Gar W. Rothwell

Fossils and Ferns in the Resolution of Land Plant Phylogeny

Fifty-two taxa of living and extinct vascular plants were evaluated in an unconstrained numerical cladistic analysis using 101 morphological characters to simultaneously resolve the phylogenetic relationships of ferns sensu lato. Included in the analysis were ferns assignable to the Cladoxylales, Stauropteridales, Rhacophytales, Zygopteridales, Ophioglossales, Marattiales, Filicales, and Hydropteridales, as well as a rhyniophyte, a trimerophyte, equisetophytes, lignophytes, and the psilotophytesPsilotum andTmesipteris. The results placed ferns and fernlike plants in three distinct clades, indicating that ferns s.l. are a polyphyletic grade group. Fern clades consist of extinct stauropterids; extinct cladoxylaleans, rhacophytaleans, and zygopteridaleans; and eusporangiates and leptosporangiates with living and extinct species. Psilotophytes occur near the base of the tree rather than nesting with the Filicales, as hypothesized by some. These results place Ophioglossales as the sister group to Marattiales plus the leptosporangiates, supporting the hypothesis that Ophioglossales represent ferns rather than progymnosperms. These analyses are a first attempt, which includes extinct plants, to develop cladistic hypotheses for the overall topology of fern phylogeny and to lay the groundwork for more detailed analyses of relationships among the homosporous leptosporangiates.

New interpretations of the earliest conifers

Abstract New evidence from pre-Permian strata provides an opportunity to enhance our knowledge of the earliest conifers, and together with previously known information forms the basis for evaluating both the time and mode of origin for the group. The earliest co-occurrence of eustelic stems with needle-like leaves, saccate pollen and compound ovulate strobili with bilaterally symmetrical ovules, suggests that the conifers originated at about the beginning of the Westphalian. The first extensive evidence for conifer anatomy from Pennsylvanian sediments of North America reveals remarkably modern-appearing internal features, and confirms Florins general interpretation of the morphology of ovulate Lebachia cones. The specimens also exhibit a more specialized structure than previously suspected. Based on a basic similarity in reproductive features among conifers, cordaites and some pteridosperms, and upon the developmental variations in the shoot systems of tracheophytes, it is proposed that the origin of conifers may have involved a change in shoot structure among either pteridosperms or cordaites. If true, then archaeopteridalean progymnosperms may not be as closely related to gymnosperm origins as presently believed.

