See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/284504789 Some comments on the origin and evolution of conifers Article in Canadian Journal of Botany · October 1981 Impact Factor: 1.4 · DOI: 10.1139/b81-254 CITATIONS READS 15 16 1 author: R. A Stockey Oregon State University 174 PUBLICATIONS 3,409 CITATIONS SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: R. A Stockey letting you access and read them immediately. Retrieved on: 09 July 2016 Some comments on the origin and evolution of conifers1 RUTHA. STOCKEY Department of Botany, University of Alberta, Edmonton, Alta., Canada T6G 2E9 Received December 16, 1980 STOCKEY,R. A. 1981. Some comments on the origin and evolution of conifers. Can. J. Bot. 59: 1932-1940. During the last several years there has been an increased emphasis on the study of conifer evolution including various aspects of their reproductive biology. The occurrence of similar stelar features as well as trends in the evolution of reproductive structures still point to cordaitalean affinities for the Coniferales. The evolutionary history of such families as the Pinaceae, Araucariaceae, and Taxodiaceae is beginning to be elucidated as well as that of certain extinct families including the Cheirolepidaceae and the Voltziaceae. Systematic investigations of fossil cone vasculature and resin canal distribution, leaf cuticles, seed integuments, and embryo structure not only have increased our knowledge of conifer evolution but also have led to changes in our views on the systematics of extant conifer groups. Introduction Beck (1970, 1971, 1975) and others (Scheckler 1978; In the last few years there has been an increased Scheckler and Banks 1971) have noted that characters emphasis on the study of conifer evolution. Since the common in conifer xylem are present in the Late pioneering work of Rudolf Florin from the 1920's to the Devonian among the progymnosperms. This relation- early 1960's (Lundblad and Malmstrom 1966) much has ship, however, has been questioned by Rothwell(1975) been learned about the evolution and reproductive biol- on the basis of primary vasculature. The group, how- ogy of these organisms. The presence of similar mor- ever, that most paleobotanists regard as the oldest in the phological and anatomical characters, in combination coniferophyte line is the Cordaitales. These plants are with comparative studies of the reproductive structures, mostly Pennsylvanian and Permian in age. still indicates possible cordaitalean affinities for the Cordaites, now known to have been a more diverse conifers. Our knowledge of primitive conifer groups, group than once thought (Whiteside 1974), may have however, has been expanded in recent years as a result of reached 30 m in height and bore large strap-shaped investigations by a great number of workers. leaves up to 1 m long. They bore their reproductive The purpose of this paper is to review some thoughts structures in loose stobili called inflorescences. The with respect to conifer origins and consider how these inflorescences (Cordaianthus) are monosporangiate, For personal use only. have changed since the time of Florin, and to discuss bearing either ovules or pollen sacs. The inflorescence some ideas dealing with the early evolution of certain consists of an axis with two or four rows of bracts fossil and modem conifer families. The main emphasis (Rothwell 1977; Daghlian and Taylor 1979). In the axil will be comparative morphology of the reproductive of each bract is a short shoot or dwarf shoot with structures. helically arranged scales. Pollen cones have from four to The plants we know as coniferophytes (Coniferophy- six pollen sacs borne at the tip of each scale. Recently, a tina) are currently divided into three orders: Voltziales, new cordaitean pollen organ has been described from Taxales, and Coniferales. Of these three orders, the Kentucky; Gothania lesliam Daghlian and Taylor Voltziales, sometimes called the transition conifers, are (1979) contains pollen referable to the dispersed spore thought to be ancestral to other conifer groups. Their genus Felixipollenites, while the pollen cones of Cor- geologic range is Pennsylvanian to Cretaceous. The daianthus contain grains of the Florinites type (Millay Taxales, represented by five living genera, have a fossil and Taylor 1974). The ovulate cones or inflorescences record extending back to the L~werJurassic and (also Cordaianthus) exhibit radially symmetrical short continuing to the recent. The third order, the Conifer- shoots in the axils of bracts. Ovules attached to the short Can. J. Bot. Downloaded from www.nrcresearchpress.com by Simon Fraser University on 11/15/14 ales, has fossil representatives by the Triassic period. shoot are stalked, orthotropous, and roughly heart The Coniferales are divided into six or seven families: shaped. Araucariaceae, Podocarpaceae, Cupressaceae, Taxo- diaceae, Pinaceae, Cephalotaxaceae, and Palissyaceae. Voltziales The Cupressaceae and Taxodiaceae are sometimes put During the time that cordaites