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The "new approach to Corystospermales" and the antarctic fossil record: A critique

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Much is now known about the Mesozoic pteridosperm order Corystospermales based on over seventy years of research. However, several concepts of corystosperm evolution have been proposed that avoid well-established phylogenetic methodology and arbitrarily discount evidence of the group from Antarctica. Here, we focus on the "New approach to Corystospermales" published by Artabe and Brea, which establishes a phylogeny for corystosperms based mainly on a restricted concept of anomalous wood anatomy in this group, and assumed relationships with cycads and medullosan pteridosperms. These authors also question evidence regarding attached corystospermalean organs from Antarctica. Based on a reconsideration of the Antarctic record, we reaffirm the evidence for a Dicroidium leaf in attachment to branches that also bear distinctive short shoots. Umkomasia cupulate organs occur on identical attached short shoots. We also emphasize that wood morphotaxa attributed to the corystosperms from Antarctica, including Jeffersonioxylon and Kykloxylon, indicate that anomalous secondary growth is not necessarily basic to the corystosperms. We conclude that a greater understanding of the corystosperms will require a consideration of all relevant evidence, and will not be advanced by single organ phylogenies based on fossils from one region.
A-B. Attached Dicroidium odontopteroides leaf. Alfie's site, Antarctica. Specimen number T12-1002 / hoja de Dicroidium odontopteroides en conexión a la rama. Localidad fosilífera Alfie, Antártida. Ejemplar número T12-1002. A, view of entire branch section with lateral short shoot (SS) and attached D. odontopteroides leaf. Arrow indicates severe bend in leaf petiole X1.0 / aspecto de la rama con un braquiblasto (SS) lateral y hojas de Dicroidium odontopteroides en conexión. La flecha indica una marcada curvatura en el pecíolo de la hoja. B, Detail of the D. odontopteroides leaf attachment point in figure 1.A. Arrow indicates branch margin. Note that the line of sediment (S) occurs within this margin, not between the petiole and branch X1.6 / detalle del punto de inserción de la hoja de Dicroidium odontopteroides señalado en A. La flecha indica el margen de la rama. Nótese que la línea de sedimento(S) se encuentra dentro de este margen y no entre el pecíolo de la hoja y la rama. C, Branch of a living Ginkgo biloba specimen from the University of South Alabama campus showing a long shoot (LS) with attached leaves of the current season in close proximity to short shoots (SS) growing out from last season's section of the long shoot X0.5 / rama de la especie actual Ginkgo biloba, tomada de los terrenos de la Universidad del Sur de Alabama, mostrando ramas de crecimiento indefinido (LS) con hojas de la reciente temporada en conexión y en las cercanías de braquiblastos originados durante la estación anterior en las ramas de crecimiento indefinido.
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... From 'Alfie's Elbow' ridge southeast of Schroeder Hill in the Shackleton Glacier region (Fremouw or Falla Formation), Antarctica, Axsmith et al. (2000Axsmith et al. ( , 2007 reported D. odontopteroides as attached to a stem (see discussion herein). ...
... By comparison with extant Ginkgo biloba, all the leaves grow from new season long shoots or in fascicles from short shoots. Axsmith et al. (2007) defended the attachment and provided further photographs (different orientation) to which Artabe & Brea (2007) responded and were still not convinced of the attachment. A strange reconstruction of a branch, bearing leaves and cupulate organs, was provided by Axsmith et al. (2007, fig . ...
... The most convincing example of leaves attached to short shoots and stems in the Gondwanan Triassic are the numerous Dicroidium leaves described from the Ipswich Flora, Australia . This Australian find raises further questions regarding the reported attached single Dicroidium leaf to a mature long shoot stem from Antarctica (Axsmith et al. 2000(Axsmith et al. , 2007, which was previously questioned by Artabe & Brea (2003. ...
Article
Anderson, H.M., Barbacka, M., Bamford, M.K., Holmes, W.B.K. & Anderson, J.M., XX. 2019. Dicroidium (foliage) and affiliated wood; Part 3 of a reassessment of Gondwana Triassic plant genera and a reclassification of some previously attributed. Alcheringa XXX, X–X. ISSN 0311-5518 Dicroidium belonging to Umkomasiaceae (Corystospermaceae) in the polyphyletic pteridosperms (seed-ferns) is reassessed comprehensively worldwide and emended. All records are analysed and some attributed to the genus previously are reclassified. Dicroidium leaves are clearly affiliated with Umkomasia ‘megasporophylls’ and Pteruchus ‘microsporophylls’. The attachments of Dicroidium leaves to stems and associated wood genera are reviewed. Dicroidium is shown to be restricted to the Triassic of Gondwana, where it is by far the most prominent and diverse genus with 23 accepted species. It is well represented in collections from South America, Antarctica, India, Australia, New Zealand and southern Africa from where the Molteno Formation is the most comprehensively sampled stratigraphic unit, yielding numerous species from 75 assemblages. The problems of defining the limits of Dicroidium and its species are addressed. The records of leaf fragments from the Indian Nidpur Flora, Early Triassic, are questionably referable to Dicroidium, whereas the multiple forking leaves from the Cisuralian of India await description as a new peltasperm genus. The forked leaves from the (?)Lopingian of Jordan, previously classified as Dicroidium, are reassessed and placed in the new genus Jordaniopteris. Heidi M. Anderson [hmsholmes@googlemail.com], Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg 20150, South Africa; Maria Barbacka [maria.barbacka@gmail.com], W. Szafer Institute of Botany, Polish Academy of Sciences, Lubicz 46, Kraków 31-512, Poland, Botanical Department, Hungarian Natural History Museumartmem H-1431, Budapest, Pf. 137, Hungary; Marion K. Bamford [marion.bamford@wits.ac.za], Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg 20150, South Africa; W. B. Keith Holmes [wbkholmes@hotmail.com], 46 Kurrajong Street, Dorrigo, NSW 2453, Australia, University of New England, Armidale, NSW 2351, Australia; John M. Anderson [jmanderson.gondwana@googlemail.com], Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg 20150, South Africa.
... En cuanto a los datos adicionados por Axsmith et al. (2000) que aportan información acerca del hábito de la " planta de Dicroidium de la Antártida " , los mismos se obtuvieron a partir de impresiones/compresiones colectadas en la localidad " Alfie " (sección superior de la Formación Fremouw-sección inferior de la Formación Falla), en el glaciar Shackleton. Los autores describen órganos cupulados: Umkomasia uniramia y hojas: Dicroidium odontopteroides en conexión orgánica con ejes cortos que se desarrollan sobre ramas semejantes a las que presenta Ginkgo biloba L. La conexión orgánica de Dicroidium odontopteroides a un eje maduro con braquiblastos revalidada por Axsmith et al. (2007) en Ameghiniana, fue cuestionada por Artabe y Brea (2003). Uno de los revisores (G. ...
... Parece entonces bastante claro que Umkomasia uniramia no comparte los caracteres diagnósticos del género Umkomasia. En respuesta al trabajo de Axsmith et al. (2007, p. 224) se especifica que no se malinterpretó la información de la Antártida. En este sentido, se reitera que no se acepta la evidencia " inequívoca " de conexión orgánica de ramas con braquiblastos con hojas de Dicroidium odontopteroides (Axsmith et al., 2007, p. 225, Fig. 1A-B). ...
... Grado de sustento de la crítica de Axsmith et al. (2007) al trabajo publicado en Alcheringa por Artabe y Brea (2003) Axsmith et al. (2007) ...
