More evidence for a slender growth habit in Mesozoic cycadophytes

Review of Palaeobotany and Palynology, 21(1976): 93--100 @Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MORE EVIDENCE FOR A S L E N D E R GROWTH HABIT IN MESOZOIC CYCADOPHYTES

T. DELEVORYAS and R. C. HOPE Department of Botany, University of Texas, Austin, Texas (U.S.A.) Department of Geology, Campbell College, Buie's Creek, IV.C. (U.S.A.) (Accepted for publication February 11, 1975)

ABSTRACT Delevoryas, T. and Hope, R. C., 1976. More evidence for a slender growth habit in Mesozoic cycadophytes. Rev. Palaeobot. Palynol., 21: 93--100. From the Pekin Formation (Upper Triassic) of the Deep River basin in central North Carolina, U.S.A., originate remains of a slender cycadeoidalean (bennettitalean) stem with leaves of a type combining features of the form genera Otozamites and Zamites. The plant, placed in the new genus and species Ischnophyton iconicum, is additional evidence that the common growth habit of Triassic and Jurassic cycadophytes was one involving a slender stem, without closely spaced, persistent leaf bases.

INTRODUCTION

In r e c e n t papers (Delevoryas, 1968, 1 9 7 5 ; Delevoryas and H o p e , 1971) we c o m m e n t e d o n the g r o w t h h a b i t o f certain Mesozoic c y c a d o p h y t e s and we i n d i c a t e d t h a t t h e usual p a t t e r n o f g r o w t h was o n e involving a slender stem, b r a n c h e d or u n b r a n c h e d , with loosely arranged leaves t h a t were periodically d e c i d u o u s . Certainly the bulk o f the k n o w n c y c a d o p h y t e s f r o m the Mesozoic, especially f r o m the Triassic and Jurassic periods, p o i n t e d t o such a h a b i t t h a t differs c o n s i d e r a b l y f r o m t h a t o f m o s t o f the e x t a n t m e m b e r s o f t h e Cycadales. T h e latter generally have s t o u t stems with closely spaced, persistent leaf bases. F u r t h e r m o r e , leaves o f living cycads d o n o t d r o p o f f while still e x p a n d e d , b u t usually shrivel u p b e f o r e separating f r o m t h e stem. A d m i t t e d l y , t h e r e is n o t m u c h in the way o f d i r e c t evidence to d e m o n strate the h a b i t o f fossil c y c a d o p h y t e s . In fact, v e r y f e w r e c o n s t r u c t i o n s are based o n the availability o f a t t a c h e d organs. Much o f the s p e c u l a t i o n is based o n the a p p e a r a n c e o f isolated stem fragments, and the size, a b u n d a n c e , and d i s t r i b u t i o n o f leaf remains in the sediments. S o m e r e c o n s t r u c t i o n s p r e s e n t e d in t h e past t h a t n e e d to be regarded with skepticism are t h e following: Bjuvia simplex, Williamsonia gigas, Williamsonia sewardiana, and Weltrichia mirabilis. Bjuvia simplex, described b y Florin ( 1 9 3 3 ) is based o n l y o n isolated m e g a s p o r o p h y l l s called Palaeocycas integer

