Devonian in situ spores: a survey and discussion

Review of Palaeobotany and Palynology, 30 (1980): 101--132 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

101

DEVONIAN IN SITU SPORES: A SURVEY AND DISCUSSION

PATRICIA G. GENSEL Department of Botany, University of North Carolina, Chapel Hill, N.C. 27514 (U.S.A.) (Received March 16, 1979; revised version accepted September 6, 1979)

ABSTRACT Gensel, P.G., 1980. Devonian in situ spores: a survey and discussion. Rev. Palaeobot. Palynol., 30: 101--132. Current information about in situ spores of Devonian plants is discussed and summarized. Spores are known from sporangia of about 40 species and 30 genera of Devonian plants and compare in morphology to approximately 17 genera of dispersed spores. While many gaps in our knowledge of in situ Devonian spores exist, those of some plant groups (e.g. trimerophytes) axe now well characterized and some directions for future investigations can be discerned. INTRODUCTION I n t e r e s t in t h e m o r p h o l o g y o f spores c o n t a i n e d in s p o r a n g i a o f D e v o n i a n p l a n t s has increased c o n s i d e r a b l y in r e c e n t years. T h e i m p o r t a n c e o f in situ s p o r e s t r u c t u r e is well r e c o g n i z e d as c o m p l e m e n t a r y t o m o r p h o l o g i c a l charact e r i z a t i o n s o f t h e s p o r o p h y t e . At t h e s a m e t i m e b o t h the p a r e n t a l origin o f spores and t h e i r n a t u r a l v a r i a t i o n p r o v i d e an i m p o r t a n t basis f o r i n t e r p r e t i n g sporae dispersae in D e v o n i a n p a l y n o l o g y . C o n t i n u i n g studies suggest t h a t r e c o g n i t i o n o f s o m e aspects o f t h e e v o l u t i o n o f s p o r e f e a t u r e s - - m o r p h o l o g i c or u l t r a s t r u c t u r a l - - c o u l d c o m p l e m e n t available i n f o r m a t i o n o n t h e evolut i o n a r y diversification o f vascular plants as well as indicate d e t a i l e d changes o c c u r r i n g d u r i n g o r as a result o f t h e e v o l u t i o n o f h e t e r o s p o r y or p r o d u c t i o n o f p o l l e n a n d seeds. A d d i t i o n a l l y , studies o f in situ s p o r e s m i g h t e n a b l e b o t a n i s t s to utilize dispersed s p o r e d a t a to learn m o r e a b o u t floras in areas w h e r e megafossils are n o t preserved. S u m m a r i e s o f D e v o n i a n p l a n t sources o f in situ spores include c h a r t s a n d lists in Balme and Hassell ( 1 9 6 2 ) a n d P o t o n i 6 ( 1 9 6 5 ) a n d an a c c o u n t b y Banks ( 1 9 6 8 ) in w h i c h he r e c o r d e d 10 g e n e r a o f D e v o n i a n p l a n t s w i t h n a m e d in situ spores a n d 12 w i t h u n n a m e d ones. R i c h a r d s o n ( 1 9 6 9 ) discussed s o m e s p o r e - p a r e n t p l a n t r e l a t i o n s h i p s in his s u r v e y o f D e v o n i a n spores a n d p o i n t e d o u t t h a t m a n y d i s p e r s e d s p o r e t y p e s o f s t r a t i g r a p h i c i m p o r t a n c e o r o f biological i n t e r e s t such as E m p h a n i s p o r i t e s or A n c y r o s p o r a have n o t y e t b e e n f o u n d in sporangia. M c G r e g o r ( 1 9 7 3 ) and M c G r e g o r a n d C a m f i e l d ( 1 9 7 6 ) discussed possible p a r e n t p l a n t s f o r s o m e o f t h e dispersed s p o r e t a x a d e s c r i b e d 0034-6667/80/0000-0000/$02.25

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102 in their accounts o f palynofloras f r om Gasp~, Quebec, and Melville Island, Canada. Chaloner (1967, 1970) examined the general morphological features o f sporae dispersae as a supplemental source of information on the early evolution and diversification of vascular plants, especially in regard to such major events as the advent of vascular land plants and the evolution of h e t e r o s p o r y and seed habit. Research since 1969 has p r o d u c e d n o t only considerable information a b o u t Devonian megafossils and dispersed spores, but also m uch additional data on in situ spores o f Devonian plants. To date, spores have been described from sporangia o f a b o u t 40 species and 30 genera o f the approxi m at el y 120+ genera of named Devonian plants and are referable to about 17 of the ca. 100 genera of Devonian dispersed spores ~ . The purpose o f this paper is to discuss and summarize current data on spores f r o m sporangia of Devonian plants based on specific studies carried out by ot her workers and myself, and to evaluate this information in the c o n t e x t of major phylogenetic groups o f Devonian plants for possible trends, major advances, or areas in need of additional study. The plants for which in situ spores are known are assignable, in order, to the following subdivisions (after Banks, 1968, 1975b,c): Subdivisions Rhyniophytina, Zosterophyllophytina, Trimerophytina, Lycophytina, P t e r o p h y t i n a - - Class Progymnospermopsida. Pre-ferns and ot her plants of u n d eter min ed affinities are discussed lastly. Tables for each of the groups are given with parent plants, sporae dispersae and reference sources. All groups are discussed in some detail 2 except the p r o g y m n o s p e r m s for which the reader is referred to K.C. Allen's f o r t h c o m i n g contribution. It has been the practice to c o m p a r e in situ spores to those sporae dispersae most like them, instead of renaming a dispersed spore entity once spores resembling it are f ound in situ. Rarely in situ spores carry the name of their parent plant, usually in a modified f or m such as Aneurospora (parent plant -A n e u r o p h y t o n ) or Enigmophytospora (parent plant - - E n i g m o p h y t o n ) . Since it is k n o wn that more than one plant t axon m ay produce spores of a given morphological t y p e (see later sections), it seems advisable to continue the established practice m e n t i o n e d above, despite problems resulting from attempting to compare entities exhibiting natural variation to ones categorized by a p r e d o m i n a n t l y artificial classification scheme. Certain factors influence accumulation of information concerning in situ spores o f Devonian plants. Preservation of fossil plants is n o t always conducive to preserving spores in sporangia; for example plants from several localities such as the late Early Devonian T r o u t Valley F o r m a t i o n of n o r t h e r n Maine (Andrews et al., 1977) or the Early Devonian localities in England or Scotland are usually preserved as impressions so no information on spore m o r p h o l o g y 1These figures are based on a general literature survey and most probably are not totally accurate. The count does not include consideration of the possibility that some genera may need to be merged and others split. 2Some plants, including many listed by Banks (1968, table 10) as plants for which in situ spores are known but unnamed, are not dealt with in this paper.

