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Brief Communication |
2Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9 3Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701 USA
Received for publication November 7, 2000. Accepted for publication February 9, 2001.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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Key Words: compound pollen cone • conifer • Paleozoic • seed plant phylogeny • walchian
A diverse Upper Pennsylvanian fossil biota preserved at the 7–11 Mine in northern Ohio includes two plant-rich assemblages, one of which contains a single species of primitive walchian conifers (McComas, 1988, 1989
; Hernandez-Castillo, 2000
). This plant is represented by numerous vegetative shoots, ovulate fructifications, and one pollen cone with a unique structure and a level of complexity heretofore unknown for fossil or living conifers (Hernandez-Castillo, 2000)
. The purpose of this communication is to report, characterize, and document this novel pollen cone morphology. These new data are also related to ongoing efforts to resolve the pattern of phylogeny for seed plants as a whole. Detailed description and naming of this new conifer plant will be presented separately.
The fossil conifer material was collected from a thinly laminated, argillaceous, organic black shale in the abandoned 7–11 strip mine near East Liverpool, Columbiana County, Ohio (McComas, 1989
). This shale is stratigraphically constrained between the Mahoning Coal and the Brush Creek marine unit near the base of the Conemaugh Group in the Appalachian Basin (fig. 1 of McComas, 1988
), which is considered by most workers to be Late Desmoinesian/Stephanian A (Phillips, Peppers, and DiMichele, 1985
; McComas, 1988
; Hernandez-Castillo, 2000
). Floristic composition of the shale includes a dominant cordaitean plant that bore Cordaites cf. principalis (Germar) Geinitz leaves, with the lycophyte Sigillaria brardii Brongniart and the walchian conifer as subdominant elements. Marattiaceous tree ferns are less common; calamiteans, equisetophytes, and medullosan seed ferns comprise relatively rare elements of the flora (McComas, 1989
).
The pollen cone was initially exposed on a split rock surface. One side of the specimen was macerated in dilute hydrofluoric acid to expose details of morphology and to free cuticles and pollen for examination. To document cone morphology, maceration was stopped in a saturated sodium bicarbonate solution at several stages, and the specimen was neutralized and photographed. Some macerated parts were mounted on microscope slides, while others were prepared for examination with the scanning electron microscope (SEM). Specimen preparations are housed in the Ohio University Paleobotanical Herbarium.
In general appearance, the pollen cone is remarkably similar to an ordinary ultimate vegetative shoot of a walchian conifer (Fig. 1). The specimen is roughly cylindrical, 8.2 cm long and 0.9 mm in maximum width, and appears to have closely spaced, helically arranged simple leaves (Fig. 1). However, careful inspection reveals additional structures within the shoot (Fig. 2) that are attached in an axillary position (Figs. 2 [at arrow points] and 4). This demonstrates that the specimen is a compound shoot system, and the leaves are actually bracts that measure 3–5 mm long and 1–2 mm wide.
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; Mapes and Rothwell, 1991
), and other coniferophytes (Rothwell and Mapes, 2001)
.
More complete cone macerations reveal that the axillary dwarf shoots also bear upright, ellipsoidal pollen sacs with a rounded tip (Figs. 3–5). Pollen sacs measure 0.8–1.2 mm long and 0.3–0.6 mm wide. Five to eight sterile scales and 3–4 sporophylls with a terminal pollen sac are produced by each axillary dwarf shoot, and these have a consistent arrangement. Sterile scales are borne on the side of the dwarf shoot axis that faces the bract and laterally (Figs. 2 and 4). They overlap each other as if borne in a helical arrangement (Fig. 3). Pollen sacs occur only on the side of the dwarf shoot axis that faces the cone axis (Figs. 3 and 4). Pollen sacs consist of an outer cellular layer and an internal noncellular substance that encloses the grains (Fig. 3, at arrows). Grains are monosaccate (Fig. 6), with a maximum diameter of 62–130 µm in polar views, and have a proximal, bent monolete suture. In light microscopy each grain shows two parallel folds over the body (Fig. 6) and conforms to the sporae dispersae genus Potonieisporites Bhardwaj (Bharadwaj, 1964
; Rothwell and Mapes, 2001
).
This specimen provides the first clear evidence for compound pollen cones in conifers, and it displays a structural equivalence to the basic morphology of ovulate conifer cones. Florin (1951)
demonstrated that the ancestral morphology of ovulate conifer cones comprises an axis that bears helically arranged bracts with axillary ovuliferous dwarf shoots. The ovuliferous dwarf shoots produce both sterile scales, and sporophylls, each with a terminal ovule (Florin, 1951
), and they are bilaterally symmetrical to varying degrees. As in the pollen cone described here, ovules occur only on the side of the ovuliferous dwarf shoot that faces the cone axis (Florin, 1951
; Kerp and Clement-Westerhof, 1991
).
