New Insights and Evolutionary Significance of the Megasporangiate Strobilus of Minostrobus Chaohuensis (Lycopsida) from the Upper Devonian of South China

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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

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Meicen Meng Deming Wang Chinese Academy of Geological Sciences Peking University

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Available from: Jinzhuang Xue Retrieved on: 12 October 2015 Review of Palaeobotany and Palynology 190 (2013) 20–40

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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.

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 about 1.0–3.0 mm Type species: Minostrobus chaohuensis Wang 2001 emend. Wang intervals. These serial sections were incrementally ground and polished et al. 2012 emend. with carborundum to yield multiple serial surfaces, which were Holotype: PB18751 (Fig. 8 in Wang, 2001). photographed under reﬂected light. Finally, transverse and longitudinal Repository: Nanjing Institute of Geology and Palaeontology, Chinese (radial and tangential) thin sections of strobili were prepared as slides Academy of Sciences, Nanjing, China. 22 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Type locality: The south slope of Beishan Hill, Shizikou Section, which have been replaced by mineral (Plate II,2–3, arrows). Sporophyll 3 km north of Chaohu City, Anhui Province, China. traces enclosed by carbonate (Plate III, 4) occur as linear protrusions on Other specimens examined herein: PKUB12001–12005, PKUB12009, the carbonate column of strobilar axes (Plate I, 7 and 8); such traces de- PKUB12011, PKUB12012, PKUB12014, PKUB12023, and PKUB12027 part at about 10° from the carbonate column and then curve to become (see Plates I–X). nearly horizontal (Plate III,2,5,and6,arrow).Theprimaryxylemis Repository: Department of Geology, Peking University, Beijing, China. 0.2–0.3 mm in diameter and exarch in maturation (Plate II,6,7;Plate Locality: Fenghuangshan Section, Chaohu City, Anhui Province, China. IV, 5 and 6). The protoxylem ridges are absent or unclear in number. Stratigraphy: Leigutai Member of the Wutong Formation. The protoxylem tracheids are circular in cross section and about Age: Late Devonian (Famennian). 5.0–10 μm in diameter. The metaxylem tracheids are polygonal and Emended diagnosis: (Emended and additional specific characters are about 20–35 μm in diameter, with the wall thickenings being visible in brackets) As for generic diagnosis. Aerial axes dichotomizing at least but unclear in detail (Plate IV,7). eight times. Sterile leaves, 12–30 per gyre, with smooth margin. Leaf bases acute toward both ends and with a circle vascular bundle scar. 4.3. Phyllotaxy Separate monosporangiate strobili attached at apices of the axes and [possibly] monoecious. [Megasporangiate strobilus about 5 mm wide. The bases of sporophyll pedicels may superficially appear to be borne Megasporophyll trace arising from stele at acute angles. Megasporo- in pseudowhorls (Plate I, 5), but in longitudinal section are shown to be phylls arranged in 2/9 helix, with angles between parastichies and hor- borne helically (Plate II, 8 and 9). Serial transverse sections of a strobilus izontal line being about 35°. Pedicel about 1.8 mm long, proximally “V” show different stages of sporophyll traces and pedicels arising from the form in transverse view, distally expanding to form alations. Lamina strobilar axis on the same plane (Plate II,1–5). Tangential sections of a long-triangular in face view.] Smooth megasporangium [about 2 mm strobilus reveal that the sporophylls are helical in arrangement (Plate long, 1.5 mm wide and 1.5 mm high. Attachment between megaspo- IV,2–3 and 8). One carbonate column of the strobilar axis with enclosed rangium and megasporophyll about 1.0 mm long and 0.2 mm wide. traces (Plate I, 1, upper arrow, 7 and 8) is selected to demonstrate phyllo- Megaspores range from about 370 μm to about 1490 μm in diameter, taxy. The traces on both sides of this column are marked (Fig. 3). The res- with distinct gula,] more or less circular amb, pear shape in equatorial toration demonstrates that they are helically arranged in a clockwise view. [Spiny ornamentation of spore about 20 μm wide at base. Spore direction. Nine traces in two gyres give rise to a cycle. Thus, a 2/9 helical wall with thin inner exospore and spongy outer exospore.] Microspo- arrangement has been determined, with angles between the parastichies rangiate strobilus over 8 cm long; microsporophylls up to 3.0 mm and a horizontal line of 30°–40°. long. Microspores Lycospora-type, with circular amb, granulated ornamentations, and narrow cingulum. Both sterile and fertile axes with sin- 4.4. Megasporophylls gle, solid primary exarch xylem strand surrounded by cortex. Twelve protoxylem ridges with annular tracheids around the metaxylem core. Megasporophylls have smooth margins (Plate I,1–5; Plate III,1–2; Metaxylem with scalariform and reticulate tracheids; Williamson's stri- Plate V,1–3), with a horizontal pedicel, a downturned heel and an ations in apertures of reticulate tracheids. Cortical cells rectangular in upturned lamina (Fig. 2B–E). The pedicel is about 1.8 mm long and outline. inserted at about 90° onto the strobilar axis (Plate II, 1 and 2; Plate III, 6), with the pedicel base being slightly expanded (Plate II,9;Plate III, 4. Description 5, arrows). Proximally, the pedicel shows a “V” form in transverse section (Plate II,8)orfraction(Plate V, 4), 0.7–1.2 mm wide and 4.1. Megasporangiate strobili 0.6–0.8 mm high. Distally, the pedicel gradually expands laterally to form the alations or lateral laminae (Fig. 4C–D; Plate IV, 2 and 3), and Overall, the megasporangiate strobili are cylindrical, sometimes becomes up to 2.5–3.0 mm wide where it joins the lamina (Fig. 5A; slightly curved (Fig. 2A; Plate I,1–5), and appear elliptical in transverse Plate V, 1, 3, and 5). The alations extend horizontally, and then bend up- view when compressed (Plate I,6;Plate II,1–5). Strobili are 4.5–6.0 mm ward to surround the base of sporangium (Figs. 2C–E, 4C; Plate VI,1).In in width, excluding the distal part of sporophylls, and up to 12.5 cm transverse section, the distal part of pedicel shows an abaxial keel long (Plate I, 1). Strobili are typically broken at both ends, but one stro- 0.2–0.6 mm high, tapering downwards (Plate VI, 1 and 5). One heel is bilus preserves the base (Plate I, 3) and another, the tip (Plate I, 4). The about 0.4 mm long and curves slightly toward the strobilar axis (Plate vegetative axis below the strobilus in Plate I, 3 is 1.3 mm wide and VI, 4). Other two heels are 0.5–0.8 mm wide and connect with the about 12 mm long, with some preserved sterile leaves departing at keel (Plate VI,6). acute angles. One strobilus permineralized by limonite is 3.0 cm long In lateral view, the lamina appears linear in shape, turns upward (Plate I, 4, with transverse fracture shown in Plate I, 6), was transversely from the pedicel at an angle of 40°–70° and then more or less curves sectioned and serially ground to show 70 surfaces, five at three levels adaxially (Plate I,1–3; Plate III,1–2; Plate V, 2). In face view, however, illustrated in Plate II,1–5. it is long-triangular in outline and distally acuminate, 2.5–3.0 mm wide at the base and 6.0–7.0 mm long (Plate I,4;Plate V, 1 and 3, 4.2. Strobilar axes upper left part). In transverse section, the lamina is 0.1–0.2 mm thick and thins laterally (Plate II,1–5, lower and upper parts), although thick- Strobilar axes are 0.9–1.3 mm wide, with carbonate or limonite filling ness may be exaggerated by compression in preservation (Plate II,1–5, the space between the epidermis-outer cortex and xylem strand (Plate I, left and right parts; Plate VII,1and2). 5; Plate II,1–6). Some sporophylls have broken off to expose the white In some hand specimens (Plate V, 1 and 3) and many sections (Plate carbonate column (0.7–0.9 mm wide, Plate I,1,3,and7–8; Plate III, VI,1,4–6; Plate VII,1–7), the carbonaceous layer on the sporophylls 1–3) containing the xylem (Plate I,3;Plate III, 3), with the black seg- possibly represents the adaxial epidermis. This layer was occasionally ments of epidermis–outer cortex remaining (Plate III, 3, arrow, and 5, ar- lost in the procedure of grinding (Plate IV,8;Plate VII, 5). It is invisible rows). One sporophyll trace (Plate IV, 1 and 4), 1.2 mm long and about on the abaxial side of the sporophylls, perhaps implying a thinner abaxial 20 μm wide, is observed in the radial section of a strobilus and enclosed epidermis. Mesophyll cells are exposed when the carbonaceous layer par- by carbonate, and diverges at an acute angle from the xylem of the tially broke off (Plate VI,3)andareobservedinsection(Plate VI,1–2, strobilar axis. In transverse section, another trace (Plate II, 6) is seen at 4–6). Each sporophyll has a single vein, about 20 μmindiameterandvis- the periphery of the strobilar axis, surrounded by limonite. Some trans- ible within the pedicel (Fig. 4C; Plate VI, 1) and lamina (Fig. 4A; Plate VI,2; verse sections of the strobili show the position of sporophyll traces Plate VII, 1). A single vein is enclosed in a sheath-like structure M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 23

