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    Wyświetlanie 1-4 z 4
    Tytuł:
    New and unusual upper Llandovery graptolites from Arctic Canada
    Autorzy:
    Lenz, A C
    Kozlowska, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/20163.pdf
    Data publikacji:
    2007
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Tematy:
    graptolite
    Mirorgraptus arcticus
    Silurian
    Llandovery
    skeleton
    morphology
    Arctic Canada
    Pseudoplegmatograptus cf.obesus
    stratigraphy
    systematics
    Giganteograptus giganteus
    biostratigraphy
    paleontology
    Graptoloidea
    Pileograptus pileatus
    Opis:
    Three isolated and well−preserved retiolitid taxa, two of them new, and one taxon of unknown affinity, are described from the upper Llandovery of Arctic Canada. All taxa display unusual characteristics The three retiolitids display extraordinary morphological features: Pseudoplegmatograptus cf. obesus preserves very delicate and lacey or cobweb−like list structures external to the main skeletal lists, structures previously seen only rarely on flattened material; Pileograptus pileatus gen. et sp. nov. possesses a thecal framework typical of retiolitines, particularly Stomatograptus, but its thecal orifices possess broad, meshwork genicular hoods similar to those in the plectograptines; and Giganteograptus giganteus, formerly attributed to Pseudoplegmatograptus, with its very coarse and relatively simple skeletal framework, well−developed paired thecal apertural spines, and a pustulose micro−ornamentation on the lists. The latter feature suggests an assignment to the subfamily Plectograptinae, rather than to the Retiolitinae as has been previously presumed. The presence of pustulose lists in Giganteograptus adds a further complication to understanding of retiolitid evolution. Mirorgraptus arcticus gen. et sp. nov., the fourth taxon and of unknown systematic affinity, lacks the proximal end, but preserves lists with seams and some fusellar material suggestive of some type of new retiolitid. Unlike known retiolitids or other graptolites, however, the skeletal list development is apparently confined to a narrow region along one side of the nema.
    Źródło:
    Acta Palaeontologica Polonica; 2007, 52, 3
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Convergent evolution of two Silurian graptolites
    Autorzy:
    Lenz, A C
    Melchin, M.J.
    Powiązania:
    https://bibliotekanauki.pl/articles/21551.pdf
    Data publikacji:
    2008
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Tematy:
    taxonomy
    biostratigraphy
    paleontology
    Graptoloidea
    nomenclature
    graptolite species
    convergent evolution
    Cochlograptus veles
    Testograptus testis
    Silurian graptolite
    Monograptidae
    Opis:
    The two graptolite speciesCochlograptus veles(Telychian, Upper Llandovery) and Testograptus testis(lower Homerian, upper Wenlock) are remarkably and uniquely similar in being strongly ventrally and planispirally coiled and in demonstrating an abrupt deflection in the immediate post−sicular regions of their rhabdosomes. The two species, however, are separated by a relatively large biostratigraphic gap and a global mass extinction, and they differ morphologically in the proportion of thecal overlap, different angles of inclination of the interthecal septa, relative proportions of the widths occupied by the free metathecae, the position of the sharp dorsal flexure relative to the tip of the sicula, and the presence of a distinctive, keel−like structure on T. testis. It is suggested therefore, that in spite of the strong proximal morphological parallelism between the two species, their origin is best explained as a remarkable example of convergent evolution. It is suggested that C. veles perhaps evolved from some modified monograptid such as Stimulograptus, whereas the small Testograptus group may have derived from some monograptid such as Monograptus flemingii. Cladistic analysis fully supports the independent derivation the two species. If correct, this hypothesis supports the validity of separate generic names for the two species, despite the close and unique rhabdosomal similarities, including proximal metathecal form.
    Źródło:
    Acta Palaeontologica Polonica; 2008, 53, 3
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Silurian retiolitid graptolites: Morphology and evolution
    Autorzy:
    Bates, D E B
    Kozlowska, A.
    Lenz, A.C.
    Powiązania:
    https://bibliotekanauki.pl/articles/22802.pdf
    Data publikacji:
    2005
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Opis:
    Developmental mode and proximal structures are commonly accepted as the best for the recognition of high−level taxonomic categories within the Graptoloidea. The petalolithids and retiolitids are unique in possessing a virgellar ancora and in the latter, distal ancora development. The ancora structures are considered homologous, and the ancorate petalolithids are considered to be the direct ancestors to the retiolitids. The Retiolitidae are unique among the diplograptoids in possessing (1) outer, lateral, ancora sleeve walls (derived from distal extension of the ancora), and (2) a skeletal framework of bandaged lists between which are a succession of very thin and rarely preserved fusellar layers. Retiolitids possess different kinds of thecal profiles and two types of micro−ornamentation on the lists, and these have served to distinguish between the subfamilies Retiolitinae and Plectograptinae. Complete retiolitid morphological terminology is clarified and explained. Cladistic analysis of the retiolitids provides some measure of a better understanding of retiolitid evolution, but adds only modest support for the retention of the two subfamily categories.
    Źródło:
    Acta Palaeontologica Polonica; 2005, 50, 4; 705-720
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-4 z 4

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