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Paleozoic Origin of the Cycads

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Paleozoic Origin of the Cycads Paleozoic Origin of the Cycads GEOLOGlCAL SURVEY PROFESSIONAL PAPER 934 Paleozoic Origin of the Cycads By SERGIUS H. MAMAY GEOLOGICAL SURVEY PROFESSIONAL PAPER 934 M egasporophylls and associated plant parts from the Upper Pennsylvanian and Lower Permian Tacks of the southwestern United States are interpreted as a lineage of cycads) derived from, the pteridosper1ns. Cycadean foliage is thought to be derived from Paleozoic taeniopterids UI~.~ITED STATES GOVERNMENT PRINTING OFFICE.. WASHINGTON 1976 UNITED STATES DEPARTMENT OF THE INTERIOR THOMAS S. KLEPPE, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Mamay, Sergius H. 1920- Paleozoic origin of the cycads. (Geological Survey professional paper; 934) Bibliography: p. Includes index. 1. Cycadales, Fossil. 2. Paleobotany--Paleozoic. 2. Paleobotany--Southwest, New. I. Title. II. Series: United States. Geological Survey. Professional paper; 934. QE976.2.M35 561 '.591 76-14384 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-02831-0 CONTENTS Page Page ~bstract ----------------------------------------- 1 Systematic descriptions-Continued Introduction -------------------------------------- 1 ?Cycadean male cones ------------------------­ 23 ~cknowledgments _ ___ _ ___ _ ___ _ __ __ __ _ ___ _ __ _ ___ _ __ 3 Cycadospadix yochelsoni Mamay, n. sp ---------- 24 26 Ter.nUnology ------------------------------------- 4 Associated plants --------------------------------­ D:istribution of the fossils ------------------------­ 28 Sources of material ------------------------------- 4 Morphologic and evolutionary considerations -------- 29 Previous work ------------------------------------ 5 Interrelationships of the ~merican fossils ------­ 29 Systematic descriptions --------------------------- 6 ~n alternative interpretation of Archaeocycas --­ 33 Spermopteris sp _:_____________________________ 7 Relationships of the ~merican fossils to other Archaeocycas Mamay -------------------------- 8 fossil cycads ------------------------------­ 36 Archaeocycas whitei Mamay ------------------- 8 Evolution of the pinnate habit in cycadalean Phasmatocycas Mamay ------------------------ '12 foliage ------------------------------------ 39 Phasmatocycas kansana Mamay ---------------- 12 Conclusions -------------------------------------- 43 ?Phasmatocycas sp ---------------------------- 21 References cited ---------------------------------- 44 ?Phasmatocycas spectabilis Mamay, n. sp -------- 22 Index -------------------------------------------- 47 ILLUSTRATIONS [Plates follow index] PLATE 1. Archaeocycas whitei Mamay. 2. Archaeocycas whitei Mamay, ?Phasmatocycas sp., Spermopteris sp., Cycadospadix yochelsoni Mamay, n. sp., and ?cycadean male cone. 3. Phasmatocycas kansana Mamay. 4. ,Seed cuticles of Phasmatocycas kansana Mamay. 5. ?Phasmatocycas spectabilis Mamay, n. sp., and Taeniopteris sp. Page FIGURE 1. Reconstruction of part of seed-bearing leaf of Spermopteris coriacea -------------------------------- 2. 2. Diagrams showing hypothetical reduction of the megasporophyll and evolution of the ovulate cone im- plied by living cycads ----------------~------------------------------------------------------ 3 3. Semidiagrammatic reconstruction of megasporophyll of Archaeocycas ------------------------------ 11 4. Semidiagrammatic reconstruction of megasporophyll of Phasmatocycas ------------------------------ 18 5. Stratigraphic correlations showing vertical distribution of late Paleozoic cycads and related plants ____ 2.9 6. Diagrams showing hypothetical evolutionary development of Cycas-like megasporophyll from spermop- terid pteridosperm ___ _ ___ _ _ ___ _ __ _ _ ___ _ _ _ _ __ _ _ ___ _ ___ _ _ __ _ ____ ___ _ _ __ __ __ __ __ __ _ _ __ ___ _ ___ __ 31 7. Semidiagrammatic representation of hypothetical evolution of Cycas-like megasporophyll from Archaeo- cycas -------------------------------------------------------------------------------------- 33 8. Semidiagrammatic representation of hypothetical evolution of carpellike organ from Archaeocycas ____ 35 9. Reconstruction·s of Dioonitocarpidium ------------------------------------------------------------- 37 10. Reconstructions of Palaeocycas-Bjuvia ----------------------------------------------------------- 39 11. Diagrams depicting stUggested evolution of cycadalean leaves from taeniopterid ancestral forms -------- 43 III PALEOZOIC ORIGIN OF THE CYCADS By SERGIUS H. MAMAY ABSTRACT either and that the two orders may have been distinct as long ago as the Early Permian. Among other plant associ­ Upper Paleozoic rocks in the southwestern United States ates, only Taeniopteris is common to all the cycad-bearing contain