Plate I. Carbonate (1–3, 5, and 7–8) and limonite (4 and 6) permineralized megasporangiate strobili of Minostrobus chaohuensis.

1. The longest strobilus. Arrows indicating white carbonate column of strobilar axis. PKUB12009a. Scale bar=10 mm. 2. A strobilus with sporophylls. PKUB12001. Scale bar=5 mm. 3. Basal part of a strobilus with sterile axis. Me, megaspore. PKUB12014. Scale bar=5 mm. 4. Distal part of a strobilus showing abaxial view of sporophyll laminae. Upper three arrows indicating positions of sections in Plate II,1–5. Lowermost arrow, fracture of strobilus enlarged in Plate I, 6. V, vein. PKUB12023. Scale bar=5 mm. 5. A strobilus with strobilar axis and bases of sporophyll pedicel exposed. PKUB12002. Scale bar=5 mm. 6. Enlargement of Plate I, 4 (lowermost arrow) showing transverse fracture of strobilus. Me, megaspore. PKUB12023. Scale bar=1 mm. 7–8. Two sides of carbonate column of strobilar axis from Plate I, 1 (upper arrow). PKUB12009a. Scale bars=1 mm.

Plate II. Limonite (1–7) and carbonate (8–9) permineralized megasporangiate strobili of Minostrobus chaohuensis. (see on page 6)

1–3. Transverse sections of strobilus in Plate I, 4 (uppermost arrow). Arrows indicating sporophyll traces replaced by limonite. L, sporophyll lamina; P, sporophyll pedicel. PKUB12023-04. Scale bars=1 mm. 4. Transverse section of strobilus in Plate I, 4 (middle-upper arrow). L, sporophyll lamina; Me, megaspore. PKUB12023-05. Scale bar=1 mm. 5. Transverse section of strobilus in Plate I, 4 (middle-lower arrow). Al, alation; L, sporophyll lamina; Me, megaspore. PKUB12023-09. Scale bar=1 mm. 6. Transverse section of strobilar axis from lower part of strobilus in Plate I, 4. Quadrangle indicating portion enlarged in Plate II, 7. St, sporophyll trace; V, vein. PKUB12023-08. Scale bar=0.5 mm. 7. Enlargement of Plate II, 6 (quadrangle) showing exarch primary xylem. PKUB12023-08. Scale bar=0.1 mm. 8. Tangential section of strobilus in Plate I, 5 showing bases of sporophyll pedicels arranged in helix. PKUB12002-03. Scale bar=1 mm. 9. Radial section of strobilus in Plate I, 5 showing expansion of sporophyll pedicel bases. PKUB12002-02. Scale bar=1 mm.

