Rothwell, G. W. (Department of Environmental and Plant Biology, Ohio University, Athens, OH 45701) and M. T. Dunn (Department of Biological Sciences, Cameron University, Lawton, OK 73505). A century of seed ferns: Introduction to the symposium. J. Torrey Bot. Soc. 133: 4–6. 2006.—More than one hundred years have now elapsed since the reconstruction of Lyginopteris oldhamia documented the existence of the previously unrecognized major group of extinct seed plants that we now call seed ferns (Oliver and Scott 1904). This recognition by Oliver and Scott (1904) may well have been the single most important element ushering in a new era of botanical inquiry by placing fossils at the center of plant phylogenetic studies. Originally envisaged as an evolutionary link between ferns and modern seed plants, seed ferns are united by the combination of large and often highly dissected leaves, and seeds that are borne on the leaves (Stewart and Rothwell 1993, Taylor and Taylor 1993).

Galtier, J. and B. Meyer-Berthaud. (UMR Botanique et Bioinformatique, CIRAD, TA40/PS2, 34398 Montpellier, France). The diversification of early arborescent seed ferns. J. Torrey Bot. Soc. 133: 7–19. 2006.—Seed plants of the lowermost Carboniferous time (Tournaisian) display important differences in morphology and habit with at least two size classes. On one side are plants of modest stature with protostelic stems, manoxylic wood and large leaves, interpreted as calamopityan, buteoxylalean and lyginopteridalean seed ferns, and on the other side are arborescent plants, with trunks up to 2 metres in diameter, of which the systematic position remains unclear. We summarize present information on these trees characterized by a thick development of generally dense wood, a broad eustele consisting of a large number of discrete primary xylem strands, short internodes and deciduous medium-sized fronds. New data have been recently obtained on Pitus, the best known member of this group, on Eristophyton, Bilignea, Stanwoodia, Aporoxylon and on several new taxa which exhibit a broad circular parenchymatous pith surrounded by numerous sympodial xylem strands, but which differ in secondary xylem and leaf trace features. Of particular interest is a new plant that has leaf traces originating as a double strand and a petiole base of the Kalymma-type, therefore showing characteristics of calamopityan seed ferns but being quite distinct in features of the stele, secondary xylem and phloem. Emphasis is placed on the evolutionary dynamics of this important diversification of arborescent plants which included: i) a rather abrupt increase in overall diameter, ii) increase in primary xylem size, with regard to other contemporaneous early seed plants; iii) very different types of wood, instead of a single dense/pycnoxylic wood as generally assumed; iv) distinct periderm types; v) deciduous leaves. The last feature may be interpreted as an innovation, related to the tree habit, within the lignophytes. The origin and systematic position of these arborescent plants remain problematical.

Dunn, M. T. (Department of Biological Sciences, Cameron University, Lawton, Oklahoma 73505). A Review of Permineralized Seed Fern Stems of the Upper Paleozoic. J. Torrey Bot. Soc. 133: 20–32. 2006.—Stem morphotaxa continue to be the primary line of evidence for pteridosperm phylogenetic analyses, and the current state of our knowledge of Upper Paleozoic permineralized medullosan and manoxylic lyginopterid stems is reviewed here. Manoxylic Lyginopteridaceae include Lyginopteris, Heterangium, Rhetinangium, Schopfiastrum, Microspermopteris, Trivena, Tetrastichia, Tristichia, Lyginopitys, and Laceya. This group is poorly defined, and cannot be grouped by any as yet known synapomorphy. Four genera are generally included in the Medullosaceae; Medullosa, Colpoxylon, Sutcliffia, and Quaestora, however Colpoxylon should be included in Medullosa. This family is characterized by rachis bases that are vascularized by numerous traces produced at more than one node.

Krings, M. (Bayerische Staatssammlung für Paläontologie und Geologie und GeoBio-Center^LMU, Richard-Wagner-Straße 10, D-80333 Munich, Germany), S. D. Klavins, T. N. Taylor, E. L. Taylor (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, The University of Kansas, Lawrence, KS 66045), R. Serbet (Division of Paleobotany, Natural History Museum and Biodiversity Research Center, The University of Kansas, Lawrence, KS 66045), and H. Kerp (Forschungsstelle für Paläobotanik am Geologisch-Paläontologischen Institut, Westfälische Wilhelms-Universität Münster, D-48143 Münster, Germany). Frond architecture of Odontopteris brardii (Pteridospermopsida, ?Medullosales): new evidence from the Upper Pennsylvanian of Missouri, U.S.A. J. Torrey Bot. Soc. 133: 33–45. 2006.—Exceptionally well-preserved adpressions of the Late Pennsylvanian seed fern Odontopteris brardii (Brongniart) Sternberg are described from the Bonner Springs Shale (Kansas City Group, middle Missourian) of western Missouri, U.S.A. The fossils indicate that O. brardii fronds are bipinnate and similar in architecture to those seen in Lescuropteris (= Odontopteris) genuina (Grand'Eury) Remy et Remy from the Stephanian of France. This seems to contradict the widely accepted opinion that O. brardii is conspecific with Odontopteris minor f. zeilleri Potonié, because fronds of the latter taxon are asymmetrically tripinnate. We suggest that heteroblastic development occurred in O. brardii and O. minor f. zeilleri, and bipinnate fronds were produced by juvenile plants. On the other hand, intraspecific differences in frond architecture may also have been a mechanism of adaptation. As a result, frond architecture can be used only in a limited sense for species circumscription in odontopterid seed ferns until the mechanisms underlying heterophylly in these plants are more fully understood.