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réplica tiene por objeto contestar algunas de las afirmaciones realizadas por Axsmith et al. (2007) en la revista Ameghiniana en donde los autores com-plementan una fuerte crítica del trabajo publicado en Alcheringa por Artabe y Brea (2003), con la revalida-ción de un estudio efectuado por Axsmith et al. (2000) sobre material de la Antártida. Con el objeto de seguir una línea de razonamiento coherente, se analizarán, en primer lugar, los datos controvertidos de la Antártida y en una segunda sección, el grado de sustento de la crítica al trabajo publicado en Alcheringa (2003). AMEGHINIANA (Rev. Asoc. Paleontol. Argent. Réplica al trabajo de Axsmith et al. (2007) publicado en Ameghiniana 44: 223-230: "The 'New Approach to Corystospermales' and the Antarctic Fossil Record: A Critique"
... Archangelsky and Brett (1961), have long been of interest to botanists and paleobotanists. Some, like Kykloxylon Meyer-Berthaud et al. (1993), a Triassic axis from Antarctica, lacks anomalous secondary growth, and has pycnoxylic wood with attached Dicroidium leaves and reproductive structures, thus indicating an affinity with the Corystospermales (Axsmith et al., 2007;Taylor et al., 2009). Jeffersonioxylon Del Fueyo et al. (1995, also with pycnoxylic wood, from the Triassic of Antarctica, Cuneumxylon Artabe and Brea (2003), Elchaxylon Artabe and Zamuner (2007), Tranquiloxylon Herbst and Lutz (1995) all from the Triassic of Argentina, and Rhexoxylon from the Triassic of Africa and South America have also been assigned to this order. ...
... Jeffersonioxylon Del Fueyo et al. (1995, also with pycnoxylic wood, from the Triassic of Antarctica, Cuneumxylon Artabe and Brea (2003), Elchaxylon Artabe and Zamuner (2007), Tranquiloxylon Herbst and Lutz (1995) all from the Triassic of Argentina, and Rhexoxylon from the Triassic of Africa and South America have also been assigned to this order. Based on the assignment of the above taxa to the Corystospermales, Axsmith et al. (2007) concluded that anomalous secondary growth is not necessarily basic to the corystosperms. ...
Article
A new genus and two new species of an incertae sedis spermatophyte are erected based on large, petrified and permineralized axes from Middle to Upper Jurassic strata of central Queensland and northeastern New South Wales in Australia and the South Island of New Zealand. Specimens of this genus were previously considered by some to be a form of the Indian taxon, Pentoxylon. The new genus, Donponoxylon, however, differs from Pentoxylon and other spermatophytes in having very small, round (in cross section) sympodial strands characterized primarily by centrifugal secondary xylem development in individual segments. Furthermore, these segments form a complex anastomosing system that branches and coalesces throughout the stem. Donponoxylon consists of two species: Donponoxylon bennettii and Donponoxylon jacksonii. D. bennettii is diagnosed by atypical secondary growth with continuous or discontinuous concentric outer vascular rings, and by a generally irregular arrangement of vascular segments around the pith. D. jacksonii differs from D. bennettii in having the vascular segments regularly arranged around the pith and the absence of the outer vascular rings. In the absence of attached foliage or reproductive structures, the phylogenetic relationships of Donponoxylon remain uncertain beyond its assignment to Spermatopsida. Donponoxylon was an arborescent component of Middle to Late Jurassic high-latitude forests along the southeast coast of East Gondwana where it thrived in moist, volcaniclastic-derived soils.
... It is, however, possible that the long-lived traces vascularized short shoots, a common feature in both extant and fossil gymnosperms (Little et al., 2013). For the corystosperms in particular, the presence of short shoots is well documented based on stem compressions from Antarctica (Axsmith et al., 2000(Axsmith et al., , 2007 and Australia (Anderson et al., 2008) that bear short shoots with attached Dicroidium leaves. Occurrences of short shoots have also been suggested for the South African plant reconstructed from Rhexoxylon tetrapteridoides and Dicroidium odontopteroides by Retallack and Dilcher (1988). ...
... Some authors interpreted this latter feature as included phloem produced by supernumerary cambia (e.g., Artabe and Brea, 2003;Bodnar, 2012). While the taxonomic value of such characters is disputable (Artabe and Zamuner, 2007;Axsmith et al., 2007), they have been used to reconstruct hypothetical phylogenies of the corystosperms (Artabe and Brea, 2003;Bodnar, 2008). However, the present work and a reinvestigation of previous literature demonstrate that in some taxa two of these characters (unequal cambial activity and dilatation parenchyma) only occur in mature stems. ...
Article
Anatomically preserved trunks and young stems of corystosperm seed ferns are described from the Triassic of Fremouw Peak, Beardmore Glacier area, Antarctica. Based on characters of the primary and secondary vascular system, these new specimens are assigned to Kykloxylon, a genus that was established based on young stems with attached Dicroidium leaf bases. The largest specimens illustrate how some secondary growth characters, such as unequal cambial activity, appeared during later development, which enables a better comparison of Kykloxylon with trunks assigned to other corystosperm genera. Jeffersonioxylon from the Gordon Valley, Antarctica, and Cuneumxylon from South America show strong similarities with the newly described larger Kykloxylon trunks from Fremouw Peak, and might be considered congeneric. Our results provide further support for the presence of two anatomically and morphologically distinct kinds of Dicroidium-bearing trees in the Triassic vegetation of Gondwana, one with a palm-like habit and Rhexoxylon stems and the other with a more Ginkgo-like habit and Kykloxylon/Cuneumxylon-type stems.
... The cupules attached to short shoots ( Fig. 24 ) and leaves attached to long shoots from the Upper Triassic of Antarctica has perplexed some who challenge whether long shoots would retain leaves (e.g., Artabe and Brea, 2003 ;Holmes and Anderson, 2005 ). In modern Ginkgo L., however, leaves are borne on relatively large stems as well as on short shoots ( Axsmith et al., 2007 ). It is also important to note that this Dicroidium plant grew at high polar latitudes, and thus the parameters affecting growth were no doubt much different than they are in temperate regions today ( Axsmith et al., 2007 ). ...
... In modern Ginkgo L., however, leaves are borne on relatively large stems as well as on short shoots ( Axsmith et al., 2007 ). It is also important to note that this Dicroidium plant grew at high polar latitudes, and thus the parameters affecting growth were no doubt much different than they are in temperate regions today ( Axsmith et al., 2007 ). The report of Umkomasia cupules ( Figs. 19,20 ) in association with Thinnfeldia -type foliage from the Upper Triassic of China ( Zan et al., 2008 ) further expands the diversity within the corystosperms. ...
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Five orders of late Paleozoic-Mesozoic seed ferns have, at one time or another, figured in discussions on the origin of angiosperms, even before the application of phylogenetic systematics. These are the Glossopteridales, Peltaspermales, Corystospermales, Caytoniales, and Petriellales. Although vegetative features have been used to suggest homologies, most discussion has focused on ovulate structures, which are generally interpreted as megasporophylls bearing seeds, with the seeds partially to almost completely enclosed by the megasporophyll (or cupule). Here we discuss current information about the reproductive parts of these plants. Since most specimens are impression-compression remains, homologizing the ovulate organs, deriving angiospermous homologues, and defining synapomorphies remain somewhat speculative. Although new specimens have increased the known diversity in these groups, a reconstruction of an entire plant is available only for the corystosperms, and thus hypotheses about phylogenetic position are of limited value. We conclude that, in the case of these seed plants, phylogenetic analysis techniques have surpassed the hard data needed to formulate meaningful phylogenetic hypotheses. Speculation on angiosperm origins and transitional stages in these fossils provides for interesting discussion, but currently it is still speculation, as the role of these groups in the origin of angiospermy continues to be cloaked in Darwin's mystery.