Plate III. Carbonate permineralized megasporangiate strobili of Minostrobus chaohuensis. (see on page 7)

1. A strobilus with carbonate column of strobilar axis and sporangia exposed. Quadrangle indicating the portion enlarged in Pate III, 6. PKUB12011. Scale bar=5 mm. 2. Strobilus showing carbonate column of strobilar axis with enclosed sporophyll traces, sporangia containing megaspores, and sporophylls in lateral view. PKUB12009a. Scale bar=5 mm. 3. Strobilar axis showing carbonate column containing xylem. Arrow indicating segments of epidermis-outer cortex. Xy, xylem. PKUB12004b. Scale bar=0.2 mm. 4. Enlargement of strobilar axis in Plate I, 1 (lower arrow). It is embedded in epoxy resin and shows xylem and sporophyll traces enclosed by carbonate. St, sporophyll trace. PKUB12009a. Scale bar=0.5 mm. 5. Strobilar axis showing carbonate column and enclosed sporophyll traces. Arrows indicating black segments of epidermis-outer cortex and expansion of sporophyll pedicel bases. PKUB12004b. Scale bar=1 mm. 6. Enlargement of Plate III, 1 (quadrangle) with the arrow indicating sporophyll trace enclosed by carbonate. L, sporophyll lamina; Me, megaspore; P, sporophyll pedicel. PKUB12011. Scale bar=1 mm.

Plate IV. Sections of a carbonate permineralized megasporangiate strobilus of Minostrobus chaohuensis.5,1–3, and 8 in sequence indicate three parts of the strobilus. (see on page 8)

1. Oblique radial section of strobilus showing xylem in strobilar axis and sporangia mostly lacking megaspores. Quadrangles indicating portions enlarged in Plate IV, 4 and Plate VI, 2. Me, megaspore. PKUB12003-02-02. Scale bar=2 mm. 2. Tangential section of strobilus showing sporophyll pedicels in transverse view and numerous empty sporangia. Note the helical arrangement of sporophylls. Me, megaspore. PKUB12003-02-03. Scale bar=2 mm. 3. Tangential section of strobilus showing pedicels with wide alations and numerous empty sporangia. Quadrangles indicating portions enlarged in Plate VI, 4 and 5. Me, megaspore. PKUB12003-02-04. Scale bar=2 mm. 4. Enlargement of Plate IV, 1 (upper quadrangle) displaying sporophyll trace arising from xylem of strobilar axis. Quadrangle indicating portion enlarged in Plate IV, 7. St, sporophyll trace. PKUB12003-02-02. Scale bar=0.5 mm. 5. Transverse section of strobilus showing megaspores and strobilar axis. Quadrangle indicating the portion enlarged in Pate IV, 6. Me, megaspore. PKUB12003-01-05. Scale bar=1 mm. 6. Enlargement of Plate IV, 5 (quadrangle) showing protostele with exarch primary xylem. PKUB12003-01-05. Scale bar=0.1 mm. 7. Enlargement of Plate IV, 4 (quadrangle) showing tracheids with thickenings. PKUB12003-02-02. Scale bar=50 μm. 8. Tangential section of strobilus showing empty sporangia. Note carbonaceous layer on adaxial side of sporophyll. PKUB12003-03-02. Scale bar=2 mm.

Plate V. Carbonate permineralized megasporangiate strobili of Minostrobus chaohuensis. (see on page 9)

1. Strobilus with some remained sporangia showing sporophylls mostly in adaxial view. Quadrangles indicating portions enlarged in Plate V, 4 and Plate VI,3. PKUB12005. Scale bar=5 mm. 2. A strobilus with the mid-upper region being counterpart of specimen in Plate V, 3. Quadrangle indicating the portion enlarged in Plate V, 6. PKUB12012a. Scale bar=5 mm. 3. Specimen showing sporophylls mostly in adaxial view. Quadrangle indicating portion enlarged in Plate V, 5. PKUB12012b. Scale bar=5 mm. 4. Enlargement of Plate V, 1 (lower quadrangle) displaying sporophyll pedicel in transverse fracture. PKUB12005. Scale bar=0.2 mm. 5. Enlargement of Plate V, 3 (quadrangle) displaying part of sporophyll pedicel and lamina. Arrow indicating sunken area interpreted as sporangium attachment. Al, alation. PKUB12012b. Scale bar=1 mm. 6. Enlargement of Plate V, 2 (quadrangle) showing sporangium attachment represented by a depression (arrow). PKUB12012a. Scale bar=1 mm.

Plate VI. Carbonate permineralized megasporangiate strobili of Minostrobus chaohuensis. (see on page 10)

1. Megasporangium-sporophyll unit in tangential section of strobilus showing subarchesporial pad and attachment between sporangium and sporophyll. Al, alation; K, keel; V, vein. PKUB12001-02-02. Scale bar=0.2 mm. 2. Enlargement of Plate IV, 1 (lower quadrangle), longitudinal section of sporophyll lamina displaying single vein and mesophyll cells. V, vein. PKUB12003-02-02. Scale bar=0.5 mm. 3. Enlargement of Plate V, 1 (upper quadrangle) displaying mesophyll cells. PKUB12005. Scale bar=0.2 mm. 4. Enlargement of Plate IV, 3 (upper quadrangle) showing heel and mesophyll cells of sporophyll. H, heel. PKUB12003-02-04. Scale bar=0.2 mm. 5. Enlargement of Plate IV, 3 (lower quadrangle) displaying keel, alations and mesophyll cells of pedicel. K, keel. PKUB12003-02-04. Scale bar=0.5 mm. 6. Tangential section of strobilus, lower part of the one in Plat IV. Showing heel connected with keel. H, heel; K, keel. PKUB12003-03-04. Scale bar=0.5 mm.

Plate VII. Serial longitudinal sections of a limonite permineralized megasporangiate strobilus of Minostrobus chaohuensis. Arrows in 1–4 indicating portions drawing in Fig. 4. Quadrangles in 5–7 indicating portions enlarged in Plate IX,8;Plate X, 6 and 7. (see on page 11)

1–3. Serial grinding surfaces of a tangential section. V, vein. PKUB12027-04. Scale bars=1 mm. 4–5. Two sides of a section representing tangential and radial view. PKUB12027-03. Scale bars=1 mm. 6–7. Serial grinding surfaces of a radial section. PKUB12027-02. Scale bars=1 mm. 24 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 25

Plate II (caption on page 4) 26 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Plate III (caption on page 4) M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 27

Plate IV (caption on page 4) 28 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Plate V (caption on page 4) M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 29

Plate VI (caption on page 4) 30 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Plate VII (caption on page 4) M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 31

