2. College of Earth Sciences, Jilin University, Changchun 130061, China; Key-Lab for Evolution of Past Life and Environment in Northeast Asia, Ministry of Education, Jilin University, Changchun 130026, China
Horsetails, extending back to the Late Devonian (Taylor et al., 2009), were an important component of the Carboniferous and Early Permian coal-forming swamp forest where they obtained the greatest diversity and became the dominant group of Arthrophyta (Feng et al., 2012a; Rößler et al., 2012; Tian et al., 1996a; Li et al., 1995; DiMichele and Hook, 1992). The decline of the once-booming taxon began in the Late Permian. Its herbal types became the majority of the group in Mesozoic (Li et al., 2004; Yang, 1994). The genus Equisetum, which consists of about 29 species, is the only extant relict (Wu and Ching, 1991; Hauke, 1979, 1963).
The Paleozoic arborescent horsetails, represented by calamites, are among the most common and frequent elements in Carboniferous and Permian continental strata (Taylor et al., 2009; Hilton et al., 2001; DiMichele and Phillips, 1994). However, compared to the large amount of impressions/compressions, the axes with anatomical details preserved remain rather rare (Rößler and Noll, 2010). The term "calamitean" is usually used to refer to axis remains of plants ascribable to the Calamitaceae (Andrews, 1952).
Anatomically preserved calamitean axes are usually divided into three genera: i.e., Arthroxylon Reed, 1952, Calamitea (Cotta) emend. Rößler and Noll, 2007 and Arthropitys Goeppert, 1864–1865. As the most common genus, Arthropitys contains up to 27 species (the varieties are not counted) which have previously been recognized (Neregato et al., 2015; Feng et al., 2012a; Rößler et al., 2012; Rößler and Noll, 2010; Wang et al., 2006, 2003; Cichan and Taylor, 1983; Eggert, 1962; Anderson, 1954; Andrews, 1952; Renault, 1896). While there are only two species of Arthropitys, A. junlianensis Wang, Hilton, Li and Galtier, 2003 and A. yunnanensis (Tian and Gu) ex Wang, Hilton, Galtier and Tian, 2006, which have been reported from the Permian of China and they are both from the Upper Permian of South China.
The Bogda Mountain area of Xinjiang Uygur Autonomous Region belongs to the Subangaran phytoprovince during the Late Permian (Meyen, 1982, 1981). There are abundant previous studies about the permineralized axes from the Upper Permian of China (e.g., Wei et al., 2015; Feng et al., 2013, 2012b, 2011, 2010, 2008; Feng, 2012; Wang et al., 2006, 2003; Wang J, 2000; Tian et al., 1996b; Tian and Li, 1992; Wang Z Q, 1985). Yet, a few studies involved the Subangaran Province (Wei et al., 2016; Shi et al., 2014; Wan et al., 2014; Sze, 1934), and permineralized calamitean axes have never been reported in that region. Recently, we found two anatomically preserved calamitean axes from the Upper Permian of the Taoshuyuan area in the southern Bogda Mountains for the first time. These specimens provide important materials for the anatomy of calamiteans and the paleoecology reconstruction of the Late Permian Bogda Mountains.1 MATERIALS AND METHODS
The terrestrial facies Taoshuyuan Section lies in the southern foothills of Bogda Mountains, which contains whole exposed continuous Permian–Triassic strata that distribute as a syncline in space (Fig. 1). These strata were deposited in the Tarlong-Taodonggou half graben, which was located near the northwestern coast of the paleo-Tethys at the easternmost Kazakhstan Plate (Yang et al., 2010; Scotese, 2001; Ziegler et al., 1997; Şengör and Natal'in, 1996). The specimens were collected in the bottom of the Wutonggou Formation at the Taoshuyuan Section (XTT-A-06: 43°13'54.70"N, 88°58'24.27"E; XTT-BW-03: 43°13'54.90"N, 88°59'38.26"E). The Wutonggou Formation is about 56 m thick, mainly composed of uneven interbed of yellowish green mudstone and medium to fine sandstone and intercalated with dark gray mudstone, coal seams and three thin-bedded limestone layers, and it contains plant fossils: Callipteris changi, C. zeilleri, Comia sp., etc. (WGRSTXUAR, 1981). Yang et al.(2010, 2007) defined the Wutonggou low-order cycle, which includes the Wutonggou, Guodikeng, and the lower part of Jiucaiyuan formations and spans the Wuchiapingian, Changhsingian and early Induan stages, on the basis of interpreted depositional environments and paleoclimatic conditions. The age of the fossil-bearing interval is Wuchiapingian, as indicated by cyclostratigraphic correlation of the Taoshuyuan Section with the northeastern Tarlong Section where three U-Pb zircon radiometric ages of Wutonggou low-order cycle are available (Yang et al., 2010).