103

can be obtained. Where carbonified remains exist they often are too altered to yield spores at all, or the spores obtained are very poorly preserved. At times the spores obtained from sporangia are immature and thus do n o t provide any definitive information. Among earlier described in situ spores, such as those of Lang (1931, 1937), preparation of spores from sporangia was by means of transfers. These often are n o t clear, and because of the thickness of the mounts, careful examination of fine details or rephotography at high magnifications is often not possible. Sometimes earlier photographs might be misleading; for example, the spores illustrated by Lang for Cooksonia pertonii (Lang, 1937, plate 8, figs.10, 11, 13--15) appear to be nearly triangular and to possess ornament of coni. Examination of Lang's preparations shows that the spores are circular to subtriangular, but often folded so that they appear strongly triangular, and unornamented. The " c o n i " actually are pieces of debris lying near the spore in question. More will be said about these spores in a later section. And lastly, although a steadily increasing awareness of the importance of examining in situ spores is evident in Devonian paleobotany today, n o t all workers a t t e m p t to extract information on spore morphology. METHODS OF PREPARATION Plants preserved as compressions or petrifactions yield in situ spores when preservation permits. Methods of preparation and study of in situ spores include the same basic techniques that have been used for over fifty years with modification resulting from improved technology or instrumentation. The use of the SEM, and more recently TEM, to supplement light microscopy has proved to be of great value in understanding features of some of the spores examined. The techniques employed by the author for removal and preparation of in situ spores are as follows: Attached sporangia of each plant are located and removed from the matrix of a specimen or transfer preparation mechanically with a brush or needle, or by maceration in HF. The sporangia are then treated with Schulze's solution and dilute NH4 OH to clear away the carbonified portions. An inner cuticular layer of the sporangium wall remains (as described in Pettitt, 1965; Hueber, 1968; Andrews et al., 1974; Gensel et al., 1975; Gensel, 1976, 1979). If dehiscence and shedding of spores had not occurred prior to fossilization, spores are found within the cuticle. Sporangial cuticles are isolated and the cuticle dissected away and spores separated. Some are m o u n t e d on slides for light microscopy and others m o u n t e d on stubs, coated with gold or gold-palladium and examined with a SEM. Spores also have been dehydrated, embedded in Spurr's low-viscosity resin, sectioned, post-stained, and studied with a TEM. Information about the relative state of m a t u r i t y of spores contained within a sporangium or the quality of preservation of spores often can be determined during preparation. In some plants, spores practically fall out without much manipulation, which I interpret as indication that t h e y were close to

104 being shed at the time of fossilization; in other plants whose sporangia were probably less mature, spores adhere to one another and to the inner wall cuticle of the sporangia. Some preparations have yielded decidedly immature spores in sporangia; these are small, thin-walled, transparent and difficult or impossible to separate. RHYNIOPHYTINA Plants included in the subdivision Rhyniophytina of Banks (1968, 1975b, c) are diverse, fragmentary and in some cases n o t well known. Genera included in this group with relative certainty are Rhynia, Horneophyton, Cooksonia, Salopella, Steganotheca, Dutoitea, and Eogaspesiea (Banks, 1975b, c). More problematical taxa include Taeniocrada, Hicklingia (Edwards, 1976, suggests on the basis of new evidence that this plant might be more closely related to Zosterophyllum), Nothia, Yarravia, and Hedeia (Banks, 1975b, c). Most probably all of these genera are n o t closely related and investigation of larger numbers of better preserved specimens is needed to clarify their relationships. As presently known, rhyniophytes are small plants with dicho~ t o m o u s l y divided axes and terminally borne sporangia whose walls are several cell layers thick. Where anatomical evidence is available, sporangia are thickwalled and lack any evidence of dehiscing and axes consist of centrarch protosteles. Spores illustrated or described from sporangia of several genera of rhyniophytes are listed in Table I. They often are n o t well preserved or else not completely described or well illustrated. In some instances the spores cannot be compared with any existing dispersed-spore taxon. The spores of Rhynia major were illustrated by Kidston and Lang (1917) and briefly described as trilete, often found in tetrads, and average 40 pm in diameter. Bhutta (1973a) described spores of Rhynia major and illustrated stages of germinating spores. According to his observations they are circular, trilete and either smooth or ornamented with tubercles. He compared them to the sporae dispersae taxon Geminospora tuberculata Allen partly because he observed a possible mesosporoid (or inner b o d y ) in one germinating spore. Evidence of inner bodies in other spores of Rhynia is lacking (Bhutta, 1973a; Plate I, 2). Additionally, the spore walls have become badly corroded by preservation in chert so that any descriptions and comparisons are severely limited (Plate I, 1). Bhutta also examined spores of Rhynia gwynne-vaughanii (1969, unpublished thesis, University of Wales), which are characterized as being curvaturate. According to Edwards and Richardson (1974), spores of the two species of Rhynia resemble most closely the sporae dispersae taxon Perotrilites microbaculatus Richardson and Lister (1969). Obtaining better preserved Rhynia spores might result in a different interpretation of their morphology. Spores from sporangia of Horneophyton lignieri have been described and illustrated by Bhutta (1973b). The majority are trilete, apiculate, and curvaturate and were compared by him to the sporae dispersae taxon Apiculiretu-

Bhutta (1973b), see also Eggert (1974) Lang (1937)

cf. Apiculiretusispora and ?Emphanisporites decoratus illustrated, not compared

illustrated, not compared Apiculiretusispora cf. plicata Retusotrilites

illustrated, described, not comparable to existing dispersed spore taxon

Horneophyton lignieri

Cooksonia pertoni

Cooksonia crassiparietalis

Renalia hueberi

Salopella allenii

Edwards and Richardson (1974)

Gensel (1976)

Iurina (1964) McGregor (1973)

(a) Bhutta (1973a) (b) Edwards and Richardson (1974)

?cf. a) Geminospora or b) Perotrilites microbaculatus

Rhynia

Reference

Dispersed spore type

Taxon

Rhyniophytes

TABLE I

¢j1

I

-3

y~

107

sispora plicata (Allen) Streel ( 1 9 6 7 ) . S o m e , smaller in size t h a n the p r e c e d i n g ones, were regarded b y B h u t t a as similar t o Emphanisporites decoratus Allen ( 1 9 6 5 ) in having radially arranged cones o n the p r o x i m a l surface. Eggert ( 1 9 7 4 ) illustrated spores f r o m Horneophyton lignieri using light and scanning e l e c t r o n m i c r o s c o p y and n o t e d t h a t a l t h o u g h m o s t spores o c c u r in t e t r a h e d r a l tetrads, some are isobilateral. I p r e f e r t o describe the latter as o c c u r r i n g in decussate t e t r a d s (Plate I, 4). It is evident f r o m Eggert's description and illustrations t h a t the spores are apiculate and t h a t some d e g r a d a t i o n o f the spore walls had t a k e n place a l t h o u g h it is n o t as extensive as in Rhynia. Chaloner and Orbell ( 1 9 7 1 ) illustrated a spore o f H. lignieri (their fig.l-C) which shows possible faint p r o x i m a l ribs and p r o b a b l e apiculate o r n a m e n t . Based on these several a c c o u n t s and personal o b s e r v a t i o n o f the original p r e p a r a t i o n s o f K i d s t o n and Lang, spores o f Horneophyton lignieri are r o u n d e d , trilete, c u r v a t u r a t e and apiculate (see Plate I, 3) and generally c o m p a r a b l e t o the sporae dispersae t a x o n Apiculiretusispora Streel. A d d i t i o n a l specimens o f in situ spores o f Horneophyton are n e e d e d t o c o n f i r m the o c c u r r e n c e on s o m e o f t h e m o f p r o x i m a l radial ribs and a n y r e s u l t a n t similarity to the dispersed spore genus Emphanisporites. Lang ( 1 9 3 7 ) illustrated spores o f Cooksonia pertoni b u t did n o t c o m p a r e t h e m to a n y t y p e . T h e y are s m o o t h , r o u n d t o r o u n d e d - t r i a n g u l a r , trilete and some e x h i b i t a slight t h i c k e n i n g o f the wall at the margins. T h e y c a n n o t be c o m p a r e d to any o n e dispersed spore t a x o n a l t h o u g h t h e y resemble Arnbitisporites H o f f m e i s t e r and Retusotriletes warringtonii R i c h a r d s o n and Lister 1969. If the e q u a t o r i a l region o f these spores is t r u l y t h i c k e n e d , t h e y w o u l d be m o s t similar to Arnbitisporites; if, however, the t h i c k e n i n g is the result o f t h e p r e s e n c e o f c u r v a t u r a e coinciding w i t h the e q u a t o r i a l margins (and t h u s n o t visible), t h e spores w o u l d be m o s t similar to R. warringtonii. Iurina ( 1 9 6 4 ) illustrated spores o f Cooksonia crassiparietalis, f r o m the late Early D e v o n i a n o f K a z a k h s t a n and d e s c r i b e d t h e m as being r o u n d or r o u n d triangular, 5 0 - - 6 5 p m and c o v e r e d w i t h spines up t o l p m high. McGregor ( 1 9 7 3 ) suggested t h a t t h e y c o m p a r e well with Apiculiretusispora cf. plicata, PLATEI Spores of Rhynia major Kidston and Lang. 1. Light mierograph of proximal surface, showing trilete mark. Exine pattern is probably result of corrosion of wall. Kidston Slide #2433 (Hunterian Museum). x 645. 2. LM, section view of spore wall, part of trilete ray is visible at top. No evidence of a mesosporoid. Kidston Slide #2433 (Hunterian Museum). x 578. Spores of Horneophyton lignieri (Kidston and Lang) Barghoorn and Darrah. 3. LM proximal view showing curvaturae; apiculate sculpture just barely visible at margin. Kidston Slide #2438 (Hunterian Museum). X 661. 4. LM of one tetrahedral and one decussate tetrad. Kidston Slide #2454 (Hunterian Museum). x 443. Spores of Renalia hueberi Gensel. 5. LM of two spores, showing well-developed curvaturae, some tapetal residue, G.S.C. #43200. x 686. 6. SEM of spore, proximal view. x 1890.

