See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/256850881 New insights and evolutionary significance of the megasporangiate strobilus of Minostrobus chaohuensis (Lycopsida) from the Upper Devonian of South China ARTICLE in REVIEW OF PALAEOBOTANY AND PALYNOLOGY · MARCH 2013 Impact Factor: 1.94 · DOI: 10.1016/j.revpalbo.2012.11.007 CITATIONS READS 4 104 4 AUTHORS, INCLUDING: Meicen Meng Deming Wang Chinese Academy of Geological Sciences Peking University 12 PUBLICATIONS 18 CITATIONS 46 PUBLICATIONS 293 CITATIONS SEE PROFILE SEE PROFILE Jinzhuang Xue Peking University 39 PUBLICATIONS 167 CITATIONS SEE PROFILE Available from: Jinzhuang Xue Retrieved on: 12 October 2015 Review of Palaeobotany and Palynology 190 (2013) 20–40 Contents lists available at SciVerse ScienceDirect Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo Research paper New insights and evolutionary significance of the megasporangiate strobilus of Minostrobus chaohuensis (Lycopsida) from the Upper Devonian of South China Meicen Meng, Deming Wang ⁎, Jinzhuang Xue, Xiao Zhu Key Laboratory of Orogenic Belts and Crustal Evolution, Department of Geology, Peking University, No. 5 Yiheyuan Road, Haidian District, Beijing 100871, China article info abstract Article history: Heterosporous lycopsids with monosporangiate strobili are highly diverse in the Carboniferous, but their early Received 16 June 2012 evolution is poorly understood. The Late Devonian Minostrobus chaohuensis was included in this plant group, Received in revised form 3 November 2012 but features of the strobili were unclear in detail. Permineralized material of M. chaohuensis was sectioned and Accepted 5 November 2012 ground in series to reveal details of megasporangiate strobili. The megasporophylls are smooth and borne in a Available online 5 December 2012 2/9 helical phyllotaxy. The megasporophyll consists of a pedicel which bears a keel and alations, lamina and heel. The alations extend horizontally and then bend upward to surround the sporangial base. A single megaspo- Keywords: Isoёtales sensu lato rangium with a subarchesporial pad is inserted onto the sporophyll pedicel through a narrow attachment. Each Lagenicula sporangium contains four Lagenicula-type spiny megaspores that may be of variable size. On the basis of these Late Devonian new fertile traits, M. chaohuensis is assigned to the Isoёtales sensu lato. This plant is proved to be monosporangiate, megasporangiate strobilus and thus Devonian lycopsids are shown for the first time to have possessed megasporangiate strobili. It is confirmed Minostrobus that phylogenetically advanced heterosporous lycopsids with monosporangiate strobili had evolved by the Late monosporangiate-strobilus lycopsid Devonian. The type of alations and size variation of the four megaspores per sporangium indicate that Minostrobus may represent an evolutionary form in transition toward the Carboniferous isoёtaleans with monosporic megasporangia more fully enclosed by alations. Evidence suggests a free megaspore dispersal mechanism in M. chaohuensis. © 2012 Elsevier B.V. All rights reserved. 1. Introduction (Wang et al., 2003b)andMinostrobus (Wang et al., 2012), but further evidence from strobili is required to confirm the monosporangiate As one of the earlier lineages of land plants, the lycopsids nature of the latter two genera. Both groups of heterosporous lycopsids, appeared by the Late Silurian–Early Devonian, and attained maxi- those with bisporangiate or with monosporangiate strobili, usually in mum diversity and significance in the Carboniferous and evolved arborescent habit, were the most conspicuous plants of the Carboniferous till now (Phillips, 1979; Gensel and Andrews, 1984; Stewart and landscape (Taylor et al., 2009). Rothwell, 1993). Heterospory in lycopsids, one of the most impor- Minostrobus chaohuensis was established by Wang (2001), on the tant evolutionary innovations, was first encountered in Middle basis of material from the Upper Devonian–Lower Carboniferous Devonian taxa with bisporangiate strobili (a single strobilus containing Wutung (Wutong) Formation, at Shizikou Section, Chaohu District, both mega- and microspores), i.e., Mixostrobilus Senkevitsch et al. Anhui Province of China. He described the morphological characters (1993) and Yuguangia Hao et al. (2007). Heterosporous lycopsids with of the strobili containing megaspores, and observed the megaspores monosporangiate strobili (separate strobili containing either mega- or and xylem of strobilar axes by scanning electron microscope microspores) are proposed by cladistic analysis to be the most derived (SEM). Recently, Wang et al. (2012) reported the sterile axes, and clade (Bateman, 1992; Bateman et