(Fig. 4B–C), but this structure was sometimes missed (Plate II,8; 3) and distal (Plate I, 4 and 6; Plate II,4–5) parts of the strobilus. Most Plate VI,5)orﬂattened (Plate I,4). sporangia in a strobilus contain spores (Plate III,1–2; Plate VII,1–7; Plate V, 2 and 3 represent the part and counterpart of the specimen Plate VIII,1–5; Plate IX,1–6), whereas nearly all sporangia of one stro- of a strobilus, with the latter corresponding to the mid-upper region bilus are empty and their walls are amorphous or discontinuous of the former. In adaxial view, the overlapped sporophylls demon- (Plate IV,1–3 and 8). Each sporangium contains a single tetrad (Plate strate the base of lamina and distal part of pedicel with alations II,4;Plate VIII, 5, arrow). Within a sporangium, one to three megaspores (Plate V, 3 and 5). An elongate sunken area (Plate V, 5, arrow) is locat- are sometimes visible in one section, however, four spores are revealed ed at the pedicel. Accordingly, a similarly shaped depression (Plate V, through serial sections (Plate VIII,1–4; Plate IX,1–6; Plate X,1 –3).The 6, arrow) occurs on the carbonate layer that occupied the space be- spores may be equal in size (Plate IX,1–6, lower right and upper right tween the sporophyll and sporangium. This structure is interpreted sporangia; Plate X,1–3), but in many cases they are unequal, with one as the attachment of sporangium (also see Section 4.5) on the sporo- large and three much smaller megaspores (Plate VIII,1–4, upper right phyll pedicel. In the region of the sporophyll beyond the attach- sporangium; Plate IX,1–6, upper left sporangium), or two large and ment, where a ligule should normally be situated, no evidence two small (Plate VIII,1–4, lower right sporangium). was found in all hand specimens (Plate V 1, 3, and 5) and (serial) The laesurae are raised as a distinct gula, which is 300–700 μmlong sections (Fig. 5A–C; Plate II,1–5; Plate IV,1–3; Plate VII,1–7; and observed in both longitudinal (Plate X, 2, 4, and 6) and oblique Plate VIII,1–5; Plate IX,1–6). transverse (Plate X, 5) sections of the megaspore. Spiny ornamentations are very clear on some megaspores, including the gula (Fig. 5B–C; Plate 4.5. Megasporangia VIII,5;Plate IX, 1 and 10). Spines are about 20 μm wide at the base, 50–90 μm long, and sometimes recurved distally. However, in some Each sporophyll bears one adaxial sporangium. The sporangia cases the spines are very short (Plate VIII,5;Plate IX, 1 and 8; Plate X, are smooth and spherical to ellipsoidal in shape, 1.8–2.2 mm long, 4) or invisible (Plate II, 4 and 5; Plate VII, 4 and 5; Plate X,1–3and6) 1.0–1.9 mm wide and 1.3–1.8 mm high (Plate III,1and2;Plate as the result of poor preservation. The spore wall is 20–60 μm thick, VII,1–7; Plate VIII,1–5; Plate IX,1–6). The sporangial wall is about and in a few cases the inner (Plate IX,8;Plate X,3)andspongyouter 30 μm thick and consists of a single layer of columnar cells (Plate exospore (Plate X, 7) are discerned. The inner exospore is about IX, 7 and 8). Possible dehiscence region of the sporangial wall is 5.0 μm thick and somewhat detached from the outer exospore. shown in Plate VIII,5. Cell-like contents are preserved within some megaspores, possibly A subarchesporial pad of parenchyma cells is often visible inside and representing remnants of megagametophytes (Plate X,6and8). at the basal part of carbonated sporangia (Plate VIII,5;Plate IX, 7 and 9), but is poorly preserved in the limonitized ones (Plate II,1–5; Plate VII, 5. Comparison 1–7). The pad extends along the sporangial wall up to about half the height of sporangia containing spores (Fig. 5C; Plate VIII,5),butwas 5.1. Minostrobus chaohuensis Wang, (2001) not observed in sporangia without spores (Plate IV,1–3and8).Thespo- rangium is connected to the sporophyll pedicel by a narrow attachment On the basis of the following similarities with the holotype, we con- about 1.0 mm long and 0.2 mm wide (Figs. 4C, 5A; Plate V,5–6; Plate VI, clude that our material represents the megasporangiate strobilus of 1; Plate IX, 8), where the subarchesporial pad lies just above the vein Minostrobus chaohuensis: 1) correlation of locality and horizon; 2) size (Plate VI,1). of strobilus and anatomy of strobilar axis; 3) margin of sporophyll, length of sporophyll lamina; 4) shape and dimension of sporangium; 5) number 4.6. Megaspores and type of megaspores (Table 1). In this paper, the emended important characters of megasporangiate Because the megaspore wall is preserved as carbonaceous fragments strobilus of Minostrobus chaohuensis include the phyllotaxy, length of (e.g., Plate VIII,5;Plate IX, 1 and 10), spores could not be obtained by sporophyll pedicel, shape and width of lamina, thickness of sporangial acid maceration, but they are visible in hand specimens and sections wall, and size of spore (Table 1). The “sporophyll base” (Wang, 2001, of sporangia. The megaspores range from 370 μm to 1490 μmindiam- e.g., his Fig. 15) is here emended as sporophyll trace enclosed by carbon- eter (excluding the laesura). They were observed in both basal (Plate I, ate (e.g., Plate I,7and8;Plate III,1and2,seeSection 4.2). From the

Plate VIII. Sections of the strobilus in Plate I,2.

1–4. Serial grinding surfaces of a transverse section (about 1.0 mm thick) of strobilus showing sporangia containing megaspores with variational size. PKUB12001-01-06. Scale bars=1 mm. 5. Oblique radial section of strobilus showing strobilar axis containing xylem, and megaspores and subarchesporial pads in sporangia. Arrow indicating a sporangium with a tetrad. Quadrangle indicating portion enlarged in Plate IX, 7. D, possible dehiscence region. PKUB12001-02-03. Scale bar=1 mm.