Pigg, K. B. (School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ, 85287-4501) and H. Nishida (Faculty of Science and Engineering, Chuo University, 1–13–27 Kasuga, Bunkyo, Tokyo 112– 8551 Japan). The significance of silicified plant remains to the understanding of Glossopteris-bearing plants: an historical review. J. Torrey Bot. Soc. 133: 46–61. 2006.—Anatomically preserved fossils from the Late Permian basins of Antarctica and eastern Australia have played a pivotal role in our understanding of Glossopteris-bearing plants since their discovery in the late 1960s. The first studies, from the Bowen Basin of Queensland by Gould and Delevoryas, showed that permineralized glossopterid ovules are borne on fertile leaf-like structures, suggesting to these authors a “seed fern” affinity for them. Based on vascular bundle orientation, it is now clear that ovules are borne on the adaxial surface of fertile structures, and more recent study has documented that they are often borne on small stalks. Plectilospermum, an Antarctic seed, was shown to exhibit simple polyembryony. With the recent discovery of evidence for swimming sperm in Australian ovules, our understanding of glossopterid reproduction continues to develop. Although whole plant reconstructions are not yet known for individual glossopterid plants, we can now document, in part, information about the plants that bore G. schopfii, G. skaarensis, and G. homevalensis leaves.

Taylor, E. L., T. N. Taylor (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045), H. Kerp (Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster, Hindenburgplatz 57, D-48143 Münster, Germany), and E.J. Hermsen (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045). J. Torrey Bot. Soc. 133: 62–82. 2006.—Mesozoic seed ferns represent a grade of gymnospermous plants whose affinities remain problematic. The three major orders recognized today include the Caytoniales (Triassic-Cretaceous), Peltaspermales (Carboniferous-Triassic) and Corystospermales (Triassic-Cretaceous). A number of genera described from Mesozoic rocks have also been included broadly in the Mesozoic seed ferns, but their frequency, distribution, and affinities render their assignment to specific orders equivocal. The morphotypes in the three principal orders have been important in phylogenetic analyses of seed plants and have been implicated as angiosperm progenitors at various times in the past. All three groups were originally described only from compression/impression fossils, but anatomically preserved corystosperms are now known from Argentina and Antarctica. Since their original description, the geographic and stratigraphic ranges of all three major groups have been expanded, and they are now known from both the northern and southern hemispheres. An additional order of Mesozoic seed ferns, the Petriellales, has been described from the Triassic of Antarctica. This paper will summarize our current knowledge of the Mesozoic seed ferns and comment on the phylogenetic position of several orders, focusing especially on permineralized and compressed corystosperms from the Triassic of Antarctica. Recent studies of well-preserved material from the central Transantarctic Mountains have provided information about the three-dimensional morphology and anatomy of pollen organs and ovulate cupules, as well as the first evidence of the attachment of reproductive organs to the parent plant. These discoveries offer new information that can be used in phylogenetic analyses to provide increased resolution of seed plant relationships.

DiMichele, W. A. (Department of Paleobiology, NMNH, Smithsonian Institution, Washington, DC 20560), T. L. Phillips (Department of Plant Biology, University of Illinois, Urbana, IL 61801), and H. W. Pfefferkorn (Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19106). Paleoecology of Late Paleozoic pteridosperms from tropical Euramerica. J. Torrey Bot. Soc. 133: 83–118. 2006.— Late Paleozoic pteridosperms are a paraphyletic group of seed plants that were prominent elements of tropical ecosystems, primarily those of wetlands or the wetter portions of seasonally dry environments. Because the group is more a tradition-based, historical construct than a well circumscribed phylogenetic lineage, the wide variance in ecological roles and ecomorphological attributes should not be surprising. Pteridosperms can be the dominant canopy trees in local habitats, prominent understory trees, scrambling ground cover, thicket-formers, or liana-like plants and vines. Some species appear to have been weedy opportunists, although this ecological strategy seems to be a minor part of the wide spectrum of pteridosperm life habits. Most pteridosperms appear to have preferred wetter parts of the landscape, though not standing water, and relatively nutrient-rich settings (in comparison with groups such as tree ferns or lycopsids). Of the Paleozoic pteridosperms as traditionally circumscribed, only the peltasperms survived to become major elements in the Mesozoic. However, these plants may have been part of a derived seed-plant clade that also includes the corystosperms and cycads (see Hilton and Bateman, this volume), indicating that only the most derived of the Paleozoic pteridosperm lineages, those that appear to have evolved initially in extrabasinal settings, persisted into the Mesozoic.

