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(American Journal of Botany. 2002;89:1468-1477.)
© 2002 Botanical Society of America, Inc.

Paleobotany

An anatomically preserved arborescent lycopsid, Sublepidodendron songziense (Sublepidodendraceae), from the Late Devonian of Hubei, China1

Qi Wang2,4, Shou-Gang Hao5, De-Ming Wang2 and David L. Dilcher3,5

2Department of Geology, School of Earth and Space Sciences, Peking University, Beijing 100871, P. R. China; 3Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611-7800 USA

Received for publication February 1, 2002. Accepted for publication April 25, 2002.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Sublepidodendron is a common megafossil plant in the Late Devonian of China, but historically the generic delimitation based on leaf bases masked its true systematic position. A reinvestigation of S. songziense from the Late Devonian Hsiehchingssu Formation, Hubei, China, provides new insights into its internal anatomy and reproductive morphology. This arborescent lycopsid is characterized by small, vertically elongated leaf bases arranged in spirals, presence of false leaf scars, possibly bearing separate cones, and association with a stigmarian rhizomorph. The potential for organic connections of these detached organ genera has been noted for other Sublepidodendron species. The anatomy of S. songziense axes from two levels reveals that the thinner axis may bear an ectophloic siphonostele with a filamentous pith and an outer cortex. The thicker axis has a siphonostele with a branch gap, two-zoned pith with secondary thickenings, multiseriate rays across secondary xylem, a thick periderm, and primary and secondary tracheid walls characterized by "Williamson's striations." Similarities to synapomorphies of Diaphorodendraceae and Lepidodendraceae suggest that S. songziense bears a closer affinity to Lepidodendrales rather than Protolepidodendrales, as formerly thought. Widespread occurrence of Sublepidodendron implies that phylogenetically advanced arborescent lycopsids must have diverged by the Late Devonian.

Key Words: arborescent lycopsid • China • Late Devonian • Lepidodendrales • Sublepidodendron songziense


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Arborescent lycopsids are one of the most conspicuous groups in Late Devonian floras. In recent years, it has become apparent that this group is in need of further study. Two widely distributed genera, Lepidodendropsis Lutz (1933) and Sublepidodendron (Nathorst) Hirmer (1927) , had a very early debut in the Devonian of China as compared with coeval floras in Europe, Africa, and America (Sze, 1936a , 1943 , 1956 ; Banks, 1960 ; Danzé-Corsin, 1960a , b ; Li and Cai, 1978 ; Chaloner and Sheerin, 1979 ; Cai and Wang, 1995 ; Li, 1997 ). Consequently, the Famennian floral assemblage of China was named after Sublepidodendron mirabile-Lepidodendropsis hirmeri-Hamatophyton verticillatum (Cai and Li, 1995 ), which did not conform with Banks' Archaeopteris (zone VI)/Rhacophyton (zone VII) scheme (1980). However, relatively little is known about the genus Sublepidodendron.

Here we present an anatomical study of previously described stem impressions of Sublepidodendron songziense based on the permineralized axes from the Hsiehchingssu Formation (sensu lato) of Songzi City, southwestern Hubei Province, China. In this formation, the upper nonmarine Clasolite Member is intercalated with black, fine, arenaceous mudstone containing abundant lycopsids (Feng, 1984 ; Chen and Jin, 1996 ; Cai, 2000 ). The Clasolite Member belongs to the Famennian because it is correlated with brachiopods (Cyrtospirifer and Tenticospirifer), bivalves (Buchiola), and conodonts (Polygnathus brevis and Icroides alternatus), which also occur in the upper part of this member (Zhang, Liao, and Feng, 2001 ). Since Sze's (1952) pioneering work on the Devonian plants from this area, abundant fossil plants have been recorded (in Feng et al., 1977 ; Chen, 1984 ; Feng, 1984 ), but most of them, especially arborescent lycopsids, were mainly based on compression-impression material. In our collection, Sublepidodendron songziense is preserved as compressions and petrifications, including large and small axes in various states of preservation and some distal shoots showing organic connection with the cones on the tips (Fig. 1). Usually, such cones are loosely assigned to a commonly dispersed organ species, Lepidostrobus grabaui (Sze, 1936b ), from the Upper Devonian of South China (Feng, 1984 ; Cai and Wang, 1995 ; Chen, 1999 ; Jin and Wu, 2001 ). Two kinds of cones (Figs. 1, 3, 4) can be identified from isolated cones and may have been detached from the same parent plant. Sublepidodendron songziense was distinguished from other species within Sublepidodendron by Chen (in Feng et al., 1977 ) on the basis of its very small, fusiform or oval leaf bases with obtuse corners (Fig. 2). However, Chen's syntype specimens are too fragmentary to cover all variations of the leaf bases so that megafossils of this species have been variously treated (Feng, 1984 ; Li and Lan, 1984 ; Cai and Li, 1995 ). An overall emendation to S. songziense will not be undertaken here, but rather new data will be presented for its anatomy.