In all respects this compound pollen cone conforms to the general morphological pattern of primitive ovulate conifer cones (Florin, 1951
), except that the axillary dwarf shoots bear sporophylls with a single, erect pollen sac, instead of a single recurved ovule (Mapes and Rothwell, 1991
). Prior to the discovery of the specimen described here, all known Paleozoic conifer pollen cones were simple shoots in which the cone axis bears sporophylls (Florin, 1951
; Mapes and Rothwell, 1998
; Rothwell and Mapes, 2001
). Pollen cones of most living conifers are also simple shoot systems (e.g., Owens and Molder, 1979
; Owens and Singh, 1982
) that differ from one another primarily by the position and number of pollen sacs produced, shape of the sporophyll lamina, and overall size and shape of the cone (Chamberlain, 1935
). A few members of the Taxaceae/Cephalotaxaceae and selected fossil conifer pollen cones have been interpreted as compound shoots by some authors (Wilde, 1975
; Kerp et al., 1990
; Grauvogel-Stamm, and Galtier, 1998
), but evidence for those interpretations is equivocal (Mapes and Rothwell, 1998
).
The discovery of an unequivocal compound pollen cone among ancient walchian conifers dramatically increases the morphological variation documented for conifer species, and documents a basic structural equivalence between the ovulate and pollen cones in at least one primitive species. This discovery supports hypothesized systematic relationships of conifers to the extinct Cordaitales (Florin, 1951
) and to fossil and living gnetophytes (Florin, 1951
; Shindo et al., 1999
). Like the conifer species that produced the compound pollen cone described here, Euramerican cordaitaleans and gnetophytes display a basic structural equivalence of their ovulate and pollen cones (Crane, 1985
). Although details of structure differ among these major groups (Stewart and Rothwell, 1993
; Taylor and Taylor, 1993
), all of these plants have both ovulate and pollen cones that are compound shoot systems with fertile dwarf shoots in the axils of bracts (Crane, 1985
).
Traditionally, conifers have been treated as a well-defined clade, but currently there are no synapomorphies for a group that includes all fossil and living species (Crane, 1985
; Rothwell and Serbet, 1994
), and the phylogenetic status of both conifers and the more broadly defined coniferophytes remain poorly resolved (e.g., Nixon et al., 1994
; Rothwell and Serbet, 1994
; Rothwell and Mapes, 2001
). As summarized by Donoghue and Doyle (2000)
, some phylogenetic analyses using nuclear, mitochondrial, chloroplast, and/or MADS genes strongly ally gnetophytes with living conifers, suggesting that they are either sister groups or that gnetophytes are nested within a conifer clade (Shindo et al., 1999
; Winter et al., 1999
; Bowe, Coat, and dePamphilis, 2000
; Chaw et al., 2000
). These studies yield strongly supported but discordant results that have a high probability of representing spurious patterns, rather than real phylogenetic relationships (Sanderson et al., 2000)
. They also differ dramatically from the results of morphological analyses and other molecular studies (Donoghue and Doyle, 2000)
. Nevertheless, at least some of the results from the molecular analyses could be detecting a true phylogenetic signal.
Given the large percentage of seed plants that now are extinct (Crane, 1985
), new data from the fossil record are crucial for testing these and other hypotheses of seed plant relationships (Donoghue and Doyle, 2000)
. The compound pollen cone reported here reveals previously unknown similarities between gnetophytes and primitive conifers, thereby supporting hypotheses of a conifer/gnetophyte clade. Although pollen cone morphology contributes only a small percentage of the total morphological characters for phylogenetic analyses of seed plants (e.g., Doyle, 1996
), extinct species with novel combinations of characters can be extremely important for resolving cladistic patterns (Huelsenbeck, 1991
). In this regard, the fossil plant that produced the compound conifer pollen cone described in this paper makes an important contribution to the resolution of land plant phylogeny.
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FOOTNOTES |
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4 Author for reprint requests (Tel: 780-492-5518; FAX: 780-492-9234; e-mail: genaro{at}ualberta.ca
).