Stelar morphology and the primary vascular system of seed plants

This paper deals primarily with the morphology, anatomy, and evolution of the eustele in seed plants. Introductory sections treat stelar terminology, problems of representation and interpretation of stelar diagrams, and the history of studies on the stele. Also included is a classification of stelar types. A significant part of the paper consists of descriptions and illustrations of the primary vascular systems of the stems of all major seed plant taxa (and their progymnosperm precursors) for which data were available. In a critical analysis of recent studies, the stele of monocotyledons is interpreted as a eustele that has become modified in relation to the distinctive morphology and modes of development of this group. Our viewpoint contrasts with that of Zimmermann and Tomlinson who consider the monocotyledon stele to be fundamentally different from that of dicotyledons. In a section on nodal anatomy the emphasis by some systematists on characters of nodal structure is decried because, as is demonstrated, taxa with similar nodal anatomy may differ significantly in their internodal structure. An original statistical study, based on characters of the primary vascular systems of 102 species of dicotyledons and data from other sources, provides the basis for a model of the primitive eustele in seed plants, for a discussion of the adaptive value of certain characteristics of the eustele, and for recognizing probable trends of specialization in the eustele. The primitive eustele is characterized as an open primary vascular system with helical trace departure, and consisting of five sympodia. It is suggested that during the course of evolution of the eustele there has been an increase in the number of vascular bundles in the system. This, apparently, has been accomplished in the gymnosperms (as reflected in the conifers) by an increase in the number of axial bundles, but in the angiosperms by an increase in the number of traces per leaf and an increase in the number of internodes traversed by leaf traces prior to their entry into leaves. There seems to have been a concomitant establishment of connection between the sympodia in the vascular system. Both the increase in number of vascular bundles and their interconnection seem to be adaptive because they probably enhance the survivability of individuals whose vascular systems are damaged by herbivores or other biotic or physical agents. Because diversity among stelar types is relatively limited, stelar morphology seems to have systematic significance primarily at or above the ordinal level. The paper closes with a set of recommendations designed to encourage the future production of comparable, useful data on the stele.PrécisCe travail traite surtout de la morphologie, de l’anatomie, et de l’évolution de la stèle des plantes à graines. Les parties initiales introduisent la terminologie stélaire, les problèmes de représentation et d’interprétation des diagrammes stélaires, et l’histoire des études faites sur la stèle. Aussi inclue est une classification des types stélaires. Une très grande partie de ce travail est consacrée aux descriptions et aux illustrations des systèmes vasculaires primaires des tiges de tous les groupes majeurs des plantes à graines (aussi bien que les progymnospermes qui les ont précédées) pour lesquels les donées soient disponibles. Dans une analyse critique des études récentes, la stèle de monocotylédones est considérée comme une eustèle modifiée vis-à-vis de la morphologie distinctive et des modes de développement de ce groupe. Notre point de vue diffère de celui de Zimmermann et Tomlinson qui considèrent la stèle monocotylédoné différente fondamentalement ce celle des dicotylédones. Dans une partie sur l’anatomie nodale, on décrie l’emphase mise sur des aspects de structure nodale par quelques taxonomistes car, comme constaté, des taxons d’une anatomie nodale similaire peuvent se différer d’une manière significative en ce qui concerne leur structure internodale. Une étude statistique originale, basée sur les caractères de systèmes vasculaires primaires de 102 espèces de dicotylédones, aussi bien que des donées d’autres sources, fournit la base d’un modèle de l’eustèle primitive des plantes à graines, d’une discussion des valeurs adaptives de certains caractères de l’eustèle, et d’une reconnaissance des tendances probables de spécialisation dans l’eustèle. L’eustèle primitive se caractérise en système vasculaire primaire ouvert qui comprend un écart de trace hélicoidale et cinq sympodies. On suggère qu’à travers l’évolution de l’eustèle il y avait une augmentation de nombre de faisceaux vasculaires dans le système. Ceci a été apparamment accompli dans les gymnospermes (par exemple, parmi les conifères) par une augmentation du nombre de faisceaux axiaux, maix dans les angiospermes par une augmentation du nombre des traces dans chaque feuille et par une augmentation du nombre d’internodules traversés par les traces de feuille avant leur entrée dans les feuilles. Il semble avoir été un établissement concomitant de liaison entre les sympodies du système vasculaire. Tant l’augmentation du nombre de faisceaux vasculaires que leur liaison semblent adaptives à cause de leur capacité probable d’améliorer la capacité de survivre chez les individus dont les systèmes vasculaires sont endommagés par des herbivores ou par d’autres agents biotiques ou physiques. Parce que la diversité des types stélaires est relativement limitée, la morphologie stélaire semble avoir de la signification systématique au niveau ou au-dessus de niveau ordinal. On termine le travail en présentant une série de recommandations destinées à encourager la future production des données utile, d’un ordre scientifiquement comparable sur la stèle.Kurze ÜbersichtDiese Arbeit befasst sich hauptsächlich mit der Morphologie, Anatomie und Evolution der Eustele in Samenpflanzen. Einleitende Abschnitte behandeln Stelenterminologie, Probleme der Veranschaulichung und Interpretation von Stelendiagrammen, und die Geschichte der Forschung über die Stele. Auch enthalten ist eine Klassifizierung von Stelentypen. Ein bedeutender Teil dieser Arbeit besteht aus Beschreibungen und Abbildungen der Primärgefässsysteme von den Stämmen aller wichtigen Samenpflanzengruppen (und ihrer Progymnosperm Vorläufer) für welche Daten vorhanden waren. In einer kritischen Analyse der vor kurzem entstandenen Studien, wird die Stele der Monocotyledonen als eine Eustele interpretiert, die in Beziehung zu der eigentümlichen Morphologie und den Arten der Entwicklung dieser Gruppe modifiziert worden ist. Unser Gesichtspunkt kontrastiert mit dem von Zimmermann und Tomlinson, die die Stele der Monocotyledonen als grundsätzlich verschieden von der Stele der Dicotyledonen ansehen. In einem Abschnitt über Knotenanatomie wird die von manchen Systematikern auf Charaktere der Knotenstruktur aufgelegte Betonung abgelehnt, weil, wie gezeigt wird, Gruppen mit ähnlicher Knotenanatomie in ihrer inwendigen Knotenstruktur wesentlich verschieden sein können. Eine statistische Originalstudie, die sich auf Merkmale der Primärgefässsysteme von 102 Arten Dicotyledonen und auf Daten von anderen Quellen basiert, ergibt die Grundlagen für ein Modell der ursprünglichen Eustele in Samenpflanzen, für eine Diskussion über den Anpassungswert von bestimmten Merkmalen der Eustele, und für das Erkennen von vermutlichen Tendenzen der Spezialisierung in der Eustele. Die ursprüngliche Eustele wird als ein offenes Primärgefässsystem mit schneckenförmigem Spurabgang charakterisiert, welches aus fünf Scheinachsen besteht. Es wird behauptet, dass während der evolutionären Entwicklung Eustele eine Erhöhung in der Anzahl der Gefässbündel stattgefunden hat. Diese Erhöhung wurde anscheinend in den Gymnospermen vollzogen (wie bei spielsweise bei den Koniferen), und zwar mit einer Erhöhung in der Anzahl von achsenförmigen Bündeln, in den Angiospermen jedoch mit einer Erhöhung in der Anzahl der Spuren pro Blatt und einer Erhöhung in der Anzahl von inwendigen Knoten, die vor ihrem Eingang in die Blätter von Blattspuren durchquert worden sind. Es scheint dort eine begleitende, gegenseitige Verbindung der Scheinachsen gegeben zu haben. Sowohl die Erhöhung in der Anzahl der Gefässbündel als auch deren gegenseitige Verbindung scheinen anpassungsfähig zu sein, weil sie wahrscheinlich die Überlebensfahigkeit der Individuen, deren Gefässsysteme von Pflanzenfressern oder anderen biotischen oder physischen Agenten beschädigt worden sind, steigern. Weil Mannigfaltigkeit unter Stelentypen relativ beschränkt ist, scheint die systematische Bedeutung der Stelenmorphologie hauptsächlich an oder über der Ordnungstufe zu liegen. Die Arbeit schliesst mit einer Reihe von Vorschlägen, die es beabsichtigen, die kunftige Hervorbringung von vergleichbaren, nützlichen Daten über die Stele anzuregen.

