A new zosterophyll from the Lower Denovian (Siegenian) of Yunnan, China

Review of Palaeobotany and Palynology, 57 (1989): 155-171

155

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

A NEW ZOSTEROPHYLL FROM THE LOWER DEVONIAN (SIEGENIAN) OF YUNNAN, CHINA HAO SHOU-GANG

Geology Department of Peking University, Beijing (China) (Received July 7, 1986; revised and accepted May 2, 1988)

Abstract Hao Shou-Gang, 1989. A new zosterophyll from the Lower Devonian (Siegenian) of Yunnan, China. Rev. Palaeobot. Palynol., 57:155 171. A new genus and species, Discalis longistipa, is described from the Posongchong Formation (Siegenian), Wenshan district of Yunnan, China. The plant has creeping axes with numerous K- or H-shaped branches. Aerial branches are either trailing, or upwardly growing, dividing sterile axes with circinately coiled tips and fertile axes with laterally born, disc-like round sporangia. The rather large sporangia, about 3.7 mm in diameter with rather long stalks (up to 5 mm), are sparsely or somewhat compactly scattered along the fertile axes, sometimes spirally, forming loose spikes. It is noteworthy that variable spines with expanded oblate apices, which possibly relate to glands, occur irregularly on the plant, along the stalks, and on the sporangium walls. Discalis longistipa is superficially similar to the genus Sawdonia, but it differs from the latter in the arrangement of its sporangia, the long sporangial stalks and the mode of branching. The discovery of this new plant contributes to our knowledge of early vascular plants. It is of interest to recognize the diversity of them and to speculate about the origins of the microphyll and sporophyll.

Introduction

Protopteridiurn minutum. The validity of this

In China, Y u n n a n p r o v i n c e has some extensively c o n t i n e n t a l fossiliferous exposures of L o w e r a n d Middle D e v o n i a n age. T h e r e are m a n y i n t e r e s t i n g a n d r e m a r k a b l y well-preserved p l a n t fossils in these c o n t i n e n t a l or p a r a l i c deposits. The first detailed d e s c r i p t i o n of D e v o n i a n p l a n t fossils from this a r e a was made by T.G. Halle in 1927. S u b s e q u e n t l y he described Drepanophycus spinaeformis, Protopteridium minutum a n d one l y c o p o d species Protolepidodendron scharyanum from the L o w e r a n d Middle D e v o n i a n beds, the L o n g h u a s h a n , Zhanyi, Q u j i n g City (1936). Also Sze HsinChien (1941) a n d Hsfi J e n (Xu Ren, 1946) described some p l a n t f r a g m e n t s of D e v o n i a n beds in Y u n n a n . It looks as if some fossils will need to be f u r t h e r studied, for example,

g e n u s is still c o n t r o v e r s i a l in p a l a e o b o t a n i c a l circles. In t h e past two decades the s t u d y of Devon i a n p l a n t s in Y u n n a n h a s established some new fossil g e n e r a or species a n d discussed and clarified others. (Hsfi, 1966; Li a n d Cai, 1977, 1978, 1979; Li, 1982; Geng, 1983, 1985; Cai and Schweitzer, H.J., 1983; Hao, 1985). A s t r o n g l y i n c r e a s e d i n t e r e s t in this field is obvious. M y new p l a n t was f o u n d in Z h i c h a n g village, Gumu, W e n s h a n district situated in the southeast p a r t of Y u n n a n . The first w o r k here on D e v o n i a n p l a n t s was made by Li Xing-Xue a n d Cai C h o n g - Y a n g , w h o published t h e i r r e p o r t in 1977, a n d i l l u s t r a t e d a few species of ZosterophyUum. R e c e n t l y G e n g Bao-Yin (1983, 1985) added to the c o l l e c t i o n s two i n t e r e s t i n g

0034-6667/89/$03.50

© 1989 Elsevier Science Publishers B.V.

156 genera, Stachyophyton and Huia. The former possesses planated lateral branches and strobili born at the tips of fertile branches which bear the helically arranged sporophylls. When the sporophylls fall, they leave scars on the axes of the strobili. The approximately oblong sporangia are attached on the adaxial surface of the sporophyll. The latter plant, Huia recurvata, seems to be the largest of the Zhichang flora with main axes up to 14 mm in diameter. It possesses pseudomonopodial and dichotomous axes with terminal spikes in which the sporangia were born terminally and laterally. Sometimes one occurs below the branching point. It should be noted that the long-ovate sporangia recurve adaxially downwards and the plant has centrarch protostele. The present account reports another new plant, first found in the summer of 1983 as a few fragments consisting of a broken fertile axis with some sporangia. In August 1984 and 1985, I went to the same locality again and obtained rather rich and complete collections. This material gives us much more information. In the same horizon there are fragments of different plants which will be studied one after another. Undoubtedly, more plant fossils will be discovered in this area and added to our knowledge of early land vascular plants.