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LITERATURE CITED |
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TOPABSTRACTLITERATURE CITED |
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Bowe L. M. G. Coat C. W. dePamphilis 2000 Phylogeny of seed plants based on all three genomic compartments: extant gymnosperms are monophyletic and Gnetales closest relatives are conifers. Proceedings of the National Academy of Sciences, USA 97: 4092-4097
Chamberlain C. J. 1935 Gymnosperms, structure and evolution. University of Chicago Press, Chicago, Illinois, USA
Chaw S. M. C. L. Parkinson Y. Cheng T. M. Vincent J. D. Palmer 2000 Seed plant phylogeny inferred from all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers. Proceedings of the National Academy of Sciences, USA 97: 4086-4091
Crane P. R. 1985 Phylogenetic analysis of seed plants and the origin of angiosperms. Annals of the Missouri Botanical Garden 72: 716-793[CrossRef][ISI]
Donoghue M. J. J. A. Doyle 2000 Demise of the anthophyte hypothesis?. Current Biology 10: R106-R109[CrossRef][ISI][Medline]
Doyle J. 1996 Seed plant phylogeny and the relationships of Gnetales. International Journal of Plant Sciences 157(6, Supplement): S3-S39[CrossRef]
Florin R. 1951 Evolution in cordaites and conifers. Acta Horti Bergiani 15: 285-388
Grauvogel-Stamm L. J. Galtier 1998 Homologies among coniferophyte cones: further observations. Comptes Rendus de l'Academie des Sciences, Serie II, Sciences de la Terre et Planets Paris, 326: 513-520
Hernandez-Castillo G. R. 2000 Reconstruction of a walchian conifer from the Upper Pennsylvanian of Ohio. Master's thesis, Ohio University, Athens, Ohio, USA
Huelsenbeck J. P. 1991 When are fossils better than extant taxa in phylogenetic analysis?. Systematic Zoology 40: 458-469[CrossRef][ISI]
Kerp H. J. A. Clement-Westerhof 1991 Aspects of Permian palaeobotany and palynology. XII. The form-genus Walchiostrobus Florin reconsidered. Neues Jahrbuch für Geologie und Palaeontologie Abhandlungen 183: 257-268
Kerp J. H. F. R. J. Poort H. A. J. M. Swinkels R. Verwer 1990 Aspects of Permian palaeobotany and palynology. IX. Conifer-dominated Rotliegend floras from the Saar–Nahe Basin (?Late Carboniferous–Early Permian; SW-Germany) with special reference to the reproductive biology of early conifers. Review of Palaeobotany and Palynology 62: 205-248
Mapes G. G. W. Rothwell 1991 Structure and relationships of primitive conifers. Neues Jahrbuch für Geologie und Palaeontologie Abhandlungen 183: 269-287
———, and ———. 1998 Primitive pollen cone structure in Upper Pennsylvanian (Stephanian) walchian conifers. Journal of Paleontology 72: 571-576[Abstract]
McComas M. A. 1988 Upper Pennsylvanian compression floras of the 7–11 mine, Columbiana County, Ohio. Ohio Journal of Science 88: 48-52[ISI]
———. 1989 Floristics and depositional ecology of two Upper Pennsylvanian (Lower Conemaugh) floras from the 7–11 Mine in northeastern Ohio. Master's thesis, Ohio University, Athens, Ohio, USA
Nixon K. C. W. L. Crepet D. Stevenson E. M. Friis 1994 A reevaluation of seed plant phylogeny. Annals of the Missouri Botanical Garden 81: 484-533[CrossRef][ISI]
Owens J. N. M. Molder 1979 Bud development in Larix occidentalis. II. Cone differentiation and early development. Canadian Journal of Botany 57: 1557-1572[CrossRef]
Owens J. N. H. Singh 1982 Vegetative development and the time and method of cone initiation in subalpine fir (Abies lasiocarpa). Canadian Journal of Botany 60: 2249-2262
Phillips T. L. R. A. Peppers W. A. DiMichele 1985 Stratigraphic and interregional changes in Pennsylvanian coal-swamp vegetation: environmental inferences. International Journal of Coal Geology 5: 43-109
Rothwell G. W. G. Mapes 2001 Barthelia furcata gen. et sp. nov. from the Late Pennsylvanian of midcontinent North America, with a review of Paleozoic coniferophytes. International Journal of Plant Sciences, in press
———, and R. Serbet 1994 Lignophyte phylogeny and the evolution of spermatophytes: a numerical cladistic analysis. Systematic Botany 19: 443-482[CrossRef][ISI]
Sanderson M. J. M. F. Wojciechowaki L.-M. Hu T. Ser Khan S. G. Brady 2000 Error, bias, and long-branch attraction in data for two chloroplast photosystem genes in seed plants. Molecular Biology and Evolution 17: 782-797
Shindo S. M. Ito K. Ueda M. Kato M. Hasebe 1999 Characterization of MADS genes in the gymnosperm Gnetum parvifolium and its implication on the evolution reproductive organs in seed plants. Evolution and Development 1: 180-190
Stewart W. N. G. W. Rothwell 1993 Paleobotany and the evolution of plants. Cambridge University Press, New York, New York, USA
Taylor N. T. E. L. Taylor 1993 The biology and evolution of fossil plants. Prentice-Hall, Englewood Cliffs, New Jersey, USA
Wilde M. H. 1975 A new interpretation of microsporaangiate cones in Cephalotaxaceae and Taxaceae. Phytomorphology 25: 434-450[ISI]
Winter K.-U. A. Becker T. Münster J. T. Kim H. Saedler G. Theissen 1999 MADS-box genes reveal that gnetophytes are more closely related to coniferts than to flowering plants. Proceedings of the National Academy of Sciences, USA 96: 7342-7347
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