al., 1992; DiMichele and Bateman, megasporangiate and microsporangiate strobili of this plant, from 1996; Xue, 2011). This monosporangiate-strobilus clade is known material collected near the type locality. However, they gave no from only three representatives in the Late Devonian, i.e., Lepidostrobus further description of the megasporangiate strobili. We have since Brongniart (Wang et al., 2003a), Sublepidodendron (Nathorst) Hirmer obtained numerous well-preserved specimens of permineralized megasporangiate strobili from the same formation and district, but from the Fenghuangshan Section. New data allow the megasporangiate strobilus of Minostrobus to be compared with the ⁎ Corresponding author at: Department of Geology, Peking University, No. 5 Yiheyuan strobili of related heterosporous lycopsids of Devonian and Road, Haidian District, Beijing 100871, China. Tel.: +86 10 62761095; fax: +86 10 Carboniferous age, to confirm the systematic position and verify the 62751187. fi E-mail addresses: [email protected] (M. Meng), [email protected] monosporangiate character, to discuss the evolutionary signi cance, (D. Wang), [email protected] (J. Xue), loofl[email protected] (X. Zhu). and to interpret the megaspore dispersal mechanism of Minostrobus. 0034-6667/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.revpalbo.2012.11.007 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 21 Fig. 1. Map showing fossil localities of Minostrobus chaohuensis. 2. Material and methods for examination under transmitted light. All specimens and slides are housed at Department of Geology, Peking University, China. To describe About 200 specimens were obtained from the Wutong Formation, the strobilus, we adopt the terminology used by Bateman et al. (1992). Fenghuangshan Section in Chaohu City, Anhui Province, South China. This section (31°37′51″Nand117°50′54″E) is located southwest of the Shizikou Section (31°38′08″N and 117°51′08″E) from which 3. Systematics Minostrobus chaohuensis was reported by Wang (2001), and northeast of another section (31°37′42″Nand117°50′43″E) where Wang et al. Class: Lycopsida Pichi-Sermolli 1958. (2012) collected additional material of this plant (Fig. 1). The Wutong Order: Isoёtales sensu lato Meyen 1987. Formation consists of the Guanshan and Leigutai members, in ascend- Suborder: Dichostrobiles DiMichele and Bateman 1996. ing order. According to Hou and Qi (2006), the middle part of the Family: Incertae sedis. Leigutai Member in Chaohu District consists of lower and upper clay Genus: Minostrobus Wang 2001 emend. Wang et al. 2012 emend. layers intercalated with quartz sandstone; these two clay layers are Emended diagnosis: (Emended and additional generic characters characterized by latest Devonian spore and plant assemblage and earli- are in brackets) [Monosporangiate-strobilus lycopsid.] Aerial axes multi- est Carboniferous invertebrate assemblage, respectively. Strobili of dichotomous. Leaf persistent, with single mid-vein. Megasporangiate M. chaohuensis in the current study came from the gray-black mudstone and microsporangiate strobili [possibly] monoecious. Single spherical to of the lower clay layer, and are thus latest Famennian in age. On the elliptical sporangium inserted to adaxial side of sporophyll with smooth basis of stratigraphic data and fossil assemblages provided by Wang margin and pointed apex. [Megasporophylls attached to strobilar axis at (2001) (see his Fig. 2), we suggest that his collection is from the lower approximately right angles, borne in clockwise helix, consisting of pedicel clay layer of the Leigutai Member, at Shizikou Section. Material with alations and keel, heel, and upturned lamina. Megasporangium described in Wang et al. (2012) was from the Leigutai Member and connected to sporophyll by narrow attachment, with sporangial base also dated as Famennian. surrounded by alations. Sporangial wall comprising one layer of columnar Under a dissecting microscope, the strobili were prepared with steel cells. Subarchesporial pad along inner surface of sporangial wall.] Each needles. Two strobili have been permineralized by limonite, others by megasporangium with four [Lagenicula-type megaspores bearing spiny carbonate. Plant tissues and structures (e.g., the leaf epidermis, walls ornamentation]. Axis with solid exarch primary xylem. Protoxylem con- of sporangium and megaspore, and (possible) the outer cortex and vas- fined to ridges at periphery of primary xylem strands. Metaxylem cular xylem of strobilar axis) are preserved as black carbonaceous seg- tracheids bearing Williamson's striations. ments. Permineralized strobili, strobilar axes and clusters of sporangia were embedded in epoxy resin, and sectioned at