Plate IX. Carbonate (1–7and9–10) and limonite (8) permineralized megasporangiate strobili of Minostrobus chaohuensis.1–6 indicate sections of the strobilus in Plate I, 2. (see on page 14)

1–6. Serial grinding surfaces of a transverse section (about 2.0 mm thick) of strobilus showing sporangia containing megaspores with variational size. Quadrangles indicating portions enlarged in Plate IX, 9 and 10. PKUB12001-01-11. Scale bars=1 mm. 7. Enlargement of Plate VIII, 5 (quadrangle) displaying sporangial wall and subarchesporial pad. PKUB12001-02-03. Scale bar=0.2 mm. 8. Enlargement of Plate VII, 6 (quadrangle) showing longitudinal section of the lower part of a megasporangium-sporophyll unit. At, sporangium attachment; Ie, inner exospore; Spw, sporangial wall. PKUB12027-02. Scale bar=0.5 mm. 9. Enlargement of Plate IX, 1 (lower quadrangle) showing section through base of sporangium. Note the parenchyma cells of subarchesporial pad. PKUB12001-01-11. Scale bar=0.2 mm. 10. Enlargement of Plate IX, 1 (upper quadrangle) displaying four spiny megaspores in a sporangium. PKUB12001-01-11. Scale bar=0.2 mm.

Plate X. Carbonate (1–5 and 8) and limonite (6–7) permineralized megasporangiate strobili of Minostrobus chaohuensis. (see on page 15)

1–3. Serial sections of a sporangium with a tetrad from strobilus in Plate III, 1. Arrow indicating gula of megaspore. Ie, inner exospore. PKUB12011. Scale bars=0.5 mm. 4. Longitudinal section of megaspore with gula. PKUB12011. Scale bar=0.2 mm. 5. Oblique transverse section of megaspore gula. PKUB12011. Scale bar=0.2 mm. 6. Enlargement of Plate VII, 7 (quadrangle) showing longitudinal section of a megaspore. Note the gula and cell-like contents. PKUB12027-02. Scale bar=0.2 mm. 7. Enlargement of a megaspore in Plate VII, 5 (quadrangle) showing spongy outer exospore. PKUB12027-03. Scale bar=0.1 mm. 8. Megasporangium-sporophyll unit in tangential section of strobilus displaying possible megagametophyte in spore. PKUB12001-02-04. Scale bar=0.2 mm. 32 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 33

Plate IX (caption on page 12) 34 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Plate X (caption on page 12) M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 35

Fig. 2. Reconstruction of megasporangiate strobilus of Minostrobus chaohuensis.(A)Strobilus with helically arranged units. Scale bar=5 mm. (B) Sporophyll lamina and heel in abaxial view. (C–E) Megasporangium-sporophyll unit in adaxial, oblique adaxial and lateral view, respectively. Scale bar=1 mm. observation of hand specimens, Wang (2001) proposed that four megaspores occurred in each sporangium. By SEM examination, he regarded the biform ornamentation of spores to consist of a tubercle-shaped base and apical spine, the gula smooth, and the spores similar to the dispersed megaspore genus Lagenicula (Bennie and Kidston) Potonié and Kremp in shape and ornamentation. It is through serial section that we verify one tetrad per sporangium, and that the ornamentation consists of simple spines also occurring on the gula. The spores are assigned to Lagenicula because their characters (i.e., gula, spiny ornamentation and two-layered exospore with the spongy outer layer) are consis- Fig. 3. Phyllotaxy of megasporophylls of Minostrobus chaohuensis, which is based on tent with the generic diagnosis as emended by Spinner (1969). sporophyll traces in Plate I, 7 and 8; full lines are from Plate I, 7, dotted lines on axis We also recognize new traits of the megasporangiate strobilus, includ- from Plate I,8.● and ■ represent two entire cycles. Scale bar=1 mm. ing the alations, keel and heel of the sporophyll, wall structure of sporangium and spore, subarchesporial pad, and attachment of sporangium (Table 1). Furthermore, the sporangial base is shown to be surrounded by the alations and the spores in the same tetrad are revealed to be varied The Middle Devonian (Givetian) Longostachys Zhu et al. and the Late in size at different extents. Devonian (Famennian) Hefengistrobus Xu and Wang and Cyclostigma Haughton contain in situ megaspores, although microspores are un- 5.2. Devonian lycopsid strobili known or uncertain. Longostachys latisporophyllus Zhu et al. (Cai and Chen, 1996)fromSouthChinaandHefengistrobus bifurcus Xu and Wang TheMiddleDevonian(Givetian)Mixostrobilus and Yuguangia and the (2002) from Northwest China resemble Minostrobus chaohuensis in Late Devonian (Famennian) Bisporangiostrobus Chitaley and McGregor their spherical to elliptical sporangia, each containing (probably) four have bisporangiate strobili. Mixostrobilus givetensis Senkevitsch et al. megaspores. As in M. chaohuensis, Cyclostigma kiltorkense Haughton (1993) from Kazakhstan has sporophylls inserted perpendicularly and (Chaloner, 1968) from Ireland has the sporophyll pedicel inserted at Lagenicula-like megaspores. In Yuguangia ordinata Hao et al. (2007) and 90° to the axis and Lagenicula-type megaspores. Nevertheless, the spo- Bisporangiostrobus harrisii Chitaley and McGregor (1988) from South rophylls of these three species are not smooth at margins, and are much China and the U.S., respectively, the sporophyll consists of a horizontal longer in L. latisporophyllus and C. kiltorkense (up to 3.0 and 8.0 cm, pedicel (bearing alations and sporangium attached along its length in respectively) than in M. chaohuensis;thestrobilusofH. bifurcus is dichot- B. harrisii), lamina and heel (uncertain in Y. ordinata); the sporangial omous; and the strobilus of C. kiltorkense is much wider (2.0–3.5 cm) and wall possesses a single columnar cell layer. In contrast to Minostrobus the sporangium is radially elongate and contains at least 32 megaspores. chaohuensis, however, these plants are characterized by bisporangiate Sublepidodendron songziense Chen (Wang et al., 2003b) and strobili. In addition, M. givetensis has wider strobili (2.0–3.5 cm), short- Sublepidodendron grabaui (Sze) Wang and Xu (2005) were collected stalked sporangia and dozens of spores per megasporangium; Y. ordinata from the Upper Devonian of South China. Sublepidodendron songziense possesses isotomous strobili, toothed sporophylls and Triletes-type mega- probably bears monosporangiate strobili, but its sporophylls lack heel spores; and B. harrisii has wider (1.4 cm) and shorter (7.2 cm) strobili, and its megaspores are Triletes-type and 250–550 μm in diameter. The shorter laminae (2.5 mm), sporangia apparently elongate in a radial di- strobilus of S. grabaui was previously named Lepidostrobus grabaui Sze, rection (3.5–4.0 mm long, 1.4–2.0 mm wide and 0.8–0.9 mm high) and an organ genus and species widespread in the Upper Devonian and 22–30 spores per megasporangium. Lower Carboniferous of South China (Gu and Zhi, 1974). This species 36 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Fig. 4. Line drawing of serial transverse sections of a megasporangium-sporophyll unit abaxial to adaxial. (A) Sporophyll lamina containing vein from Plate VII, 1 (arrow). (B) Sporophyll lamina and sporangium from Plate VII, 2 (arrow). (C) Sporophyll pedicel bearing upturned alations and sporangium from Plate VII, 3 (arrow). (D) Basal part of sporophyll pedicel and sporangium from Plate VII, 4 (arrow). Al, alation; V, vein. Scale bar=1 mm. differs from Minostrobus chaohuensis in having 18 sporophylls per gyre, DiMichele and Bateman, 1996; Feng et al., 2008). Some species of pedicel 3.0–4.0 mm long and 0.2 mm wide, and more elongate Flemingites are considered as the reproductive organs of Paralycopodites sporangia (4.0 mm long and 0.8 mm high). Morey and Morey (Phillips and DiMichele, 1992). Like Minostrobus, Flemingites possesses keel, alations and heel of sporophyll, a narrow 5.3. Carboniferous lycopsid strobili attachment between sporangium and pedicel (Brack-Hanes and Thomas, 1983), a subarchesporial pad in species such as Flemingites The Early Carboniferous Oxroadia Alvin from Scotland (Alvin, 1965; schopﬁi (Brack) Brack-Hanes and Thomas (Brack, 1970)andFlemingites Bateman, 1992) was assigned to the Isoёtales sensu lato by DiMichele arcuatus Stevens et al. (2010),andLagenicula-type megaspores in some and Bateman (1996).AsinMinostrobus, it has a subarchesporial pad species (see Stevens et al., 2010, Table 1). Nevertheless, the strobilus of in the sporangium, but Oxroadia possesses bisporangiate strobili Flemingites is bisporangiate and wider (6.0–70 mm), the alations are (1.0–1.7 cm wide), alations which are narrower than the ﬂask- narrower than the sporangium, and each megasporangium contains shaped sporangia and Setispora-type spores in one or four tetrads per 12 or more spores. megasporangium. Nudasporestrobus ningxicus Feng et al. from the Lower Pennsylva- Flemingites Carruthers, Mazocarpon (Scott) Benson, Nudasporestrobus nian of northwestern China was thought to be sigillarian strobilus Feng et al., Achlamydocarpon Schumaker-Lambrey and Lepidocarpon (Feng et al., 2008). It is megasporangiate strobilus and has horizontal Scott represent the strobili of some members of Isoёtales sensu lato pedicel, heel and lamina with entire margin. In contrast to Minostrobus which were widespread in the Carboniferous of Euramerica and also oc- chaohuensis, N. ningxicus is wider (15–25 mm), contains more (32) curred in this period of China (Gu and Zhi, 1974; Bateman et al., 1992; Sublagenicula-type megaspores per megasporangium.