To examine the anatomy of the fossils, we made thin slides of three plans (transversal, longitudinal and tangential) in the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, and Shenyang Institute of Geology and Mineral Resources. The slides were examined and photographed under light microscope Leica DM4000 B and SPOT Flex Color with SPOT advanced microscope software. All the specimens and the slides are housed in the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan.2 RESULTS
Genus: Arthropitys Goeppert, 1864–1865
Type species: Arthropitys bistriata (Cotta) Goeppert emend. Rößler, Feng and Noll, 2012
Arthropitys taoshuyuanensis sp. nov.
Holotype: Specimen XTT-A-06.
Type locality: Taoshuyuan C Section, Turpan, Xinjiang Uygur Autonomous Region, China
Geological horizon: Wutonggou Formation
Stratigraphic position: Early Late Permian (Wuchiapingian)
Etymology: The species name refers to the fossil locality.
Diagnosis: Plant axis possesses pith, primary and secondary vascular tissues. Pith consists of a large central cavity and a narrow perimedullary zone in internodal region and diaphragm in nodal region. Carinal canals are circular and surrounded by a single layer of metaxylem tracheids. Secondary xylem consists of interfascicular rays and fascicular wedges. Fascicular wedges comprise thick-walled tracheids and thin-walled fascicular ray cells. Interfascicular rays are initially four to five cells wide, gradually diminished distally. Radial tracheid walls have uniseriate or biseriate circular pits, or scalariform pits. Fascicular rays parenchymatous, vary from five to more than 45 cells high and one to up to seven cells wide in the middle of the secondary xylem. Fascicular ray cells are rectangular in cross section, polygonal in tangential section.
General features The specimen XTT-A-06 is 250 mm in length and 58 mm in diameter. It is composed of pith, primary xylem and secondary xylem.
Detailed description Pith of the specimen XTT-A-06 has an elliptical outline, which is likely the consequence of partial compression (Fig. 2a). The diameter is about 17 mm×11 mm. The pith is filled with calcite contents. The pith diaphragms are poorly preserved, but the protuberances at the nodes are visible (Fig. 2b). At the internodes, the pith constitutes a large cavity and a narrow perimedullary zone (Figs. 2c and 2d, black arrows). Between the perimedullary zone and xylem, some dark cell contents are preserved.
A discrete ring of primary xylem strands surround the pith (Figs. 2d and 3). There are approximately 55 primary xylem strands. The carinal canals are surrounded by a single layer of metaxylem tracheids (Figs. 2c, 2d and 3). These tracheids are 16–34 μm in diameter.