108 thus differing from those of C. pertoni. The megafossil remains of C. crassiparietalis are few and incomplete, and it is possible t h a t C. crassiparietalis after further study might be more closely related to Renalia or the zosterophyllophytes. A compression fossil which appears similar in general organization to the petrifaction remains of Rhynia was described by Edwards and Richardson (1974) as Salopella allenii from the Early Devonian (Dittonian) of the Welsh Borderland. Spores isolated from its sporangia are circular, azonate, thinwalled, with faint trilete sutures and apparently covered with tapetal residue. The authors suggest the spores might be immature, partly because of their general appearance and the presence of globules interpreted as tapetal residue on their surface and partly because they do n o t compare well with any known types of dispersed spores. A plant which is tentatively allied with the rhyniophytes, but possesses features of both rhyniophytes and zosterophyllophytes and may be intermediate between the two groups, is Renalia hueberi Gensel (1976) from the Emsian of Gasp~, Quebec, Canada. Spores from sporangia of this plant (Plate I, 5,6) are smooth-walled, trilete, curvaturate and 40--70 pm in diameter. Many are covered with globules of possible tapetal residue (Plate I, 1) and it is possible t h a t t h e y were slightly less than completely mature at the time of preservation as it is very difficult to break the spores apart. They compare best to the sporae dispersae genus Retusotriletes (Nauru) Streel. Thus, in general, spore morphology is quite varied among the morphologically diverse plants included in the rhyniophytes or regarded as being closely allied to t h a t group. The oldest known vascular plant, Cooksonia, possesses small, trilete spores (25--35 pm), lacking o r n a m e n t and possibly having curvaturae. These spores are comparable in general size, organization, and complexity to some of the sporae dispersae c o m m o n l y obtained from Late Silurian--earliest Devonian sediments (Hoffmeister, 1959; Richardson and Lister, 1969; Richardson and Ioannides, 1973). Salopella has a similar type spore and is of the same age. Other rhyniophytes possess spores of 40--70 pm which are either smooth or apiculate and may be of an Apiculiretusispora type, a c o m m o n l y occurring taxon in dispersed spore assemblages t h r o u g h o u t the Early Devonian. Most of these spores have clearly defined contact areas -- is this significant or could it be indicative of length of time they have been separated from the tetrad at the time of preservation? Additional information is needed on the morphology of rhyniophytes as a whole and on the morphologic nature and variation of their spores. ZOSTEROPHYLLOPHYTINA Zosterophyllophytes are plants which have more or less dichotomously divided axes on which rounded to reniform sporangia are borne laterally. The sporangia are either sessile or are borne on short stalks, with walls several cell layers thick, and m a n y of them possess a distinctly modified dehiscence region. Stelar a n a t o m y is known for some parts of some zosterophyllophytes

109 and typically consists of an exarch protostele. The genera included in this group are very similar to one another, differing in details such as presence or absence of enations, arrangement of sporangia, and way in which sporangia are borne. The considerable degree of similarity suggests that these plants might represent an almost natural group as proposed by Banks (1975a) in his survey of the current status of the zosterophyllophytes. Table II lists the several zosterophyllophyte genera for which in situ spores are known. Often the spores are n o t well characterized because of poor preservation. The following discussion will deal first with the better preserved spores, especially those of Sawdonia, and then some less well understood. Two species of Sawdonia are known at present, S. ornata (Hueber, 1971; Ananiev and Stepanov, 1968) and S. acanthotheca (Gensel et al., 1975). Spores have been described in some detail for the latter species by Gensel et al. (1975) based on both light and scanning electron microscopy. The spores (Plate II, 1, 2) are subcircular, often folded and thin-walled, trilete and 40--74 pm in diameter. They possess a darkened triangular apical area at the juncture of the trilete marks, the trilete rays extend 3/4 the spore radius, and the exine lacks ornamentation. All but a small percentage of the spores lack curvaturae. Many of the spores are covered with spherical globules, often quite densely, which are 0.5--1.0 pm in diameter (Plate II, 2, 3). Again the globules may represent tapetal residue as occurs on spores of Salopella alleni (Edwards and Richardson, 1974) and Renalia hueberi (Gensel, 1976). These also may be comparable to similar entities, referred to as Ubisch bodies, found on pollen grains of some Carboniferous plants (Taylor, 1976) and of certain extant plant genera. Spores of Sawdonia acanthotheca are similar to the sporae dispersae genus Calarnospora Schopf, Wilson and Bentall 1944, especially C. atava (Naum) McGregor and to some extent, C. pannucea Richardson 1965. Those that possess curvaturae are more comparable to Retusotriletes rotundus (Streei) Streel. McGregor (1973) notes that spores referable to these sporae dispersae taxa resemble the inner bodies of dispersed spores called Apiculiretusispora brandtii Streel after the outer sculptured exine layer has become detached. Spores of Sawdonia ornata have been briefly described (Hueber, 1971) but so far only one has been illustrated (McGregor, 1973; plate 2, fig.20) and it resembles spores of S. acanthotheca except that it is obviously curvaturate. It is apparent from the above discussion that spores from sporangia of a given plant or even one sporangium often are quite variable and could be assigned to more than one sporae dispersae taxon (genus or species). For example, in situ spores of several fossil plant taxa show curvaturae present on some but n o t all spores from a single sporangium. Possibly the presence of curvaturae, while useful in a taxonomically descriptive sense with dispersed spores, has a different significance in a biological sense. Spores are also known from several other genera of zosterophyllophytes. Those obtained from sporangia of Rebuchia ovata by Hueber (1972) appear to be poorly preserved or possibly immature as they are extremely thinwalled and folded. The only recognizable (haptotypic) features present are

Reference Edwards (1969a)

Edwards (1969b) Edwards (1970) Hueber (1971) McGregor (1973) Gensel et al. (1975) McGregor and Camfield (1976) Hueber (1972)

Dispersed spore type illustrated, described, not referable to existing sporae dispersae taxon Retusotriletes cf. R. dubius

illustrated, not compared described, not illustrated or compared

cf. Calamospora atava, C. pannucea Retusotriletes rotundus illustrated, described, not compared, poorly preserved or immature?