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  		Figs. 1–5. The external morphology of Sublepidodendron songziense. 1. A distal, leafy shoot shows at least three orders of subequal or unequal dichotomous branching and its organic connection to a Lepidostrobus-like cone. Note that there are two other detached cones on the same bedding plane, and the lower cone and the upper two ones are quite different in shape, which may be identified as two separate cones types. Arrowhead refers to the enlarged part in Fig. 2 . Specimen: WQ-001A. Bar = 2 cm. 2. The shoot shows that the small leaf bases are arranged in spirals. Arrowhead refers to the false leaf scars as defined by some authors (al. "Bogenlinie," Nathorst, 1920 ; Hirmer, 1927 ; "arc foliaire," Danzé-Corsin, 1958a , b ; "fausse cicatrice foliaire," Chaloner and Boureau, 1967 ; Corsin et al., 1973 ; "false leaf scar," Meyen, 1976 ; Thomas and Meyen, 1984 ). Some leaf bases show the median keels. These are the major diagnostic characters of the genus Sublepidodendron. Bar = 2 mm. 3–4. Two types of cones are often preserved isolated on the same bedding plane. They are thought to correspond to those cones in Fig. 1 . Specimens: WQ-018A and WQ-014A. Bars = 2 cm. 5. A fragmentary part of the stigmarian rhizomorph shows some elliptical root scars and wrinkled surface. Specimen: WQ-012D. Bar = 2 cm

 

   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
All the specimens were collected from one of the syntype localities, Huangkuang at Liujiachang, about 30 km southwest of Songzi City, where the Devonian strata is subdivided into three formations. These are, in descending order: the Hsiehchingssu Formation (Famennian), the Huangchiateng Formation (Frasnian), and the Yuntaikwan Formation (Givetian) (Sze, 1952 ; Feng, 1984 ). The plant material was collected from a black, fine, arenaceous mudstone horizon ranging from 20 to 52 cm thick within the lower part of the Clasolite Member of the Hsiehchingssu Formation.

Anatomical information is gleaned from two invaluable permineralized axes. One petrified axis (axis A) is <10 mm long, which was laying parallel in the same bedding plane as an incomplete cone (Lepidostrobus grabaui). These two organs may be from the same source plant, Sublepidodendron songziense, which is the only megafossil identified from these sediments. The other one (axis B) is about 30 mm long with partially exposed leaf bases on the surface of an impression, which can be identified as S. songziense in one part, and then was gradually fusainized and permineralized towards another end. The contour of axis B is not intact. They were prepared by standard procedures of embedding, sectioning, grinding, and polishing for examination with the light microscope and reflected light (Stein, Wight, and Beck, 1982 ). In addition, the freshly cleaved surfaces of the petrified and fusainized axes were prepared for examination with scanning electron microscopy (SEM; AMARY 1910FE, Electron Microscopy Laboratory, Peking University). All specimens and slides are housed in the Palaeontological and Stratigraphical Section, Department of Geology, Peking University, China.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Description
Transverse sections of axis A show a circular primary xylem strand with a nearly smooth or undulate margin (Fig. 6) because the peripheral protoxylem does not form conspicuous tooth-like ridges or coronae, but is distributed continuously. The tracheids are scalariform with minute anastomoses or vertical fimbrils connecting the horizontal bars (Figs. 10, 11). The leaf traces are indiscernible, and the mode of their emission is superficial (Bateman, DiMichele, and Willard, 1992 ). The primary xylem is exarch (Figs. 6, 7). The stelar center either lacks cellular preservation or bears an irregularly filamentous core (Figs. 6–9). Immediately adjacent to the primary xylem is a poorly preserved narrow band, 2–4 cells thick, of extraxylary tissue, which may be primary phloem. It is homogeneous, parenchymatous tissue, and the cell lumina are largely circular or elliptical in transverse view, elongated in longitudinal view with horizontal end walls, and possible sieve cells ill-defined (Figs. 10, 11). Therefore, the primary vascular system may be an ectophloic siphonostele (Beck, Schmid, and Rothwell, 1982 ). In addition, there are patches of cells that are separated from the stelar tissues by a wide circumstelar lacuna, which probably belong to the outer cortex. The patches of cells are thick-walled, yellowish-colored, possibly with resinous contents, rounded in transverse section and elongate with horizontal or bluntly tapered end walls in longitudinal section (Figs. 12, 13). The outer cortex has a discontinuous layer of cells with periclinal divisions, probably the result of early stages of secondary cortical initiation (Fig. 12). Under the SEM, the transversely broken surface of partially decorticated axis A shows a stele, about 1 mm in diameter, with some irregular cavities in the center, which may represent a pith (Fig. 23).