94 and leaves of the Macrotaeniopteris type. The reconstruction shows a stout, erect stem, at the apex of which are borne the macrotaeniolSterid leaves and megasporophylls. The stem, however, is purely hypothetical. Although the reconstruction of Williamsonia gigas Williamson, 1870 utilized leaves actually attached to stem fragments, only a very small part of the stem is known and a little uncertainty exists with regard to the appearance of the whole plant. The reconstruction of Williamsonia sewardiana Sahni, 1932 is well known and often reproduced, but attachment of leaves to a stem in the fossil record has not been demonstrated. Schuster (1911) portrayed the habit of Weltrichia mirabilis, but it is purely imaginary. Occasionally, however, specimens appear in which organs are organically connected and the resulting reconstructions are considerably more accurate. Wielandiella angustifolia Nathorst, 1909, from the Rhaetian of Sweden, is known from stems, leaves, and cones, and the reconstruction shows a slender, branching plant with leaves borne in tufts at the points of branching. The Jurassic Williamsoniella coronata from Yorkshire, England, is also known from stem fragments, leaves, and reproductive structures. Both Thomas (1915) and Zimmermann (1933) show it as a slender, branched plant. They differ in their ideas concerning the position of cones. Although the various pieces were not connected, Harris (1961), using a number of pieces of evidence, reconstructed a Jurassic member of the Cycadales with parts consisting of leaves of the Nilssonia tenuinervis type, pollen cones called Androstrobus wonnacottii, and seed cones known as Beania mamayi. No stem is known, but because the leaf appears to have separated from the stem cleanly, it is presumed that no persistent leaf bases were left on the stem that bore them. Another cycadophyte, this one a cycadeoidalean (bennettitalean), was reconstructed by Harris (1969) on the basis of material from the Jurassic of Yorkshire. Consistent association of stems of Bucklandia pustulosa, leaves of Ptilophyllum pecten, and ovulate cones of Williamsonia leckenbyi led Harris to suggest a much branched plant with some of the distal axes quite slender. More positive evidence for the existence of at least one type of slender cycadalean with loosely spaced leaves and a fairly smooth stem was presented by Delevoryas and Hope (1971) when we showed a reconstruction of Leptocycas gracilis from the Upper Triassic of central North Carolina. This is the oldest member of the Cycadales that allows a fairly complete restoration. Other evidence exists for the presence of slender cycadophytes during the Triassic and Jurassic periods in the form of abundant stem remains. For example, Bose (1953a, 1953b) described a number of cycadophytic stems that are quite slender and often branched. Wieland (1914--1916) also showed a number of stem fragments from the Jurassic of Oaxaca, Mexico, that he assigned to Williamsonia. Most of these stem pieces are slender, the largest one only about 6 cm in diameter. Professor T. M. Harris, a long time student of Mesozoic cycadophytes, is inclined to agree that at least the distal parts of these plants were slender, but

95 felt t h a t the plants were actually large trees (personal communication, 1974). Obviously, the only way to be certain of the habit of plants preserved as fossils is to have all parts attached. With periodic finds such as that of Leptocycas we continue to make progress along that direction. DESCRIPTION OF NEW MATERIAL Further evidence for the prevalence of cycadophytes with slender growth habit is described in this paper. It is in the form of compression material from the Pekin Formation~ Upper Triassic~ in central North Carolina (Hope and Patterson, 1969; Delevoryas and Hope, 1971). It consists of a stem fragment to which are attached at least six pinnately c o m p o u n d leaves (Plate I). These deposits also yield remains of an abundance of isolated leaves of the same type. Most likely the part of the stem fragment preserved is near the apex because there appears to be a t u f t of leaves attached. The largest leaf fragment attached is about 20 cm long, with a petiole about 5 cm in length. Petiole bases measure about 5 mm in diameter, and the rachis in the most nearly distal part of a leaf preserved tapers to less than 2 mm. Pinnae near the basal part of the frond are 5--6 mm long and about 3 mm wide. Distally pinna size increases to about 2.6 cm in length and about 4 m m in width. Pinnae have parallel, or only slightly tapering, margins, with the tip rounded. A t t a c h m e n t of the pinnae is on the adaxial surface of the rachis and tends to be auriculate. A number of veins enter the pinna base, flare, and dichotomize. It is difficult to determine where the stem tip is in view of the several petioles attached near the stem apex. The estimated length of the stem fragment is 6--7 cm. It is only about 1 cm in thickness in a region where there do n o t appear to be decurrent leaf bases. Phyllotaxy appears to be lax, in spite of the clustering of y o u n g leaves at the t i p The stem surface is slightly wrinkled, but obviously lacking are persistent leaf bases that are so c o m m o n among m a n y cycadophytes. Because the material presents some insight as to the habit of a whole plant, it is deemed advisable to place this plant into a new genus, rather than to place it in an existing form genus for Mesozoic cycadophyte leaves. SYSTEMATIC PALEOBOTANY

Class Cycadeoidopsida Order Cycadeoidales Family Williamsoniaceae Genus Ischnophyton gen. nov. (diagnosis same as for type species)

Type species: Ischnophyton iconicum sp. nov. Diagnosis: Plant with slender stem with slightly wrinkled surface and

