Rotafolia
A sphenopsid from the Upper Devonian (Famennian) Xiejingsi Formation, south-western Hubei Province, China, previously named as various species in Sphenophyllum, Hamatophyton, Bowmanites and Sphenophyllostachys, is now reinvestigated and assigned to a new taxon, Rotafolia songziensis gen. et comb. nov. Its ribbed axes are anisotomous and possess slightly expanded nodes. Lateral axes are inserted at nodes on main axes. Whorls of much divided vegetative leaves are attached at nearly right angles to nodes of basal axes, and at acute angles to nodes of terminal axes. There are six leaves per whorl. The terminal strobilus includes a central axis and verticils of fertile units. Each fertile unit consists of a bract and numerous sporangia. The margin of the elongate-cuneate bract bears a distal and many lateral elongate segments. Clusters of elongate sporangia are abaxially attached to the base of the bract at the same level. The axis has an actinostele, composed of a three-ribbed, exarch primary xylem and radial secondary xylem. Although Rotafolia songziensis closely resembles Hamatophyton verticillatum in axis character, leaf morphology and primary xylem type, they are quite different in strobilar structure. Taxonomically, Rotafolia is placed in the order Sphenophyllales by three well-defined characters: 1) whorled appendages; 2) ribbed protosteles; 3) exarch primary xylem maturation.
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The sphenopsids arose and began to diversify as herbaceous plants in the Devonian and reached a climax in abundance and diversity during the Carboniferous. The genus Equisetum is usually accepted as the only extant representative of this group (Stewart & Rothwell, 1993: 180), although some authors have suggested that it should be subdivided into two genera, Equisetum and Hippochaete, mainly based upon the differences in growth habit, chemical components and antheridia (Ching, 1978). Both fossil and living sphenopsids are morphologically characterized by axes with well-defined nodes bearing verticils of leaves or branches and internodes with longitudinal ribs. Fertile organs, termed strobili or cones, comprise an axis possessing whorls of bracts, or sporangiophores with adaxially attached sporangia. Anatomically preserved axes demonstrate a vascular system varying from actinostele to siphonostele. The primary xylem maturation order is either exarch or endarch/mesarch. Secondary xylem is sometimes present.
In China, some Upper Devonian sphenopsids have been documented (Table 1). Among them, Xihuphyllum megalofolium (Chen, 1988), Eviostachya hoegii (Wang, 1993) and Hamatophyton verticillatum (Li, Cai & Wang, 1995) have been studied morphologically and/or anatomically in detail. Other simply described sphenopsids still need further study. In the spring of 2002, abundant and well-preserved sphenopsid fossils were collected from Songzi and Yidu districts, south-western Hubei Province. By careful comparison with the formerly described sphenopsids, in this paper we combine material from the same horizon and localities previously referred to Sphenophyllum yiduense (Feng et al., 1977), Hamatophyton verticillatum, H. yiduense, Bowmanites songziensis (Feng, 1984), Sphenophyllostachys huangkuangensis, S. songziensis, S. oblongifolius, S. sp.1 and S. sp.2. (Feng & Ma, 1991) into a new taxon, Rotafolia songziensis gen. et comb. nov., and make comparisons with other sphenopsids.
Some specimens herein were collected from the Upper Devonian Xiejingsi Formation, Huangkuang section (a coal-mining locality on top of a mountain), Liujiachang town, Songzi district, south-western Hubei. From the same horizon and section, two specimens were loaned by courtesy of Professor Feng Shao-Nan (Institute of Geology and Mineral Resources of Yichang). Others, including most of the strobili were obtained from the same formation, in the Tizikou section (another coal-mining locality on the top of a mountain), Maohutang village, Yidu district, c. 22 km north-west of Huangkuang section. A detailed description of the locality and stratigraphy of Huangkuang and Tizikou sections was given in Feng (1984) and Wang et al. (2003).
Rotafolia songziensis D.-M. Wang, Hao & Q. Wang gen. et comb. nov. Fig. 10. An axis with leaves at nodes. Hu-12. Scale bar = 5 mm. Fig. 11. A longitudinally fractured axis showing a whorl of leaves at a node. Hu-08. Scale bar = 5 mm. Fig. 12. Single wedge-shaped leaf with two longer (L) and shorter (S) segments. Arrow indicating leaf base. Hu-20. Scale bar = 5 mm. Fig. 13. Spiny anisotomous axis. Hu-04. Scale bar = 5 mm. Fig. 14. Axis transversely fractured at node showing six leaves per whorl. Arrow indicating a symmetrical leaf along the mid-longitudinal line through leaf base. Hu-10. Scale bar = 5 mm. Fig. 15. Longitudinally fractured axis showing three leaves at node. Arrows indicating indistinct rounded leaf tips. Hu-15. Scale bar = 5 mm. Fig. 16. Parallel leaf segments below division of axis. Hu-01. Scale bar = 5 mm. Fig. 17. Axis transversely fractured at node showing six leaves per whorl. Hu-16. Scale bar = 2 mm. Fig. 18. Axis transversely fractured at node showing six leaf bases. Hu-01. Scale bar = 2 mm. Fig. 19. Fragmentary axis attached to a wedge or fan-shaped leaf. Arrow indicating leaf base. Hu-04. Scale bar = 2 mm. Figs 20, 21. Higher magnification of, respectively, upper and middle parts of a ribbed axis in Fig. 8, showing tiny, hair-like spines on internodes. Hu-06. Scale bar = 5 mm.