flourished (Carboni- into one large family (Eckenwalder 1976). ferous-Permian) the voltzialean conifers are also present paper was presented in a symposium entitled ~~d-in the fossil record. In current classification schemes the mark Events in the Evolution of Plants, co-sponsored by the Voltziales contains three families: the Lebachiaceae, Canadian Botanical Association and the Canadian Association Voltziaceae, and Cheirolepidiaceae. The imbricate, of Palynologists at Carleton Unive'rsity on June 21, 1979. needlelike foliage of some of these conifers (Lebachia, 0008-40261811101932-09$01.00/0 01981 National Research Council of CanadalConseil national de recherches du Canada STOCKEY 1933 Walchia) looked similar to that of living Araucaria ceae. The structure of the integuments too, with an heterophylla (Mirb.) Franco, the Norfolk Island pine. In extensive system of tightly compacted sclereids as general, members of the Lebachiaceae had more com- described by Schweitzer (1963) is similar to some of the pact cones than the cordaites. Lebachia has one erect living and fossil araucarians (Stockey 1975, 1978). ovule that is borne terminally on each radially symmet- In addition to these genera there are a large number of rical short shoot in the axil of a bifurcate bract (Fig. 1). other conifers in the Voltziaceae that all show bilaterally Another genus in this family, Ernestiodendron, has three symmetrical, reduced short shoots, or flattened lobed to seven ovules per short shoot; and the scales or sterile scales in the axils of bracts. Aethophyllum from the elements of the short shoot are few to none in different Triassic of France has recently been placed in the species of the genus (Fig. 3). The ovules are either erect Voltziales by Grauvogel-Stamm and Grauvogel (1975). or inverted. There is, in addition, a noted difference in Young plants, pollen and seed cones, and foliage are symmetry of the short shoot which is flattened or known. There are five sterile lobes and five inverted bilaterally symmetrical (Florin 1951). Both Lebachia ovules in the axil of a bract (Fig. 7). Seedlings are and Ernestiodendron are Upper Carboniferous to Lower known to have epigeal germination and two cotyledons. Permian in age. Both juvenile and mature foliage are known for the A recent discovery from the Permian of Texas (Miller genus that is now considered by Grauvogel-Stamm and Brown 1973) has expanded our knowledge about the (1978) to belong to its own family. The ovulate cones diversity of reproductive structures within the Lebach- are most closely compared with Cryptomeria D. Don iaceae. The ovulate cone of Moyliostrobus texanum and the fossil Swedenborgia Nathorst. Miller and Brown combines several features of Le- Another interesting Triassic genus Cycadocarpidium, bachia and some of those of Ernestiodendron (Fig. 2). also redescribed by Grauvogel-Stamm (1978), has One seed per scale and numerous sterile scales per short cones up to 30 mm in length with cone-scale complexes shoot are like those found in Lebachia. The bilaterally inserted nearly at right angles to the axis. The ovulifer- symmetrical short shoot is similar to the cones of ous scale is trilobed (each lobe coming to a fine point) Ernestiodendron. fused to and borne in the axil of a long (11 mm) In addition to these types of reproductive structures lanceolate bract (Fig. 8). There are usually two to three there are a large number of late Paleozoic vegetative inverted seeds per scale that are roughly rectangular in remains that are unlike those of Lebachia or Ernestio- shape with lateral wings and resemble seeds of Sciado- dendron. One of these is Carpentieria frondosa pitys or Taiwania of the Taxodiaceae. (Goepp.) Florin (Fig. 4) from the Lower Permian, the Florin documented several intermediate stages be- For personal use only. lateral shoot system of which resembles "the walchias" tween the short shoot or dwarf shoot of cordaites and the (Florin 1951). Leaves of this plant are bifurcate and may ovuliferous scale of conifers. Changes that have taken indicate that the entire conifer leaf has arisen from the place involve: the general reduction of the inflorescence forked type by reduction. axis, a flattening to the short shoot, a fusion of the sterile Members of the family Voltziaceae have been used by parts of the dwarf shoot, fusion of the ovule axis to the Florin to show intermediate stages between the Le- scale, and eventual elimination of the ovule axis, total bachiaceae and modern conifers. Pseudovoltzia (Per- reduction in the number of fertile parts, and the mian age), recently reinvestigated by Schweitzer reduction in number of ovules from many to few. There (1963), has a bilaterally symmetrical short shoot in the has also been a change in ovule position from ortho- axil of a bract with five sterile lobes that are fused to the tropous to anatropous in some conifer groups. It is bract at their bases (Fig. 9). There are three stalked important to note that Cordaianthus and early forms ovules, the stalks fused to the short shoot.