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97 lacking persistent leaf bases; leaves simply compound, petiolate; petiole 4--5 cm in length, 5 mm in diameter; frond exceeding 20 cm in length, with pinnae short proximally, larger distally, with generally parallel margins, constricted bases, rounded, with symmetrical or asymmetrical auricles and several veins entering each pinna; pinnae attached to adaxial surface of rachis; veins flaring distally, dichotomously branched, especially near pinna base. Stomata syndetocheilic, lower epidermal cells papillate, with wavy cell walls; upper epidermis cells elongated, smooth walled. Type material: Plate I; Plate II, 1 (holotype, University of Texas Paleobotanical Collection 135). Plate II, 2--5 (UTPC 136) is a paratype. Stratigraphic occurrence: Pekin Formation, Upper Triassic. Source o f name: Ischnos (Gr. slender); p h y t o n (Gr. plant); eiconicos (Gr. typical, referring to the presumed usual habit of Mesozoic cycadophytes). Stem: The stem of Ischnophyton iconicum lacks distinguishing characters, and without attached leaves it would be impossible to identify it as belonging to the genus. It is slender, iacking persistent leaf bases, and has widely spaced leaves attached to it (Plate I). If found without attached leaves, it would be virtually indistinguishable from stems of Leptocycas gracilis. Foliage: Leaves of Ischnophyton iconicum bear some resemblance to those of Otozamites on one hand, as well as to Zamites on the other. Pinnae are attached perpendicularly or slightly acropetally directed to the upper surface of the rachis and are constricted and rounded at the base (Plate I, Plate II, 1). There is a tendency for auricles to occur at the base, and these are occasionally asymmetrical. Several veins enter the base of the pinna and radiate distally as they do in both Otozamites and Zamites. There are a few dichotomies near the basal part of a pinna. Thus, these leaves cannot be placed with certainty in either genus, although Zamites, of the two, looks closer. These leaves resemble in some aspects those of Otozamites hespera Wieland, 1914--1916 from the Middle Jurassic of Oaxaca, Mexico. This species, however, also seems to have the same uncertainty as to generic assignment as do the leaves of Ischnophyton. Attachment of the pinna is near the middle in O. hespera, with a tendency for auricles to develop; these auricles may be symmetrical or slightly asymmetrical. There is, then, the question as to whether O. hespera is rightly placed within the genus Otozamites. Leaves of the holotype of Ischnophyton lack retrievable cuticle, so no epidermal details were available. Another leaf specimen (Plate II, 2), however, matches the base of leaves on the type specimen and is u n d o u b t e d l y of the same species. Cuticle was well preserved on it and allowed observation of epidermal characters. Cells of the epidermis on the abaxial surface are roughly rectangular in surface view, 20--25 X 30--35 pm, with sinuous walls (Plate II, 3). The undulations, however, are n o t deeply developed. Lower PLATE I

Ischnophyton iconicum gen. et sp. nov. Holotype; × 0.75. UTPC 135.

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PLATE II

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epidermal cells are also papillate (Plate II, 4). Cuticle is extremely brittle and fragmentary, so it was not possible to observe the relationship between stomata and veins. Stomata, however, appear to be arranged generally perpendicular to the axis of alignment of ordinary epidermal cells; suggesting that they were at right angles to the veins. Stomata are syndetocheilic, with subsidiary cells lacking wavy walls (Plate II, 3, 5). Guard cells are not highly cutinized, and the stomatat apparatus does not appear to be sunken. Trichome bases are also scattered among the epidermal cells (Plate II, 3). Leaves of the type attached to I s c h n o p h y t o n are by far the most abundant type of foliage in the shale quarry. There exists a tremendous range in size and form among these leaves suggesting that if, indeed, they were all borne on the same kind of plant, then the generic diagnosis would need expansion. Until more certainty exists, however, concerning the type of plant that bore them, it is premature to include this expanded range in size and structure within the present concept of the genus. Cuticular preservation among these leaves is inconsistent, but a program devoted to cuticular analysis and comparison among all of these leaves would be one step toward trying to determine their affinities. DISCUSSION

Ischnopyton iconicum and Leptocycas gracilis are two contemporary cycadophytes, the former cycadeoidalean, the latter cycadalean, with a remarkably similar growth habit. They co-existed in the same geographic region during the Late Triassic. An obvious initial interpretation would be that both kinds of plants had responded in a similar manner to the same environmental stimulus. Yet the kind of growth habit exhibited by these two only distantly related plants appears to be more universal, not only geographically, but through time as well. Triassic and Jurassic cycadophytes from such widely separated places as Yorkshire, England; Rajmahal Hills, India; Sweden; Oaxaca, Mexico; and eastern North America all display a similar pattern of development. As emphasized earlier (Delevoryas and Hope, 1971; Delevoryas, 1975), this common type of growth habit would suggest that it is the primitive one for cycadophytes, in which case it would be logical to look for seed plants of the late Paleozoic that would provide P L A T E II