Locality and stratigraphy The plant material described in this paper was obtained from a horizon only 20 cm thick. Its outcrop appears along the southeast slope of a mountain approximately 150m west of Zhichang village, Gumu, Wenshan district, Yunnan province (Fig.l). The collections occur in the Posonchong Formation. This formation was established by Liao et al. (1978) who considered the unit to be Early Devonian in age, corresponding to the Siegenian. In 1983, three students of Peking University, Leng Guang-Dong, Liu Zhen-Feng and Zhou Xiao-Yuan and I made essential research on the age of the fossiliferous strata

of Posongchong Formation and Pojiao Formation. According to our data Liao's opinion can be accepted in principle. This unit is also called Cuifengshan Formation. The Posongchong Formation (Fig.2) attains a total thickness of about 158 m and consists mainly of continental origin deposits characterized by white-grey, yellow-grey siltstones or sandstones and dark mudstones or shales, which yield abundant plant fossils and fish remains. It unconformably overlies the quartzose sandstones of the Meitan Formation in which the brachiopod Sinorthis sp. indicates a Lower Ordovician age. Its overlying unit, the Pojiao Formation, consists of neritic or paralic mudstone and siltstone, which contain corals and brachiopod fossils. In the area adjacent to our section, i.e. in Guangnan and Funing district, the Pojiao Formation yields Polygnathus dehiscens (Bai et al., 1982), a zonal conodont species of the Early Emsian (Klapper and Ziegler, 1979). Based on marine megafossil data, here the Pojiao Formation can be correlated well to the lower Yujiang Formation of Guangxi province. The lower Yujiang Formation yields also Polygnathus dehiscens, and its underlying strata, i.e. the Nagaoling Formation yields Eognathodus sulcatus, a late Siegenian conodont species (Klapper and Ziegler, 1979). Thus the Pojiao Formation may be correlated to the lower Emsian, while its underlying Posongchong Formation may be assumed to be late Siegenian in age. Figure 2 lists these two formations, their lithology, the main fossils and the position of my collections. All fossils collected will be deposited in the Geology Department, Peking University.

Material and m e t h o d s The fossil plants are preserved as compressions or impressions in grey-dark arenaceous mudstone which is rather hard. The fossils are black or red-brown, often with white outer edges that increase their contrast greatly. The

157

N~.....~WENSI IAN

WUTUCItONG o

11

"2 I

4 km I

Area enlarged above

/ J

? 105

I10

----imimmmmm~

115

Fig.1. Map showing the Wenshan area. Filled line indicates section locality.

colour was possibly caused by oxidization during the diagenetic stage. Most of the plant compressions are converted into charcoal. When subjected to chemical preparation they fall apart into thin pieces that do not reveal details of the epidermis and spines. Very occasionally the epidermis may be preserved. Generally, impressions of plants in a matrix are of limited value for study, but because my material has a sufficiently compact texture, it forms a consolidated mould on which epidermal features can be shown. Therefore, this material is dug from the surface of the rock. A scanning electron microscope

(SEM) was used. To reveal the characters of the spines, petrological thin sections and cellulose acetate peels have been made. Because permineralized material was not obtained, I do not know the form of the stele, but bulk maceration with hydrofluoric acid revealed some fragments of tracheids. Some sporangia were uncovered by steel needles and removed in a small plastic container and then treated with hydrofluoric acid and hydrochloric acid for 24-48 h. After that, these fragments were washed and mounted on slides with mounting medium. Spores were obtained by this procedure.

158

S~ )nbols

q~ d

dark green

Alicroo'c[u.~ bm~wai M. inh'r.dar~ (Yin)

gr g

(~

R~Jstrosptrlt~

d%ech)

[,mklnen~i~

(Mansuy)

Dk'oelostrophb, (IH~I(I~II[I(([ (Mansuyl

Fr

gre~

\~

white

!,

yellow

'y

@

V

id

brachiopod

Dic~w/o,~trophaz fltowtata Wang E/vmvspirtfi,r lewa~,~ " ~ts IHou) Z.~h'rophylllcm

cf

az~Dw[icz~m~¢ Lang

el

Y d

V

grl

Cookson

~'~

corals

~

fish

~7

plant

fossil

fossil f(~ssil fossil

shale

DiscaOs longlstlpa gen. find sp. nov. HMa recurvata Geng

~-_~__

muds, one sandstone

g w

grq

61t ~

t

Y1o~nanolcp/.~

sp. arenaceous

~y

Zo.~tt rophylho~ subvcrlictlkllum Li et .b'hwkyophylon vunnammse Geng

Cai

sh.

;tF('T]~tCO()US coils tnlconforrllit y

(I tla

Fig.2. Stratigraphical column of Posongchong Formation and Pojiao Formation in Zhichang. Systematics Class ZOSTEROPHYLLOPSIDA Order ZOSTEROPHYLLALES Family ZOSTEROPHYLLACEAE

into two equal valves, scattered, sometimes spirally along the axis, sparsely or compactly, forming loose spikes. The plant is homosporous.

DiscMis H a o g e n u s n o v u m

Type species: Discalis longistipa Hao

Derivation: from the Greek "disc", and the

Discalis longistipa Hao sp. nova (Plate I, 1-4;

suffix "-alis", meaning disc-like round sporangium. Creeping axes have K- or H-shaped branches repeatedly, and are either trailing or upwardly growing; the latter are divided into sterile axes with circinate tips and fertile axes with laterally borne sporangia. Multicellular spines of varying shape and size projecting from the entire surface, are tapered, usually at an acute angle, or bend distally. Sporangia, attached on long stalks, are disc-like and round in abaxial view, dehisce completely along convex margin

Plate II, 1-6; Plate III, 1--11; Plate IV, 1 16)

Derivation: longistipa (Latin), meaning long sporangial stalk. Diagnosis: This plant has horizontal, creeping "main axes", up to 5.0 mm in diameter; sterile axes, 3.2-4.0 mm wide, at least 15 cm long. Spines are multicellular in length 0.6-2.5 mm and in width 0.3-1.2 mm, and round or elliptic in transverse section at their bases; they range from slender or curved cones to straight cylinders in shape, with expanded heads