Rotafolia songziensis D.-M. Wang, Hao & Q. Wang gen. et comb. nov. Fig. 22. Fertile axis with a terminal strobilus. Arrows indicating fragmentary leaves at nodes. Hu-17. Scale bar = 1 cm. Fig. 23. Fertile axis with a terminal strobilus. White arrows indicating fragmentary leaves at a node, black arrow a piece of lateral axis. Hu-18. Scale bar = 1 cm. Fig. 24. Higher magnification of Fig. 23 showing lateral fertile units consisting of a bract with segments and sporangia attached at the same level. White arrows indicating elongate sporangia in abaxial view, black arrows distal segment of bract. Hu-18. Scale bar = 5 mm. Fig. 25. Strobilus bearing up to 16 whorls of fertile units. Hu-23. Scale bar = 1 cm. Fig. 26. Single bract with bifurcate distal segment (arrow) and many elongate lateral segments. Hu-B1. Scale bar = 2 mm. Fig. 27. Strobilus showing whorls of fertile units. White arrows indicating fertile units in abaxial view, black arrows two fertile units enlarged in Fig. 28. Hu-19. Scale bar = 1 cm. Fig. 28. Higher magnification of two fertile units from Fig. 27 showing bracts in side view with elongate lateral segments. White arrows indicating distal segment of bract, black arrow sporangia at base of bract. Hu-19. Scale bar = 2 mm. Fig. 29. Bract in abaxial view showing lateral segments (upper arrow) and basal sporangia (lower arrow). Hu-A1. Scale bar = 2 mm.
Rotafolia songziensis D.-M. Wang, Hao & Q. Wang gen. et comb. nov. Fig. 35. Oblique abaxial view of two fertile units. Hu-B2. Scale bar = 5 mm. Fig. 36. Higher magnification of an upper fertile unit in Fig. 35 showing bract with marginal segments (white arrows) and basal sporangia (black arrow). Hu-B2. Scale bar = 2 mm. Fig. 37. Abaxial view of a single fertile unit beside a strobilar axis showing bract bearing marginal segments and basal sporangia (arrow). Hu-B1. Scale bar = 2 mm. Fig. 38. Higher magnification of a fertile unit in Fig. 30 (lower arrow) attached to a strobilar node. Side view of a bract with lateral segments and basal sporangia. Arrow indicating attaching position of sporangia. A-076. Scale bar = 2 mm. Fig. 39. Higher magnification of a fertile unit in Fig. 30 (upper arrow). Side view of a bract showing many lateral segments and basal sporangia (arrow). A-076. Scale bar = 2 mm. Fig. 40. Leafy vegetative axis providing anatomical information. Arrow indicating xylem strand. Hu-A8. Scale bar = 1 cm. Fig. 43. Transverse section of axis in Fig. 40 showing an actinostele with a central triarch primary xylem and a surrounding zone of radial secondary xylem. Arrows indicating protoxylem strands at tip of primary xylem. H-02. Scale bar = 200 µm. Fig. 44. Higher magnification of Fig. 43 showing boundary between metaxylem with larger tracheids and secondary xylem with files of smaller tracheids. Arrows indicating secondary xylem tracheids near metaxylem tracheids. H-02. Scale bar = 100 µm.
Rotafolia songziensis D.-M. Wang, Hao & Q. Wang gen. et comb. nov. Fig. 30. Holotype. Strobilus showing whorls of fertile units attached at distinct nodes. Fertile units consisting of a bract with elongate segments and basal sporangia. Upper and lower arrows indicating two fertile units separately enlarged in Figs 39, 38. A-076. Scale bar = 1 cm. Fig. 31. Abaxial view of elongate sporangia attached to base of an indistinct bract at the same level. Hu-21. Scale bar = 2 mm. Fig. 32. Anisotomous division of two fertile axes with terminal strobili. Arrows indicating fragmentary vegetative leaves at nodes. A-056. Scale bar = 1 cm. Fig. 33. Adaxial view of a bract attached to a strobilar axis showing leaf blade bearing a distal segment (white arrow) and many marginal elongate segments (black arrow). Hu-22. Scale bar = 2 mm. Fig. 34. Higher magnification of bract in Fig. 33 showing lines of depressions possibly representing epidermal cells. Hu-22. Scale bar = 2 mm.