Fig. 5. Line drawings of megasporangiate strobili of Minostrobus chaohuensis. (A) Transverse section of strobilus from Plate II, 5. (B) Transverse section of strobilus from Plate VIII, 4. (C) Oblique radial section of strobilus from Plate VIII, 5. Al, alation; At, sporangium attachment; Xy, xylem. Scale bars=1 mm. M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 37

Table 1 Comparison of chief characters of Minostrobus chaohuensis megasporangiate strobilus described by Wang (2001) and in this study.

Wang (2001) This study

Strobilar dimension (L×W, mm) 120 at least ×6.0 125 at least ×4.5–6.0 Phyllotaxy Six sporophylls per gyre 2/9 helix Axis Width (mm) 1.2 0.9–1.3 Stele Exarch protostele with 12 ridges of protoxylem Exarch protostele, ridges of protoxylem absent or unclear in number Sporophyll Structure Simple Complex Pedicel Perpendicular to axis, b0.5 mm long Perpendicular to axis, about 1.8 mm long Alations – Present Keel – Present Heel – Present Lamina shape Circular, smooth margin and pointed apex Long-triangular, smooth margin Lamina dimension (L×W, mm) 6.0–8.0×1.5–2.2 6.0–7.0×2.5–3.0 at base Sporangium Shape Spherical to spherical–ellipsoidal, without stalk Spherical to ellipsoidal, without stalk Dimension (mm) 2.0–3.0 in diameter 1.8–2.2×1.0–1.9×1.3–1.8 (L×W×H) Wall 0.2–0.4 mm thick 30 μm thick with one columnar cell layer Subarchesporial pad – Present Attachment – Elongate area (half length of sporangium) between sporangium and pedicel Megaspore Number Four per sporangium Four per sporangium Size 600–800 μm in equatorial width 370–1490 μm in diameter Type Very similar to Lagenicula Lagenicula Ornamentation Biform, base (tubercle in bulbiferous shape, Spiny, about 20 μm wide at base, 50–90 μm about 10–18 μm in diameter and 10–20 μm high) long and apical spine (about 6.0–8.0 μm wide at base, 18–25 μm long) Wall – 20–60 μm thick, with inner exospore of about 5.0 μm thick and spongy outer exospore

Note:–, lack of information; H, height; L, length; W, width.