... Their attribution to Umkomasia and the reported organic connection has been questioned by Anderson and Anderson (2003), Holmes and Anderson (2005), and Anderson et al. (2008) on the basis of long and short shoot development. However, their reasoning was shown to be doubtful using an example from a living Gingko biloba L. (Axsmith et al. 2007); that species has complex and highly variable shoot development that is strongly influenced by environmental conditions (Leigh et al. 2011, Little et al. 2013. The suggestion by Artabe and Brea (2003) Anderson and Anderson (2003, p. 240) noted the diversity of branching mode that has been included in the genus. ...
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The plant fossil genera Umkomasia Thomas 1933 and Pteruchus Thomas 1933 emend. Townrow 1962 are known chiefly from the Middle and Upper Triassic of Gondwana. The structure of these fructifications has been conjectural, some being identified as pinnate, others as helically arranged. Specimens from the Ladinian-lower Norian of Queensland (northeastern Australia) show that the female and male fructifications - U. geminata (Shirley 1898) Rigby in Playford et al. 1982 emend. nov. and P. dubius Thomas 1933 emend. Townrow 1962, respectively - have a bipinnate structure. Those fructifications and the bipinnate leaf, Dicroidium feistmantelii (Johnston 1894) Gothan 1912, probably all belonged to the same parent plant. It was first suggested by John Townrow in 1962 that the sporangial heads of P. dubius have a pinnate structure; this character is confirmed herein. Pteruchus is recorded for the first time from the Carnian Tarong Basin, Queensland. The holotype of Stachyopitys simmondsii Shirley 1898 is shown to be ovuliferous. The species is recombined as Umkomasia simmondsii (Shirley 1898) comb. et emend. nov. Genuine male fructifications, previously identified as S. simmondsii, and later as Pteruchus simmondsii (Shirley 1898) Jones and de Jersey 1947, are assigned to P. minor Thomas 1933. That species comprises the smallest fructifications of the genus. Townrovia polaris Bomfleur et al. 2011 and Stachyopitys lacrisporangia Anderson & Anderson 2003 are identified as junior synonyms to P. minor. The diagnosis of Townrovia Retallack 1981 is inaccurate and the genus is insufficiently distinguished from Pteruchus.
... This association was disputed by Anderson and Anderson (2003), Holmes and Anderson (2005a) and Anderson et al. (2008) on the basis of shoot development (Holmes and Anderson, 2005a, p. 2). However, their arguments were refuted by Axsmith et al. (2007) based on shoot development in a living Ginkgo Linnaeus, 1771. The diagnosis of Umkomasia uniramia Axsmith, Taylor, Taylor and Cúneo, 2000 included foliage considered conformable with Dicroidium odontopteroides (Morris, 1845) Gothan, 1912. ...
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Pteridosperms are preserved abundantly in the Gondwanan Triassic, with many species exhibiting considerable morphological variation that has been attributed to a hybridization model of speciation. This is an improbable explanation given that hybridization is very rare in gymnosperms. Allopatric speciation resulting from geographic and climatic provincialism is a more likely explanation for the morphological diversity which is well represented in Anisian–Norian (Middle and Upper Triassic) floras of Australasia and elsewhere in Gondwana. Most specimens are distributed among three families: Umkomasiaceae, Peltaspermaceae and Matatiellaceae. These families, together with other possibly pteridospermous genera, are reviewed herein. Diversity in these families apparently declined by the Rhaetian and they did not persist into the Gondwanan post-Triassic. Australasian post-Triassic strata contain remarkably different floral assemblages to those of the Triassic. No fructifications are clearly pteridospermous and no remains show any obvious relationship with pteridosperms of the Gondwanan Triassic. Caytonialean fructifications are not known in Australasian strata; however, associated foliage has been reported from the Eastern Gondwanan Upper Triassic through Middle Jurassic including Australia. Much fern-like foliage, claimed to be pteridospermous from the Lower Jurassic through Eocene of Eastern Gondwana, lacks supporting evidence of such affiliation. © 2015, Instituto Geologico y Minero de Espana. All rights reserved.
... In our opinion, Umkomasia is a validly erected genus with well-defined characters and should be retained as such. This approach is shared by Artabe & Brea (2007), who wrote that U. uniramia should be assigned to another taxon. In conclusion, this Antarctic material does not belong in Umkomasia, nor in any other described genus and is thus placed in the new genus Axsmithia as A. uniramia. ...
Article
The genus Umkomasia, a megasporophyll, belonging to the pteridosperms (seed ferns) in the family Umkomasiaceae (Corystospermaceae), is reassessed comprehensively worldwide. All previous records are analysed. Certain fertile structures previously attributed are reclassified. Umkomasia is shown to be restricted to the Triassic of Gondwana where it is associated with the genus Pteruchus, a microsporophyll, and the genus Dicroidium, a vegetative leaf. It is well represented from Argentina, Australia and southern Africa where the Molteno Formation is by far the most comprehensively sampled with eight species described. Two specimens from the upper Permian of India attributed to Umkomasia are reclassified as cf. Arberiopsis sp. A whorled fertile structure from Antarctica, previously assigned to Umkomasia, is reclassified in a new genus as Axsmithia uniramia. Another compression fossil and the permineralized Umkomasia resinosa remain as valid records from Antarctica. The material described as Umkomasia from the Triassic of China is reclassified as Stenorachis asiatica. The Lower Jurassic record from Germany is placed in a new genus as Kirchmuellia franconica. The records of Umkomasia sp. from the Rhaetic of Germany are reclassified as cf. Kirchmuellia sp. and the single specimen from the Jurassic of Libya as genus et sp. indet. The Lower Cretaceous record from Mongolia has been reclassified by other researchers as Doylea mongolica. A pictorial key to Umkomasia species is provided, geographic and stratigraphic distributions are tabulated.
... In these aberrant forms, the lateral ovule-bearing structures have two collateral vascular bundles similar to those of corystosperms. (u) Umkomasia uniramia, modified from Axsmith et al. (2000Axsmith et al. ( , 2007) showing five reflexed cupules. Successive transverse sections from the base to the chalaza showing position and orientation of vascular bundles (black, xylem; white, phloem). ...
... De acuerdo a Artabe & Brea (2003), las especies argentinas de Rhexoxylon se ajustan a una serie evolutiva (R. cortaderitaense, R. piatnitzkyi, y R. brunoi) que presenta un incremento en la polixilia centrípeta y centrífuga, y la adquisición progresiva de tallos de mayor tamaño. Elchaxylon representaría una rama que se inicia con R. cortaderitaense (Artabe & Zamuner, 2007), ya que ambos poseen un xilema secundario centrípeto que no forma verdaderos haces perimedulares. Un modelo evolutivo alternativo fue hipotetizado por Bodnar (2008), quien incorporó los taxones no argentinos e interpretó los caracteres caulinares desde un punto de vista diferente. ...