Basionym
Bowmanites songziensis (Feng, 1984. In: Institute of Geology and Mineral Resources of Yichang, ed. Biostratigraphy of the Yangtze Gorge Area (Vol. III), Late Palaeozoic Era. Beijing: Geological Publishing House, p. 302–303, plate 48: fig. 4. Note: Although one of the following synonyms –‘Sphenophyllum yiduense’ was named by Feng et al. (1977), it originally denoted vegetative axes or leaves; ‘Bowmanites songziensis’ has priority as the basionym in that it referred to a strobilus)
Synonyms
Sphenophyllum yiduense (Feng et al., 1977.In: Hubei Institute of Geoscience et al. eds. Palaeontological atlas of central and south China (Vol. II). Beijing: Geological Publishing House, 638, plate 235: figs 1–4)
(Li et al., 1995. In: Palaeontographica 235B: 4)
Hamatophyton verticillatum (Feng, 1984. In: Institute of Geology and Mineral Resources of Yichang, ed. Biostratigraphy of the Yangtze Gorge Area (Vol. III), Late Palaeozoic Era. Beijing: Geological Publishing House: 10, 302, plate 48: fig. 9)
Hamatophyton yiduense (Feng, 1984. In: Institute of Geology and Mineral Resources of Yichang, ed. Biostratigraphy of the Yangtze Gorge Area (Vol. III), Late Palaeozoic Era. Beijing: Geological Publishing House: 10, 302, plate 48: figs 11, 12)
(Li et al., 1995. In: Palaeontographica 235B: 4, Synonymy Hamatophyton verticillatum (Chen) Feng et al.‘1982’: 302; plate 48: figs 11, 12. Note: according to Feng (1984), here the year ‘1982’ should be changed to ‘1984’, and the species name ‘verticillatum’ to ‘yiduense’)
Sphenophyllostachys songziensis (Feng & Ma, 1991. In: Acta Botanica Sinica 33(2): 142–143, plate I: fig. 1, plate II: fig. 1, text-fig. 2)
Sphenophyllostachys huangkuangensis (Feng & Ma, 1991. In: Acta Botanica Sinica 33(2): 141–142, plate I: figs 2, 2a, text-fig. 1)
Sphenophyllostachys oblongifolius (Feng & Ma, 1991. In: Acta Botanica Sinica 33(2): 143–144, plate II: fig. 2, text-fig. 3)
Sphenophyllostachys sp.1 (Feng & Ma, 1991. In: Acta Botanica Sinica 33(2): 144, plate II: fig. 3a, text-fig. 4)
Sphenophyllostachys sp.2 (Feng & Ma, 1991. In: Acta Botanica Sinica 33(2): 144, plate II: fig. 3b, text-fig. 5)
Hamatophyton (Wang et al., 2003. In: Review of Palaeobotany and Palynology 127 (3–4): 280)
Generic diagnosis: Plant with anisotomous axes characterized by slightly expanded nodes and ribbed internodes. Lateral axes attached at nodes of main axes. Whorls of much divided vegetative leaves inserted at nearly right angles to nodes of basal axes, at acute angles to nodes of terminal axes. Terminal strobilus possessing a central axis and verticils of fertile units; each consisting of a bract and numerous sporangia; elongate-cuneate bract bearing a distal and many lateral elongate segments. Pendulous elongate sporangia abaxially attached to base of bract at the same level. Actinostele comprising three-ribbed primary xylem and radial secondary xylem. Primary xylem maturation exarch, with protoxylem strands at tips of xylem ribs.
Etymology: Name from Latin words ‘rotalis’ and ‘folium’, respectively, meaning whorled and leaved. When combined, they refer to whorls of leaves at nodes of axes.