Taxon

Zosterophyllum llanoveranum

Zosterophyllum cf. fertile

Gosslingia breconensis

Sawdonia ornata

Sawdonia acanthotheca

Rebuchia ovata

Zosterophyllophytes

TABLE II

111

trilete rays and curvaturae. Spores of Gosslingia breconensis illustrated by Edwards (1970, plate 38, figs.48--53), while also not well preserved, are mostly subtriangular, 36-(41.6)-50 pm in diameter, with no visible triradiate marks. Exine ornament is variable, consisting of small spinae, coni, and baculi ranging in height from 0.5 to 2.0 pm, with most being 1.0 pm. Difficulty in distinguishing between true ornament and structures produced by erosion of the exine was noted. Some spores appear to have large areas of u n o r n a m e n t e d wall. Comparison to sporae dispersae was n o t made. Spores have been described and illustrated for two species o f Zosterophyllum, also by Edwards (1969a, b). Those of Z. cf. fertile from the Lower Devonian of South Wales are circular to subcircular, 50--59 pm in diameter, trilete, curvaturate, smooth and with a two-layered exine. The spore masses were enclosed in a granular material possibly representing tapetal residue which disappeared on further oxidation. Z. fertile spores are most similar to the dispersed spore taxon Retusotriletes dubius (Eiseneck) Richardson 1965. Zosterophyllum llanoveranum spores (Plate II, 7) range from 30 to 50 pm in diameter and are covered with spinae or coni up to 10 pm high (Edwards, 1969a). Possible wall ornamentation is difficult to discern; globules representing ornament, remains of o r n a m e n t or possible tapetal residue occur on both proximal (to a limited extent) and distal hemispheres. The spores are apparently filled with mineral and extremely difficult to examine at high magnifications. No comparisons to sporae dispersae were made by Edwards. A plant from New Brunswick whose affinities are closest to Zosterophyllum or Rebuchia has been examined in this laboratory; its spores (Plate II, 4--6) are trilete, curvaturate, 54--65 pm in diameter and smooth or with a chagrenate to finely granulose sculpture (on some). Again, globules of probable tapetal material cover the spores; at high magnifications with the SEM these are observed to be covered with tiny bumps (Plate II, 6). These spores are presently compared to the sporae dispersae taxon Retusotriletes (Naum) Richardson. Further t h e y compare favorably to the spores of the Zosterophyllum species just discussed as much as can be determined from current studies and are quite similar to Sawdonia spores, differing mainly in the presence of well-developed curvaturae on all spores examined, longer trilete rays and in the possible occurrence of minute sculptural elements. Presently, the spores of zosterophyllophytes seem to fit two categories, those of some taxa being smooth walled and of a Calamospora or Retusotriletes type and some possessing sculpture which would more closely resemble an Apiculiretusispora type. They range from 30 to 74 pm in size with most falling in the 40--60 pm size range and thus tend to be larger than those of Cooksonia or Salopella. The presence of probable tapetal material is c o m m o n on spores of zosterophyllophytes but its significance is not presently known. It might be a reflection of degree of maturfty of spores at the time the plant was fossilized, a reflection of type of tapetum in these early plants, or perhaps both. Sometimes preservation precludes clear resolution of surface features, such as determination of sculpture as opposed to tapetal residue.

pml

~r

113 TRIMEROPHYTINA Included in the subdivision T r i m e r o p h y t i n a are plants of Early and Middle Devonian age which are m o r e advanced in their m o r p h o l o g y than r h y n i o p h y t e s b u t are still comparatively simply organized. T h e y consist of d i c h o t o m o u s l y to p s e u d o m o n o p o d i a l l y divided axes, with lateral d i c h o t o m o u s branches, either vegetative or terminating in clusters of fusiform sporangia. Sporangial dehiscence is longitudinal and where know n, sporangial walls are several cell layers thick. Vascular a n a t o m y of the aerial axis is well k n o w n for one of the more complex species o f Psilophyton, P. dawsonii (Banks et al., 1975), and consists of a centrarch protostele. Some similar anatomical data have been obtained for two o t h e r species of Psilophyton (Gensel, 1979). T r i m e r o p h y t e s have m a n y features in c o m m o n and exhibit an almost continuous range of variation. Three genera comprise the best k n o w n t r i m e r o p h y t e s , including Psilophyton (Dawson) emend. Banks et al. (1975) with seven described species, Pertica (Kasper and Andrews, 1972 emend. Doran et al., 1978) with three k n o w n species, and the m o n o t y p i c Trimerophyton robustius (Hopping, 1956). Spores have been obtained f r om sporangia of m any of these species, as listed in Table III, and all are quite similar to one another. Most are referable to the sporae dispersae genus Apiculiretusispora Potoni~ and Kremp. T r i m e r o p h y t e spores all exhibit a partially detached out er sculptured layer ( ex cep t possibly Trimerophyton robustius) and those spores in which the sculptured layer is lost entirely are referable to Retusotriletes or Calamospora. Spores f r om a single sporangium have been observed to range from sculptured to co mpl e t e l y smooth. Spores o f the genus Psilophyton, based on observations of four species, are f r o m 40 to 100 p m in diameter, r ound to r o u n d e d triangular in outline, oft en folded and trilete with simple rays t h a t e xt e nd 1/2 to 1/3 the spore radius (Plate III, 1--7). Often a darkened triangular region surrounds the trilete rays at their p o in t of juncture, and it appears t ha t this area is thicker-walled than the remainder (Plate III, 1, 2, 4) as d e m o n s t r a t e d by Streel (1967) for spores o f P. dawsonii. O r n a m e n t occurs on the distal surface and outside the c o n t a c t areas of the proximal surface (Plate III, 1, 4, 6) and consists of coni,

PLATE II Spores of Sawdonia acanthotheca Gensel et al. 1. LM of two spores, note abundant tapetal residue. G.S.C. #35432. x 604. 2. SEM of spores with abundant tapetal globules. G.C.S. #35431. x 1805. 3. SEM of tapetal globules enlarged. X 4165. Spores of new zosterophyllophyte from New Brunswick. 4. LM showing trilete mark, curvaturae, tapetal globules, x 536. 5. SEM, note curvaturae, chagrenate exine, x 759. 6. SEM, tapetal globules with minute bumps on their surface, x 9383. Spores of Zosterophyllum llanoveranum from slides loaned by Dianne Edwards. 7. LM, proximal view showing smooth contact areas, trilete mark. X 666.

Retusotriletes sp. Apiculiretusispora brandtii same

Retusotriletes cf. R. triangulatus Apiculiretusispora plicata/arenorugosa A. brandtii/A, plicata illustrated, described, not compared

Psilophy ton princeps

Psilophyton forbesii

Psilophyton charientos

Psilophyton dawsonii

Pertica varia

Pertica dalhousii

Trimerophyton robustius

Hopping (1956) McGregor (1973)

Doran et al. (1978)

Granoff et al. (1976)

Banks et al. (1975), Streel (1967)

same

Gensel (1978)

Hueber (1968)

Reference

A feature of the spores of all of the above plants is that the outer sculptured layer of exine tends to slough off to varying degrees (least so in P. forbesii, most in P. princeps and P. dawsonii).