The secondary xylem of XTT-A-06 is between 13 and 25 mm wide. It consists of interfascicular parenchymatous rays and fascicular wedges. Interfascicular rays start at the outer periphery of the pith at a width of four to five cells (ca. 0.3 mm) (Figs. 2d and 3). After a short distance, they gradually diminished. The fascicular wedges start from the external side of the primary xylem at a width of four to six rows of tracheids and rays. The widths of the fascicular wedges rapidly increase as new files of tracheids and rays are added. In the inner part of the secondary xylem, the tracheids and the ray cells are both rectangular in the cross section and the former is smaller in dimension. In the outer part of the secondary xylem, the fascicular ray cells are always rectangular, 29 μm×47 μm–49 μm×97 μm. Tracheids are nearly square and smaller, 31 μm×38 μm–48 μm×57 μm. In radial section the ray cells are rectangular, 76–117 μm high (Fig. 2f). The radial walls of tracheids possess uniseriate or biseriate circular pits, or scalariform pits (Figs. 2e and 2g). In tangential section through the inner part of the secondary xylem, the interfascicular ray cells are rectangular or polygonal, 57–106 μm high (Fig. 2h). The fascicular rays are one or two cells wide and the ray cells are rectangular. In tangential section through the middle region of the secondary xylem, the fascicular ray cells are polygonal, and the walls are smooth (Fig. 2i). The rays vary from five to more than 45 cells (400–3 125 μm) high and up to seven cells (~500 μm) wide.
The nature of the secondary xylem, particularly the interfascicular rays, acts as the basis to distinguish the three fossil genera from anatomically preserved calamitean axes (e.g., Rößler and Noll, 2007; Cichan and Taylor, 1983; Eggert, 1962; Andrews, 1952). Arthropitys and Arthroxylon both show a high portion of parenchyma and uniform type of tracheids (Rößler et al., 2012; Rößler and Noll, 2010; Hass, 1975; Andrews, 1952; Reed, 1952). In Arthropitys, the interfascicular rays consist of parenchymatous cells with rectangular to isodiametric shape and horizontal cell end walls (Rößler et al., 2012; Rößler and Noll, 2010; Wang et al., 2003; Andrews, 1952). In Arthroxylon, the interfascicular rays are mainly composed of vertically elongated prosenchymatous cells with tapering end walls (Reed, 1952). In contrast, the wood of Calamitea contains a relatively small proportion of parenchyma and possesses two types of tracheids in different diameter (Rößler and Noll, 2007).
The studied specimens show a high portion of parenchyma. Interfascicular rays consist of only parenchymatous cells with rectangular or polygonal shape and horizontal cell end walls. Tracheids are of uniform diameter. These characteristics all conform to those of Arthropitys.
For the delimitation of different species of Arthropitys, the constant characteristics such as interfascicular rays, nature of tracheid pitting, size of carinal canals and development patterns in the primary and secondary body could be taken into account (Rößler and Noll, 2007; Wang et al., 2003; Andrews, 1952). And secondary xylem contains most of the features that are considered to be important in classification of anatomically preserved calamitean axes (Wang et al., 2003).
About 27 species of Arthropitys have previously been reported as far as we know. Of these species, A. jongmansi Hirmer, 1927 (in Knoell, 1935), A. herbacea Hirmer and Knoell (in Knoell, 1935), A. bistriatoides Hirmer and Knoell (in Knoell, 1935) and A. renaultii Boureau, 1964 have very thin or absent secondary xylem, which makes it difficult to compare with other species. Arthropitys hirmeri Knoell, 1935 is an unique species in Arthropitys that lacks interfascicular rays (Anderson, 1954).
Nine species possess well-developed interfascicular rays: Arthropitys kansana Andrews, 1952, A. versifoveata Anderson, 1954, A. felixi Hirmer and Knoell (in Knoell, 1935), A. bistriata (Cotta) Goeppert emend. Rößler and Noll, 2010, A. gallica Renault, 1896, A. deltoides Cichan and Taylor, 1983, A. yunnanensis (Tian and Gu) ex Wang, Hilton, Galtier and Tian, 2006, A. isoramis Neregato, Rößler, et Noll, 2015 and A. iannuzzii Neregato, Rößler, et Noll, 2015. Their interfascicular rays extend throughout the secondary xylem. While the interfascicular rays of Arthropitys taoshuyuanensis sp. nov. are only four to five cells wide and abruptly taper centrifugally. Arthropitys major (Weiss) Renault, 1896 and A. approximata (Schlotheim) Renault, 1896 have very broad interfascicular rays that are different from those of the new species.