159

0.06-0.28 mm high and 0.18-0.63 mm wide, perhaps related to a secretory function. Usually spines are at an acute angle in the direction of the distal end of the plant. They are irregularly arranged on the axes; the density of their distribution ranges from 80 to 430 per cm 2 of axes. Round sporangia are laterally scattered along the axes forming loose spikes, about 3.7 mm in diameter (3.2-4.3 mm); stalks 2.0-5.0 mm long and 0.6-1.2 mm broad depart from axes at about 20o-50 ° angle and are covered with spines similar to those of the axes. On one valve of a sporangium there are 7-15 spines. Dehiscence occurs along the entire margin of sporangia which have a thickened rim, about 0.12 mm high, on the convex margin. Spores are 30-50 ~ n in diameter, subcircular to circular in outline; trilete rays simple, 1/2-3/4 radius of spore; exine thin, smooth, often folded. Tracheids with annular, spiral thickenings are 14-32 ~m across. Epidermal cells are elongate to isodiametric, dominantly elongate, 78-310 ~tm long and 16-56 ~m wide. Holotype: P.U.H. 3 0 1 ; Paratypes: P.U.H. 302-306. The illustrated specimens will be deposited in the Geological Department of Peking University. Type locality: Mountain outcrop approximately 150 m west of Zhichang Village.

1

Age: Late Siegenian. Formation: Posongchong Formation. Description: Axes are densely aggregated parallel to the bedding plane (Plate I, 1; Plate II, 1). The longest axis found is 15 cm in length, and is broken at both ends, axes are generally 3.2-4.0 mm in width. It seems possible that the plant attained a height of at least 20-30 cm. The presumably creeping axis branches repeatedly in K- or H-shaped fashion (Plate II, 5, 6). Figures 3 and 4 are drawn from the specimens on Plate II, 5, 6. The diagram (Fig.4), shows t h a t one axis A-A' (a branch of one H-shaped branching) gave rise to a very short lateral B t h a t forked immediately to form axes C and D running in opposite directions (about a 180 ° angle) parallel to A-A'. Within 5.0 mm, axis C produced a short branch E at about 90° angle. Branch E gave off two branches and axis F continued branching to produce G and a short lateral branch H. The rest may be deduced by analogy, forming repeated K- or H-shaped branchings. The oblique extending "axis" in Fig.4, which is together the branches A, B, E and H, seems to be a creeping "main axis" with numerous "lateral branches". It may be up to 5 mm in width. The same branching pattern is fairly obvious in Fig.3, and very possibly these axes form part of one plant.

cm

Fig.3. Discalis longistipa, diagram of the b r a n c h i n g pattern, based on the specimen shown in Plate II, 5.

160

~

5

A

(} Cl*l~

Fig.4. Discalis longistipa, diagram of the branching pattern, based on the specimen shown in Plate II, 6. Spines p r o j e c t i n g f r o m t h e e n t i r e s u r f a c e of the p l a n t a r e m u l t i c e l l u l a r (Plate III, 4, 6-8), u s u a l l y at a n a c u t e a n g l e ( a b o u t 30o-60 ° angle), p o i n t i n g distally s o m e t i m e s at a r i g h t a n g l e or r a r e l y twisted a n d h a n g i n g down. T h e i r size, s h a p e a n d t h e d e n s i t y on the p l a n t v a r y (Plate II, 1-4; P l a t e III, 1-3, 5; Fig.5). To s u m up, t h e y r a n g e f r o m 0.6-2.5 m m long (usually 1.4-2.0 mm) a n d 0.3-1.2 m m wide at t h e i r b a s e s w h i c h are r o u n d or elliptic in t r a n s v e r s e section. S o m e t i m e s v e r t i c a l l y elong a t e d ridges c a n be seen at t h e i r bases on the impressions. P r o b a b l y this r e s u l t s from c o m p r e s s i o n d u r i n g p r e s e r v a t i o n (Plate III, 3, 5, 9, 10). M o s t spines on t h e m a t u r e a x e s a r e curved, c i r c u l a r c o n e s or s t r a i g h t cylinders, t a p e r i n g g r a d u a l l y f r o m a widened b a s e

i/

f Fig.5. Diagrams of spines, Discalis longistipa, show variation in shape, size and direction of spines. (The top of diagram is direction of distal end of plant). t o w a r d s the apex. Spines a r e a r r a n g e d irregularly, occasionally showing a roughly " s p i r a l " a r r a n g e m e n t . T h e i r d e n s i t y is variable, r a n g i n g f r o m 80-430 per cm 2 (Plate III, 3, 5). On t h e t e r m i n a l r e g i o n of t h e axes (i.e. g r o w i n g region), t h e spines are dense, hairlike or s l e n d e r c o n e s (Plate IV, 3), b u t at t h e b a s e of t h e p l a n t (the H - s h a p e d b r a n c h i n g region) t h e y a r e sparse, r a t h e r long, c i r c u l a r cones. M o s t of the spines seen h a v e m i n o r e x p a n d e d o b l a t e or disc-like apices w h i c h a r e a b o u t 0.06-0.28 m m h i g h or 0.18-0.63 m m wide. S o m e t i m e s t h e apices a p p e a r forked or hook-

PLATE I Discalis longistipa. Fertile axes with sporangia. One unit of scale is 5 mm.

1. 2. 3. 4.