Article
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Evolutionary developmental studies in fossil stems of Corystospermaceae. A methodological critera to study the development of permineralized stems is proposed, which is applied in the analysis of the evo-devo patterns of Corystospermaceae axes, the most important Mesozoic seed fern family in gondwanic Triassic paleofloras. This group accounts for an excellent case study because their stems are preserved permineralized, showing cellular and histological details, and it is possible to examine their whole ontogeny in a single individual since the cells of the secondary xylem and phloem are produced in successive layers retaining permanently their position. In the ontogenetic studies of pteridosperms, regulating mechanisms of development must be discerned comparing with ontogenies of comparable living plants. This extrapolation is possible since it is known that regulating processes of secondary growth are homologous in all lygnophytes. Most of corystosperm axes present a dissected secondary vascular cylinder, and tissues with unusual development and position, which is caused by four classes of cambial deviations: 1-differential activity throughout the stem circumference; 2-remnant activ-ity; 3-inverse or centripetal cambium; and 4 -successive inverse and/or normal cambia (poyxyly). The origin of these cambium deviations is intimately linked to heterotopic events, including neoheterotopy and homeosis. By comparison with living plant studies, it is theorized that homeotic genes implied with these events could be those related to the beginning of cambial activity, cell differentiation, and vascular bundle arrangement. Resumen: Se propone un criterio metodológico para estudiar el desarrollo de tallos permineralizados, aplicado en el análisis de los patrones evolutivos-del desarrollo de los ejes caulinares de las Corystospermaceae, familia de "pteridospermas" dominante en las paleofloras triásicas gondwánicas. Este grupo representa un excelente caso de estudio dado que sus tallos se preservan permineralizados, por lo que exhiben detalles celulares e histológicos, y, asimismo, su ontogenia entera puede ser examinada en un solo ejemplar debido a que las células del xilema y floema secundario son producidas en capas sucesivas que retienen su posición de manera permanente. En los estudios ontogenéticos de pteridospermas los mecanismos reguladores del desarrollo deben ser discernidos por medio de la comparación con ontogenias de plantas vivientes comparables. Esta extrapolación es posible ya que se sabe que los procesos reguladores del crecimiento secundario son homólogos en todas las lignofitas. Los troncos de las corystospermas presentan un cilindro vascular secundario disecto y tejidos vasculares de desarrollo y posición inusuales, lo que es causado por cuatro clases de desviación de la actividad cambial típica: 1-actividad diferencial; 2-actividad remanente; 3-cámbium inverso o centrípeto; y 4-cámbiumes sucesivos inversos y/o normales (polixi-lia). El origen de estas variantes del cámbium está intimamente vinculado con eventos heterotópicos, incluyendo la homeosis. Al comparar con estudios de plantas vivientes, se teoriza que los genes homéoticos implicados en estos eventos podrían ser aquellos relacionados con el inicio de la actividad cambial, la diferenciación de las células vasculares y la organización de los haces vasculares.
... Doyle & Donoghue, 1986). Peltasperms represent an enigmatic and isolated fossil lineage (Fig. 1) that has been variously placed in different morphological phylogenetic analyses, most recently as closely related to the Mesozoic pteridosperm order Corystospermales (Doyle, 2006;Axsmith, Taylor & Taylor, 2015). Remarkably clear leaf-surface images of the Mesozoic corystosperm leaf-genus Dicroidium illustrated by Hamad et al. (2008) show at least some stomata possessing two distinct LSCs, although this feature appears highly variable. ...
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Stomata play a critical ecological role as an interface between the plant and its environment. Although the guard‐cell pair is highly conserved in land plants, the development and patterning of surrounding epidermal cells follow predictable pathways in different taxa that are increasingly well understood following recent advances in the developmental genetics of the plant epidermis in model taxa. Similarly, other aspects of leaf development and evolution are benefiting from a molecular–genetic approach. Applying this understanding to extinct taxa known only from fossils requires use of extensive comparative morphological data to infer ‘fossil fingerprints’ of developmental evolution (a ‘palaeo‐evo‐devo’ perspective). The seed‐plant order Bennettitales, which flourished through the Mesozoic but became extinct in the Late Cretaceous, displayed a consistent and highly unusual combination of epidermal traits, despite their diverse leaf morphology. Based on morphological evidence (including possession of flower‐like structures), bennettites are widely inferred to be closely related to angiosperms and hence inform our understanding of early angiosperm evolution. Fossil bennettites – even purely vegetative material – can be readily identified by a combination of epidermal features, including distinctive cuticular guard‐cell thickenings, lobed abaxial epidermal cells (‘puzzle cells’), transverse orientation of stomata perpendicular to the leaf axis, and a pair of lateral subsidiary cells adjacent to each guard‐cell pair (termed paracytic stomata). Here, we review these traits and compare them with analogous features in living taxa, aiming to identify homologous – and hence phylogenetically informative – character states and to increase understanding of developmental mechanisms in land plants. We propose a range of models addressing different aspects of the bennettite epidermis. The lobed abaxial epidermal cells indicate adaxial–abaxial leaf polarity and associated differentiated mesophyll that could have optimised photosynthesis. The typical transverse orientation of the stomata probably resulted from leaf expansion similar to that of a broad‐leaved monocot such as Lapageria, but radically different from that of broad‐leafed eudicots such as Arabidopsis. Finally, the developmental origin of the paired lateral subsidiary cells – whether they are mesogene cells derived from the same cell lineage as the guard‐mother cell, as in some eudicots, or perigene cells derived from an adjacent cell lineage, as in grasses – represents an unusually lineage‐specific and well‐characterised developmental trait. We identify a close similarity between the paracytic stomata of Bennettitales and the ‘living fossil’ Gnetum, strongly indicating that (as in Gnetum) the pair of lateral subsidiary cells of bennettites are both mesogene cells. Together, these features allow us to infer development in this diverse and relatively derived lineage that co‐existed with the earliest recognisable angiosperms, and suggest that the use of these characters in phylogeny reconstruction requires revision.
... Consequently, the only stem attached to Dicroidium-like foliage with anatomy is Kykloxylon, and the similarities of the new trunks described in this paper to this genus are a good indication of their corystosperm affinities. Artabe and Brea (2003) base their concept of corystosperm stems on the presence of anomalous secondary growth (see also Axsmith et al., 2007;Artabe and Brea, 2007 for a discussion of this concept), because Rhexoxylon, the first genus of stems assigned to the group, has a dissected cylinder with both centripetal and centrifugal secondary xylem present in each segment of the stele, as well as perimedullar bundles. Centripetal secondary xylem has also been described in Elchaxylon (Artabe and Zamuner, 2007) from the Upper Triassic of Mendoza province, Argentina, although in this taxon, the centrifugal secondary xylem is a continuous cylinder. ...
Article
Permineralized gymnosperm axes with pycnoxylic wood from the Middle Triassic Fremouw Formation of the Central Transantarctic Mountains, Antarctica, are assigned to the corystosperms (seed ferns) and conifers. Both groups have been previously described from this formation based on juvenile stems with attached leaf bases and decorticated trunks. Here we describe large axes with preserved bark from the Fremouw Peak permineralized peat locality. The specimens are characterized by a small parenchymatous pith with clusters of sclereids, a thick cylinder (>10cm) of pycnoxylic wood, and 1–2cm of bark containing distinctive clusters of sclereids and a complex system of cortical vascular bundles. Comparison with axes previously described from the Middle Triassic of Antarctica shows that the new specimens are most similar to Kykloxylon, a corystosperm genus based on young stems bearing Dicroidium leaves, and with a portion of axis previously described as Rhexoxylon like. We suggest that both the new specimens and the Rhexoxylon-like axis represent proximal parts of a Dicroidium/Kykloxylon plant that possibly had a fluted trunk base, and we discuss the problem of delimiting features in corystosperm axes.
... N. Phipps et al. 1998), subterranean organs of sphenophytes (Bomfleur et al. 2013b), and abundant debris of leafy and thallose bryophytes (Bomfleur et al. 2014a). At the Alfie's Elbow site, petriellalean remains occur only in the level 2 bed, which also yielded (1) the only known occurrence of corystosperm reproductive organs attached to short shoots Axsmith et al. 2000Axsmith et al. , 2007, (2) one of only three known occurrences worldwide of attached Dicroidium leaves (Axsmith et al. 2000), and (3) the only known record of dipterid ferns in the Antarctic Triassic (Escapa et al. 2011). We interpret this rich assortment of otherwise rare plant taxa and organs and the extraordinary proportion of attached organs to reflect high-energy depositional events (e.g., catastrophic river flooding or riverbank collapse after heavy rainstorms) that caused traumatic removal of living plants and plant parts, especially cryptogamic ground cover (Bomfleur et al. 2014a). ...