Calamospora atava/pannucea

Dispersed spore type

Taxon

Trimerophytes

TABLE III

115 grana, or baculi less than I pm high. The outer sculptured layer may be intact or partially (Plate III, 1, 3, 4, 6) to completely detached (Plate III, 2). Curvaturae are sometimes observed; where not, they may be truly absent or obscured by sculpturing. A b o u t 20% of spores from sporangia of P. forbesii from Gasp~ and P. charientos from New Brunswick are smaller, darker, thicker-walled (Plate III, 5) and m a y be aborted (Gensel, 1979). Spores of P. princeps appear to vary the greatest in exhibiting a broader size range (up to 95 pm) than those found in other species (up to 70--80 um) and in having a variably shaped darkened area at the juncture of the trilete rays (sometimes triangular and sometimes more circular). Spores of P. princeps obtained by Hueber (1968) were smooth and compared b y him to Retusotriletes and Phyllothecotriletes (= Retusotriletes according to Richardson, 1969). My preparations of spores of P. princeps include types referable to b o t h of the above-mentioned sporae dispersae taxa (Plate III, 2) as well as sculptured ones (Plate III, 1, 3) nearly indistinguishable from those of P. forbesii, P. dawsonii or P. charientos from New Brunswick. Spores of Psilophyton dawsonii that were obtained from compressions (Abitibi River) are identical to those o f P . forbesii from Gasp~ (Gensel, 1979). In spores of P. dawsonii obtained from petrifactions the outer sculptured layer is more often detached, or the sculpturing present is somewhat more irregular in pattern than in the compression material. This could represent further variation in spore morphology or perhaps these differences are caused by preservational factors. Further investigation is needed. Spores of Dawsonites arcuatus from the Strathmore Beds (Emsian) of Scotland (Lang, 1932) are identical to spores of the above Psilophyton species as are spores found adhering to tapetal membranes of sporangia referred to Dawsonites b y Chaloner et al. (1978) from the Emsian of the Apley Barn Borehole, Oxfordshire. Detaching of the sculptured layer was not observed, however. Ornamented spores of Psilophyton are most closely comparable to the sporae dispersae species Apiculiretusispora brandtii Streel 1964 or A. plicata Streel 1967 whereas smooth ones have been compared most frequently to Retusotriletes triangulatus Streel 1967. Spores are k n o w n for t w o species of Pertica; those of P. varia (Plate III, 8, 9) differ from spores of Psilophyton in lacking any evidence of a darkened triangular area and most closely resemble Apiculiretusispora plicata or A. arenorugosa, depending on their size (Granoff et al., 1976). Spores of Pertica dalhousii from New Brunswick, although poorly preserved, are closely comparable to those of P. varia (Doran et al., 1978). Spores of Pertica also exhibit a detaching outer sculptured layer. Hopping (1956) briefly described and illustrated spores of Trimerophyton robustius b u t did n o t compare them to any dispersed spore genus. They are 40--63 pm (ave. 52), smooth-walled, with apparently fine ornamentation. Hopping noted that a few larger spores -- up to 82 pm, were associated with the smaller ones. He compared them all to spores of Psilophyton princeps

13

117

(sensu Sawdonia ornata) d e s c r i b e d b y L a n g ( 1 9 3 1 ) f r o m Gasp~. M c G r e g o r ( 1 9 7 3 ) suggests t h a t spores o f T. robustius r e s e m b l e t h e d i s p e r s e d s p o r e species Calamospora atava a n d C. pannucea. T h e n a t u r e o f t h e o u t e r , o f t e n d e t a c h i n g , s c u l p t u r e d l a y e r in t r i m e r o p h y t e spores is p r o b l e m a t i c a l . I t has b e e n suggested b y B a n k s et al. ( 1 9 7 5 ) and Streel ( 1 9 6 7 ) t h a t this l a y e r m a y r e p r e s e n t a p o s s i b l e p e r i s p o r e or t h a t it b e c o m e s d e t a c h e d as t h e s p o r e s r e a c h m a t u r i t y . P r e l i m i n a r y findings o f ultras t r u c t u r a l studies o f s p o r e s o f Psilophyton forbesii s h o w t h a t t h e o u t e r l a y e r is identical to the i n n e r o n e (Plate I I I , 1 0 b a n d Gensel, 1 9 7 8 ) , s o m e t i m e s continuous with the underlying layer and sometimes separated. These data plus t h e o b s e r v a t i o n s t h a t t h e o u t e r s c u l p t u r e d l a y e r d o e s n o t e x t e n d c o m p l e t e l y a r o u n d t h e s p o r e b o d y (Plate III, 3, 6, 8, 9) d o n o t s u p p o r t interp r e t i n g it as a p e r i s p o r e , at least as f o u n d in living ferns. In the latter, perispores are a c o n t i n u o u s l y s u r r o u n d i n g l a y e r c o m p o s e d o f t a p e t a l residue d i f f e r i n g in c o m p o s i t i o n a n d u l t r a s t r u c t u r e f r o m t h e r e s t o f t h e s p o r e wall (Lugardon, 1972, 1974). D i f f e r e n c e s in p r e p a r a t i o n t e c h n i q u e and p r e s e r v a t i o n a p p e a r t o a f f e c t t h e e x t e n t to w h i c h t h e o u t e r l a y e r is d e t a c h e d . P r e p a r a t i o n s o f P. princeps spores involving b o t h S c h u l z e ' s s o l u t i o n a n d dilute base y i e l d e d s m o o t h spores while b o t h s m o o t h a n d o r n a m e n t e d spores w e r e r e m o v e d f r o m u n t r e a t e d s p o r a n g i a b y m e c h a n i c a l l y b r e a k i n g t h e m a p a r t w i t h a needle. S p o r a n g i a p r e s e r v e d in m o r e c a l c a r e o u s s e d i m e n t s (P. princeps, P. dawsonii) s e e m to lack t h e o r n a m e n t e d l a y e r m o r e f r e q u e n t l y t h a n t h o s e f r o m less c a l c a r e o u s s e d i m e n t s . S o m e biological f a c t o r s ( e x t e n t o f m a t u r i t y as i n d i r e c t l y suggested b y B a n k s et al., 1 9 7 5 ) m i g h t also be i m p o r t a n t since d e t a c h i n g is o b s e r v e d in all t r i m e r o p h y t e s f o r w h i c h s p o r e s are k n o w n ( e x c e p t p e r h a p s T. robustius). I t also is o b s e r v e d a m o n g sporae dispersae (Apiculiretusispora) and is illus-

PLATE III Spores of Psilophyton princeps (Dawson) Hueber. 1. LM of spore with intact sculptured layer, x 410. 2. LM of spore having lost all ornamentation; note darkened area adjacent to trilete rays. x 361. 3. SEM of spore showing partially detaching ornamentation, x 926. 4--7, 10. Spores of Psilophyton forbesii from Gasp~. 4. LM of spore with intact ornamentation, darkened area adjacent to trilete rays. G.S.C. #57883. x 339. 5. LM of two spores, lower dark one is probably aborted. G.S.C. #57883. x 356. 6. SEM proximal view showing smooth contact areas, x 856. 7. SEM of distal view, ornament is intact, x 875. 10. TEM of spore section, a = spore wall; b = outer sculptured layer of spore wall; c = tapetal material. The region interpreted as tapetal material appears darker immediately adjacent to the spore wall in this photo but not in every section examined; it therefore may be artifact, x 4463. Spores of Pertica varia Granoff et al., from Gasp4. 8. LM proximal view, showing smooth contact areas, sculpture outside. G.S.C.#43229.X 561. 9. SEM of spore with partially detached ornament, smooth contact areas, x 712.

118

trated by Richardson (1969, plate II-2, fig.12; plate II-3, fig.7) and Streel (1967, plate II, figs.18--20). Both authors compare their dispersed spores to those of P. dawsonii. F u r t h e r Richardson and Rasul (1978) describe an assemblage of spores from Lower Devonian sediments obtained from the Apley Barn Borehole in which the d o m i n a n t species is Apiculiretusispora brandtii showing degrees of detaching of the outer layer. Sporangia referable to Dawsonites and vegetative axes of Sawdonia ornata have been described from the Apley Barn sample by Chaloner et al. (1978) and spores o f the former taxon were compared to Apiculiretusispora. In summary, spores of trimerophytes may be characterized as an Apiculiretusispora type which frequently loses its external sculptured layer; if this is completely lost, the spores are more similar to Calamospora or Retusotriletes, depending on absence or presence of curvaturae. The similarity of spore morphology among the three trimerophyte genera supports the suggestion of Banks (1975b) and other workers that the trimerophytes are very closely related plants. In m a n y instances it seems probable that certain dispersed spores, such as Apiculiretusispora spp., were produced by trimerophytes, but caution must be used since apparently some zosterophyllophytes, Krithodeophyton Edwards, and C. crassiparietalis possess a quite similar apiculate spore. LYCOPHYTINA