The radial tracheid walls of Arthropitys taoshuyuanensis sp. nov. possess uniseriate or biseriate circular pits or scalariform pits. But A. lineata Renault, 1896 and A. rochei Renault, 1896 are characterised by elongate bordered pits on their tracheid walls; Arthropitys gigas (Brongniart) Renault, 1896 and A. cacundensis Mussa, 1984 (in Coimbra and Mussa, 1984) have round pits; Arthropitys illinoensis Anderson, 1954 possesses multiseriate oval to elongate pits; Arthropitys sterzelii Rößler and Noll, 2010 shows equally spaced reticulate wall thickenings. The tracheid radial walls of A. porosa Renault, 1896 are composed of circular reticulate, elongate reticulate and scalariform thickenings. These pitting patterns are all different from those of the new species.
The carinal canal of Arthropitys taoshuyuanensis sp. nov. is lined by a single row of elements. In A. communis (Binney) Hirmer and Knoell (in Knoell, 1935) and A. junlianensis Wang, Hilton, Li and Galtier, 2003 the carinal canal and protoxylary tracheids are surrounded by an arch of multiple layers of isodiametric cells.
The new species possesses multiseriate fascicular rays, similar with A. medullata Renault, 1896. While the pith of A. medullata is weakly developed, in comparison, A. taoshuyuanensis sp. nov. has a large pith. The secondary rays of A. ezonata Goeppert, 1864–1865 are only two cells wide.
Therefore, the specimen should be designated to a new species that is here described as Arthropitys taoshuyuanensis sp. nov.
Locality: Taoshuyuan B Section, Turpan, Xinjiang Uygur Autonomous Region, China
Geological horizon: Wutonggou Formation
Stratigraphic position: Early Late Permian (Wuchiapingian)
General features XTT-B-03 is 100 mm long, and only secondary xylem was preserved. In cross section, the outline is sub-rectangular, 53 mm×26 mm.
Detailed description In cross section, the fascicular ray cells are rectangular, 53 μm×102 μm–85 μm×169 μm (Fig. 4a). The tracheids are smaller, rectangular or square, 22 μm×49 μm–70 μm×101 μm (Fig. 4a). In radial section, the fascicular ray cells are rectangular, 60–142 μm high (Fig. 4b). The radial walls of tracheids possess reticulate pits (Fig. 4c), or scalariform thickenings (Fig. 4d). There are many simple pits in the cross-field units (Fig. 4e). In tangential section, the fascicular ray cells are polygonal with smooth walls (Figs. 4f and 4g). The rays vary from three to more than 39 cells (350–3 225 μm) high and up to seven cells (~425 μm) wide.
The absence of pith and primary xylem makes it impossible to determine which species the specimen belongs to. Although the width and height of fascicular rays are similar to that of Arthropitys taoshuyuanensis sp. nov., the tracheid pitting pattern of this specimen is different. Besides, the cross-fields show many simple pits in the present specimen, which is also different from those of Arthropitys taoshuyuanensis sp. nov.3 DISCUSSION
Paleozoic horsetail trees preferred wetland environments as the extant Equisetum. They formed hygrophilous strands surrounding lakes or grew in swamps and along river banks (Pfefferkorn et al., 2001; Scott, 1979). There are many geological facts that confirm the ecological preference of the horsetail trees. In Euramerica, it is well-documented that the Late Carboniferous coal swamp ecosystems collapsed near the Permo-Carboniferous boundary (DiMichele et al., 2001; DiMichele and Hook, 1992). The geographical extinction of certain plant taxa in this event is primarily related to the more seasonal and dry climate, and the lycopsids and sphenopsids (including the calamitean plants) who cannot adapt to the changed environments in Euramerica (DiMichele et al., 2001; Gastaldo et al., 1996; DiMichele and Hook, 1992). In Cathaysian flora, these taxa lived in the waterlogged swamp environments during the Early Permian of North China (Hilton et al., 2002, 2001) and the Late Permian of South China (Wang et al., 2006, 2003; Li et al., 1995). Accordingly, the calamitean axes found in the Upper Permian of the southern Bogda Mountains indicate the presence of waterlogged environments in that region during the Late Permian in which calamitean plants lived.