How Does the Inclusion of Fossil Data Change Our Conclusions about the Phylogenetic History of Euphyllophytes

Recent phylogenetic analyses have yielded conflicting results regarding relationships among ferns and other major groups of vascular plants and have prompted some authors to propose novel patterns of tracheophyte phylogeny based on analyses that include only living taxa. The results of one such study resolve seed plants as the sister group to all nonlycophyte pteridophytes and place equisetophytes and psilotophytes within a clade that also includes all of the living ferns. That hypothesis of phylogeny differs markedly from the results of a recent analysis that utilizes morphological data from both living and extinct taxa, which resolves ferns as a polyphyletic assemblage. To evaluate these competing hypotheses and to increase our understanding of the differing results, several tests were performed. Tests identify a high level of character conflict among the various gene sequence data matrices used in the analysis of living taxa. Contrary to the expectations of some, inclusion of extinct taxa in analyses of morphological characters produces striking changes in the topology of the resulting vascular plant tree when compared with the analysis of living taxa only. Together, these tests reveal that relationships among major groups of vascular plants are far less completely understood than claimed by some and that ferns s.l. are probably a polyphyletic assemblage. They also suggest that the impressive successes of gene sequence data in resolving relationships within smaller clades may not be easily duplicated when addressing deep internal nodes of the polysporangiophyte tree. Additional refinement of morphological characters for a combination of living and well‐reconstructed extinct taxa, improved character/taxon ratios, fuller sampling of extinct clades, and tests that utilize rare genetic markers and developmental pathways may hold the greatest promise for ultimately resolving the overall pattern of vascular plant phylogeny.