Specific diagnosis: As for generic diagnosis. Axes up to 20.0 cm long. With internodes 1.5–(2.2)−4.8 cm long, vegetative axes 0.8–(3.0)−8.3 mm in diameter and branching at angles of 45–85°. Axial spines 1.0–(2.0)−2.8 mm long and c. 0.2 mm wide at base. Wedge or fan-shaped vegetative leaves six per node of axis, 5.0–(14.5)−24.0 mm long and 2.0–(11.0)−18.0 mm wide as a whole. Each vegetative leaf equally or unequally dividing at angles of 20–80°, 2–4 times. Below first division, leaf bases 2.7–(6.0)−9.0 mm long and 0.4–(0.8)−1.5 mm wide. Fertile axes below strobili anisotomous at 30–45°, up to 7.8 cm long and 2.7–(3.8)−6.4 mm in diameter. Strobili 2.9–(4.9)−8.5 cm long and 1.0–(1.6)−2.2 cm wide, with internodes of strobilar axes 0.5–(0.8)−1.6 cm long. Each strobilus bearing up to 16 whorls of fertile units. Fertile units 6.4–(8.8)−11.0 mm long by 2.0–3.2 mm wide and inserted at c. 60° or 90° to strobilar axis. Bract 5.0–6.7 mm long and c. 2.0 mm wide, with distal and lateral segments, respectively, 1.5–8.0 mm long by c. 0.2 mm wide and 2.0–(3.1)−3.8 mm long by 0.2–0.3 mm wide. 10–18 lateral segments attached to bract at 65–85°. Sporangia 1.4–(2.5)−3.8 mm long and 0.2–0.4 mm wide, and 6–10 per bract.
Holotype: A-076 (Fig. 30; also sensuFeng & Ma, 1991; plate I: fig. 1, plate II: fig. 1).
Paratypes: Hu-27 (Fig. 2), Hu-13 (Fig. 4), Hu-20 (Fig. 12), Hu-10 (Fig. 14), Hu-06 (Fig. 20), Hu-18 (Fig. 23), Hu-B1 (Fig. 26), Hu-19 (Fig. 27), A-056 (Fig. 32), Hu-22 (Fig. 33), Hu-B2 (Fig. 35), Hu-B1 (Fig. 37), H-02 (Figs 43, 44).
Repository: Holotype A-076 and paratype A-056, Institute of Geology and Mineral Resources of Yichang, Hubei Province, P. R. China; other paratypes and figured specimens, Department of Geology, Peking University, Beijing, P. R. China.
Type locality: Huangkuang section, Liujiachang town, c. 30 km south-west of Songzi City, south-western Hubei Province, P. R. China.
Horizon: Upper Devonian (Famennian) Xiejingsi Formation.
DESCRIPTION
Morphology
The description is based on about 90 specimens of incomplete leafy axes (possible vegetative ones) and 18 specimens with strobili. There is as yet no specimen indicating the basal portion of the plant. Figures 2, 5 and 8, 9 separately represent the part and counterpart of a specimen.
Vegetative main and lateral axes: The anisotomous axes consist of slightly enlarged nodes and ribbed internodes (Figs 2, 5, 7–9, 13). The first-order lateral axes depart at 65° from the nodes of the assumed main axes (Figs 2, 5, 7–9), with diameter of the former axes c. half that of the latter; the second-order lateral axes at 85° from the nodes of the first-order (Fig. 2, arrow), with the diameter of the former axes c. 2/3 of the latter. The lateral axes are slightly enlarged at the point of emission. Lacking preserved base or an apex, the biggest main axis noted is up to 16.5 cm long and c. 8.0 mm in diameter (Figs 2, 5). Two first-order lateral axes occur on either side. The distance between their insertions is 5.0 cm and one lateral axis extends up to 10 cm long; the internodes of the main axis range from 2.0 to 2.5 cm long and those of the left first-order lateral axis vary from 1.5 to 1.7 cm long. Two longitudinal ribs, c. 1 mm wide, are present on the exposed surface of the axes and are continuous at the internodes and nodes (Figs 2, 5, 7–9, 20, 21). The ribs reflect the position of internal vascular strands.
The axes bear distinct tiny spines (Figs 8, 9, 13, 20, 21). About eight spines are visible along the side of an axis 1 cm long. The leaves are attached at nearly right angles to the nodes of the main and lateral axes (Figs 2, 5, 7–10).
Vegetative terminal axes: Measuring 0.8–1.8 mm in diameter, the terminal vegetative axes (Figs 3, 4, 6) are much more slender than the main and lateral ones. They are composed of indistinct nodes and internodes. Without clear longitudinal ribs and spines, the internodes are 1.0–3.6 cm long and 1.0–2.0 mm in diameter. In contrast with the attachment to the main and lateral axes, the leaves are inserted at c. 45° to each node of the terminal axis.