Devonian fossils referable to the lycophytes are k n o w n from Siegenian -Famennian and include a number of genera preserved to varying extents and quality. Features such as small univeined leaves, adaxial reniform or ellipsoidal sporangia with distal dehiscence, and a solid terete to lobed exarch xylem strand characterize these plants. Sufficient evidence is available to suggest that lycophytes have been a distinct group since Early Devonian times and have the "longest u n b r o k e n record in time of any group of vascular plants" (Banks, 1975c, p.732). Spores are k n o w n from sporangia of only a few Devonian lycophytes, shown in Table IV, and of these only one taxon is homosporous. This will be dealt with first. The best known in situ Devonian l y c o p h y t e spores are those of Leclerqia cornplexa described by Banks et al. (1972), Streel (1972), and Bonamo and Grierson (1973) from the late Middle Devonian of New York State. These spores are extremely abundant, exhibit several ontogenetic stages and are morphologically distinctive. The spores are subcircular to subtriangular in shape, trilete and 60--85 pm in diameter exclusive of ornamentation. The trilete rays are bordered by raised folds. Sculpture on the proximal and distal faces consist of verrucae and biform spines 5--9 pm high, with those on the proximal contact areas being reduced in size. The verrucae on the distal surface may fuse to produce muri. A thickened or rigid band may parallel curvaturae (Streel, 1972) and a few spines are terminated by a cupshaped element. Streel (1972) named these spores Aneurospora cf. heterodonta

Banks et al. (1972) Streel (1972) Fairon-Demaret ( 1977) Chaloner (1968)

Aneurospora (Acanthotriletes) cf. heterodonta not compared; trilete, verrucate megaspore Lagenicula-type megaspore

Leclercqia complexa

Barsostrobus famennensis

Cyclostigma k iltork ense

Reference

Dispersed spore type

Taxon

Lycophyta

TABLE IV

CD

120 (Naum) Streel. They also strongly resemble Acinosporites lindlarensis Riegel 1968 although some possible significant differences may exist (McGregor and Camfield, 1976). Banks et al. (1972) described y o u n g spore tetrads 70 pm in diameter, so that each spore was about 1/4 the typical size. Crowded ornament smaller than on mature spores was observed on their distal surface. Nearly mature spores in tetrads, still n o t fully ornamented, approach the size range of fully mature spores that had separated from their tetrads. Some lycophyte strobili of Late Devonian age contain megaspores; for example, megasporangia of Cyclostigma kiltorkense (Chaloner, 1968) produce a Lagenicula-type megaspore. Another lycophyte cone fragment named Barsostrobus famennensis (Fairon-Demaret, 1977) contains megaspores which u n f o r t u n a t e l y are preserved so that they can only be examined with reflected light or SEM. They are trilete, with slightly elevated rays, and are ornamented with low verrucae. Ornamentation is reduced on the contact areas. No comparisons to sporae dispersae could be made. Megasporangia containing megaspores and microsporangia containing microspores of a cone fragment of presumed lycophyte affinities from the Upper Devonian of Petino, U.S.S.R. was named Kryshtofovichia africani by Nikitin (1934). McGregor (1969) described the megaspores and microspores in more detail, and recorded t h e m from other localities. Megaspores with prominent spines called Nikitinsporites Chaloner occur in megasporangia while monolete microspores referable to the sporae dispersae genus Archaeoperisaccus (Naum) Potoni~ 1958 have been found adhering to some megaspores and in microsporangia. Monolete spores occur only rarely in Devonian dispersed spore assemblages (McGregor, 1977; Gensel, 1977) and this is the only record of in situ ones. Additionally, Archaeoperisaccus has only been found in Devonian rocks occurring in extreme northern latitudes and may be geographically restricted. It would be of considerable interest to obtain data on spore morphology of other Devonian lycophytes, as well as of plants which are either very primitive lycophytes or putative pre-lycophytes such as Asteroxylon or Kaulan. giophyton. Such information would n o t only add to our knowledge of these plants but also might show trends in spore morphology in lycophytes, including earliest occurrence of heterospory in that group, and perhaps aid in elucidating evolutionary relationships in this long-lived and distinctive group of plants. CLASS PROGYMNOSPERMOPSIDA Table V includes a list of progymnosperm taxa for which in situ spores are known. Spores of these taxa exhibit a two-layered wall which may possibly be of some significance, perhaps as an evolutionary stage presaging the advent of pollen grains. Spores of these plants will be discussed in more detail in a forthcoming paper by K.C. Allen (pers. comm.) so they will n o t be dealt with further here.

Rhabdosporites langi Rhabdosporites langi Aneurospora

Tetraxylopteris schmidtii

Rellimia thomsoni

Aneurophyton germanicum

Rhacophyton ceratangium

Perotrilites cf. perinatus

PROTOPTERIDIALES sensu Cornet, Phillips and Andrews 1976 (pre-ferns)

L ycospora svalbardeae

Svalbardia polymorpha

A. halliana

A. macilenta Microspores: Cyclogranisporites/Geminospora type

Megaspores: Biharisporites

Archaeopteris cf. jacksoni

A. latifolia

Dispersed spore type

Taxon

Progymnospermopsida

TABLE V

Andrews and Phillips (1968)

Streel (1964)

Leclercq and Bonam o ( 1971 )

Bonamo and Banks (1966)

Vigran (1964)

Phillips et al. (1972)

Pettitt (1965)

Reference

122 PROTOPTERIDIALES (Hoeg, 1942) emend. Cornet et al., 1976 The order Protopteridiales has recently been re-established with slight emendation by Cornet et al. (1976) to include plants with a more or less fern-like habit whose branch systems are frond-like but exhibit little or no distinction between stems and leaves. Sporangia are borne terminally on ultimate non-laminate divisions. Cornet et al. (1976) suggest that this category should be useful for plants that are transitional between ~rimerophytes and ferns or pteridosperms, or that m a y resemble progymnosperms in external morphology but are either n o t k n o w n anatomically or may n o t be in a direct line of gymnosperm evolution. Included in this order are Protocephalopteris, Cephalopteris, and Rhacophyton. Quite possibly other plants could be included as well, such as the Carboniferous taxa Chlidanophyton dublinensis Gensel and Stauropteris spp. The spores of Rhacophyton ceratangium from the Upper Devonian of West Virginia were described by Andrews and Phillips (1968). They are broadly ovoid, 50--70 pm in diameter, trilete and typically possess an outer wrinkled transparent wall that possibly could be termed a perispore. The perispore is ornamented with grana and some coni, all not greater than 1 pm high. The spores compare welt with the sporae dispersae taxon Perotrilites (Erdtman) Couper, especially P. perinatus (Hughes and Playford, 1961) or P. minor (Owens, 1971). TEM sections of Rhacophyton spores might better elucidate the nature of the probable perispore and determine if the wall differentiation is different from the pseudosaccate or mesosporoid condition of progymnosperm spores such as Rhabdosporites (the spores of Tetraxylopteris and Rellimia). It is also of interest that a plant considered by some as related to the zygopterid ferns (Leclercq, 1951; Leclercq and Bonamo, 1971) and by others as a "prefern" (Cornet et al., 1976) possess probably perinate spores, since most or all living ferns possess perinate spores (Lugardon, 1972, 1974).