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  		Figs. 6–11. The internal anatomy of Sublepidodendron songziense as seen through the light microscope. 6–7. Two serial transverse sections of axis A. The pith is destroyed or appears to be filamentous. Protoxylem exarch, distributed in a continuous layer at the margin of the tracheary cylinder without conspicuous ridges or coronae. There are some circumstelar lacunae and residual outer tissues. Slides: Pku-wq-Tr2 and Pku-wq-Tr1. Bars = 500 µm. 8. Enlarged stelar periphery of axis A. The layer appears to be made up of thin-walled cells, which may be explained as the primary phloem (left arrowhead), and are distinctively larger than the inner protoxylem elements (right arrowhead). Slide: Pku-wq-Tr2. Bar = 100 µm. 9. Enlargement of the stelar pith of axis A. It is filamentous or lacunose with some secondary thickenings. Slide: Pku-wq-Tr1. Bar = 100 µm. 10. A longitudinal section of axis A shows scalariform thickened tracheids with tapering end walls (right arrowhead) and possible phloem elements with horizontal end walls (left arrowhead). Slide: Pku-wq-L3. Bar = 100 µm. 11. Enlarged stele of Fig. 10 in longitudinal view showing tracheids with scalariform thickenings that have minute anastomoses or vertical fimbrils connecting the horizontal bars (al. "Williamson's striations"). Slide: Pku-wq-L3. Bar = 100 µm

 


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  		Figs. 12–18. The internal anatomy of Sublepidodendron songziense as seen through the light microscope. Fig. 12–13 . Outermost portion of axis A showing patchy thick-walled cells, which are rounded in transverse section and elongate with horizontal or bluntly tapered end walls in longitudinal section. They are interpreted as the outer cortex. Arrowheads refer to a discontinuous layer of cells that appear to initiate an early stage of secondary cortex development. Slides: Pku-wq-Tr2 and Pku-wq-L3. Bars = 100 µm. 14–15. Secondary xylem divided into sections by radial rays in axis B. There are some discontinuous layers at the secondary xylem periphery, which is composed of distinctively smaller tracheids than most of the inner secondary xylem. Slide: Pku-wq-Tr4. Bars = 500 µm. 16. A tangential section of the secondary xylem of axis B. Arrowheads refer to two elliptical leaf traces in a low spiral. Slide: Pku-wq-Ta8. Bar = 100 µm. 17–18. A transverse section of axis B. It shows the partly preserved pith and extrastelar tissues. The lower part is enlarged in Fig. 18 , which shows the outer zone of pith, narrow primary xylem, and wide secondary tissues. Slide: Pku-wq-Tr11. Bars = 500 µm

 


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  		Figs. 23–30. The internal anatomy of Sublepidodendron songziense, examined with scanning electron microscopy (SEM). 23. A transversely broken stele of axis A before embedding. It shows a possible central pith and column of exarch primary xylem. Bar = 100 µm. 24. The freshly cleaved transverse surface of the secondary xylem of axis B before embedding. It shows radially aligned tracheid elements divided by the rays. Bar = 100 µm. 25–26. Enlargements of the ray in transverse view. It indicates that the multiseriate rays are 4–6 cells wide as they transverse the secondary xylem. Bars = 100 µm (left), 10 µm (right). 27–28. Enlargements of the ray in radial view. It shows the multiseriate rays are 4–5 cells high with what appear to be scalariform thickenings, which imply that the ray cells may have the same origin as the xylary tracheids. Bars = 10 µm. 29. Partial enlargement of the secondary xylary tracheids. It shows there is a compound middle lamella between the neighboring tracheid walls and the scalariform thickened tracheids. Bar = 10 µm. 30. Enlargement of the tracheidal horizontal bars. There are anastomoses and vertical threads connecting the bars, which are generally termed "Williamson's striations." Bar = 10 µm