Specimen showing some parallel fertile axes. Cross or oblique axes in photo belong to another plant. P.U.H. 301. Distal region of a fertile axis. Arrows indicate immature sporangia. P.U.H. 306. Part of a fertile axis showing compact arrangement of sporangia. P.U.H. 305. Part of a fertile axis showing sparse arrangement of sporangia. Sporangium indicated by arrow enlarged in Plate III. Fig.1. P.U.H. 304.

161

PLATE I

162 l i k e ( P l a t e IV, 2-9). P l a t e III, 4, 6 - 8 s h o w a cylinder-shaped spine. U n d e r SEM, it has n u m e r o u s e p i d e r m a l cells, m o r e t h a n 35 r o w s ( o n e s u r f a c e of t h e s p i n e ) a n d a n e x p a n d e d h e a d 130 ~m h i g h a n d 420 ~ w i d e s h o w i n g a neat, palisade-like epidermis. E p i d e r m a l cells are g e n e r a l l y e l o n g a t e runn i n g p a r a l l e l to t h e axis. T h e y a r e f r o m 7 8 - 3 1 0 ~m i n l e n g t h a n d f r o m 1 6 - 5 6 llm i n w i d t h . T h o s e l o c a t e d n e a r t h e d e p a r t u r e of b r a n c h e s or a r o u n d t h e s p i n e b a s e a r e u s u a l l y r h o m b o i dal o r n e a r l y i s o d i a m e t r i c a l . T h e e n d w a l l s a r e o b l i q u e ( P l a t e III, 9-11). E p i d e r m a l c e l l s o n t h e s p i n e s a r e e l o n g a t e s i m i l a r t o t h o s e o f t h e axes,

but usually shorter. The longest one measures 262 × 22 ~m ( P l a t e III, 4, 6-8). T h e p l a n t c o n s i s t s of s t e r i l e a n d f e r t i l e axes, m o s t of w h i c h a r e u n i f o r m i n w i d t h ( a v e r a g e 3.6 m m ) n a r r o w i n g s l i g h t l y u p w a r d s . S o m e d i s t a l s e g m e n t s of s t e r i l e a x e s h a v e c i r c i n a t e l y c o i l e d t i p s a n d a p p e a r t o b e d e l i c a t e ( P l a t e II, 1, 2). A few t e r m i n a l f r a g m e n t s of f e r t i l e a x e s a r e f o u n d ( P l a t e I, 2). E x c e p t for s p o r a n g i a , t h e fertile axes are n o t different from sterile ones. The sporangia are laterally scattered along t h e axis. S o m e t i m e s t h e y s e e m to be s p i r a l , some are more distantly spaced and others in the middle p a r t of the axis are q u i t e close

PLATE II Discalis longistipa. One unit of scale is 5 mm. 1. Specimen showing numerous parallel sterile axes. Arrow indicates a circinately coiled tip. P.U.H. 302. 2. A circinately coiled tip. Arrow indicates broken position. P.U.H. 307. 3, 4. Parts of specimen in Fig.l, enlarged, showing spines on axes. 5. Specimen showing characteristic branches on creeping axes. This position is drawn in Fig.3. P.U.H. 303. 6. Specimen showing repeated K-shaped branches. Indicated by arrows are branching points. This position is drawn in Fig.4. P.U.H. 308.

PLATE III (see p.164) Discalis longistipa. Scale line, of Figs.1 5. is 1 mm, of Figs.6 11 is 30 ~m. 1, 2. Stalked sporangia with spines, in abaxial view slightly oblique (Fig.l) and in lateral view (Fig.2); arrows indicate spines or scars of broken spines. P.U.H. 304, 309. 3. Part of a axis with spines. Note shape and arrangement of spines, elongate scars of inner surface because of compression. P.U.H. 310. 4, Cylinder-like spine. P.U.H. 311. Piece of a axis with numerous scars of spines, inner surface, note their irregular arrangement. P.U.H. 311. 5. 68. SEM. Same spine of Fig.4. Figure 6 is an expanded apex with neat palisade-like epidermal cells. Figure 7 is the middle part of the spine with numerous lines of epidermal cells which are similar to those of the axes. Figure 8 is the base of spine. H.S. 3-4, 3-5, 3-6. 9-11. SEM of inner surface of cuticle. Figure 9 shapes of epidermal cells on axis, arrows indicate two spine scars. Figure 10 circular base of a spine that stretches into the matrix, note various shapes of epidermal cells around base of spine. Figure 11 shapes of epidermal cells near a spine. Arrow indicates a joint filled. H.S. 4-2, 4-3, 4-6.

PLATE IV (see p.165) longistipa. Scale line, of Figs.l-10 is 1 mm, of Figs.ll-13 is 20 ~m, of Figs.14-16 is l0 ~m. Film peel of sporangium showing complete marginal dehiscence (arrows), H.S. 6-1. Film peels of spines showing their shapes and dense arrangement near apex of axes. H.S. 6-2, 6-3. Section through spines showing various apices of spines. Figure 9 arrow indicates a probable rupturing cuticle. H.S. 7-1, 7-2, 7-3, 7-4, 7-5, 7-6. Section through axis. Arrow indicates position of tracheids enlarged in Fig.ll. H.S. 7-10. 10. 11. Annular (?) tracheids. 12, 13. Tracheids revealed from bulk maceration showing annular and spiral thickenings. H.S. 8-2, 8-4. Mass of spores revealed from sporangium H.S. 8-5. 14. 15, 16. Spores. H.S. 9-1, 9-2.

Discalis 1. 2,3. 49.

163

PLATE II

164

PLATE III

(for explanation see p.162)

165

PLATE IV

~8 !