The callistophytales (Pteridospermopsida): Reproductively sophisticated paleozoic gymnosperms

Abstract The Callistophytales nov. order is proposed for a small family of Pennsylvanian and possibly Permian pteridosperms. Plants conform to two species of Callistophyton and their reproductive organs. The small, eustelic sporophytes have a scrambling, shrub-like habit, profuse branching, adventitious roots and pinnately compound fern-like leaves with sphenopterid pinnules. Reproductive organs are borne on the abaxial surface of the pinnules, and consist of cardiocarpalean ovules and synangiate pollen organs with saccate pollen. Gametophyte development and reproductive biology conform closely to those of extant gymnosperms in the Coniferales. Anatomically preserved reproductive organs of the major groups of Paleozoic gymnosperms are compared, and a closer than currently recognized relationship of Paleozoic seed plants is proposed.

Chaloneria Gen. Nov.; Heterosporous Lycophytes from the Pennsylvanian of North America

Several hundred permineralized fragments of a single type of plant occur in upper Pennsylvanian coal balls from the Appalachian Basin, and these provide evidence for the description and complete reconstruction of a new heterosporous lycophyte. Chaloneria cormosa gen. et sp. nov. is unbranched, ca. 2 m tall, and has a rounded rooting base. Secondary xylem is abundant at the base, diminishes distally, and is absent from the apical region. The stem has ligulate microphylls that are vegetative at proximal levels and grade upward into a fertile apex. Leaf cushions are not produced. In the fertile zone there are alternating regions of megasporophylls and microsporophylls. A second species, Chaloneria periodica sp. nov., has alternating vegetative and fertile zones and is based on Middle Pennsylvanian specimens that were previously referred to Polysporia mirabilis. Spores of both species are assignable to Valvisisporites and Endosporites. Plants with these sporomorphs comprise several genera rather than the single species P. mirabilis.

Thucydiaceae Fam. Nov., with a Review and Reevaluation of Paleozoic Walchian Conifers

Abundant fossils of a single conifer species occur in a Pennsylvanian‐age deposit of eastern North America, providing the opportunity to describe a biological taxon of primitive conifers as well as to clarify the uncertain taxonomy and systematics of walchian conifers. Thucydia mahoningensis gen. et sp. nov. is represented by up to three orders of interconnected vegetative and fertile shoots that are preserved as coalified compressions with cuticles; some also display internal anatomy. The plant has an orthotropic stem, with two orders of plagiotropic lateral branches that all bear helically arranged simple leaves. Ovulate fructifications consist of compact zones of bracts with axillary ovuliferous dwarf shoots on otherwise vegetative branches. Pollen cones are compound shoots comprising helically arranged dwarf shoots in the axils of bracts on a main axis. Polliniferous dwarf shoots produce sterile scales, sporophylls with a terminal pollen sac, and in situ Potonieisporites grains. Stems have an endarch eustele with dense wood, periderm, and resin canals in the pith. There are two adaxial stomatal zones on vegetative leaves, but stomata are distributed over the entire adaxial surface of bracts and sterile scales. Thucydia is the only conifer with ovuliferous fertile zones, compound pollen cones, and dissimilar stomatal distributions on vegetative and fertile leaves. This novel combination of features characterizes the Thucydiaceae fam. nov. The currently confused state of primitive conifer taxonomy is reviewed, nomenclature is clarified, and revised approaches for inferring relationships are proposed. Thucydia provides a benchmark for developing sound taxonomic concepts and useful criteria for identifying specimens of walchian species and for resolving phylogenetic relationships among fossil and living conifers.