INCERTAE SEDIS Some plants whose affinities are uncertain contain spores which merit consideration (Table VI). Oocampsa catheta, described by Andrews et al. (1976) from the late Early Devonian of New Brunswick, Canada, is regarded as a possible intermediate between trimerophytes and progymnosperms, as its branching system is clearly more complex than those of trimerophytes and somewhat reminiscent of, yet n o t as complex as, the branching systems of progymnosperms such as Tetraxylopteris or Archaeopteris (see Andrews et al., 1976). Spores of Oocampsa are quite different from spores of any plants discussed thus far; t h e y are trilete, 90--120 pm in diameter and are zonate or possibly pseudosaccate (Plate IV, 1--3). Ornamentation of cones up to 8 pm long, often biform, occurs on the distal hemisphere. The trilete mark of the inner body is simple and folds of the outer layer overlie the trilete rays. The folds extend nearly to the equator. Oocampsa spores are

Samarisporites praetervisus or Grandispora macrotuberculatus large: Apiculiretusispora gaspiensis and Apiculatisporis microconus small: Camarozono triletes sex tan tus

Retusotriletes simplex

Oocampsa catheta

Chaleuria cirrosa

Cathaiopteridium (Protopteridium ) minutum Halle (Obhrel)

Vigran (1964)

Bonamo and Banks (1966) Leclercq ( 1957 )

cf, Calamospora (large ones); small ones n o t referable to any dispersed spore taxon, without wrinkled layer are like Calamospora

Calamospora atava like microspores of B. richardsonii Calamospora pannucea mega -- Enigmophytospora simplex micro -- Re tuso triletes (Phyllothecotriletes microgranulatus)

Dibolisporites Acanthotriletes

Barinophyton richardsonii

Protobarinophyton obrutschevii

Barinophyton-like plant of Hueber (illustr:, in Pettit, 1970)

Enigmophyton superbum

Calamophyton bicephalum

Eviostachya hf~egii

Pettitt (1970) McGregor (1973 )

McGregor (1973)

Pettitt (1965, 1970)

cf. Apiculiretusispora brandtii

Edwards (1968)

McGregor (1973)

Andrews et al. (1974), also McGregor and Camfield (1976)

Andrews et al. (1976)

Reference

Krithodeophyton croftii

Barinophytaceae:

Dispersed spore type

Taxon

Incertae sedis or plants of various affinities

TABLE VI

b*z

¢,J1

<

....-I

.-3

125 most closely comparable to the sporae dispersae t axon Samarisporites praetervisus Allen 1965, which is placed in the genus Grandispora by some workers (McGregor, 1973). Oocampsa spores are the first k n o w n occurrence of in situ zonate spores. It was suggested by Andrews et al. (1976) t hat these spores may exhibit a wall structure t h a t might presage the two-layered construction o f p r o g y m n o s p e r m spore and g y m n o s p e r m pollen grain walls. Chaleuria cirrosa, a n o t h e r plant o f uncertain affinities from the late Early Devonian o f New Brunswick, was described b y Andrews et al. (1974) and is of considerable interest in a n o t h e r c o n t e x t . Spores obtained from sporangia o f this plant are o f t w o sizes and morphologies (Plate IV, 4--7), suggesting an early or incipient stage in the evolution of heterospory. The larger spores (Plate IV, 4, 5) are f r om 60 to 156 pm in diameter, nearly circular, with trilete rays either simple or with raised or folded lips. The trilete rays are straight to sinuous, c o n t a c t areas are s m o o t h and delineated by o r n a m e n t of grana up to 1.5 pm high; o r n a m e n t also covers the distal surface. These spores are referable to the sporae dispersae genus Apiculiretusispora, especially A. gaspianus (McGregor and Camfield, 1976). Some o f the largest spores (see Andrews et al., 1974, plate 56, fig.4) lack distinct c o n t a c t areas or curvaturae and are comparable to Apiculatisporis microconus. The small spores (Plate IV, 6, 7) are 30--48 # m in diameter, trilete, sm oot h proximally and o r n a m e n t e d distally with baculi and coni 1--3 pm high. T h e y were originally co m pa r ed to the sporae dispersae taxa Procoronospora B u t t e r w o r t h and Williams and Anapiculatisporites burnotense Streel 1967, b u t do n o t c o n f o r m well to either; McGregor and Camfield (1976) have since described identical spores as Camarozono triletes sex tan tii. Chaleuria s o mew hat resembles taxa such as Aneurophyton (progymnosperms), and also has some features comparable to genera of the Cladoxylales such as Ibyka, Calamophyton, and Hyenia. Its a n a t o m y is u n k n o w n and presently Chaleuria is regarded as Incertae sedis. A plant o f quite different affinities, Barinophyton richardsonii from the Upper Devonian of Perry Basin, Maine, was described by Pettitt (1965) as possessing spores o f two sizes. Both large spores, 220--250 pm in diameter, and small spores, 48--62 pm in diameter, resemble the sporae dispersae t axon Calamospora Schopf, Wilson and Bentall. The smaller ones may differ in possessing a thin, wrinkled o u t e r covering which is n o t k n o w n in any PLATE IV Spores of Oocampsa catheta Andrews et al. 1. LM, general view. G.S.C. #41702. x 475. 2. SEM showing smooth proximal surface, raised folds along trilete. G.S.C. #41703. x 595. 3. SEM,showing spines on distal surface. G.S.C. #41703. × 858. Spores of Chaleuria cirrosa Andrews et al. 4, 5. Large spores, one with straight and one with sinuous trilete rays. 4. G.S.C. #33266. x 484.5. G.S.C. #33267. x 603. 6, 7. Small spores, two levels of focus. G.S.C. #33265. x 756.

126 dispersed spore of t h a t morphological type -- could this perhaps be tapetal residue? Pettitt (1965) noted that both megaspores and microspores apparently occur together in one sporangium, but he suggested that this most probably was the result of preservation of megasporangia and microsporangia occurring closely adjacent to one another. A second Upper Devonian species, B. citrulliforme, is also cited as being heterosporous (Pettitt, 1970; Brauer, 1977, 1978) and possibly here too large spores occur in the same sporangium as small ones (Brauer, 1978). A plant tentatively referable to Protobarinophyton (Brauer, 1978) is also heterosporous. Spores resembling the sporae dispersae taxon Calamospora were obtained from a Barinophyton-like plant, collected by F.M. Hueber and illustrated by Pettitt (1970), which also exhibit a considerable range in size (97--240 pm). Further information on spores of these plants will be of considerable interest in regard n o t only to the occurrence of heterospory but to the possible occurrence of spores of two sizes (? mega- and micro-) in one sporangium. Another plant exhibiting spores of two sizes is Enigmophyton superbum from the Upper Devonian of Spitsbergen (H4eg, 1942; Vigran, 1964). As has been noted by Pettitt (1970), the reproductive organs attributed to this genus have not been found in a t t a c h m e n t with the leaves and are quite similar to the fertile region of Barinophyton. Enigmophyton spores were named by Vigran (1964) as follows: large ones - - E n i g m o p h y t o s p o r a simplex; and small ones -- Phyllothecotriletes microgranulatus (now Retusotriletes). The information provided by spores of the several above-mentioned plants, plus evidence from dispersed spores that spore size had reached 200 pm by Emsian times (Chaloner, 1967; Richardson, 1967; Pettitt, 1970; McGregor, 1973) points to the strong probability that the evolution of heterospory was well underway and perhaps accomplished by the end of the Early Devonian and possibly in more than one plant group or evolutionary line (Chaleuria, barinophytes, progymnosperms?, possibly lycophytes). New finds or further investigation might provide further insight on the advent of and mechanisms involved in the establishment of heterospory. SUMMARY Although m a n y questions remain and gaps exist, progress has clearly been made in learning more about the morphology and variability of spores within taxonomic categories ranging from species to subdivisions of Devonian plants. It is now possible to m o d i f y to some extent earlier surveys of in situ spores and to begin to evaluate spore morphology of certain plants in terms of morphologic construction, such as trilete--apiculate--curvaturate (trimerophytes, large spores of Chaleuria, other plants) or trilete--zonate--echinate (Oocampsa). In some cases spores may ultimately provide more clues concerning the accuracy of taxonomic designations of megafossils, especially above the generic level, although some plant groups may continue to be heterogeneous in terms of spore types. Specific comments follow.