13

14 (for explanation see p.162)

166 together forming a loose spike (Plate I, 1-4). The mature sporangia are round in abaxial view, 3.2-4.3 mm in diameter (average 3.7 mm), but usually ovoid or nearly so. This results from compression during preservation. They dehisce along a median line, dividing the sporangia into two equal valves (Plate IV, 1). Thickening rims, about 0.12 mm high on the whole convex margin, can be observed. They narrow slightly towards both ends. The sporangia are singly attached by a rather long stalk which is 2.0-5.0 mm long and 0.6-1.2 mm wide. The stalk departs from the axes at an acute angle (about 20°-50 ° angle). Spines, identical with those of axes, occur along stalks and on sporangium walls. There are about 7-15 spines on one valve. They are also curved at an acute angle similar to that of the axes, sometimes "attached", in an irregular arrangement, the longest one measured is 1.1 mm long (Plate III, 1, 2; Plate IV, 1; Fig.7). Because permineralized material of the axes was not obtained, the form of the stele is unknown. Thin sections and macerations revealed fragments of tracheids with annular and spiral thickenings. These tracheids are 14-32 lim across (Plate IV, 10-13). Although not well preserved, the spores were obtained from some of the sporangia. They are subcircular to circular in equatorial outline and range from 30-50 ~m in diameter, with most being very close to 42 ~m (on the basis of measurements of 25 spores). The trilete rays are usually indistinct, simple, may be split open and extend about 1/2-3/4 of the spore radius. Sometimes the ends of the trilete rays were stopped by folding near the equator (possible curvaturae?). The exine is thin, often folded. The surface in most cases appears smooth but may have some sparsely scattered granules or indistinct material, which possibly represents tapetal material of the sporangium (Plate IV, 14-16).

Discussion and comparison A construction of Discalis longistipa - - Fig.6 A construction of D. longistipa is drawn in Fig.6. I believe that this plant was a relatively

Fig.6. Discalis longistipa. Restoration of a portion of the plant. small herbaceous organism with prostrate rhizomatous axes consisting of repeated K- or H-shaped branchings, from which fertile and sterile axes divided immediately. They are trailing or erect. Some indirect evidence that supports this

167

Fig.7. Discalis longistipa. Restoration of part of a fertile axis.

life habit have been seen on my specimens, as follows: (1) Some terminal fragments of both fertile and sterile axes have been found separately (Plate I, 2; Plate II, 1, 2), which indicate their differentiation. (2) Some folded traces of aerial axes also have been observed (Fig.3, dotted line); they occurred near the branching points. It is clear that these traces are the result of compression during deposition, and show t h a t the plant was three-dimensional when it lived.

Spines of D. longistipa The spines of this plant are remarkable. Their size, shape and distribution varied in the different parts of the plant and have not a regular phyllotaxy, but most of them bend

distally. They have expanded multicellular apices possibly related to a secretory function (Plate IV, 2-9). I have also found t h a t on young shoots they often show a convex-round oblate form (Plate IV, 3), but on mature shoots usually they show minor forks or hooks (Plate IV, 4-8). For this reason I suppose the secreted substances probably were accumulated in these heads. At maturity the epidermal cells or the cuticles ruptured (Plate IV, 9), the secreted substances reached the surface, and then the vestiges of the cuticle formed those various shapes of the apices of the spines. Plate III, 4, 6-8 show a cylinder-shaped spine around which there are more than 70 rows of epidermal cells similar to those of the axes in morphology. Although I can not determine whether a vascular trace and stomata occur in such spine, I suggest that it is unvascularized. It is obvious that "they may represent an increase in the surface available for photosynthetic activity" (Chaloner, 1970, p.360). Sawdonia ornata has glandular spines (Hueber, 1971; Zdebska, 1972; Chaloner et al., 1978; Edwards et al., 1983; Rayner, 1983). Its spines have an apex with a vesicular end or multicellular dark tip. The spines average about a millimeter in length. Zdebska gave a particularly detailed description of the spines. My material is similar to that described by Zdebska 1972 in shape, in spine bases and in their probably secretory character, especially to her plate IV, fig.3; plate V, figs.2, 3. The present material differs from hers which has apices with a smaller vesicular end, and fewer epidermal cell rows on spines. Based on the description of Zdebska, spines of S. ornata are usually at an acute angle similar to those of D. longistipa. Nevertheless, in the reconstructions shown by Ananiev and Stepanov (1968, fig.7) and Rayner (1983, fig.6) the spines project irregularly. The presence of spines along the stalks allows a comparison of D. longistipa with Sawdonia acanthotheca Gensel, Andrews and Forbes (1975). The latter differs in t h a t the sporangia are sessile or with very short stalks. The presence of spines on the sporangia and

168 along the long stalks distinguishes D. longistipa from other spiny plants.

Spores of D. longistipa In situ spores obtained are not well-preserved. The triradiate mark is obscure and it seems likely that the present spores were immature at the time of fossilization. They are subcircular to circular, thin-walled and folded. The exine lacks ornamentation and the trilete rays are usually longer t h a n half of the radius. Thus they are comparable to the dispersed spore genus Punctatisporites Ibrahim emend. Potonie a n d Kremp 1954 [or Retusotriletes (Naumova) Streel, 1964, if they have curvaturae]. Gensel et al. (1975) described and illustrated spores of Sawdonia acanthotheca based on LM and SEM observations, and considered that for the most part they appear similar to those of S. ornata (McGregor, 1973, plate II, 20) except that the latter has a more obvious curvature. But Rayner (1983, fig.2, j m) illustrated spores of S. ornata, from the Strathmore Group, Scotland, and described spores with a crescent-shaped halo around the triradiate mark but no curvature. In general, the spores of D. longistipa appear similar to those of Sawdonia, except for the length of the trilete rays.