Hilton J. (School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK) and R. M. Bateman (Natural History Museum, Cromwell Road, London, SW7 5BD, UK). Pteridosperms are the backbone of seed-plant phylogeny. J. Torrey Bot. Soc. 133: 119–168. 2006.— Using Doyle (1996) as a starting point, we compiled a morphological cladistic matrix of 54 coded taxa (31 wholly extinct, and 23 at least partly extant) and 102 informative characters in order to explore relationships among gymnosperms in general and pteridosperms in particular. Our core analysis omitted six supplementary fossil taxa and yielded 21 most-parsimonious trees that generated two polytomies in the strict consensus tree, both among pteridosperms; the first affected several hydraspermans, and the second affected the three peltasperm/ corystosperm taxa analyzed. The resulting topology broadly resembled topologies generated during previous morphological cladistic analyses that combined substantial numbers of extant and extinct higher taxa. Each of the five groups that include extant taxa was relatively well resolved as monophyletic and yielded the familiar Anthophyte topology (cycads (Ginkgo (conifers (Gnetales, angiosperms)))), strongly contradicting most recent DNA-based studies that placed Gnetales as sister to, or within, conifers. These five extant groups were embedded in the derived half of a morphologically diverse spectrum of extinct taxa that strongly influenced tree topology and elucidated patterns of acquisition of morphological character-states, demonstrating that pteridosperms and other more derived “stem-group” gymnosperms are critical for understanding seed-plant relationships. Collapses in strict consensus trees usually reflected either combinations of data-poor taxa or “wildcard” taxa that combine character states indicating strongly contradictory placements within the broader topology. Including three progymnosperms in the analysis and identifying the aneurophyte progymnosperm as outgroup proved crucial to topological stability. An alternative progymnosperm rooting allowed angiosperms to diverge below cycads as the basalmost of the extant groups, a morphologically unintuitive position but one that angiosperms have occupied in several recent molecular studies. We therefore believe that such topologies reflect inadequate rooting, which is inevitable in analyses of seed plants that use only extant taxa where the outgroups can only be drawn from ferns and/or lycopsids, groups that are separated from extant seed-plants by a vast phylogenetic discontinuity that is bridged only by wholly fossil groups. Given the rooting problem, and the poverty of the hypotheses of relationship that can be addressed using only extant taxa, morphology-based trees should be treated as the initial phylogenetic framework, to subsequently be tested using molecular tools and employing not only molecular systematics but also evolutionary-developmental genetics to test ambiguous homologies. Among several possible circumscriptions of pteridosperms, we prefer those that imply paraphyly rather than polyphyly and exclude only one monophyletic group, providing one cogent argument for the inclusion of extant cycads in pteridosperms. Although pteridosperms cannot realistically be delimited as a monophyletic group, they remain a valuable informal category for the plexus of gymnosperms from which arose several more readily defined monophyletic groups of seed-plants. The ideal solution of recognizing several monophyletic groups, each of which combines a “crown-group” with one or more pteridosperms, is not yet feasible, due to uncertainties of relationship and difficulties to satisfactorily delimiting the resulting groups using

Doyle, J.A. (Section of Evolution and Ecology, University of California, Davis, California 95616, USA). Seed ferns and the origin of angiosperms. J. Torrey Bot. Soc. 133: 169–209. 2006.—If molecular analyses are correct in indicating that Gnetales are related to conifers and no other living gymnosperm group is directly related to angiosperms, studies on the origin of angiosperms must focus on fossil taxa, including “seed ferns.” Some authors have homologized the angiosperm carpel with the cupule of seed ferns, but because angiosperm ovules have two integuments rather than one, cupules are more likely to be homologous with the outer integument. Cupules of the earliest seed ferns may be derived from fertile appendages of “progymnosperms,” but those of later taxa appear to be modified leaves or leaflets, with ovules borne on the abaxial surface in some (peltasperms, corystosperms), the adaxial surface in others (glossopterids, Caytonia). Positional relationships and developmental genetic data suggest that the bitegmic ovule is comparable to a cupule with adaxial ovules. Analysis of a critically revised morphological data set for seed plants indicates that trees in which Gnetales are nested in conifers, as in molecular analyses, are almost as parsimonious as those in which Gnetales are linked with angiosperms, suggesting that the molecular arrangement should be accepted. When living taxa are constrained into the molecular topology, angiosperms are linked with glossopterids, Pentoxylon, Bennettitales, and Caytonia, supporting the homology of the cupule and the bitegmic ovule. Origin of the carpel poses more problems; it could correspond to the leaf portion of the glossopterid leaf-cupule complex, but its homologies in Caytonia are more obscure. New data on currently unknown characters of glossopterids, “Mesozoic seed ferns,” and Bennettitales are needed to test these hypotheses.