 
The original specimen of axis B was serially sectioned and is estimated to be about 5–8 mm thick, but its surface is so irregular that the anatomical characters observed are often discontinuous. Most sections possess distinctive secondary xylem quite different from the sections of axis A (Figs. 14–22). Remarkably, the division of the stele occurs by the separation into unequal columns of stelar tissues. It shows that there is a lateral branch of the axis with a smaller stele partly surrounded by secondary xylem or periderm that has the same pith as the larger stele (Fig. 19). Under reflected light, the piths have well-defined heterogeneous central tissue, which consists of irregularly oriented cells and can be subdivided into two zones. The outer zone, 4–12 or more cells wide and in contact with the metaxylem, is composed of a mass of larger parenchymatous cells than those of the inner zone, and their cell walls appear to have secondary thickenings. The inner zone, elliptical in cross section, is often crushed, degraded, or partially separated from the outer zone (Fig. 19). Under transmitted light, parenchyma cell walls of the inner pith apparently are secondarily thickened (Fig. 21). Immediately external to the pith there is a narrow, undulate band of the primary xylem whose peripheral protoxylem is inconspicuously similar to that of axis A (Figs. 20, 22). The primary xylem only accounts for a small area of the total stele, and the tracheal lumina are generally smaller than those of the parenchymatous pith cells (Figs. 19, 20, 22). The primary vascular system can be characterized as a siphonostele with a branch gap (see Beck, Schmid, and Rothwell, 1982 ). External to the primary xylem, there is a wide zone of the secondary xylem, which is divided into wedges by distinctive radially parting along the ray cells (Figs. 19, 20). Freshly cleaved surfaces of the partly fusainized axis B show under SEM hive-like secondary xylem crossed by radial vascular rays. The tracheids have scalariform thickenings with vertical threads connecting the horizontal bars, generally termed "Williamson's striations," as described by many authors (Evers, 1951 ; Fry, 1954 ; Smith, 1962 ; Alvin, 1965 ; Taylor, 1981 ; Geng, 1990 ) (Figs. 24–30). The vascular rays are multiseriate, 4–6 cells wide, and 4–5 cells high, with scalariform thickening (Figs. 25–28). A transverse section shows some discontinuous layers of small, secondary tracheids peripheral to the inner secondary xylem (Figs. 14, 15). Such layers at the edge of the secondary xylem are not thought to be natural, and they appear to be part of the process of degradation and crushing (W. A. DiMichele, Smithsonian Institution, personal communication, 2001). They do not show features sufficient to confidently identify them as a vascular cambium, and they may instead represent sites where the initial tracheids ceased developing at the end of secondary growth. The leaf traces are elliptical in shape and composed of isodiametric cells. They are arranged in a low spiral (Fig. 16). The tracheids are elongated with tapered end walls. External to the secondary xylem, the cortical zonation is either lacking or difficult to recognize due to the extreme narrowness and compression of the inner zones and the obliqueness of the sections. The primary cortex is rarely preserved, in association with patches of leaf traces, or possibly replaced by the periderm (Figs. 19, 20, 22). The periderm is often split radially into large segments, which appear to lack distinct differentiation into phellem and phelloderm. It consists of radially aligned areas of tangentially elongated cells, which appear to contain a kind of yellowish resinous substance (Fig. 22). The leaf bases are absent.



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  		Figs. 19–22. The internal anatomy of Sublepidodendron songziense as seen through the light microscope. 19. Transverse section of axis B at a different level. Arrowheads indicate the divided pith. The pith can be distinctively defined as two zones with secondary thickenings. Due to the obliqueness and compression of the section, the secondary xylem is seen mainly on the left. External to the pith, there is a narrow sheath of the primary xylem with a slightly undulating margin. The periderm splits radially into large segments on the right. Slide: Pku-wq-Tr10. Bar = 500 µm. 20. Partial enlargement of the upper left of Fig. 19 . Arrowheads show two leaf traces at the periphery of the secondary xylem. Slide: Pku-wq-Tr10. Bar = 500 µm. 21. Enlargement of the inner pith of axis B under transmitted light. It shows irregularly oriented cells with secondary thickenings on all walls. Slide: Pku-wq-Tr10. Bar = 500 µm. 22. Enlargement of the upper part of axis B. From the top to the bottom it shows periderm, secondary xylem with rays, narrow primary xylem, and a two-zoned pith. Note that the protoxylem does not form conspicuous coronae, and the cells of the outer zone of the pith are distinctively larger in size than the associated metaxylem elements. Slide: Pku-wq-Tr10. Bar = 500 µm

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Overall, axis A may be part of a distal branch attached to a cone or part of a deciduous lateral branching system while axis B is possibly originated from the terminal part of a trunk with a lateral branch or from a larger deciduous lateral branch. As far as the primary vascular systems of the two axes are concerned, their anatomical characters are extremely similar. Thus, these two axes may respectively represent different developmental stages and different levels in the branching system of the source plant, Sublepidodendron songziense Chen.