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Premise of research. Well-preserved Triassic plant fossils from Antarctica yield insights into the physiology of plant growth under the seasonal light regimes of warm polar forests, a type of ecosystem without any modern analogue. Among the many well-known Triassic plants from Antarctica is the enigmatic Petriellaea triangulata, a dispersed seedpod structure that is considered a possible homologue of the angiosperm carpel. However, the morphology and physiology of the plants that produced these seedpods have so far remained largely elusive. Methodology. Here, we describe petriellalean stems and leaves in compression and anatomical preservation that enable a detailed interpretation of the physiology and ecology of these plants. Pivotal results. Our results indicate that the Petriellales were diminutive, evergreen, shade-adapted perennial shrubs that colonized the understory of the deciduous forest biome of polar Gondwana. This life form is very unlike that of any other known seed-plant group of that time. By contrast, it fits remarkably well into the “dark and disturbed” niche that some authors considered to have sheltered the rise of the flowering plants some 100 Myr later. Conclusions. The hitherto enigmatic Petriellales are now among the most comprehensively reconstructed groups of extinct seed plants and emerge as promising candidates for elucidating the mysterious origin of the angiosperms.
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This book provides up-to-date coverage of fossil plants from Precambrian life to flowering plants, including fungi and algae. It begins with a discussion of geologic time, how organisms are preserved in the rock record, and how organisms are studied and interpreted and takes the student through all the relevant uses and interpretations of fossil plant. With new chapters on additional flowering plant families, paleoecology and the structure of ancient plant communities, fossil plants as proxy records for paleoclimate, new methodologies used in phylogenetic reconstruction and the addition of new fossil plant discoveries since 1993, this book provides the most comprehensive account of the geologic history and evolution of microbes, algae, fungi, and plants through time. * Major revision of a 1993 classic reference * Lavishly illustrated with 1800 images and user friendly for use by paleobotanists, biologists, geologists and other related scientists * Includes an expanded glossary with an extensive up-to-date bibliography and a comprehensive index * Provides extensive coverage of fungi and other microbes, and major groups of land plants both living and extinct.
The present paper contains an account of three new species of the corystosperm pollen organ Pteruchus from the Middle Triassic beds of Nidpur, M.P. India. The new species P. glandulatus, P. lobatus, and P. truncatus differ from each other in the nature of central axis, disposition and length of the microsporophyll stalk, angle of divergence of the stalk from the central axis, lobing of head margin, number and shape of microsporangia per head and cuticular details. In all the species, the central axis has square outline in cross section and bears spirally arranged microsporophylls. In P. glandulatus, the stalked microsporophylls appeared to arise in low spirals from nodal regions leaving wide, naked internodal areas. The central axis also shows secretory body like structures. In P. lobatus, the microsporophylls are closely placed, short stalked and borne almost at right angles to the central axis. The head is hemispherical and has a deeply lobed margin. The pollen sacs are “cigar”- shaped and contain bisaccate pollen grains in pollen masses. In P. truncatus, the short stalked microsporophylls are borne in low to high spirals on the central axis at angles ranging between 45º–90º. The elliptical to orbicular head has an entire to occasionally shallowly lobed margin and the pollen sacs are large, “banana”-shaped with untapered truncated apices. The species of Pteruchus described here are very different from all other species so far reported from the Nidpur beds.
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Pteridosperms are preserved abundantly in the Gondwanan Triassic; in particular, the umkomasiaceans are the most commonly recorded floral element but they are not represented subsequently in that region or beyond. Indeed, nearly all umkomasiacean fronds are unknown above the Norian. However, representatives of one genus, Zuberia Frenguelli 1943 emend. Artabe 1990, persisted almost to the close of the Triassic. Umkomasiacean fronds probably belong to at least four genera. Several commonly occurring umkomasiacean species are re-assessed with particular attention to their type specimens, two of which are accurately figured here for the first time. The view that a morpho-continuum links umkomasiacean fronds is shown to be erroneous, being based on greatly enlarged circumscriptions of some species and misidentification of numerous specimens. Many species exhibit considerable morphological variation that has been attributed to a hybridization model of speciation. This is an improbable explanation given that hybridization is very rare among gymnosperms. Allopatric speciation resulting from geographic and climatic provincialism is a more likely explanation for the morphological diversity represented in Middle and Late Triassic floras of Australasia and elsewhere in Gondwana. Representatives of Umkomasia Thomas 1933 and Pteruchus Thomas 1933 emend. Townrow 1962 from the Carnian–lower Norian Ipswich Basin of Queensland are confirmed to have a bipinnate structure. The latter genus is also recorded for the first time from the Carnian Tarong Basin, Queensland. The holotype of Stachyopitys simmondsii Shirley 1898 from the Ipswich Basin, having been regarded as a male fructification for over 100 years, is shown to be ovuliferous. The species is included in Umkomasia and may be synonymous with the type species of that genus. Genuine male fructifications previously identified as S. simmondsii or P. simmondsii (Shirley 1898) Jones & de Jersey 1947a and a number of other Gondwanan species that were originally included in Townrovia Retallack 1981 or Stachyopitys Schenk 1867 belong to P. minor Thomas 1933. Townrovia was erected with an inaccurate diagnosis and the genus has not been convincingly distinguished from Pteruchus. Furthermore, Stachyopitys may not be genuinely represented in the Gondwanan Triassic. No reproductive organ has been confidently allied with Linguifolium Arber 1917 emend. Pattemore & Rigby in Pattemore et al. 2015b, its pteridospermous affiliation being based on limited cuticular evidence. The genus is known chiefly from the Middle and Upper Triassic of Gondwana but its representatives apparently extend into the lowermost Jurassic beyond Gondwana and possibly in Western Gondwana. Speciation in the genus has been largely determined statistically using leaf size; however, a number of other characters identified herein provide a basis for a more convincing separation of species. Consequently, the spatio-temporal distributions of some species have been more precisely defined. Australasian post-Triassic strata contain strikingly different floral assemblages to those of the Triassic. Eastern Gondwanan Lower and Middle Jurassic floras largely comprise ferns, lycopods, conifers, cycads and bennettitaleans. Ginkgoaleans were present in Australia until near the end-Triassic but were absent below ca 60ºS palaeolatitude during Early and Middle Jurassic time. No Eastern Gondwanan fructifications from that interval are clearly pteridospermous and no remains show any obvious relationship with pteridosperms of the Gondwanan Triassic. Fern-like fronds from the Lower Jurassic through Eocene of Eastern Gondwana, claimed to be pteridospermous, lack supporting evidence of such affiliation. Bona fide caytonialean fructifications are unrecorded from Australasia and are doubtfully represented elsewhere in Eastern Gondwana. Caytonialean leaves have been reported from the Eastern Gondwanan Upper Triassic–Middle Jurassic, albeit rarely. However, many specimens are attributable to Scoresbya Harris 1932 or to other genera exhibiting reticulate venation. Genuine caytonialean representation in Eastern Gondwana remains speculative. Australasian and Antarctic assignments to Palissya Endlicher 1847 emend. Florin 1958 are structurally distinct from those of the northern hemisphere and may belong to the Knezourocarponaceae Pattemore in Pattemore et al. 2014. The higher taxonomic placement of the family is uncertain; however, affinity with ginkgoaleans or pteridosperms is improbable.
