Kurzfassung
Die vielgestaltigen und oft vollständig erhaltenen Schalen von ordovizisch-silurischen Nautiloideen konnten wahrscheinlich nicht lange auf der Meeresoberfläche treiben, sogar wenn die Tiere dort starben. Das Lumen des Phragmacons war groß verglichen mit dem verlorenen Weichkörper, und die Kammern enthielten im lebenden Tier nicht nur Flüssigkeit sondern auch Gas mit weniger als 1 atm. Nach dem Tode und Entfernung oder teilweiser Verwesung des Weichkörpers verursachte dieser Unterdruck in wenigen Stunden oder Tagen das Fluten eines Teils der Kammern durch Meerwasser. Während oder nach dem Sinken der leeren Schale konnten die dünnen Septen von oberflächennah lebenden Nautiloideen unter umgekehrtem, zur Mündung gerichtetem, hydrostatischen Druck leicht nachgeben und brechen. Diese Art von Druckladung und Implosion der Septen geschah wahrscheinlich durch plötzlichen Bruch (?Boden-Aufschlag) des Siphos in einer apikalen Kammer, im Bereich der maximalen Lebenstiefe der jeweiligen Art. Das schnell folgende Einbrechen der adoralen Septen endete wahrscheinlich an relativ stärkeren Septen, die dicker waren und/oder durch teilweises Fluten der dahinter liegenden Kammer gestützt waren. Nach diesem Szenario betrug die minimale Ablagerungstiefe der silurischen Nautiloideen von Böhmen etwa 65 m. Alternative Szenarios, wo dieselben Septen unter (Membran-)Spannung (adapikal) brechen, wie bislang angenommen, oder wo Druckunterschiede in benachbarten Kammern durch Flutung entstanden, resultieren in 160 m minimale Wassertiefe für diese Fazies. Die Septen vieler kleinerer Silur-Nautiloideen, die in über 100–300 m Wassertiefen lebten, waren zu dick um nachzugeben und brachen auch kaum unter Spannung, weil die Gehäuse bei dem hohen Wasserdruck wahrscheinlich schneller geflutet wurden als sie absanken.
Abstract
It is unlikely that the intact or commonly preserved varieties of Ordovician-Silurian nautiloid shells were able to drift for any distance at the surface of the sea even if they died there. Their cameral capacity was much larger than the volume of the extracted or decayed body, and it would have contained a partial vacuum and cameral liquid when they were alive. The closely spaced and thin septa of the shallow-water adapted species were liable to buckle in compression and then implode in local tension during reverse hydrostatic loading by water pressure. This reverse loading and internal implosion of the septa was probably initiated by the sudden cameral refilling of an apical chamber caused by the depositional rupture of the apical siphuncle at or near the maximum habitat depth of these species. The instantaneous buckling of the more adorai septa was potentially terminated by variations in the septum thickness and cameral fill-fractions at that time, and they imply that some of the Silurian nautiloids from Bohemia were deposited at a minimum depth of about 65 m. Alternative interpretations involving the breakage of the same septa in tension, or buckling due to the difference in pressure between adjacent flooded chambers, set a maximum depth limit of about 160 m for the same facies. Many of the smaller Silurian nautiloids were unlikely to buckle during refilling, and they were potentially flooded faster than they could sink, below a depth of 100–300 m.
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Hewitt, R.A., Westermann, G.E.G. Post-mortem behaviour of Early Paleozoic nautiloids and paleobathymetry. Paläont. Z. 70, 405–424 (1996). https://doi.org/10.1007/BF02988081
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DOI: https://doi.org/10.1007/BF02988081