Vegetative leaves: All portions of axes show nodes that bear wedge or fan-shaped leaves (Fig. 2). Because of preservation, the axes generally show two rows of leaves (Figs 2, 5, 7–10), but there are actually six leaves per whorl based on transversely fractured axes at nodes (Figs 14, 17, 18). Leaves also occur at the branching points of an axis (Figs 2, 7, 8, 16). Three leaves aligned in parallel are deeply dissected in the middle (Fig. 16) as the leaves in Figure 11; their lower parts are obscured by the overlying axis. This plant is characterized by much divided leaves (Figs 2–12, 14, 15, 17, 19). The leaf base (e.g. Figs 12, arrow, 19, arrow) and leaf segments separately refer to the parts below and above the first leaf-dividing point. In one example, only leaf bases remain (Fig. 18). The ratio of the diameter of the leaf bases to that of the attached axes is c. 0.2–1.0. The leaves equally or unequally divide. In the former case, the leaf segments are somewhat symmetrical along the mid-longitudinal line through the leaf bases (e.g. Figs 12, 14, arrow, 19). The leaves divide 2–4 times, with the first division usually occurring at the mid-point of the leaf length. As illustrated in a better-preserved specimen (Fig. 12), the initial division of a leaf produces two longer (L) and shorter (S) segments; at an angle of 25°, each longer segment then equally divides twice, forming four much shorter distal segments. A few leaves show indistinct rounded tips (Fig. 15, white arrows), while most leaves taper to a sharp point (Figs 14, 19). Neither veins nor spines have been observed on leaves except for a longitudinal thin depression on the leaf base possibly representing the vein (Fig. 15, black arrow).
Strobili: Some axes bear distally incomplete (Figs 22, 23) and complete terminal strobili (Fig. 32). These fertile axes below strobili are analogous to the sterile axes in that they both possess nodes and internodes, with fragmentary leaves attached at the nodes (Figs 22, arrows, 23, white arrows, 32, arrows). As in the vegetative axes, the fertile axes are anisotomous, with the lateral fertile axis (Figs 23, black arrow, 32) thinner than the main one. The strobilus of the lateral axis is much shorter and thinner than that of the main axis (Fig. 32). Strobili consist of a central axis and lateral fertile units at nodes (Figs 24, 25, 27, 30, 41). On a single strobilar axis, the nodes are sometimes represented by distinct linear depressions on the rock matrix impression surface (Figs 30, 38, 39) and the internodes are of more or less constant length. The lateral fertile units seen on longitudinally fractured axes appear superficially to be arranged in two rows (Figs 22, 23, 30, 32), but two strobili in higher magnification show another row of compressed fertile units bearing sporangia above the strobilar axis (Figs 24, white arrows, 27, white arrows, 41). A strobilar axis possesses up to 16 whorls of fertile units (Fig. 25). Because of the lack of transversely fractured strobili and failure in dégagement, the precise number of fertile units per node could not be determined, although it may be six as is the case for vegetative leaves. Vertically adjacent fertile units are detached from each other.
Rotafolia songziensis D.-M. Wang, Hao & Q. Wang gen. et comb. nov. Line drawing of mid-lower part of the strobilus in Fig. 24, illustrating arrangement of fertile units. Side view of fertile units showing a bract with a distal segment (arrows), many lateral segments and basal sporangia. Abaxial view of sporangia along vertically central area of strobilar axis indicating their attachment at the same level (marked by dotted line).
A fertile unit consists of a bract and numerous sporangia. In adaxial view, the blade of the bract is attached to a strobilar axis and elongate wedge-shaped in outline (Fig. 33). On its surface, there are many linear depressions which may represent the remains of the epidermal cells (Figs 34, 42A, arrow 4). The blade bears a distal elongate segment (Figs 33, white arrow, 42A, arrow 1). At the lateral side of the blade near the strobilar axis, there are several lateral elongate segments that recurve toward the axis (Figs 33, black arrow, 42A, arrow 2). Possibly because of preservation, the other side of the blade lacks lateral segments. In abaxial view, lateral elongate segments are distinct on one side of the elongate wedge-shaped bract blade near the strobilar axis (Figs 29, upper arrow, 37, 42B, 42C). In these examples, elongate projections suggest lateral segments on the other side of the blade, which are shown by dotted lines in Figure 42B, C. They were folded and then pressed close to the upper surface of the bract blade. In oblique abaxial view, lateral elongate segments are observed on either one side (Fig. 42D, E) or two lateral sides (Figs 35, 36, white arrows) of the bract blade. In lateral view, the distal segment of the bract is sometimes visible (Figs 24, black arrows, 28, white arrows, 41, arrows, 42F, arrow); in one example, a distal segment is up to 8.0 mm long and dichotomous (Fig. 26, arrow); lateral segments slightly reflex towards the strobilar axis (Figs 28, 35, 36, 39).
Rotafolia songziensis D.-M. Wang, Hao & Q. Wang gen. et comb. nov. Line drawings of fertile units. A, adaxial view of a bract from Fig. 33. Arrow 1 showing a distal segment of bract, arrow 2 lateral segments of bract, arrow 3 depression marking position of sporangial attachment, arrow 4 vertical depressions possibly representing epidermal cells. B, abaxial view of a bract showing lateral segments and basal pendulous sporangia (arrow). Dotted lines indicate projections representing lateral segments preserved under bract. C, abaxial view of a fertile unit from Fig. 37 showing a bract bearing lateral segments and basal sporangia. Dotted lines indicate projections representing lateral segments preserved under bract. D, E, oblique abaxial view of fertile units showing a bract with lateral segments and basal pendulous sporangia. F, side view of a fertile unit from Fig. 28 illustrating a bract with distal segment (arrow), lateral segments and basal sporangia.
