genus distribu- The early evolutionary history of the Xenoxylon genus is still unclear, tion range apparently shifted northward during the Norian, toward cir- partly because of the recent stratigraphical reassignments of some early cumpolar regions. It is striking that Xenoxylon is documented from the data. The X.gr.meisteri data from Guangdong, southern China (Wang, Norian onward from the high paleolatitudes of the Russian Arctic and 1991a) can now be assigned to the Carnian as the Hongweikeng Forma- Xinjiang (NE China) to the mid-paleolatitudes of both Europe and tion is now better dated (Pang, 2014). The Triassic data from the Nampo Asia, circa 30°N, on the northeastern rim of the Tethys (Boucot et al., Group, Korea (Shimakura, 1936; Kim et al., 2005; Philippe et al., 2009), 2009; Oh et al., 2015; Wan et al., 2016). There, allopatric speciation are reassigned to the Early Jurassic (Egawa and Lee, 2011). New data may have taken place and initiated the partition into the three sub- and clarifications of the Triassic record of the genus in Japan were pub- groups which are subsequently observed. However, both the Carnian lished by Yukawa et al. (2012). Table 1 sums up Triassic Xenoxylon data and the pre-Carnian fossil wood records of the Arctic are not well worldwide. enough documented to unambiguously support this hypothesis. Wher- The earliest fossil records of Xenoxylon can be traced back to the ever the genus originated, the new species of X. guangyuanense sp. nov. Carnian (Late Triassic) based on evidence from Western Europe, China reinforces the idea that China played a key role in the diversification of and Japan (Fliche, 1910; Vogellehner, 1965; Rumpel, 1979; Yamazaki Xenoxylon. and Tsunada, 1981, 1982; Yamazaki et al., 1980; Wang, 1991a). Two species are known from the Carnian, i.e. X. conchylianum (Fliche, 5.2. Paleoclimatic implications 1910)andX. parvipunctatum (Vogellehner, 1965); while Chinese (X. gr. meisteri, Wang, 1991a) and Japanese (X. sp. A and B, Yamazaki and Climatically, several lines of evidence strongly suggest that the Tsunada, 1981) data are still in open nomenclature. The wood X. genus Xenoxylon was bound to cool and/or wet climate conditions. parvipunctatum is peculiar and well described, however, the type spec- Xenoxylon has a typical circumpolar distribution (in the broad sense) imens of X. conchylianum as well as Yamazaki's material are lost, and with the majority data from high paleolatitudes (Philippe and Wang's material from Guangdong of China needs reappraisal. The Thévenard, 1996; Philippe et al., 2009). Cretaceous data analysis

Table 1 List of Triassic Xenoxylon record worldwide.

Taxa Stratigraphy Locality References

X. conchylianum Schilfsandstein, Early Carnian NE France Fliche, 1910 X. parvipunctatum Early Carnian S Germany Vogellehner, 1965; Rumpel, 1979 X. gr. meisteri (as X. ellipticum) Hongweikeng Fm., Carnian Guangdong, S China Wang, 1991a; Philippe et al., 2013; Pang, 2014 X. junggarensis Huangshanjie Fm., Norian Xinjiang, NW China Wan et al., 2016 X. sp. A & sp. B Momonoki Fm., Mine Gr., Carnian SW Japan Yamazaki and Tsunada, 1981; K. Terada, pers. com. X. nariwaense and X. cf. japonicum Mogamiyama Fm., Nariwa Gr., Mid. Norian SW Japan Yamazaki et al., 1980; K. Terada, pers. com. Xenoxylon sp. Hinabata Fm., Nariwa Gr., Late Norian SW Japan Yukawa et al., 2012 X. latiporosum Upper Kokuibelsu Subsuite, Norian Central Pamir, Tajikistan Khudaiberdyev, 1993 X. phyllocladoides Upper Kokuibelsu Subsuite, Norian Central Pamir, Tajikistan Khudaiberdyev, 1993 X. guangyuanense sp. nov. Xujiahe Fm., Norian to Rhaetian Sichuan, SW China This paper

Abbreviations: Fm. = formation; Mid. = middle; Gr. = group; NE = northeast; S = south; SW = southwest; NW = northwest.

Fig. 6. Late Triassic (220 Ma) paleogeographic map Notes: The map was modiﬁed after Scotese (2004), Fig. 3; indicates the site of the present fossil wood locality in the South China Block.