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Phylogenetic relationships of 27 taxa of extant and extinct lignophytes were assessed in a numerical cladistic analysis using 65 characters. The analysis yielded 12 most parsimonious trees, where fossil taxa were found to play a crucial role in the reconstruction of spermatophyte phylogeny. The results supported hypotheses of monophylesis for spermatophytes, archaeopteridalean progymnosperms, medullosan seed ferns, cordaiteans, anthophytes, and gnetophytes. Progymnosperms, seed ferns s.l., and hydrasperman seed ferns were found to be paraphyletic. A large clade that previously has been referred to as "platysperms" or "saccate platysperms" occurs in our results, but it is not characterized by either of these features. The results reflect a hypothesized trend from large dissected leaves (the "cycadophytic" growth habit) to small leaves and aggregated sporophylls (the "coniferophytic" growth habit) in several groups, and coniferophytes (including cordaites, conifers plus taxads, and ginkgos) were found to be polyphyletic. Surprisingly, inclusion of the Late Pennsylvanian voltzialean conifer Emporia renders monophylesis for conifers (including taxads) less parsimonious than polyphylesis under most sets of assumptions. Extinct taxa provided data to clarify the order of character originations in the evolution of spermatophytes. Their inclusion in the analysis also helped clarify homologies among ovule enclosing structures that have been termed "cupules."
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Most phylogenetic analyses of morphological data agree that Gnetales are a monophyletic group related to angiosperms and Bennettitales. However. they disagree on whether these groups (anthophytes) are related to coniferopsids or to Mesozoic seed ferns, and thus on whether the flowers of Gnetales are primitively simple or reduced. Molecular analyses indicate that both Gnetales and angiosperms are monophyletic but disagree on their relationship. The conclusion of Nixon et al. (1994) that Gnetales are paraphyletic, with angiosperms nested within them, is weakly supported; when several questionable embryological characters are redefined in a neutral manner, Gnetales are inferred to be monophyletic. Jurassic reproductive structures associated with linear leaves and ephedroid pollen (Piroconites) consist of a bract and a scalelike sporophyll covered with Welwitschia-like microsynangia or ovules, recalling the bract-sporophyll complex of glossopterids. An analysis of seed plants incorporating these fossils and other new data links Gnetales with Piroconites, angiosperms with Caytonia, and both groups (plus Bennettitales and Pentoxylon) with glossopterids, making up a clade called the glossophytes. These results imply that glossophytes originally had glossopterid-like leaves and bract-sporophyll complexes, which were transformed into carpels with bitegmic ovules in angiosperms, but reduced to single, terminal ovules in Gnetales; flowers arose independently in the two lines. The common ancestor of angiosperms and Gnetales may be as old as Permian, and some of their shared advances, such as double fertilization (without endosperm formation), may have arisen as adaptations to seasonal temperate climates in Gondwana.
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A new species, Rhexoxylon piatnitzkyi is described and illustrated. It is a small stem showing the primary organization of the vascular cylinder and has enabled the authors to present a new interpretation of the development of the complex mature stem structure of Rhexoxylon. Together with further specimens of larger stems described provisionally as Rhexoxylon sp.A the material has revealed new anatomical details. The diagnosis of Rhexoxylon and of R. tetrapteridoides Walton are emended. R. waltonii Krausel is reduced to a synonym of R. africanum Bancroft. R. priestleyi (Seward) Walton is restored to Antarcticoxylon Seward because of differences judged to be of generic value. The affinities of Rhexoxylon are discussed. The present record from Triassic strata in north-west Argentina is the first occurrence of Rhexoxylon outside Africa and supports a stratigraphical correlation of the beds (Ischigualasto Formation) with the Stormberg Series of South Africa.
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The permineralized corystosperm pollen organ Pteruchus is described from the early Middle Triassic of Antarctica. Pteruchus fremouwensis consists of an axis bearing numerous, helically arranged microsporophylls, each of which terminates in a distal flattened head. The axis is 1-2 mm in diameter and eustelic. Spherical to elliptical secretory cavities are present in the ground tissue of the axis, microsporophyll, and pollen sac wall. The basal stalk of the microsporophyll is vascularized by a C-shaped strand that gives rise to a midvein and numerous lateral veins in the distal head. At least 38 pollen sacs are borne on the abaxial surface of the microsporophyll head. These are arranged in pairs on either side of lateral veins. Each pollen sac is sessile, elongated, and uniloculate. The pollen sac wall is several cell layers thick early in ontogeny, but reduced to a single layer in thickness when mature. Dehiscence is longitudinal along the inner surface. Pollen is monosulcate and bisaccate, and of the Alisporites-type. The Triassic specimens are the first structurally preserved pollen organs of the Pteruchus-type and can be related to the associated corystosperm stem and leaf genera based on the presence of unique secretory cavities. The morphology of Pteruchus and the relationship of this pollen organ with other Mesozoic and Paleozoic pollen organs is discussed.
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Anatomically preserved mature stems of late Middle Triassic corystosperms from the Paramillo Formation of Argentina are described and assigned to Cuneumxylon spallettii gen. nov. et sp. nov. The silicified specimens show features of the pith, primary xylem and successive rings of secondary xylem and phloem. The most striking characteristic is the anomalous secondary growth, represented by secondary xylem bounded by arcs of secondary phloem probably derived from successive repositioned cambia. Cuneumxylon has two kinds of unusual centrifugal secondary growth. The first shows unequal activity of different portions of the cambium on the circumference of the axes; the consequent restriction of cambial activity to certain restricted areas develops wedged stems, which often split. The second produces polyxylic stems following supernumerary cambial activity. As in extant plants growing in arid regions, the included phloem and the associated parenchyma may have had functional value avoiding desiccation of the outer tissues of the stem during droughts. Anatomical features of other Corystospermaceae were used to determine systematic affinities and to establish relationships among medullosans, corystosperms and cycads.
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The two recent theories of flowering plant evolutionary origins—the Anthophyte and Neo-Pseudanthial theories—are based on phylogenies (from morphological data) that show Gnetales as extant sister to angiosperms or as paraphyletic to angiosperms. Gnetales figured prominently in homology assessments and evolutionary scenarios of these theories. Several recent studies, including ours, provide strong evidence that extant gymnosperms are monophyletic, so Gnetales are most closely related to other gymnosperms, not to flowering plants. This removes the basis for both recent theories, leaving earlier theories that relate flowering plants to fossil “seed-fern” gymnosperms as the only active contenders. Our data from the homeotic gene Floricaula/LEAFY imply that the lineage leading to flowering plants originally had two copies of this gene, but that one copy was lost in flowering plants, which suggests a new theory: that developmental control of flower organization derives more from systems active in the male reproductive structures of the gymnosperm ancestor, rather than from the female, with ovules being ectopic in the original flower. This theory was based entirely on data from living plants, but the fossil group Corystospermales has both male and female structures that fit the theory. Corystosperms could include ancestors of the flowering plants. Communicating Editor: Paul Wilson
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Representative organs from the seed fern groups Glossopteridales and Corystospermales are commonly found in Gondwana during the Permian and Triassic, respectively. To date, both groups have been reconstructed predominantly on the basis of compression fossils and, in the case of the glossopterids, impressions. As a result, many details of their morphology remain unclear, and their taxonomic status and relationship to other groups are somewhat enigmatic. Collections of anatomically preserved fossils from permineralized peat in the central Transantarctic Mountains, Antarctica include a number of organs assignable to these two orders. Anatomical characters provide an opportunity to correlate isolated plant organs and to develop new reconstructions of these plants. Combined with paleoecological data, these fossils are beginning to provide a more accurate picture of the habitat and life history of these unique seed plants.