In side view, it is clear that many pendulous sporangia are abaxially attached at the base of a bract (Figs 28, black arrow, 39, arrow, 42F). It is also the case in abaxial (Figs 29, lower arrow, 37, arrow, 42B, arrow, 42C) and oblique abaxial (Figs 35, 36, black arrow) views of a bract. In adaxial view, about 2.0 mm above the attachment point to the strobilar axis, the bract blade possesses a linear depression (Fig. 42A, arrow 3) indicating the position where the abaxial sporangia were once attached. Corresponding to this example, abaxial sporangia are pendulous at the bract 3.3 mm away from the node of the strobilar axis (Fig. 38, arrow). Sporangia are sometimes observed along the vertically central area of the strobilar axis (Figs 24, white arrows, 27, white arrows) and, as illustrated in Figure 41, they are attached at the same level although their bract is indistinct (except the fertile unit in Fig. 27, middle white arrow) or has broken off when the specimens were split. When enlarged from the corresponding area of another strobilar axis, it is obvious that the sporangia are also inserted at the same level (Fig. 31). Sporangia are elongate in shape, showing no evident morphological differences with the lateral segments of the bract. Unfortunately, we have not found any spores or dehiscence of these sporangia.
Anatomy
An anatomically preserved sterile axis collected from the Huangkuang section (Fig. 40, arrow) shows that this plant possesses an actinostele, which is rounded-triangular shaped and c. 1.7 mm in diameter (Fig. 43). The xylem consists of a central primary xylem column and a surrounding zone of secondary xylem. The extraxylary tissues have not been preserved. In some portions of the xylem, the tracheids have broken down as a result of preservation. The primary xylem is roughly triarch in outline and has three xylem ribs with three shallow concave sides between. One rib is shorter radially than the other two, and the tips of three ribs are widely spaced. The ribs vary in radial dimension from 560 to 750 µm, and average 520 µm in tangential dimension. In this transverse section, the primary xylem is composed entirely of polygonal tracheids. The protoxylem tracheids are 12–(20)−30 µm in diameter and located at the tips of the ribs (Fig. 43, arrows). The tracheids of the metaxylem are 25–(60)−85 µm and generally centripetally increase in diameter. They are significantly larger than those of the protoxylem and secondary xylem.
The secondary xylem opposite the shorter primary rib are strongly limonitized. Rays are indistinct in the compact secondary xylem. In contrast, abundant tracheids are arranged in radial files. The tracheid files vary in tangential dimension. The tetragonal tracheids of the secondary xylem are 17–(30)−45 µm in diameter, usually with the smaller ones situated beside the periphery of the metaxylem strand (Fig. 44, arrows). From inner to outer regions, the secondary xylem tracheids either appear relatively constant or gradually increase or abruptly decrease in size (Figs 43, 44).
COMPARISONS
Hamatophyton verticillatum is an important member of the Upper Devonian (Famennian) plant assemblage in China. It was established by Gu & Zhi (1974: 38–39) and described as having clear axis nodes and ribbed internodes, whorls of leaves with occasional hooked tips and loose strobili. The plant was placed in the Hyeniales and regarded as a possible primitive sphenopsid. Later, other authors reported or briefly described this genus from many outcrops of China (Table 1). Based on well-preserved material from Jiangsu, Hunan and Anhui Provinces, Li et al. (1995) studied H. verticillatum in detail. They thought this plant represented a new family Hamatophytaceae included in the order Sphenophyllales.
In autumn 2003, one of the authors, Wang De-Ming, observed specimens of Hamatophyton verticillatum studied by Li et al. (1995) and found that they closely resemble Rotafolia songziensis in axis character, leaf morphology and primary xylem type. Both possess spiny and ribbed axes, divided leaves. The xylem is characterized by an exarch primary xylem with three ribs. However, as is found in some sphenophyllalean plants where vegetative organs are similar in appearance, these two taxa demonstrate considerable variation in strobilar structure. As to Hamatophyton, each sporangiophore at the strobilar node bifurcates into two parts. The shorter part acts as a possible bract, the longer one results in two adaxially recurved long stalks, each of which terminates in an elliptical sporangium. Two sporangia are more or less parallel to the sporangiophore before bifurcation. In Rotafolia, however, each fertile unit of the strobilus has an elongate-cuneate bract that bears a distal segment and 10–18 lateral elongate segments. 6–10 elongate abaxial sporangia are pendulous at the base of the bract. Where they are inserted on the bract, the sporangia are at the same level.