Please cite this article as: Tian, N., et al., New record of fossil wood Xenoxylon from the Late Triassic in the Sichuan Basin, southern China and its paleoclimatic implications, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.006 N. Tian et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx 9 transitional period (Parrish, 1993). Many lines of evidence support that (e.g. palynological data, sedimentological data and organic geo- the megamonsoon was still functional by the Late Triassic (Dubiel et al., chemistry data). 1991; Mutti and Weissert, 1995; Balog et al., 1999; Therrien and Fastovsky, 2000; Nakada et al., 2014), and was a major driver of global climate until the Early Jurassic period (Parrish and Peterson, 1988). Acknowledgments However, it is controversial whether the megamonsoon had com- pletely collapsed or was still functional in Southern China during the This article is a contribution to the United Nations Educational, Sci- entific and Cultural Organization—International Union of Geological Late Triassic (Early Norian). Yan and Zhao (2002) suggested that the — climate of the eastern Tethys region was not controlled anymore by Sciences International Geoscience Program (UNESCO-IUGS-IGCP) Project 632. We thank Prof. S. L. Zheng and W. Zhang (Shenyang Insti- the megamonsoon after the Carnian stage. However, Qian et al. (2010) proposed that the western Sichuan Basin was still under control of tute of Geology and Mineral Resources, Ministry of Land and Resources, the megamonsoon during the late Late Triassic on the basis of well- Shenyang, China) for suggestions on specimen analysis. Special appreci- defined growth rings in an unidentified petrified wood fragment from ations are due to Dr. X. Y. Wen (Mianyang Normal University), J. Luo the Xujiahe Formation in western Sichuan Basin. (Shehong Bureau of Land and Resources, Sichuan Province, China), In fact, the growth ring types of Mesozoic conifer wood may not be G.Y. Shui and P. Deng (Guangyuan Bureau of Land and Resources, fi controlled only by environmental factors, but may also be determined Sichuan Province, China) for their support on eld survey. Kazuo Terada by their systematic affinity, e.g. Cupressinoxylon, Phyllocladoxylon, (Fukui Prefectural Dinosaur Museum, Japan) kindly shared information Protopodocarpoxylon and Taxodioxylon (Brison et al., 2001). Therefore, about Japanese Xenoxylon stratigraphy. We also thank Dr. Mike Pole growth ring type paleoclimatological analyses have to be based on large (NIGPAS, Nanjing) for linguistic improvement and suggestions on the fi and taxonomically diverse assemblages (Brison et al., 2001). The occur- manuscript. The manuscript bene ted greatly from the comments and rence of growth rings is insufficient in itself to infer climatic seasonality. suggestions of Dr. Mihai E. Popa. and an anonymous reviewer. This For some extant wood which inhabit monsoon regions, false annual study is jointly supported by the State Key Programme of Basic Research rings or false latewood bands might result from seasonal drought in the of Ministry of Science and Technology, China (2012CB822003), National pre-monsoon period. Hence, the occurrence of false annual rings might Natural Sciences Foundation of China (NSFC 41572014, 41272010, be more indicative of monsoon than annual rings. However, no typical 41302004 and 41402004), Innovation Project of CAS (KZCX-2-YW- fi false annual rings occur in the present fossil wood specimen from 154), the Team Program of Scienti c Innovation and Interdisciplin- China. In this view, though with distinct growth rings, the Guangyuan ary Cooperation of CAS, the State Key Laboratory of Palaeobiology material referred here to Xenoxylon does not definitely support the exis- and Stratigraphy (Nanjing Institute of Geology and Palaeontology, tence of a megamonsoon during the late Late Triassic (Norian to Rhaetian) CAS) (no. 133113), Science Research Project of Liaoning Provincial Educa- in the northern Sichuan Basin. tion Department (no. L2012391) and the Talent Fund of Shenyang Nor- The end-Triassic is marked by one of the five mass extinctions of mal University (no. 91400114006). the Phanerozoic (Raup and Sepkoski, 1982; Olsen et al., 1987, 2002; Sepkoski, 1996; McElwain et al., 1999; Hesselbo et al., 2002; Akikuni References et al., 2010). However, the Late Triassic extinctions remain poorly understood (Hallam and Wignall, 1997; Vajda and Bercovici, 2014; Akikuni, K., Hori, R., Vajda, V., Grant-Mackie, J.A., Ikehara, M., 2010. 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