Comparison with Sawdonia and other members of the Zosterophyllopsida The general characteristics of the plants included in the Zosterophyllopsida are the laterally born sporangia which are either sessile or terminal on short stalks. Vascular anatomy, if it is preserved, is an exarch protostele (Banks, 1968). In this group there are some genera with spiny axes. Among them, the genus Sawdonia (Dawson) Hueber, 1971 is a significant plant. It bears a strong resemblance to D. longistipa. It has a long research history and occurs in many localities of the world. Presently there are three species of Sawdonia that have been described. They are S. ornata (Dawson) Hueber (Dawson, 1871; Hueber and Banks, 1967;

Ananiev and Stapanov, 1968; Hueber, 1971; Zdebska, 1972; Chaloner et al., 1978; Rayner, 1983), S. acanthotheca (Gensel et al., 1975) and S. spinosissima (Schweitzer, 1980a). D. longistipa is similar to S. ornata mainly in possessing round sporangia in abaxial view, which dehisce along the distal margin into two nearly equal valves and spines and in situ spores mentioned above. The differences, however, between Sawdonia and D. longistipa are quite distinct. The former described by Hueber (1971) and others shows it has pseudomonopodial aerial axes and its global sporangia are born singly on very short stalks (only 0.5-0.75 mm long) or are sessile and alternate to subopposite. The present plant has numerous wide dichotomies, i.e. K- or H-shaped branchings in the creeping parts. Its sporangia are terminal on longer stalks (up to 5.0 mm in length), scattered along the axes sparsely or more compactly, sometimes apparently spirally. Recently, Rayner (1983) described in detail an H-shaped branching of S. ornata and showed that the descending limb is clearly different from the ascending axis. The author considered it most likely to be a rhizophore or root-like structure. Among the genera of the Zosterophyllopsida, Zosterophyllum is similar to D. longistipa in having K- or H-shaped branchings and similar types of sporangia on the axes such as Z. myretonianum (Lang, 1927; Walton, 1964; Edwards, 1975), but it lacks the spines that are found on the axes of D. longistipa. Crenaticaulis (Banks and Davis, 1969), Serrulacaulis (Hueber and Banks, 1979) and Koniora (Zdebska, 1982) all have the spiny axes similar to those of D. longistipa. But the spines of Crenaticaulis and Serrulacaulis are like teeth, and are arranged in one or two rows, Koniora has narrow longitudinal wings and short and long spines on the surface of the axes. In the characteristics of the sporangia, those of Crenaticaulis dehisce into a large and a small valve; the sporangia of Serrulacaulis occur alternatively in two rows along the axes; while in the specimens of Koniora the sporangia are borne singly below the dichotomies.

169

Comparison with other plants Gensel et al. (1969) described Kaulangiophyton akatha from the upper Lower or lower Middle Devonian horizon in the south bank of Trout Brook, Maine (U.S.A.). The speicmens illustrated as K. akantha (figs.l, 2) and the reconstructions (figs.4, 8) resemble D. longistipa, especially the lateral, stalked sporangia and the branching pattern. But the former have large ovoid sporangia (5 x 8 mm) and they have thorn-like, '~reduced" leaves (Gensel and Andrews, 1984, p.127). They are "about 2 mm long with broad, decurrent base." (Gensel et al., 1969, p.269). There has been some discussion t h a t Kaulangiophyton is identical to Drepanophycus (Schweitzer and Giesen, 1980; Rayner, 1984). Drepanophycus spinaeformis Goeppert (1852) also shows a superficial resemblance to D. longistipa. According to the restoration of Schweitzer (1980b, fig.22) it had robust stems, bears true stout microphylls and a root-like structure, which can distinguish them. D. longistipa possesses some characters in common with Asteroxylon mackiei (Kidston and Lang, 1920; Chaloner and Macdonald, 1980). Both plants show similar round sporangia with stalks, although the latter's stalks are shorter, and are scattered among the leaf-like ~'spines". Their sporangial dehiscence is similar. D. longistipa differs from A. mackiei in branching and spines. Asteroxylon has upright pseudomonopodial axes bearing dichotomizing laterals. Its rhizomatous portions are naked, but aerial axes are covered with lanceolate enations which are approximately 5 mm long and spirally arranged.

The systematic position of Discalis Discalis has lateral, disc-like round sporangia and exhibits a special mode of dehiscence along the distal margin showing it has a close affinity to the zosterophylls and Asteroxylon. Its lack of true microphylls indicates that it may be allied to some member of the Zosterophyllophytina (Banks, 1968, 1975).