Mazocarpon is considered as the fertile organ of Sigillaria Brongniart Caudatocorpus Brack-Hanes (1981),fromtheUpperCarboniferous (Benson, 1918; Pigg, 1983; Phillips and DiMichele, 1992; Taylor et al., of North America, also belongs to the Isoёtales sensu lato (Meyen, 2009). The megasporophylls of both Mazocarpon and Minostrobus 1987). Megasporangiate strobili of this genus and Minostrobus share the include the pedicel (with keel and alations), heel and lamina. However, keel, alations and heel of the sporophyll, the sporangial wall composed the megasporangiate strobili of Mazocarpon are wider (1.0–2.2 cm), of one columnar cell layer, and four Lagenicula-type megaspores per spo- the alations are narrower than or equal to the sporangium, the rangium. Nevertheless, the alations of Caudatocorpus are narrower than subarchesporial pad is massive, and there are 8–12 megaspores of the sporangium, and the tetrad includes a single functional and three Laevigatosporites Ibrahim or Tuberculatisporites (Ibrahim) Spinner abortive megaspores, which are enclosed in a persistent noncellular in each sporangium. structure. The megasporangiate strobili of some species of Achlamydocarpon are attached to Diaphorodendron DiMichele, Hizemodendron Bateman and DiMichele, Lepidodendron Sternberg and Synchysidendron DiMichele and 6. Discussion Bateman (DiMichele, 1985; Bateman and DiMichele, 1991; DiMichele and Bateman, 1992; Phillips and DiMichele, 1992; Taylor et al., 2009). 6.1. Systematics Achlamydocarpon and Minostrobus share structure of megasporophylls and four spores per megasporangium. In comparison, small fleshly or The cladistic analysis of DiMichele and Bateman (1996) defined the obvious alations are narrower than or equal to the sporangium, order Isoёtales sensu lato as heterosporous lycopsids with rhizomorphs. e.g., Achlamydocarpon takhtajanii Snigirevskaya and Achlamydocarpon In the previous studies of Minostrobus chaohuensis (Wang, 2001; Wang varius Baxter (Phillips, 1979; DiMichele, 1983); in Achlamydocarpon et al., 2012), a rhizomorph has not been demonstrated, but Wang pingquanensis Zhou et al. (Zhou et al., 2006) from the Lower Permian et al. (2012) assigned this plant to the Isoёtales sensu lato and subor- of North China, the alations are wider but do not turn up to surround der Dichostrobiles on the basis of the microsporangiate strobili with the sporangium. The sporangial wall of Achlamydocarpon is multilay- Lycospora-type spores. However, their reliance upon microspores as ered (Schumacker-Lambry, 1966), and each megasporangium normally crucial evidence is problematic because several features of these contains Cystosporites Schopf spores, with one being large functional spores (amb, cingulum, trilete rays and ornamentation) are very dif- and three small abortive (Schumacker-Lambry, 1966; Leisman and ficult to recognize (see Fig. 2G in Wang et al., 2012) due to the poor Phillips, 1979; Zhou et al., 2008). preservation. Our study and comparison indicate that many taxa Lepidocarpon is considered as the megasporangiate strobilus of (e.g., Oxroadia, Flemingites, Mazocarpon, Achlamydocarpon, Lepidocarpon Lepidophloios Sternberg (Phillips and DiMichele, 1992; Taylor et al., and Caudatocorpus)oftheIsoёtales sensu lato and Minostrobus share 2009), and like Minostrobus, the sporophyll has keel, heel and features of the strobilus, including megasporophyll structure, the narrow upturned alations, and each megasporangium contains one tetrad attachment of megasporangium to sporophyll, subarchesporial pad, (Ramanujam and Stewart, 1969; Phillips, 1979). However, the strobi- and Lagenicula-type megaspore. The presence of monosporangiate lus is 3.0–7.0 cm in diameter, the megasporangium is almost complete- strobili was considered as one of the synapormophies that define the ly enclosed by alations, and three of the Cystosporites megaspores in the Dichostrobiles (DiMichele and Bateman, 1996). We include Minostrobus tetrad are abortive. in this suborder for possessing such strobili (see Section 6.2). 38 M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40