Characterizing the Most Primitive Seed Ferns. I. A Reconstruction of Elkinsia polymorpha

Abundant gymnospermous remains occur in association with Elkinsia polymorpha in Upper Devonian deposits near Elkins, West Virginia, and these provide an opportunity to characterize and further refine our concepts of the most ancient seed ferns. The sporophyte of E. polymorpha has helically arranged dimorphic fronds, with large numbers of either preovulate cupules or synangiate prepollen organs borne terminally on the fertile fronds. Vegetative fronds are planar, with rachides that fork up to four times, and pinnate laterals that terminate in sphenopterid pinnules. Fertile fronds branch dichotomously throughout and are three-dimensional in the distal regions. Stems display a centrarch, three-ribbed protostele with prominent sclerenchyma of the sparganum type in the outer cortex. The frond rachis is vascularized by two C-shaped bundles. The hydrasperman preovulate cupules are as characterized in earlier studies. Prepollen organs consist of six to eight radially arranged sporangia that are fused at the base and free distally. Elkinsia polymorpha is the first seed fern to be reconstructed from Devonian sediments and the first preovulate seed fern to be characterized as a plant. As such, it represents an important evolutionary unit for ongoing phylogenetic studies of the origin of seed plants.

Anatomically Preserved Williamsonia (Williamsoniaceae): Evidence for Bennettitalean Reproduction in the Late Cretaceous of Western North America

An anatomically preserved ovulate cycadeoid cone has been discovered in Upper Cretaceous (Campanian) sediments of Vancouver Island, British Columbia, Canada. The specimen is preserved by calcareous cellular permineralization and displays diagnostic features of the genus Williamsonia Carruthers. The cone consists of a receptacle from which tightly packed interseminal scales and ovulate sporophylls with terminal ovules diverge over an arc of ca. 300°. Adjacent interseminal scales interdigitate and form a continuous tissue. Seeds are erect and more or less round in cross sections at all levels, and a cupule is not produced. The sarcotesta consists of multicellular peglike projections. The nucellus is attached to the integument only at the chalaza and is vascularized by a shallow cup of tracheids. Apically, the nucellus narrows to a solid finger‐like projection that fits tightly into the base of the micropylar canal. A pollen chamber is not produced. Nucellar cells are often separated from each other and are associated with large, hollow structures that represent pollen tubes similar to those in living conifers. Cellular megagametophytes and immature embryos are also preserved in some seeds. Williamsonia bockii sp. nov. represents the most recent seed cone of the Williamsoniaceae and is the first anatomically preserved reproductive structure of this family to be discovered in western North America. It reveals new features for the family Williamsoniaceae and allows for the interpretation of several additional facets of reproductive biology in the Bennettitales, particularly pollen tube production, pollination biology, and mode of fertilization.

Hanskerpia gen. nov. and phylogenetic relationships among the most ancient conifers (Voltziales)

A new fossilized walchian (Voltziales) conifer plant from the Late Pennsylvanian Hamilton Quarry of Kansas, U.S.A. combines morphological characters of the morphogenus Walchia and the fossil plant genus Otovicia, with cuticular characters like those of the fossil plant genus Ernestiodendron, and seed cone morphology like that of the fossil plant genus Emporia. Internal anatomical characters are basically similar to the other walchian genera for which such features are known, but some are taxonomically informative. Numerical cladistic analysis reveals that this novel combination of characters defines a new fossil plant genus and species of walchian conifers Hanskerpia hamiltonensis. This analysis provides a test of hypotheses for relationships among walchian and other ancient conifers. Results of this analysis support some previously proposed relationships among the most primitive conifers and other Paleozoic coniferophytes. However, the analysis also calls to question the validity of all previously proposed familial circumscriptions for walchian conifers, and highlights the perplexing systematic challenges posed by the most ancient fossil conifers. The unexpected species richness of voltzialean conifers is interpreted as having resulted from the evolutionary ecology of unstable environments.