Growth rings are absent in Arthropitys taoshuyuanensis sp. nov. and A. sp. and the tracheids in the secondary xylem are of uniform size. In Septomedullopitys szei Wan, Yang et Wang, a coniferopsid wood also found in the lower part of the Wutonggou Formation, annual growth rings are absent as well but irregular growth interruptions are developed (Wan et al., 2014). The climate of the study area during the Late Permian was humid to subhumid with a wet-dry seasonality as indicated by abundant Histosols, Gleysols, and Argillisols capping thick meandering stream and freshwater deltaic deposits in the Wutonggou low-order cycle (Yang et al., 2010). Wan et al. (2014) proposed that the absence of true growth rings in S. szei indicates a subhumid to perhumid climate and the irregular growth interruptions reflect drought-induced fluctuations in groundwater table caused by short-lived, irregular droughts. The development of growth rings in a tree is affected by both intrinsic and extrinsic factors. In general, the extrinsic factors exert a greater influence than the intrinsic ones (Creber and Chaloner, 1984). Moreover, as hygrophilous taxa, calamitean plants are supposed to be more sensitive to the changes in water availability than coniferopsids. Additionally, growth rings related to droughts or changes in water availability have been previously reported in A. yunnanensis from South China and A. isoramis and A. iannuzzi from the Parnaíba Basin (Neregato et al., 2015; Wang et al., 2006). Given the above, the absence of growth rings and growth interruptions in Arthropitys taoshuyuanensis sp. nov. and A. sp. indicates that their habitats are different from where S. szei and the other trees with growth interruptions lived. Considering the ecological preference of the calamitean plants, they probably grew in the lowland area where it kept humid all the year round like lakeside and riverside, the enough supply of moisture contributed to the absence of growth rings and growth interruptions. In comparison, S. szei and the other trees grew in the higher grounds or the place away from rivers and lakes, and the short-lived droughts controlled the development of the growth interruptions. Temperature plays as another important factor that influences the development of growth rings (Creber and Chaloner, 1984). To some extent, the absence of annual growth rings in both A. taoshuyuanensis and S. szei indicates a limited range of annual temperature in the Late Permian southern Bogda Mountains. More detail work on growth rings is needed to document the annual temperature range.4 CONCLUSIONS
(1) Two calamitean axes including a new species Arthropitys taoshuyuanensis sp. nov. are firstly found in the Late Permian Subangaran flora of China. The new discovery increases diversity of the Subangaran flora and the genus Arthropitys with only two species previously found in China.
(2) The absence of growth rings and growth interruptions in the calamitean axes indicates the limited range of annual temperature and sufficient water supply of their habitats.
(3) In the Late Permian southern Bogda Mountains, the calamitean plants probably lived in the humid lowland areas, and Septomedullopitys szei and some other trees with growth interruptions grew in the higher grounds where there were short-lived droughts.
We gratefully acknowledge the anonymous reviewers for their critical comments and constructive suggestions, which have improved the quality of the paper greatly. We also thank Prof. Shucheng Xie from China University of Geosciences and Ms. Qingting Wu from Ohio State University for collecting the samples. This study was supported by the National Natural Science Foundation of China (Nos. 40972002, 41272024 and 41572005). The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0941-3.
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