Feng et al. (1977) reported Sphenophyllum yiduense in the Xiejingsi Formation, Maohutang village, Yidu district, Hubei Province. Feng (1984) briefly described Hamatophyton verticillatum, H. yiduense in the same formation, Huangkuang section, Songzi district and Tizikou section, Yidu district. These species refer to vegetative axes with either nodes/internodes or leaves. Feng (1984) also briefly described a strobilus, Bowmanites (=Sphenophyllostachys) songziensis, in the Xiejingsi Formation, Huangkuang section, Songzi district. Subsequently, according to the specimens from the same formation and section, Feng & Ma (1991) erected five species of Sphenophyllostachys, i.e. S. huangkuangensis, S. songziensis, S. oblongifolius, S. sp.1 and S. sp.2. Generally speaking, these taxa were regarded as possessing fertile axes with nodes and internodes, whorls of bracts that are decurrent and terminally curved towards the strobilar axis or occasionally bifurcate at tips; each bract has one or two variously shaped adaxial sporangia with a short stalk. One noticeable character is the so-called ‘lines of ornamentation on the surface of the sporangium’ in some species. Feng & Ma also suggested that the reproductive organ of Hamatophyton Gu & Zhi, 1974 be replaced by Sphenophyllostachys. However, Li et al. (1995) thought that ‘The descriptions (of species of Sphenophyllostachys) are too simple and fail to make a comparison with the original specimens of H. verticillatum; and according to their descriptions, the illustrations in plates and the text-figures, the features of the strobili of Sphenophyllostachys are quite different from those of the type specimens of H. verticillatum. They may represent two different taxa’. We have checked the above plant specimens described from the Xiejingsi Formation of south-western Hubei, i.e. Sphenophyllum yiduense, Hamatophyton verticillatum, H. yiduense, Bowmanites songziensis and species attributed to Sphenophyllostachys. Of these, two specimens, loaned by courtesy of Professor Feng Shan-Nan, are hereby illustrated (Figs 30, 32, 38, 39). Although their fertile features are not easy to identify, it is most likely that ‘the lines of sporangial surface ornamentation’ of Bowmanites songziensis (Feng, 1984) and species of Sphenophyllostachys (Feng & Ma, 1991) actually represent individual lateral segments of the bracts. Definite attachment of sporangia therefore has not been discovered previously. All these species show great morphological similarities with the present plant in strobili as well as axes and leaves. Furthermore, these fertile and vegetative parts are now found in an organic connection. In light of these close resemblances and connection, together with the same plant-occurring horizon in Huangkuang and Tizikou section, they are most probably synonymous. By careful comparison between these previously described fossils and our new material, we have decided to make a new taxon, Rotafolia songziensis gen. et comb. nov.
A few organ genera have been established to represent the detached strobili assigned to the Sphenophyllales. Most of these Carboniferous and Permian form genera are characterized by whorls of bracts that are usually fused laterally for part of their length to form a disk (Stewart & Rothwell, 1993: 193). Among them, Bowmanites or Sphenophyllostachys have variously constructed bracts adaxially attached by a variable number of sporangiophores with anatropous or orthotropous terminal sporangia (Good, 1978). Each bract of the Sphenostrobus strobilus has a single axillary sporangiophore ending in one sporangium (Levittan & Barghoorn, 1948; Good, 1978). In Peltastrobus (Leisman & Graves, 1964), the more complex strobilus consists of a whorl of three sterile and three fertile units at each node. Sterile and fertile units alternate. Each fertile unit comprises a bract and a cluster of five axillary peltate sporangiophores bearing several concentric cycles of sporangia. Two sporangiophores are distally, two are proximally directed and one is at 90° to the strobilar axis. The strobili of Rotafolia differ from those of Bowmanites, Sphenostrobus and Peltastrobus in that the bracts are independent and bear distal and lateral elongate segments; multiple abaxial sporangia are attached to the base of the bracts at the same level.
Recorded in the Upper Devonian, Eviostachya hoegii (Leclercq, 1957; Wang, 1993) is a sphenopsid that has whorls of divided vegetative leaves and opposite strobilar axes at nodes. The strobilus comprises whorls of sporangiophores. The sporangiophore equally trichotomizes twice to produce nine portions, each of which terminates in three adaxially recurved ellipsoid sporangia. The actinostele is three-ribbed. Although the sporangiophore bears sterile processes, it is not associated with any leafy bracts. These fertile structures differ greatly from those of Rotafolia songziensis.