127 (1) While preservation or other factors preclude thorough characterization of spores of rhyniophytes, those of Gedinnian--Siegenian age appear to have less specialized spores than geologically younger taxa, being small, smooth and curvaturate. (2) Zosterophyllophyte spores appear to be of two types; smooth ones referable to Calamospora and ornamented ones referable to (probably) Apiculiretusispora. Lack of good preservation often makes interpretation of aspects of ornamentation problematical and it is possible that the apparent sculpture of some zosterophyllophyte spores (e.g., Z. llanoveranum) might instead be tapetal residue. Better preserved zosterophyllophyte spores are covered to varying extents with rounded globules of tapetal residue or Ubisch bodies. The significance of this material is as yet u n k n o w n b u t it may reflect the nature of the tapetum in zosterophyllophyte sporangia or perhaps the extent of maturity of the spores or both. (3) Spore morphology of the trimerophytes is n o w quite well elucidated; the spores are characterized as an Apiculiretusispora type. Trimerophyte spores often show varying degrees of detachment of the outer ornamented layer and those that have lost all ornament are comparable to Calarnospora or Retusotriletes. (4) Little information is available on spores of Devonian lycophytes; one homosporous and a few heterosporous taxa are known in part. (5) Progymnosperm spores often possess two wall layers and Rhacophyton spores appear to have a perine; examination of the possible significance of this organization in terms of evolutionary potential or mode of reproduction would be of interest. (6) The zonate, possibly cavate spores of Oocampsa are quite distinctive in relation to other plants with in situ spores, although possibly of an organization that might presage the development of a two-walled spore or putative pollen grain. (7) Little is k n o w n of articulates in the Devonian, although putative ones or possible immediate precursors have been suggested (Skog and Banks, 1973; Scheckler, 1974). In situ spores are k n o w n for Calamophyton bicephalum from the Middle Devonian of New York State (Bonamo and Banks, 1966) and for the arthrophyte cone Eviostachya hdegii (Leclercq, 1957). Both possess ornament of spines over 1 pm high, thus differing from spores of many arthrophytes of CarboniferOus age. (8) Early Devonian plants of problematical b u t quite obviously different affinities appear to exhibit early stages in the development of heterospory, i.e., Chaleuria and Barinophyton. Heterospory is also known in some Middle and Late Devonian progymnosperms and lycophytes. More information, including ultrastructural data, is needed. (9) Comparatively speaking, spore morphology appears to differ little or not at all between species of some genera, such as Psilophyton, Pertica, and Archaeopteris or sometimes between genera (Tetraxylopteris and Rellimia) as presently known or defined. Conversely, plants of quite different affinities may possess similar spores; for example Krithodeophyton, Cooksonia crassi-

128

parietalis, Psilophyton and possibly some Zosterophyllum species have trilete, apiculate, curvaturate spores of an Apiculiretusispora type. More data are needed, either on spore morphology or morphology of these megafossils to determine if this similarity is consistent; obviously similarity of spore morphology presently cannot always be utilized as a criterion for uniting plants in a single genus or even species. SOME LIMITATIONS It often is difficult to compare spores from sporangia to sporae dispersae taxa as in situ spores may possess some characters of one sporae dispersae taxon and some characters of another one. Often a difference in presence or absence of curvaturae or height of sculptural elements will relegate spores from a single plant or sporangium to quite different sporae dispersae taxa. Ultimately it may be possible to better define some dispersed spore taxa because of the variability observed among in situ ones. A wide variety of dispersed spore types remains for which parent plants are unknown. Many are zonate or pseudosaccate forms or types with distinctive and/or prominent sculptural elements (e.g., Ancyrospora--Hystrichosporites types, Brochotriletes, Emphanisporites, Spelaeotriletes, etc.) and m a n y are distinctive of some mid-Late Devonian assemblages. As has been often asked, what kinds of plants produced these types and are they all spores of vascular plants? Some possible avenues for future investigation, in addition to continuing to d o c u m e n t the nature of spores produced by various types of fossil plants, include the following: (1) a survey of the range of morphology exhibited by spores obtained from one sporangium of a given taxon; (2) if preservation or available material permits, an examination of ontogenetic stages in the development of spores of Devonian plants; and (3) investigation of the ultrastructure of in situ spores in order to determine better the nature of their wall structure for comparison with living pteridophyte spores or to clarify features of spore morphology t h a t are presently difficult to interpret. In this context, it is particularly pertinent to examine the ultrastructure of Rhacophyton spores, with their probable perine; the zonate, possibly pseudosaccate Oocampsa spores, or the spores of plants such as Chaleuria and Barinophyton which seem to indicate early stages in the development of heterospory. As more information becomes available on spore morphology and ultrastructure of Devonian plants, we may be able to gain not only a better understanding of kinds of spores produced by certain kinds of plants, but also deduce more accurately evolutionary trends within and among these plant groups. Additional evidence of this nature could be applied to assessing better what types of plants contributed to the m a n y dispersed spore assemblages known for various stages of the Devonian.

129 ACKNOWLEDGMENTS

The author expresses appreciation to the following persons for helpful discussion: John Richardson, Colin McGregor, Maurice Streel, Dianne Edwards, Keith Allen, H.N. Andrews and F.M. Hueber. Grateful appreciation is extended to the following persons for making available preparations of in situ spores for study: D. Edwards, F.M. Hueber, Ian Rolfe (Hunterian Museum, Glasgow), Cedric Shute (Department of Palaeontology, British Museum Natural History, London). Thanks are also extended to R. Malcolm Brown, Jr. and Alan White for assistance with the TEM studies, and to Tom L. Phillips and William C. Dickison for critically reading the manuscript. Funds for some of the research discussed here were provided by a University of North Carolina Faculty Research Grant and NSF Grant DEB 77-02945 to Patricia G. Gensel and by several NSF grants to H.N. Andrews. REFERENCES Allen, K.C., 1965. Lower snd Middle Devonian spores of north and central Vestspitsbergen. Palaeontology, 8(4): 684--748. Ananiev, A.R. and Stepanov, S.A., 1968. Finds of sporogenous organs of Psilophyton princeps Dawson emend. Halle in the Lower Devonian of the South-Minusinsk hollow. Treatises Order Red Banner of Labor State Univ. Geol. Ser., 2 0 2 : 3 0 - - 4 6 (in Russian). Andrews, H.N. and Phillips, T.L., 1968. Rhacophyton from the Upper Devonian of West Virginia. J. Linn. Soc. (Bot.), 61(384): 37--64. Andrews, H.N., Gensel, P.G. and Forbes, W.H., 1974. An apparently heterosporous plant from the Middle Devonian of New Brunswick. Palaeontology, 17(2): 387--408. Andrews, H.N., Gensel, P.G. and Kasper, A.E., 1976. A new fossil plant of probable intermediate affinities (Trimerophyte--Progymnosperm). Can. J. Bot., 53(16): 1719-1728. Andrews, H.N., Kasper, A.E., Forbes, W.H., Gensel, P.G. and Chaloner, W.G., 1977. Early Devonian flora of the Trout Valley F o r m a t i o n of northern Maine. Rev. Palaeobot. Palynol., 23: 255--285. Balme, B.E. and Hassell, C.W., 1962. Upper Devonian spores from the Canning Basin, Western Australia. Micropaleontology, 8(1 ): 1--28. Banks, H.P., 1968. The early history of land plants, In: E. Drake (Editor), Evolution and Environment: A symposium Presented on the Occasion of the One Hundredth Anniversary of the F o u n d a t i o n of Peabody Museum of Natural History at Yale University, Yale University Press, New Haven, Conn., pp.73--107. Banks, H.P., 1975a. Paleogeographic implications of some Siluro-Early Devonian floras, in: K.S.W. Campbell (Editor), Gondwana Geology. Australian National University Press, Canberra, A.C.T., pp.71--97. Banks, H.P., 1975b. Reclassification of Psilophyta. Taxon, 24(4): 401--413. Banks, H.P., 1975c. Early vascular plants: proof and conjecture. BioScience, 25(11 ): 730--737. Banks, H.P., Bonamo, P.M. and Grierson, J.D., 1972. Leclercqia complexa gen. et sp. nov., a new lycopod from the late Middle Devonian of eastern New York. Rev. Palaeobot. Palynol., 14: 19--40. Banks, H.P., Leclercq, S. and Hueber, F.M., ]975. A n a t o m y and morphology of Psilophyton dawsonii, sp.n. from the late Lower Devonian of Quebec (Gasp~) and Ontario, Canada. Palaeontogr. Am., 8: 73--127.

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