Although leaf traces of Asteroxylon reach only to the base of the enations, they are typical leaf traces. I believe the plant to be a lycopod, not a zosterophyll. Recently, an evolutionary series involving the origin of the microphyll has been proposed with new discoveries of early land plants. This series includes such fossil plants as Sawdonia, Asteroxylon, and Drepanophycus which represent a transitional series between the zosterophylls and lycopods. Their relationships have been discussed by Banks (1968, 1975), Gensel et al. (1975, 1984), Chaloner (1970), Schweitzer (1980b, 1983) and Stewart (1983). Their common characters are lateral sporangia borne along a definite area of axis, round or reniform sporangia and dehiscence along the convex distal margin. In cross-section the xylem strand is exarch. All have a similar general habit, horizontal and erect axes and are covered with various microphyll-like spines or microphylls. I consider the new plant a member of this series. Among them, the microphylls were hypothetically initiated from nonvascularized spines e.g. Discalis and Sawdonia to Asteroxylon where a trace of vascular tissue extended only to the base of the leaves. Then the trace reached into the leaves and a complete microphyll had formed as exemplified by Drepanophycus. Because Sawdonia shows a specialized arrangement of sporangia (i.e. alternate to subopposite), very short sporangial stalks and has a root-like structure, I believe Sawdonia seems to be more advanced than Discalis. The information offered by D. longistipa adds to our knowledge of the early vascular plants and shows that within the Siegenian, plants with spiny axes are not unusual and exhibit extensive variation in shape and distribution of the spines. It is noteworthy that D. longistipa has microphyll-like spines which are not only on the axes, but also along the stalks and on the sporangial walls. If the microphylls are derived from such spines, as speculated by many morphologists, then the model of the spine distribution along the stalks and on the sporangial walls of D. longistipa would represent an early stage in the evolution of the

170

sporophyll. It also seems to offer an explanation of why lycopod sporangia are always associated with leaf organs and why the sporophylls are not different from the vegetative leaves (microphylls) in some lycopods.

Acknowledgements The author wishes to thank Professor Bai Shun-Ling, Department of Geology, for checking the marine fossil list determined and to thank Dr. Patricia G. Gensel for helpful discussions in personal communication. I also wish to thank Dr. Dianne Edwards for much good advice and discussion. I a m greatly indebted to Xue Jia and Xu Yun for aiding in the preparation of the photographs and the drawings.

References Ananiev, A.R. and Stepanov, S.A., 1968. Finds of sporiferous organs in Psilophytonprinceps Dawson emend. Halle in the Lower Devonian of the southern Minusinsk hollow. Treatises Tomsk Order Red B a n n e r Labor State Univ. Geol. Ser., 202:30-46 (in Russian). Bai Shun-Liang, Dong Zhi-Zhong and J i n Shan-yu, 1982. The Devonian conodont sequence of Yunnan. In: Bai Shun-liang, Jin Shan-Yu and Ning Zhong-Shan (Editors), The Devonian Biostratigraphy of Guangxi and Adjacent Area. Peking University Press, Beijing, pp.67-71 (in Chinese). Banks, H.P., 1968. The early history of land plants. In: E.T. Drake (Editor), Evolution and Environment: A Symposium presented on the Occasion of the 100th Anniversary of the Peabody Museum of Natural History at Yale University. Yale Univ. Press, New Haven, Conn., pp.73-107. Banks, H.P., 1975. Reclassification of Psilophyta. Taxon, 24(4): 401 413. Banks, H.P. and Davis, M.R., 1969. Crenaticaulis, a new genus of Devonian plants allied to Zosterophyllum, and its bearing on the classification of early land plants. Am. J. Bot., 56:436 449. Cai Chong-Yang and Schweitzer, H.J., 1983. l~ber Zosterophyllum yunnanicum Hsii aus dem Unterdevon Sudchinas. Palaeontographica, 185B: 1 10. Chaloner, W.G., 1970. The rise of the first land plants. Biol. Rev., 45:353 377. Chaloner, W.G. and Macdonald, P., 1980. Plants invade the land. H.M.S.O. The R. Scott. Mus., Edinburgh, 17 pp. Chaloner, W.G., Hill, A.J. and Rogerson, E.C.W., 1978. Early Devonian plant fossils from a southern England borehole. Palaeontology, 21:693 707.

Dawson, J.W., 1871. The fossil plants of the Devonian and Upper Silurian Formations of Canada. Geol. Survey Canada, Montreal, pp.1 92. Edwards, D., 1975. Some observations on the fertile parts of Zosterophyllum myretonianum Penhallow from the Lower Old Red Sandstone of Scotland. Trans. R. Soc. Edinburgh, 69:251 265. Edwards, D., Edwards, D.S. and Raynor, R., 1983. The cuticle of early vascular plants and its evolutionary significance. In: D.F. Cutler, K.L. Alvin and C.E. Price (Editors), The Plant Cuticle, Linnean Soc. Syrup., Ser. 10. Academic Press, New York, N.Y., pp.341-361. Geng Bao-Yin, 1983. Stachyophyton gen. nov. discovered from Lower Devonian of Y u n n a n and its significance. Acta Bot. Sin., 25(6): 574 579 (in Chinese, with English abstract). Geng Bao-Yin, 1985. Huia recurvata - - A new plant from Lower Devonian of Southeastern Yunnan, China. Acta Bot. Sin., 27(4): 419--426 (in Chinese, with English abstract). Genset, P.G. and Andrews, H.N., 1984. Plant Life in the Devonian. Praeger Publishers, New York, N.Y., 381 pp. Gensel, P.G., Kasper, A.E. and Andrews, H.N., 1969. Kaulangiophyton, a new genus of plants from the Devonian of Maine. Torrey Bot. Club Bull., 96:265 276. Gensel, P.G., Andrews, H.N. and Forbes, W.H., 1975. A new species of Sawdonia with notes on the origin of microphylls and lateral sporangia. Bot. Gaz., 136:50 62. Goeppert, H.R., 1852. Fossile flora des (~bergangsgebirges. Nova Acta Leopold., 22:1 299. Halle, T.G., 1927. Fossil plants from Southwestern China. Palaeontol. Sin., Ser. A. 1(2): 1 26. Halle, T.G., 1936. On Drepanophycus, Protolepidodendron and Protopteridium, with notes on the Palaeozoic flora of Yunnan. Palaeontol. Sin., Ser. A, 1(4): 1-38. Hao Shou-Gang, 1985. Further observations on Zosterophyllum yunnanicurn Hsfi. Acta. Bot. Sin., 27(5): 545-549. (in Chinese, with English abstract). Hsii Jen, 1946. P l a n t fragments from Devonian beds in Central Yunnan, China. Iyenger Commemoration volume, J. Indian Bot. Soc., pp.339--360. Hsii Jen, 1966. On plant-remains from the Devonian of Y u n n a n and their significance in the identification of the stratigraphical sequence of this region. A c t a Bot. Sin., 14(1): 50-69 (in Chinese, with English abstract). Hueber, F.M., 1971. Sawdonia ornata: a new name for Psilophyton princeps var. ornatum. Taxon. 20: 641-642. Hueber, F.M. and Banks, H.P., 1967. Psilophyton princeps: the search for organic connection. Taxon, 16: 81-85. Hueber, F.M. and Banks, H.P., 1979. Serrulacaulis furcatus gen. et sp. nov., a new zosterophyll from the lower Upper Devonian of New York state. Rev. Palaeobot. Palynol., 28: 169-189. Kidston, R. and Lang, W.H., 1920. On Old Red Sandstone plants showing structure, from the Rhynie chert bed, Aberdeenshire. III. Asteroxylon rnackiei Kidston and Lang. Trans. R. Soc. Edinburgh, 52: 643-680. Klapper, G. and Ziegler, W., 1979. Devonian conodont biostratigraphy. In: M.R. House, C.T. Scrutton and M.G.