Sublepidodendron (Nathorst) Hirmer is traditionally classified in the Protolepidophytales or Protolepidodendrales (Kräusel and Weyland, 1949 ; Banks, 1960 ; Chaloner and Boureau, 1967 ; Gu and Zhi, 1974 ; Lejal-Nicol, 1975 ; Thomas and Brack-Hanes, 1984 ). The reproductive structure of lycopsids from this order is relatively simple because their sporangia are not commonly aggregated into cones, but are interspersed among the microphylls (Gu and Zhi, 1974 ; Taylor, 1981 ). However, Sublepidodendron songziense bears cones and two other common species, S. mirabile and S. wusihense, were also recorded in possible (or organic) connection with the cones (Gothan, 1933 ; Sze, 1943 ; Cai, 2000 ; Yi Wang, Nanjing Institute of Geology and Palaeontology, personal communication, 2001). Meanwhile, Sublepidodendron stems are probably attached to Stigmaria-type root systems (Gothan, 1933 ; Gu and Zhi, 1974 ; Feng et al., 1977 ), as exemplified by S. songziense (Fig. 5). Evidence from the anatomy also prompts us to cast further doubt on the systematics of Sublepidodendron (Nathorst) Hirmer.

Sublepidodendron songziense is anatomically different from those well-known herbaceous lycopsids, such as Leclercqia complexa (Banks, Bonamo, and Grierson, 1972 ; Grierson, 1976 ), because it has a distinctive medullated stele with inconspicuous exarch points or coronae of protoxylem and a well-developed cylinder of secondary xylem. In comparison with those early arborescent forms like Atasudendron mirum (Senkevitsch, Jurina, and Arkhangelskaya, 1993 ) (syn. Lepidodendropsis kazachstanica; Iurina and Lemoigne, 1975 ) and Longostachys latisporophyllus (Cai and Chen, 1996 ), S. songziense possesses more advanced secondary vasculature showing the presence of "Williamson's striations," and a distinct two-zoned pith. Remarkably, parallel anatomical characters, to some extent, have been found in the Sublepidodendron wusihense trunk, which has a siphonostele and secondary xylem (Yi Wang, Nanjing Institute of Geology and Palaeontology, personal communication, 2001), and Leptophloeum rhombicum, which has secondary xylem with "Williamson's striations" (Cai and Qin, 1986 ; Geng, 1990 ). These two common Late Devonian arborescent lycopsids from China, and better anatomically preserved axes from slightly younger strata in the New Albany Shale (Cichan and Beck, 1987 ; Roy and Matten, 1989 ) and the Montagne Noire (Meyer-Berthaud, 1984 ), also show an evolutionary tendency towards features typically occurring in the Lepidodendrales. These arborescent lycopsids reached a level of anatomical complexity comparable to the Lepidodendrales. Grierson and Banks (1963) suggested that the large complex of Upper Devonian-Lower Carboniferous arborescent lycopsids need reexamination, and Meyer-Berthaud (1984) and Matten (1989) further support this point. Reproductively, S. songziense may bear separate cones, although available evidence is weak due to the lack of knowledge of their in situ spores. Anatomically, S. songziense axes differ from those of Leptophloeum rhombicum, Paralycopodites, and Anabathra, from the New Albany Shale and the Montagne Noire, in possessing filamentous or parenchymatous pith with secondary thickenings, multiseriate rays in the secondary xylem, and possibly sporangia confined to well-defined, separate cones. Further on, the protoxylem of S. songziense axes is distributed continuously like those of Levicaulis (Beck, 1958 ), Lepidodendron (Eggert, 1961 ; Smith, 1962 ), Lepidophloios (Andrews and Murdy, 1958 ; Eggert, 1961 ), Paralycopodites (DiMichele, 1980 ), and Diaphorodendron (DiMichele, 1985 ; DiMichele and Bateman, 1992 ). The stelar center bears an irregularly oriented or filamentous pith similar to that of Lepidophloios and Lepidodendron (DiMichele, 1979 , 1983 ). The parenchymatous pith with secondary thickenings is also described in Diaphorodendron and Synchysidendron (DiMichele, 1985 ; DiMichele and Bateman, 1992 ). Unequal division of the stele of axis B is like that of Paurodendron (Fry, 1954 ), Levicaulis (Beck, 1958 ), Phytokneme (Andrews, Read, and Mamay, 1971 ), Paralycopodites (DiMichele, 1980 ), Anabathra (Pearson, 1986 ), Diaphorodendron, and Synchysidendron (DiMichele, 1985 ; DiMichele and Bateman, 1992 ), showing a lateral branch of the axis with a smaller stele. Remarkably, Ulodendron scars have been recorded in the well-known type species Sublepidodendron mirabile (Nathorst) Hirmer (Hirmer, 1927 ; syn. Lepidodendron mirabile Nathorst, Nathorst, 1920 ; Gothan and Sze, 1933 ), which imply that the species also bear a lateral branching system. In addition, the extraxylary tissue of S. songziense immediately adjacent to the primary xylem is similar to those of Levicaulis (Beck, 1958 ), Oxroadia (Alvin, 1965 ; Long, 1971 ), Phytokneme (Andrews, Read, and Mamay, 1971 ), Lepidodendron (Eggert and Kanemoto, 1977 ; DiMichele, 1983 ), Paralycopodites (DiMichele, 1980 ), and Wexfordia (Matten, 1989 ). Similar multiseriate vascular rays are also found in Stigmaria, Paralycopodites, and Lepidodendron (Cichan, 1985 ). These characters are consistent with synapomorphies of the advanced lepidodendrids among phylogenetic lineages such as the Diaphorodendraceae and Lepidodendraceae (sensu DiMichele and Bateman, 1992 ; Felix, 1952 ; Andrews and Murdy, 1958 ; Eggert, 1961 ; DiMichele, 1979 , 1981 , 1983 , 1985 ; Bateman, DiMichele, and Willard, 1992 ; DiMichele and Bateman, 1996 ; Kenrick and Crane, 1997 ). However, S. songziense differs from these genera because its leaf bases are simpler in only having false leaf scars, the parenchymatous cells of the outer zone of the pith are larger than those of the associated metaxylem, and the periderm is almost homogeneous. As previous authors have suggested, Sublepidodendron seems to be a closer ally of the Carboniferous genera than the earlier Devonian herbaceous forms (Grierson and Banks, 1963 ; Thomas, 1978 ). We suggest that it is time to separate the genus Sublepidodendron from the Protolepidodendrales and place it in the Lepidodendrales based on the features presented here, of its stem anatomy, and its cone morphology, although little is known about the anatomy of the type species S. mirabile, except for commonly encountered stem impressions suggesting that it was arborescent. Obviously, S. songziense has a close affinity to the Lepidodendrales rather than the Protolepidodendrales, and the phylogenetically more advanced arborescent lycopsids must have evolved by the Late Devonian. This conclusion is supported by the phylogenetic analysis of DiMichele and Bateman (1992) . Therefore, more attention should be paid to Devonian taxa in order to elucidate the early diversification and phylogeny of arborescent lycopsids.


   FOOTNOTES
 
1 This paper represents a portion of Qi Wang's dissertation submitted to Peking University in partial fulfillment of the requirements of a doctoral degree.

The authors thank Terry A. Lott, Florida Museum of Natural History, for his help in the preparation of this paper; William DiMichele for his helpful suggestions; Yi Wang for recently communicating unpublished findings; and Chun-Yuan Zhou for helping with photography. This study was supported by the National Natural Science Foundation of China (49972009) and Major Basic Research Projects of the Ministry of Science and Technology, China (G2000077700). Back

4 happyking2644{at}sina.com Back

5 Authors for reprint requests (dilcher{at}flmnh.ufl.edu ;sghao{at}geo.pku.edu.cn ) Back


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Alvin K. L. 1965 A new fertile lycopod from the Lower Carboniferous of Scotland. Palaeontology 8: 281-293

Andrews H. N. W. H. Murdy 1958 Lepidophloios and ontogeny in arborescent lycopods. American Journal of Botany 45: 552-560

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Q. Wang, B.-Y. Geng, and D. L. Dilcher
New perspective on the architecture of the Late Devonian arborescent lycopsid Leptophloeum rhombicum (Leptophloeaceae)
Am. J. Botany, January 1, 2005; 92(1): 83 - 91.
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