several taxa of fossil plants that appear to repre­ assemblages; this strengthens the hypothesis that Taeniop­ sent a lineage from the Pennsylvanian pteridosperms. to the teris was the leaf of primitive cycads, as indicated by Sper­ modern cycads. The oldest genus, Spermopteris Cridland and mopteris. Morris, is a megasporophyll that has surficial seeds borne in two rows along the sides of presumably abaxial fac,es of Stratigraphic distribution of the plants supports the idea taeniopterid foliage; it was first found in the Virgilian Law­ of derivation of the cycads from the Pennsylvanian spermop­ rence Formation of Kansas, and a poorly preserved specimen terids and of a lineage to the modern cycads through the is now known from the Leonardian Wellington Formation of Triassic genera Palaeocycas-Bjuvia and Dioonitocarpidium. Oklahoma. Horizontal and vertical distribution of the Paleozoic taxa em­ The putatively derivative cycadean genera Phasmatocycas phasizet the importance of the lower Leonardian of the south­ Mam:a.y and A rchaeocycas Mamay occur in lower Leonardian western United States as a temporal and geographic locale rocks of Kansas and Texas. P hasmatocycas (type-species: P. for rapid and dramatic plant evolution. kansana Mamay) is a fertile axis that has two lateral rows Modern cycadean leaves are thought to be derived from of naked ovules. In P. kansana, the ovules are represented by taeniopterid stock in the Paleozoic and Mesozoic through well-preserved double cuticles similar to those of the Jurassic progressive incision of the laminar margin. In one line of cycad Beania; small resinoid spherules, interpreted as remains foliar evolution, shallow marginal dentation, in which each of glands, alternate with the ovules. Taeniopterid foliage as­ tooth receives only one· vein, becomes progressively deeper, sociated with P. kansana bears abundant spherules identical resulting in the cycadaceous leaf type in which each leaflet has with those on the fertile axis; on the basis of the spherules a :single vein; a parallel .series of known forms involved sev­ and the known seed habit of Taeniopteris, Phasmatocycas is eral veins to a segment and could have produced the zamia­ reconstructed as a cycadlike. megasporophyll with an elaminar, ceous leaf type·. The Stangeria leaf type could have arisen fertile base and a t31eniopterid distal lamina. The glands may from a compound taeniopterid with little modification, whereas be primitive nectaries, poss.ibly involved in an early stage in marginal incision of a compound taeniopterid might have the development of entomophily. Phasmatocycas is regarded produced the Bowenia type of leaf. as hav.ing been derived from a spermopterid ancestor through Cumulative evidence indicates that the cycads extend back reduction of numbers of seeds and loss of the basal foliar into the Late Pennsylvanian instead of only the Late. Triassic; lamina. Althoug'h the type specimen of Phasmatocycas is from' thus they rival the conifers as the oldest extant seed plants. the Wellington Formation of Kansas, a poorly preserved specimen is known from the younger Vale Formation of Texas, where it is associated with structures strongly reminiscent of INTRODUCTION male cycadean cones. Archaeocyca.s Mamay, typified by A. whitei Mamay, is ap­ The tenor of this paper is appropriately estab­ proximately contemporary with Phasmatocycas kansana, oc­ lished by reference to some of T. M. Harris' percep­ curring at two localities in the Belle Plains Formation of tive comments on the fossil history of the cycads. Texas. It resembles Phasmatocycas in having a basal s·eed­ Harris (1961, p. 322) :wrote as follows: bearing area and a sterile distal lamina. In Archaeocycas, however, the seeds are attached to the basal lamina, which The fossil Cycads consist of a good many genera of isolated is inrolled and at least partly encloses. the seed. A rchaeocycas organs classified on more or less good evidence as Cycadales, probably represents a form intermediate between Spermopteris a very few genera with two known organs and one genus and Phasmatocycas. Alternativ·ely, Archaeocycas might be a with more than two organs. If we are right in our synthesis fertile appendage evolving in the direction of a carpellike or­ of these, plants with two or more· organs, and also right in our gan through proliferation and fusion of the laminar margins; pre:sent classification of, say, half of the isolated organs (and this alternative, however, is not preferred. I think it unreasonable to hope for more), then we can say Associated with the cycadean megasporophylls are a single that by Jurassic times the family had probably completed its specimen of Cycadospadix (C. yochelsoni Mamay, n. sp.) and evolution. * * * The known genera are no more
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