Occurring from Late Carboniferous to Early Permian, Lilpopia (i.e. Tristachya) is a sphenophyllalean taxon (Taylor & Taylor, 1993: 315). Although this plant possesses sporangia below a leaf [sensu the reconstruction presented by Kerp (1984)], it differs from Rotafolia in that the fertile region is situated between the upper and lower vegetative leaves, the sporophylls lack distal or lateral divisions and the tiny ovoid sporangia are arranged in clusters.
Sphenophyllum (e.g. S. emarginatumBatenburg, 1977), a common sphenopsid of Carboniferous or Permian age, bears whorls of much divided vegetative leaves as in Rotafolia songziensis. As for the vegetative internal structure of Sphenophyllum, the primary xylem is also exarch and three-ribbed. However, the axes of some species show fascicular (opposite the primary xylem ribs) and interfascicular (between the ribs) regions of secondary xylem (Reed, 1949; Boureau, 1964: 96; Cichan & Taylor, 1982; Cichan, 1985); concentric zones of secondary xylem are sometimes present (Reed, 1949). In addition, the primary xylem of the stele in Sphenophyllum is much smaller than that in Rotafolia.
DISCUSSION
Generally speaking, the Sphenopsida (Equisetopsida) include two major orders, Sphenophyllales and Equisetales. According to Stein, Wight & Beck (1984), they share five characters, i.e. whorled appendages, leaf morphology, sporangiophores, spore features and secondary tissues. Among these synapomorphies, Stein et al. considered that only the whorled appendages are unambiguously defined. However, the leaves as in Sphenophyllum and secondary xylem were regarded to be relatively unproblematic (Kenrick and Crane, 1997: 242). According to the phylogenetic analysis on these characters, Stein et al. questioned whether the Sphenophyllales are as closely related to the Equisetales as commonly believed. We think that other anatomical differences also cast doubt on the close affinity between these two orders. The former order is characterized by ribbed protosteles with exarch primary xylem, while the latter by siphonosteles with endarch or mesarch primary xylem. In the sphenophyllalean primary xylem, a limited number of protoxylem strands are located at the tips of the ribs and the lacunae are problematic, which differ from the equisetalean multiple protoxylem marked by distinct carinal canals. In some previous works, however, the discussion of the origin of the sphenopsids concerns the Sphenophyllales and Equisetales as a whole. Skog & Banks (1973) regarded the sphenopsids as derived from the primitive trimerophytes through the Ibykales (e.g. Ibyka and Arachnoxylon) and then Hyeniales (e.g. Calamophyton), which somewhat corresponds with one of the hypotheses of Stein et al. (1984). To date, the members of these two orders have been placed in the Iridopteridales and Pseudosporochnales of the Cladoxylopsida, respectively (e.g. Berry & Stein, 2000). Two species of Hyenia (i.e. H. banksii and H. vogtii) were separated by Berry & Stein (2000) from the Hyeniales and assigned to Iridopteridales and other species with the exception of H. sphenophylloides were referred to Calamophyton (Fairon-Demaret & Berry, 2000).
The Sphenophyllales mainly consist of the vegetative organ genus Sphenophyllum, several fertile organ genera (e.g. Bowmanites, Sphenostrobus, Peltastrobus) and Hamatophyton. Other possible members include Eviostachya, Cheirostrobus and Lilpopia. Fertile organ genera might represent the strobili of Sphenophyllum indicated by leafy axes with uniform vegetative internal structure preserved. Although the Sphenophyllales have variously shaped constructed fertile parts, they possess the whorled appendages typical of the Sphenopsida. In the list of Stein et al. (1984), another distinctive character of the sphenophyllaleans is three- to four-ribbed primary xylem. Secondary xylem appears unnecessary in defining this order, and is absent in axes of Hamatophyton, but this taxon was still assigned to the Sphenopsida (Li et al., 1995), which represents a primitive member of this group and leads to a new family Hamatophytaceae within the Sphenophyllales. Rotafolia belongs to the Sphenopsida in verticils of vegetative leaves and fertile units at distinct nodes. It fits well with the sphenophyllalean plants in three-ribbed primary xylems of axes. Exarch primary xylem was not identified by Stein et al. (1984) as a character of the Sphenophyllales. In fact, this order of xylem maturation characterizes the actinosteles (protosteles with ribbed primary xylems) of Sphenophyllum (Cichan, 1985), Eviostachya (Leclercq, 1957; Wang, 1993), Hamatophyton (Li et al., 1995), Rotafolia (this paper) and sphenophyllalean strobilar axes (Levittan & Barghoorn, 1948; Stewart & Rothwell, 1993: 194). However, the fertile unit of the strobilus of Rotafolia has an elongate-cuneate bract with a distal segment and lateral elongate segments, as well as numerous elongate abaxial sporangia pendulous at the base of the bract, which are distinct characteristics from other members of the Sphenophyllales. Thus, considering classification at the family level, Rotafolia is now left as incertae sedis.