171 Basset (Editors), The Devonian System. Spec. Papers Palaeontol., 23: 199-224. Lang, W.H., 1927. Contributions, to the study of the Old Red Sandstone flora of Scotland VI. On Zosterophyllum myretonianum Penh., and some other plant remains from the Carmyville Beds of the Lower Old Red Sandstone. Trans. R. Soc. Edinburgh, 55: 443-455. Liao Wei-Hua, Xu Han-Kui, Wheng-Yuan, Ruan Yi-Ping, Cai Chong-Yang, Mu Dao-Cheng and Lu Li-Chang, 1978. The subdivision and correlation of the Devonian stratigraphy of S.W. China, In: Inst. Geol. Miner. Resour. Chin. Acad. Geol. Sci. (Editors), Syrup. Devonian System of South China. Geol. Press, Peking, pp.189-193 (in Chinese). Li Cheng-Sen, 1982. Hsiia robusta, A new land plant from the Lower Devonian of Yunnan, China. Acta Phytotax. Sin., 20(3): 331-342 (in Chinese, with English abstract). Li Xing-Xue and Cai Chong-Yang, 1977. Early Devonian Zosterophyllurn remains from southwest China. Acta Palaeont. Sin., 16(1): 12 34 (in Chinese, with English abstract). Li Xing-Xue and Cai Caong-Yang, 1978. A type-section of Lower Devonian strata in southwest China with brief notes on the succession and correlation of its plant assemblages. Acta Geol. Sin., 52(1): 1-12 (in Chinese, with English abstract). Li Xing-Xue and Cai Chong-Yang, 1979. Devonian floras of China. Acta Stratigr. Sin., 3(2): 90-95 (in Chinese). McGregor, D.C., 1973. Lower and Middle Devonian spores of Eastern Gaspe Canada. I. Systematics. Palaeontographica, 142B: 1 77. Potonie, R. and Kremp, G., 1954. Die Cattungen der

paHiozoischen Sporae dispersae une und ihre Stratigraphie. Geol. Jahrb., 69: 111-194. Rayner, R.J., 1983. New observations on Sawdonia ornata from Scotland. Trans. R. Soc. Edinburgh, 74: 79-83. Rayner, R.J., 1984. New finds ofDrepanophycus spinaeformis GSppert from the Lower Devonian of Scotland. Trans. R. Soc. Edinburgh, 75: 353-363. Schweitzer, H.J., 1980a. Die Zosterophyllaceae des rheinischen Unterdevons. Bonn. PaHieobot. Mitt., 3: 1-32. Schweitzer, H.J., 1980b. Uber Drepanophycus spinaeformis Goeppert. Bonn. PaHieobot. Mitt., 7:1 29. Schweitzer, J.J., 1983. Unterdevon-flora des Rheinlandes. Palaeontographica, 169B: 96 102. Schweitzer, H.J. and Geisen, P., 1980. t}ber Taeniophyton inopinaturn, Protolycopodites devonicus und Cladoxylon scoparium aus dem Mittel Devon Wuppertal. Palaeontographica, 173B: 1-25. Stewart, W.N., 1983. Paleobotany and the evolution of plants. Cambridge Univ. Press, New York, N.Y., pp.85-99. Streel, M., 1964. Une association de spores du Giv~tien inf~rieur de la Vesdre, Agoe (Belgique). Ann. Soc. Geol. Belg., 87(7): 1-30. Sxe Hsin-Chien, 1941. A new occurrence of the oldest land plants from the Chaotung district, Yunnan. Bull. Geol. Soc. China, 21(1): 107-110. Walton, J., 1964. On the morphology of Zosterophyllum and other early Devonian plants. Phytomorphology, 15: 155-160. Zdebska, D., 1972. Sawdonia ornata (= Pailophyton princeps var. ornatum) from Poland. Acta Palaeobot., 13: 77-97. Zdebska, D., 1982. A new zosterophyll from the Lower Devonian of Poland. Palaeontology, 25:247 263.