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    Wyświetlanie 1-3 z 3
    Tytuł:
    Aragonitic rostra of the Turonian belemnitid Goniacamax: arguments from diagenesis
    Autorzy:
    Dauphin, Y
    Williams, C.T.
    Barskov, I.S.
    Powiązania:
    https://bibliotekanauki.pl/articles/20211.pdf
    Data publikacji:
    2007
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Tematy:
    Turonian
    sea shell
    Cephalopoda
    belemnite
    calcite
    Goniocamax
    rostrum
    skeleton
    sea cephalopod
    diagenesis
    paleoenvironment
    freshwater shell
    Belemnitida
    aragonite
    Siberia
    belemnitid rostrum
    Piasina River
    paleontology
    Opis:
    The hypothesis that belemnitid rostra are formed by primary biogenic low−Mg calcite is widespread. However, the coexistence in the same rostrum of both aragonitic and calcitic components has been reported in true belemnites (Goniocamax, Turonian). A combined microstructural and chemical composition study of the comparison of shells with undisputed mineralogy from the same site as the Turonian Goniocamax, shows that these aragonitic shells display the effects of diagenetic alteration. These observations favour the hypothesis that belemnite rostra are composed of primary aragonite, rather than low−Mg calcite, and are consistent with all other cephalopod shells. Calcitic and aragonitic rostra are also known in other Dibranchiata such as Triassic Aulacocerida and Eocene Belopterina. Diagenetic changes such as shown here may clearly affect palaeo−environmental interpretations based on carbonate shells.
    Źródło:
    Acta Palaeontologica Polonica; 2007, 52, 1
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Redox conditions in the Late Cretaceous Chalk Sea: the possible use of cerium anomalies as palaeoredox indicators in the Cenomanian and Turonian Chalk of England
    Autorzy:
    Jeans, C. V.
    Wray, D. S.
    Williams, C. T.
    Powiązania:
    https://bibliotekanauki.pl/articles/139095.pdf
    Data publikacji:
    2015
    Wydawca:
    Polska Akademia Nauk. Czytelnia Czasopism PAN
    Tematy:
    chalk environments
    rare earth element (REE) anomalies
    methods
    palaeoredox indicators
    diagenesis
    history
    pierwiastki ziem rzadkich
    REE
    anomalie
    metody
    diageneza
    historia
    Opis:
    The cerium anomalies preserved in the Chalk have been investigated as possible palaeoredox indicators of the Late Cretaceous Sea and its sediment. This has been based upon over a hundred new rare earth element analyses of selected samples and grain size fractions from the Chalk. Particular attention has been given to the methodology of differentiating between the cerium anomalies preserved in the bioclastic calcite and those in carbonate-fluorapatite preserved in the acetic acid insoluble residues of chalks. Variations in the cerium anomaly of different particle size fractions of uncemented chalks suggest that fractionation of rare earth elements between the Chalk’s seawater and the various organisms that contributed skeletal material to the bioclastic calcite of the Chalk may have occurred. Post-depositional processes of calcite cementation and late diagenetic sulphidisation have had no apparent effect on the cerium anomaly of the acetic acid insoluble residues. The cerium anomalies associated with the acetic acid insoluble residues from (1) an alternating sequence of chalks and marls from Ballard Cliff (Dorset, UK) typical of Milankovitch cyclicity show a marked diagenetic pattern, whereas those from (2) non-volcanic and volcanic marls display a pattern that is best explained by the variations in the availability of phosphorus and the timing of argillisation of volcanic glass during diagenesis. The general conclusion is drawn that the cerium anomalies preserved in the Chalk can provide an insight into the changing palaeoredox conditions in the Late Cretaceous Sea as well as in the pore fluids of its sediments.
    Źródło:
    Acta Geologica Polonica; 2015, 65, 3; 345-366
    0001-5709
    Pojawia się w:
    Acta Geologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    A mysterious giant ichthyosaur from the lowermost Jurassic of Wales
    Autorzy:
    Martin, J.E.
    Vincent, P.
    Suan, G.
    Sharpe, T.
    Hodges, P.
    Williams, M.
    Howells, C.
    Fischer, V.
    Powiązania:
    https://bibliotekanauki.pl/articles/23272.pdf
    Data publikacji:
    2015
    Wydawca:
    Polska Akademia Nauk. Instytut Paleobiologii PAN
    Tematy:
    ichthyosaur
    deposit
    morphotype
    Jurassic
    Wales
    Opis:
    Ichthyosaurs rapidly diversified and colonised a wide range of ecological niches during the Early and Middle Triassic period, but experienced a major decline in diversity near the end of the Triassic. Timing and causes of this demise and the subsequent rapid radiation of the diverse, but less disparate, parvipelvian ichthyosaurs are still unknown, notably because of inadequate sampling in strata of latest Triassic age. Here, we describe an exceptionally large radius from Lower Jurassic deposits at Penarth near Cardiff, south Wales (UK) the morphology of which places it within the giant Triassic shastasaurids. A tentative total body size estimate, based on a regression analysis of various complete ichthyosaur skeletons, yields a value of 12-15 m. The specimen is substantially younger than any previously reported last known occurrences of shastasaurids and implies a Lazarus range in the lowermost Jurassic for this ichthyosaur morphotype.
    Źródło:
    Acta Palaeontologica Polonica; 2015, 60, 4
    0567-7920
    Pojawia się w:
    Acta Palaeontologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
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