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Wyszukujesz frazę "graptolite" wg kryterium: Temat

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    Wyświetlanie 1-2 z 2
    Tytuł:
    The ultrastructure and building of graptolite dissepiments
    Autorzy:
    Urbanek, A
    Mierzejewski, P.
    Powiązania:
    https://bibliotekanauki.pl/articles/22043.pdf
    Data publikacji:
    2009
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Tematy:
    paleontology
    ultrastructure
    graptolite
    building
    rhabdosome
    dendroid graptolite
    Hemichordata
    Graptolithoidea
    Dendroidea
    Dictyonema
    dissepiment
    skeletal tissue
    Ordovician
    Opis:
    Dissepiments or connecting bars between adjacent stipes in rhabdosomes of dendroid graptolites were studied by means of electron microscopy (SEM and TEM). The material, chemically isolated from rock matrix, originating from the Ordovician of Estonia and glacial boulders of Baltic origin found in Poland, is assigned provisionally to the genus “Dictyonema” sensu lato. Early growth stages of dissepiments are made only of the fusellar component. Older dissepiments are composed of the central core and the outer envelope: the central core is made of rather irregularly arranged growth units made of the fusellar tissue, whereas the outer envelope has a distinct cortical appearance. TEM observations indicate that the fusellar component is made of both typical fusellar and microfusellar tissues (the latter with complete and reduced microfuselli). The cortical component of dissepiments is made both of dependent and independent cortex. The opinion is advanced that the dissepiments were constructed externally by the mortaring activities of zooids, similar to that of Recent Cephalodiscus. Our observations indicate that bizooids were most probable dissepiment constructors. These results, in general, does not support earlier opinions that dissepiments are made of cortical tissue acquiring a fusellar aspect in some cases, and that dissepiments were produced by the extrathecal membrane surrounding the rhabdosome.
    Źródło:
    Acta Palaeontologica Polonica; 2009, 54, 2; 243-252
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Morphogenetic gradients in graptolites and bryozoans
    Autorzy:
    Urbanek, A
    Powiązania:
    https://bibliotekanauki.pl/articles/23410.pdf
    Data publikacji:
    2004
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Tematy:
    Bilateria
    clone
    graptolite
    morphogenetic gradient
    colony
    bryozoan
    paleontology
    Hox gene
    Opis:
    Despite independent evolution of coloniality in hemichordates and bryozoans, their colonies show common features. In both instances colony is a genet or clonal system composed of zygotic oozooid and a number of blastozooids (= modules) integrated by physical continuity of tissues, sharing a common genotype and subject to common morphogenetic control. In some groups of graptolites and bryozoans, colonies display a regular morphological gradient. Simple graptoloid and bryozoan colonies consist of a proximal zone of astogenetic change and a distal zone of astogenetic repetition. Observed morphological gradient may be attributed to diffusion, along the colony axis, of a morphogen produced by the oozooid; in the zone of astogenetic change the morphogen is above certain threshold level and drops below it in the zone of astogenetic repetition. This model is supported by observations on regeneration of fractured graptoloid colonies. Regenerative branch never displays astogenetic change. The same rule is valid for regeneration of fractured bryozoan colonies. While the early astogeny of simple bryozoan colonies may be explained within the framework of the gradient theory, the late astogeny of more complex ones involves multiple succession of zones of change and repetition, without analogy in astogeny of graptoloids. Thus, late astogeny in bryozoan colonies may be controlled by cyclic somatic/reproductive changes, probably independent of the primary morphogen. Evolutionary changes in the graptoloid colonies involve both the spreading of the novelties over a greater number of zooids (penetrance) and an increase in the degree of phenotypic manifestation of a given character (expressivity). In the phylogeny of bilaterian colonies morphogenetic gradient probably originated as a sort of a side effect of sexual process leading to the appearance of the oozooid. The latter contaminated the neighbouring blastozooids with the products of its own morphogenesis. The resulting morphogenetic gradient could be used by selective forces to produce various effects of adaptive significance. Morphogens responsible for patterning of bilaterian colonies are probably related to the products of genes responsible for the anteroposterior control of embryos in all solitary Bilateria (Hox, zootype genes).
    Źródło:
    Acta Palaeontologica Polonica; 2004, 49, 4
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-2 z 2

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