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A structural and ecological study of a standing Triassic forest containing 99 silicified trunks from the Fremouw Formation of Antarctica is detailed. Utilizing quantitative methods, forest density, total forest cover per hectare, mean separation of trees, and basal area per stump were obtained. This information, integrated with sedimentologic, taxonomic, fossil wood, and biomechanical data, has allowed the reconstruction of a plant community that grew at very high paleolatitudes (∼70–75°S). The paleoforest grew along river banks (levees) and within proximal floodplain environments, where a taphocoenosis of permineralized stumps and compression–impression foliage of the Dicroidium type has been preserved. In particular, the intimate association of this leaf type with dense, conifer-like wood provides additional confirmation that the Dicroidium foliage morphotype was attached to several types of stems. This riparian forest stand was apparently in a mature stage prior to the beginning of preservation. Tree ring analysis, as well as additional indirect evidence, indicates that this Middle Triassic ecosystem in Antarctica experienced a season very suitable for plant growth resulting from an overall favorable climatic regime. This conclusion contrasts with earlier paleoclimate models based on physical parameters in which temperature ranges would make plant or animal growth at these latitudes almost impossible. Finally, Antarctic climatic change from latest Permian through Late Triassic is considered in the context of tectonic and paleogeographic reconstructions of the East Antarctic craton.
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The permineralized, corystosperm, cupulate, ovule-bearing organ Umkomasia resinosa is described from the early Middle Triassic of Antarctica. This is the first description of anatomically preserved Umkomasia, which consists of a determinate cupulate branch with helically arranged, recurved, pedicellate megasporophylls, each of which bears one or two abaxially attached unitegmic ovules. Cupules are ovoid, bilobed with elongate ventral and dorsal openings or unlobed with a single ventral opening, and have a two-zoned parenchymatous cortex and abundant secretory cavities. Ovules are small, orthotropous, and possess a thin integument that contains numerous secretory cavities. The ovules are broadly attached at the base, with a bifid integumentary apex that extends past the cupule lobes. The cupulate branch displays stem-like anatomy, producing paired traces into each cupule stalk. These structurally preserved ovulate organs can be related to other corystosperm organs from Antarctica, particularly the pollen-organ Pteruchus fremouwensis. Both anatomical and morphological features support interpretation of corystosperm reproductive structures as branching systems rather than as compound sporophylls. As a result of an increased understanding of the organization of Umkomasia, it appears doubtful that any direct relationship exists between the corystosperm and angiosperm lineages.
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Developing a detailed estimate of plant phylogeny is the key first step toward a more sophisticated and particularized understanding of plant evolution. At many levels in the hierarchy of plant life, it will be impossible to develop an adequate understanding of plant phylogeny without taking into account the additional diversity provided by fossil plants. This is especially the case for relatively deep divergences among extant lineages that have a long evolutionary history and in which much of the relevant diversity has been lost by extinction. In such circumstances, attempts to integrate data and interpretations from extant and fossil plants stand the best chance of success. For this to be possible, what will be required is meticulous and thorough descriptions of fossil material, thoughtful and rigorous analysis of characters, and careful comparison of extant and fossil taxa, as a basis for determining their systematic relationships.
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We provide a brief overview of this special issue on the plant tree of life, describing its history and the general nature of its articles. We then present our estimate for the overall topology and, for land plants, divergence times of the plant tree of life. We discuss several major controversies and unsolved problems in resolving portions of this tree. We conclude with a few thoughts about the prospects for obtaining a comprehensive, robustly resolved, and accurately dated plant tree of life and the importance of such a grand endeavor.
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Recent molecular phylogenetic studies indicate, surprisingly, that Gnetales are related to conifers, or even derived from them, and that no other extant seed plants are closely related to angiosperms. Are these results believable? Is this a clash between molecules and morphology?
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The stem of a new genus and species, Tranquiloxylon petriellai with pteridosperm affinities, from the Upper Triassic of Patagonia is described. It is characterized by a parenchymatous pith with cells with dark contents surrounded by a continuous ring of primary xylem; the eccentric pycnoxylic wood is composed of closely packed wedge-shaped sections of secondary xylem with well marked growth rings, separated by vertical radial parenchymatic rays. The xylem bears uniseriate araucarioid bordered pits. Complex leaf-trace forming structures are also described. -from Authors
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The forked leaves of the morpho-genus Dicroidium are the most commonly occurring foliage in the collections from two quarries in the Middle Triassic Basin Creek Formation at Nymboida, N.S.W. The extensive Nymboida collections include leaves which, in gross morphology, range from simple with entire margins to pinnatifid, pinnate, bipinnatifid and bipinnate forms. The wide range of variation, which includes intergrading forms, creates problems in establishing species boundaries. For comparison with other Gondwanan material, the Nymboida leaves have been placed in five 'species complexes' distinguished as 'Dicroidium coriaceum', 'D. odontopteroides', 'D. lineatum', 'D. dubium' and 'D. zuberi'. In each complex there is a continuum of variation of form and there are intergrading forms that link each complex. Illustrations of over eighty leaves demonstrate the range of variation present. A single leaf only of D. elongatum has been collected. An unusual leaf is described as ?D. nymboidense sp. nov. Fertile material affiliated with Dicroidium includes three species of female strobilus, Umkomasia distans, U. sessilis and U. sp. A. together with dispersed cupules and ovules. Microsporophylls are placed in Pteruchus sp. cf. P. matatamajor and a single specimen in P. sp. A.
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Anatomically preserved one to five year-old stems are described from a Triassic site in the central Transantarctic Mountains. They are assigned to Kykloxylon fremouwensis gen. et sp. nov. and are regarded as related to the corystosperm stem Rhexoxylon on the basis of wood and pith anatomy and leaf trace organization. The bases of leaves attached to one-year-old stems of K. fremouwensis are similar to the leaves of Dicroidium fremouwensis described from the same locality in Antarctica. The "Dicroidium/Kukloxylon plant' from Antarctica is branched and more complex than the hypothetical "Dicroidium/Rhexoxylon plant' reconstructed from disarticulated remains from the Ischigualasto Formation of Argentina. It is suggested that the "Dicroidium/Rhexoxylon plant' may have been dominant in western Gondwana, whereas the Dicroidium plants with Kykloxylon stems might have had a wider geographical distribution in Gondwana. -from Authors
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Wood from an in situ permineralized forest from the Middle Triassic of Gordon Valley (Queen Alexandra Range, central Transantarctic Mountains) in Antarctica is described as a new taxon. Approximately 100 trunks in growth position are present at the site; they range from'13-61 cm in diameter and suggest that some of the trees were up to 20 m tall. Pits in the radial walls of the tracheids are of the abietinean type. Rays are uniseriate and 1-9 cells high; cross fields include one to two pits that appear to be simple. Axial parenchyma is absent. Pith and cortex are not preserved. The Antarctic wood is compared with existing fossil wood types from Antarctica and other parts of Gondwana. Although the fossil wood shares a number of characteristics with the Podocarpaceae, it differs from any existing genera and is described as a new taxon, leffersonioxylon gordonense.