Theliguleusedtobeanimportant character in classifying the first time proven in the Devonian lycopsids, confirming that the lycopsids, and as some workers have noted (e.g., Meyen, 1987; monosporangiate-strobilus clade had evolved by the Late Devonian Kenrick and Crane, 1997), previous studies divided the lycopsids (Wang et al., 2003a). into the homosporous eligulate and heterosporous ligulate lineages. However, the ligule exists on the microphylls of homosporous 6.3. Alations and megaspore number Leclercqia Banks et al. (Grierson and Bonamo, 1979). In our study, no ligule has been observed in the megasporangiate strobili of Broadening of sporophyll alations as protective structures and Minostrobus chaohuensis, a heterosporous lycopsid. This condition also reduction in the number of megaspores per sporangium are consid- occurs in the strobili of many other heterosporous genera, e.g., some ered as evolutionary trends within the monosporangiate-strobilus species of Flemingites and Lepidostrobus (Felix, 1954; Stevens et al., lycopsids (Dichostrobiles), with the most derived state represented 2010), Oxroadia (Bateman, 1992), Longostachys (Cai and Chen, 1996), by Lepidocarpon, in which the megasporangium is almost completely Cyclostigma (Chaloner, 1968), and Sublepidodendron (Wang et al., enclosed by alations and contains a single functional and three abortive 2003b). However, in many species of Flemingites and Lepidostrobus, megaspores (Abbott, 1963; Ramanujam and Stewart, 1969; Thomas, the strobili have ligules (e.g., Brack, 1970; Wang et al., 2003a) and the 1978; Brack-Hanes, 1981; Bateman, 1992; Kenrick and Crane, 1997). sterile axes bear ligule pits (Bateman et al., 1992). As Stevens et al. The variation in alations was summarized by some workers (2010) suggested, the failure to recognize a ligule on the sporophylls (e.g., Abbott, 1963; Ramanujam and Stewart, 1969) on the basis of of Flemingites may be attributed to its small size. Ligules occur on the morphology of both mega- and microsporangiate strobili. Four types sterile microphylls of Oxroadia. It is uncertain or not found on the sterile of alations in megasporangiate strobili are recognized here: 1) alations axes of Minostrobus (Wang et al., 2012), Longostachys, Cyclostigma and narrower than sporangium (e.g., Achlamydocarpon takhtajanii and Sublepidodendron, with the latter two genera still being placed in the Caudatocorpus); 2) alations equal to or wider than sporangium Isoёtales sensu lato (Pigg, 2001; Wang et al., 2003b; Xue et al., 2006). (e.g., Achlamydocarpon varius,somespeciesofMazocarpon and Given the uncertainty about the function and homology of the ligule Achlamydocarpon pingquanensis); 3) alations upturned to surround (Gifford and Foster, 1989; Pant et al., 2000), we suggest that this part of sporangium (Minostrobus); and 4) alations almost completely organ should not feature prominently in the taxonomy of the enclose sporangium (Lepidocarpon). The megasporangiate strobilus of lycopsids. Minostrobus is the only known example with the third type of alations, which was previously found in the microsporangiate strobili of 6.2. Monosporangiate strobili Lepidocarpopsis semialata Abbott and Lepidocarpopsis angusta Ab- bott (Abbott, 1963). The cladistic analyses of Carboniferous rhizomorphic lycopsids In Carboniferous lycopsids with monosporangiate strobili, the (Bateman, 1992; Bateman et al., 1992; DiMichele and Bateman, 1996) number of megaspores per sporangium varies from numerous indicate that, the monosporangiate-strobilus monophyletic clade was (Lepidostrobopsis missouriensis Abbott), 32 (Nudasporestrobus originated from bisporangiate-strobilus lycopsids, which are paraphyletic ningxicus), 8–12 (Mazocarpon)tofour(Lepidocarpon, Achlamydocarpon, and basal in the Isoёtales sensu lato. This phylogenetic framework is Lepidocarpopsis Abbott and Caudatocorpus).Inthelatterfourgenera,a supported by the inclusion of some Mid–Late Devonian heterosporous megasporangium always contains a single functional and three abortive genera (Xue, 2011). Lepidostrobus xinjiangensis Wang et al., a microspores. Each megasporangium of Minostrobus has four spores, which sporangiate strobilus species, suggests that the divergence of the may include one large and three small spores, as in e.g. Lepidocarpon,or monosporangiate-strobilus clade from lycopsids with bisporangiate two large and two smaller spores, or four of equal size. Considering the strobili occurred in the Late Devonian (Wang et al., 2003a). Howev- type of alations, and number and variablesizeofmegasporesineachspo- er, the record of this clade is sparse in the Devonian and its early rangium, we suggest that the Late Devonian Minostrobus represents a evolution is poorly known. In Cyclostigma kiltorkense (Chaloner, transitional form in the evolution toward the more highly derived Car- 1968)andHefengistrobus bifurcus (Xu and Wang, 2002), only in boniferous representatives such as Lepidocarpon, with its monosporic situ megaspores have been found but the monosporangiate charac- megasporangia almost completely enclosed by alations. ter is unclear. Wuxia bistrobilata Berry et al. (2003) hastwotypesoffer- tile organs: probable microsporangiate strobili terminating axes and 6.4. Dispersal mechanisms of megaspores unique megasporangiate strobilus-like structures borne at dichotomies of axes, but spores observed only from the latter organs. Sublepidodendron According to Phillips (1979) and Phillips and DiMichele (1992),the songziense was considered to bear two kinds of terminal strobili (possibly megaspore dispersal mechanisms of the arborescent Carboniferous het- monosporangiate) in different morphology, from which megaspores or erosporous lycopsids fall into three types. One type is represented by microspores were separately obtained through maceration (Wang et al., Mazocarpon, in which the megasporangiate strobili are shed and later di- 2003b). In L. xinjiangensis, the strobilus including the basal part con- vide in sequence into megasporangium-sporophyll units, megasporangia tains microspores of Lycospora Schopf et al. (Wang et al., 2003a). It is and megaspores. Another type includes Achlamydocarpon and thus the only unequivocal Late Devonian monosporangiate lycopsid Lepidocarpon, which have the megasporangium-sporophyll units strobilus. disseminated and megaspores germinating within the sporangium. In the study of Minostrobus chaohuensis by Wang (2001),the The third type is known from some bisporangiate-strobilus genera megasporangiate strobili could not be determined. In the current such as Flemingites and Bothrodendrostrobus Hirmer, whose mega- study of M. chaohuensis, megasporangiate strobili have been proven so spores are freely released. This free-sporing mechanism also occurs that the monosporangiate character is demonstrated. First, in numer- in the megasporangiate strobilus of Caudatocorpus,whichbearsa ous cases, most or all sporangia of a strobilus contain megaspores wing-like attachment (noncellular structure) on the spore tetrad (e.g., Plate III,1and2;Plate VII,1–7; Plate VIII,1–5; Plate IX,1–6). (Brack-Hanes, 1981). As one exception, several sporangia in the upper (Plate IV,5)and The available remains of Minostrobus chaohuensis, including the mid-lower (Plate IV,1–3) parts of a long strobilus have megaspores, branching axes, most possibly represent the distal part of an arborescent whereas other sporangia are empty (Plate IV,1–3 and 8). These empty or pseudoherbaceous lycopsid, where the secondary xylem is absent sporangia have broken walls and lack subarchesporial pads, suggesting (Wang et al., 2012). The gulate Lagenicula-type megaspores found in that the megaspores had been dispersed. Second, megaspores occur in Minostrobus have been thought adapted to dispersal from tall trees both the basal (Plate I, 3) and distal (Plate I,4and6;Plate II,4and (Hemsley et al., 1999; Glasspool and Scott, 2005; Arioli, et al., 2007). A 5) parts of the strobili. Thus, a megasporangiate strobilus is for the free-sporing mechanism of megaspores in M. chaohuensis is indicated M. Meng et al. / Review of Palaeobotany and Palynology 190 (2013) 20–40 39 by the recovery of one strobilus showing that most megasporangia are DiMichele, W.A., 1985. Diaphorodendron, gen. nov., a segregate from Lepidodendron (Pennsylvanian age). Systematic Botany 10 (4), 453–458. empty and have broken walls. Furthermore, the megasporangium- DiMichele, W.A., Bateman, R.M., 1992. Diaphorodendraceae, fam. nov. (Lycopsida: sporophyll units have never been discovered to be detached from strobili. Carboniferous): systematics and evolutionary relationships of Diaphorodendron The main characters of the Dichostrobiles sensu DiMichele and Bateman and Synchysidendron,gen.nov.AmericanJournalofBotany79(6),605–617. DiMichele, W.A., Bateman, R.M., 1996. The rhizomorphic lycopsids: a case-study in (1996) areproposedheretobeexpandedtoincludethefree-sporing paleobotanical classification. Systematic Botany 21 (4), 535–552. monosporangiate-strobilus lycopsids. Felix, C.J., 1954. 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