Ischnophyton iconicum gen. et sp. nov. 1. Counterpart o f leaf fragment just below and slightly to the right o f center o f leaf in Plate I Veins are visible at the bases o f s o m e o f the pinnae; × 2.5. 2. F r a g m e n t of another, similar leaf; × 2.5. UTPC 136. 3. Cuticular preparation of lower epidermis of leaf in Plate II, 2. Note syndetocheilic stoma, wavy epidermal walls, and t w o t r i c h o m e bases (dark spots); × 450. 4. Preparation of cuticle of lower epidermis o f leaf in Plate II, 2, p h o t o g r a p h e d at a slightly oblique angle to s h o w papillae on epidermal cells; X 450. 5. Preparation o f cuticle of lower epidermis o f leaf in Plate II, 2, with syndetocheilic s t o m a and wavy epidermal cell walls; × 450.

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growth patterns of a type that could be considered ancestral to Mesozoic plants with a slender growth pattern. Of the plants now known in the Late Paleozoic, the pteridosperms are the ones that have most of the features necessary to be considered precursors to cycads and cycadeoids. ACKNOWLEDGEMENTS

Thanks are extended to Mr. E. L. Rummage and other officials of the Boren Clay Products Company for their continued support and cooperation. Research supported by National Science Foundation Grant BMS73-01428 t o T . D. REFERENCES Bose, M. N., 1953a. Bucklandia sahnii sp. nov. from the Jurassic of the Rajmahal Hills. Palaeobotanist, 2: 41--49. Bose, M. N., 1953b. On some fossil cycadean stems from the Rajmahal Hills, Bihar. Palaeobotanist, 2: 71--74. Delevoryas, T., 1968. Some aspects of cycadeoid evolution. J. Linn. Soc. (Bot.), 61: 137--146. Delevoryas, T., 1975, Mesozoic cycadophytes. Gondwana Geology. Proc. Third Gondwana Symposium, Aust. Natl. Univ. Press, in press. Delevoryas, T. and Hope, R. C., 1971. A new Triassic cycad and its phyletic implications. Postilla, 150: 1--31. Florin, R., 1933. Studien fiber die Cycadales des Mesozoikums. K. Sven. Vetensk. Akad. Handl., Ser. 3, 12(5): 1--134. Harris, T. M., 1961. The fossil cycads. Palaeontology, 4: 313--323. Harris, T. M., 1969. The Yorkshire Jurassic Flora III. Bennettitales. Br. Mus. (Nat. Hist.), Lond., Publ. 675: 1--186. Hope, R. C. and Patterson, O. F., III, 1969. Triassic flora from the Deep River Basin, North Carolina. N.C. Dep. Conserv. Dev., Div. Min. Res., Spec. Publ., 2: 1--12. Nathorst, A. G., 1909. Pal~iobotanische Mitteilungen, 8. K. Sven. Vetensk. Akad. Handl., 45(4): 1--37. Sahni, B., 1932. A petrified WiUiamsonia (W. sewardiana, sp. nov.) from the Rajmahal Hills, India. Palaeontol. Indice, N. S., 20(3): 1--19. Schuster, J., 1911. Weltrichia und die Bennettitales. K. Sven. Vetensk. Akad. Handl., 46(11): 1--57. Thomas, H. H., 1915. On Williamsoniella, a new type of bennettitalean flower. Philos. Trans. R. Soc. Lond., Ser. 13, 207: 113--148. Wieland, G. R., 1914--1916. La flora Li~isica de la Mixteca Alta. Bol. Inst. Geol. M6x., 31: 1--165. Williamson, W. C., 1870. Contribution toward the history of Zamia gigas Lindley and Hutton. Linn. Soc. Lond. Trans., 26: 663--674. Zimmermann, W., 1933. Pal~iobotanische und phylogenetische Beitr~ige, I--V. Palaeobiologica, 5: 321--348.