1,720,973 research outputs found
Fossil Ophioglossales in the Paleocene of Western North America.
No full textVegetative and fertile frond segments of Botrychium have been recovered from Paleocene deposits of central Alberta, Canada. Specimens are preserved as coalified compressions that yield information about frond structure, sporangia, and spore ultrastructure. These fossils, described as Botrychium wightonii sp. nov., establish a megafossil record for the Ophioglossales, and demonstrate that modem-appearing species of the order were present in western North America by the earliest Tertiary. The largest vegetative fragments are up to 4.6 cm long and tripinnately compound, with opposite to subopposite branching. Ultimate segments are pinnatifid with dentate pinnules and open dichotomous venation. Fertile specimens are also tripinnately compound with a long rachis and subopposite to alternate pinnae. Sporangia are either submarginal and superficial, or marginal, and are all directed toward one surface of the pinnule. They are ovoid to subspheroidal and 0.8-2.0 mm in diameter. Some sporangia are apparently stalked, while others appear to be sessile. This variation results both from the ultimate frond segments being compressed in several different planes, and the fossils being exposed at different levels. Spores macerated from the sporangia are radial and trilete, and range 30-67 ,um in diameter. Most are psilate, but some have a densely striate surface
The Role of Hydropteris pinnata gen. et. sp. nov. In Reconstructing the cladistics of Heterosporous Ferns.
No full textLarge segments of intact plants that represent a heterosporous fern have been discovered within an aquatic plant community from the Late Cretaceous St. Mary River Formation near Cardston in southern Alberta, Canada. Branching rhizomes of Hydropteris pinnata gen. et sp. nov. are 1-2 mm wide. They produce fronds at intervals of 2-12 mm and bear numerous elongated roots. Fronds, up to approximately 6 cm long, are pinnate with subopposite to alternate pinnae that exhibit anastomosing venation. Large, multisoral sporocarps occur at the junctures of the rhizome and frond rachides. Both microsporangiate massulae and megaspore complexes occur within each sporocarp. Megaspore complexes are assignable to the sporae dispersae genus Parazolla Hall. Microspores are trilete, smooth-walled, and are embedded in episporal material of the massulae. A numerical cladistic analysis indicates that the heterosporous aquatic ferns are monophyletic, and not as closely related to either schizaeaceous or hymenophyllaceous ferns as they are to some other filicaleans. Systematic revisions are proposed to reflect newly recognized cladistic relationships within the heterosporous clade, and character originations in the evolution of heterosporous aquatic ferns are evaluated. Hydropteridaceae fam. nov. is proposed, and included with Salviniaceae and Azollaceae in the Hydropteridineae subord. nov., and the Hydropteridales Willdenow
The Aquatic Angiosperm Trapago angulata from the Upper Cretaceous (Maastrichtian) St. Mary River Formation of Southern Alberta.
No full textA floating aquatic dicot with leaves assignable to Trapago angulata has been characterized from Upper Cretaceous (Maastrichtian) deposits of the St. Mary River Formation in southern Alberta, Canada. Reconstruction of the plant is based on nearly 500 specimens of various isolated and attached organs. Prominent floating rosettes of leaves were interconnected by submerged horizontal rhizomes with a distinct, crenulate surface and formed dense mats at the surfaces of small freshwater ponds. At least eight different leaf morphologies were produced. Rosettes bore opposite pairs of simple leaves and compound leaves with three, five, seven, nine, and possibly 11 leaflets. Highly dissected, submerged leaves of differing morphologies occur on rosette-bearing axes and rhizomes. Paired bud scales at the nodes of the crenulate rhizomes subtend both rhizome leaves and elongate, branched roots. Solitary flowers on long pedicels are attached in the axils of compound leaves. These remains are compared to those described as Quereuxia angulata (Newberry) Kryshtofovich from Russia, and extant Trapa L. Specimens from Russia, while showing leaves of mor- phology similar to that of T. angulata, differ in growth and branching pattern, supporting the interpretation that they are indeed different genera. Although Trapago has commonly been placed in the Trapaceae, the similarities between extant Trapa and Trapago could be attributed to convergence, and the familial affinities of the fossil remain uncertain
Anatomically Preserved Williamsonia (Williamsoniaceae): Evidence for Bennettitalean Reproduction in the Late Cretaceous of Western North America
Get PDFAn 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
Growth Architecture of Thucydia mahoningensis, a Model for Primitive Walchian Conifer Plants.
Get PDFA large number of vegetative and fertile branching systems of Thucydia mahoningensis provide data for interpreting the growth architecture and life-history pattern of a primitive Paleozoic conifer. Internal anatomy is similar to modern conifers, indicating an arborescent life-history pattern. Three categories of vegetative plagiotropic branching systems are recognized, and these resemble developmental variations among lateral branching systems in juvenile trees of the extant conifer Araucaria heterophylla (Salisb.) Franco. Thucydia mahoningensis is considered to be a small tree on the basis of the size and branching pattern of the stem and lateral branching systems and on the basis of internal anatomy. Growth architecture of T. mahoningensis conforms to Massart’s model, but fertile remains reveal interesting differences from living species. The T. mahoningensis tree has an orthotropic stem that bears two orders of plagiotropic vegetative shoots. Fertile shoots have a compound ovulate fertile zone and lack vegetative lateral branches. Ovulate branches bear penultimate-type vegetative leaves in the basal region, a compound ovulate fertile zone in the midregion, and ultimate-type vegetative leaves in the distal region. Axillary ovuliferous dwarf shoots of the compound fertile zones correspond to ultimate branches of the vegetative systems. Compound pollen cones are terminal on branches. These and supplementary data from previously described walchian species reveal that the architecture of the most primitive conifers resembles juvenile araucarian conifers and suggest that giant conifers evolved from diminutive trees by the combination of an extended vegetative growth phase and hypermorphoses in leaf development
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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