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Principles of phylogenetic analysis (cladistics) are introduced with an examination of relationships among extant genera of Gnetales. The Gnetales can be supported as a monophyletic group, with Gnetum and Welwitschia more closely related to each other than either is to Ephedra. Characters of the progymnosperm Archaeopteris and 19 extinct and extant seed plant taxa are then reviewed as a basis for a cladistic analysis of their interrelationships. The seed plant taxa included are: medullosans, cycads, Lyginopteris, Cordaixylon, Mesoxylon, Lebachia, extant conifers, Ginkgo, Callistophyton, peltasperms, glossopterids, Caytonia, corystosperms, Bennettitales, pentoxylon, Gnetum, Welwitschia, Ephedra, and angiosperms. Preliminary analyses of relationships within conifers, glossopterids, and Bennettitales are also presented. Results suggest that seed plants are a monophyletic group, and Lyginopteris is resolved as the sister taxon to all other seed plants considered. The cordaites, Cordaixylon and Mesoxylon, along with Lebachia, extant conifers, and Ginkgo constitute a monophyletic group. Pentoxylon is the sister taxon to Bennettitales, and the Gnetales are the sister group to angiosperms. Together the Gnetales plus the angiosperms form the sister group to the Bennettitales plus Pentoxylon. If the outer integument of bennettitalean and angiosperm ovules is interpreted as homologous with the "cupule" of Caytonia and corystosperms, then the corystosperms are resolved as the sister group to the Bennettitales plus Pentoxylon plus Gnetales plus angiosperm clade. Under this interpretation all the seed plant taxa considered except Lyginopteris, cycads, and medullosans are part of a single clade in which flattened seeds and saccate pollen are primitive. The principal difficulties with the cladistic analysis concern necessary inferences on unknown characters in certain plants, and the current absence of a large base of comparative data. Results of the analysis suggest that the seed ferns as currently circumscribed are not a meaningful group for phylogenetic purposes and permit an evaluation of the possible phylogenetic position of Eremopteris, Nystroemia, Spermopteris, Phasmatocycas, Vojnovskiales, Leptostrobus, and several other enigmatic groups of plant fossils. Comparison of the phylogenetic analysis with previous theories of angiosperm origin shows that it reconciles the ideas of Arber and Parkin, that Bennettitales and Gnetales are closely related to flowering plants, with more recent hypotheses that invoke Caytonia and corystosperms as highly relevant to the angiosperm problem. The results suggest that increased understanding of Triassic Bennettitales, Gnetales, and corystosperms will be of maximum interest in further elucidating the phylogenetic relationships of flowering plants.
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The secondary xylem anatomy of trees and lianas was compared in the family Bignoniaceae. General descriptions of the family and the six woody tribes are provided. Lianas belong to the tribes Bignonieae, Tecomeae and Schlegelieae, and most have ve.ssels of two distinct diameters, many vessels per unit area, large intervascular pits, septate fibres, large heterocellular rays often of two distinct sizes, scanty paratracheal and vasicentric axial parenchyma and anomalous growth. Conversely, trees, which belong to the tribes Coleeae, Crescentieae, Oroxyleae and Tecomeae generally have narrower vessels in one diameter class, fewer vessels per unit area, smaller intervascular pits, non-septate fibres, small homocellular rays, scanty paratracheal, aliform or confluent parenchyma, and none exhibits anomalous growth. The majority of both trees and Hanas possess growth rings, are diffuse-porous, have non-solitary vessels which lack helical thickenings, and few have apotracheal parenchyma or storied structure. All species have alternate intervascular pitting and simple perforation plates.
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Seed plant phylogeny is evaluated using a data set of 46 terminals (taxa) and 103 morphological and anatomical characters. Cladistic analyses using the criterion of parsimony were performed on the complete data set as well as on subsets of the data, e.g., excluding fossils and/or combining various complex taxa into single terminals. The results support the placement of the cycads as the sister group of a monophyletic group that includes several fossil "seed ferns" as well as extant Ginkgo, conifers, gnetopsids, and angiosperms. When fossils were included, Bennettitales (cycadeoids) were part of an "anthophyte" clade that included gnetopsids and angiosperms. Pentoxylon was a sister taxon to the core anthophyte clade, in some, but not all, of the most parsimonious trees. Caytonia was not found to be closely associated with the anthophyte clade, but instead was often associated as a sister taxon of the glossopterids, and these two taxa were consistently outside of the Ginkgo-conifer-anthophyte clade. In all most parsimonious trees for all analyses, Ephedra was to the outside of a clade that included all angiosperm taxa, Gnetum, and Welwitschia, thus rendering the traditional gnetopsid clade paraphyletic. New information is provided on the morphology of Caytonia and some previous interpretations of homology of the caytonian "cupule" are rejected. The effects of sampling, compartmentalization, and polymorphism are explored in these data, showing how different results may be obtained when polymorphic or "summary" terminals are used. The need for more work on gnetopsids and fossil taxa is suggested.
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The first anatomically preserved species of Dicroidium, D. fremouwensis Piggs sp. nov. is described from the Triassic of the central Transantarctic Mountains, Antarctica. Dicroidium fremouwensis has pinnules that ranges from entire-margined to highly lobed morphologies. Entire-margined pinnules occur basally and pinnules become progressively more dissected at more distal levels of the frond. Dicroidium fremouwensis is most similar to the compression-impression forms D. dubium var. dubium (Feistmantel) Retallack and D. dubium var. tasmaniense (Anderson et Anderson) Retallack. Fronds also bear some resemblance to D. sp cf. D. feistmantelii sensu Lele, D. dubium (Feistmantel) Gothan and D. feistmantelii (Johnston) Gothan.Distal to the frond bifurcation, pinna rachides have a dorsiventral vascular anatomy composed of an abaxial ring of six to eight bundles and an adaxial linear group of five to eight bundles. Bundles of the adaxial line produce lateral strands that vascularize laminar pinnules, while the abaxial bundles remain in a ring that diminishes to three bundles distally. Pinnules have odontopterid venation, differentiated mesophyll, resinous cells and transfusin tracheids. The vascular organization and other anatomical features of D. fremouwensis are similar to those of the fronds of some extant cycads. This information provides a new level of resolution in our knowledge of Dicroidium and documents how permineralized fossils promise to refine our understanding of complex Mesozoic gymnosperms.
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A new Triassic corystosperm is described from the Shackleton Glacier region of Antarctica. The compression fossils include cupulate organs (Umkomasia uniramia) and leaves (Dicroidium odontopteroides) attached to short shoot-bearing branches. The cupulate organs occur in groups near the apices of the short shoots, and each consists of a single axis with a pair of bracts and a subapical whorl of five to eight ovoid cupules. This unique architecture indicates that the cupules are individual megasporophylls rather than leaflets of a compound megasporophyll. A branch bearing an attached D. odontopteroides leaf provides the first unequivocal evidence that Umkomasia cupulate organs and Dicroidium leaves were produced by the same plants. Although this had previously been assumed based on organ associations, the new specimens are important in demonstrating that a single species of corystosperm produced the unique cupulate organs described here and the geographically and stratigraphically widespread and common D. odontopteroides leaf. Therefore, biostratigraphic, paleoecological, and phylogenetic studies that treat Dicroidium leaf morphospecies as proxies for biological species of entire plants should be reconsidered. Phylogenetic analysis suggests that the corystosperm cupule is an unlikely homologue for the angiosperm carpel or outer integument.
On some pteridospermous plants from the Mesozoic rocks of South Africa The corystosperm pollen organ Pteruchus from the Triassic of Antarctica Recibido: 1 de febrero de Aceptado: 23 de noviembre de 2005
  • H H Thomas
  • X Yao
  • T N Taylor
  • E L Taylor
Thomas, H.H. 1933. On some pteridospermous plants from the Mesozoic rocks of South Africa. Philosophical Transactions of the Royal Society of London 222B: 193-265 Yao, X., Taylor, T.N. and Taylor, E.L. 1995. The corystosperm pollen organ Pteruchus from the Triassic of Antarctica. American Journal of Botany 82: 535-546. Recibido: 1 de febrero de 2005. Aceptado: 23 de noviembre de 2005. AMEGHINIANA 44 (1), 2007