Unter- und mittelkambrische Porifera (Chancelloriida und Hexactinellida)

Zusammenfassung

Folgende Taxa werden (ganz überwiegend anhand isolierter Nadeln) behandelt: Klasse HeteractinidaHinde 1887. Ordnung ChancelloriidaWalcott 1920. Fam. ChancelloriidaeWalcott 1920.Chancelloria maroccana n. sp.Chancelloria sp. indet.Eiffelia? hispanica n. sp. Fam. Archiasterellidae n. fam.Allonnia tripodophora Doré & Reid 1965. GenusArchiasterella n. gen.Archiasterella pentactina n. sp.Archiasterella antiqua n. sp. Klasse HexactinellidaSchmidt 1870.Protospongia? sp. A, sp. B.Calcihexactina franconica n. gen.,n. sp. Fragliche Porifera-Reste (Problematikum A, B).

Die Chancelloriida (hier zur Ordnung erhoben) unterscheiden sich von allen anderen Schwämmen in der Struktur der Nadeln: Dünne Kalk-Wände umschließen sehr weite «Achsen-Höhlen», die vielleicht mit Spongin gefüllt waren und von denen aus «Basal-Poren» die Nadel-Wand durchbrechen. Grundlegendes Symmetrie-Element der Chancelloriida-Nadel ist der «Archiaster», der bei den Archiasterellidae allein, bei den Chancelloriidae in Kombination mit einem Vertikal-Strahl auftritt. Die Zahl der Strahlen im Archiaster wechselt, kann aber innerhalb der Arten konstant sein. Von der Chancelloriida-Nadel kann die Symmetrie der Nadeln aller bekannten Schwamm-Klassen abgeleitet werden, einschließlich jener der Hexactinellida (4-strahliger Archiaster — Stauractin). Kambrische Hexactinellida hatten, wie die Chancelloriida, noch Kalziumkarbonat als Material der Nadeln. Die Chancelloriida treten mit mehreren Gattungen bereits im tiefen Unter-Kambrium auf und könnten sehr wohl die Gruppe sein, von der die meisten späteren Schwämme abzuleiten sind.

Summary

The taxa described (mostly from isolated spicules) or discussed (Chapter F) are listed above.

The Chancelloriida (treated as an order, the author of which has to beWalcott) are characterized by a peculiar structure (B 1): Each ray consists of a separate «axial cavity» enclosed by a thin calcareous wall and opening towards the base of the spicule through a «basal pore». It is thought that the axial cavities might have been filled with spongin and that strands of this material might have connected the spicules of the sponge by way of the basal pores, thus giving it resistance to shallow water conditions in spite of its seemingly fragile structure (B 3). The feature common to the spicules of all Chancelloriida is termed «archiaster» (B 2). This may be represented alone (Archiasterellidae) or combined with a vertical ray (Chancelloriidae). The number of rays in the archiaster varies within the whole group, but may be fixed for certain species. The different symmetries of existing classes of sponges may have originated in different forms of the spicule of Chancelloriida (C). This is also true of hexactinellids: A four-rayed archiaster would have the symmetry of the stauract. Middle Cambrian sponges of theProtospongia type possibly incorporated octacts (Protospongia? sp. A) and had spicules consisting of calzite (P.? sp. B,Calcihexactina franconica n. gen., n. sp.). Chancelloriida first occur in the low Lower Cambrian (D) and they might well represent the ancestors of most later sponges. Differences in organisation between the latter may partly be explained by retainment of the shallow water environment of Chancelloriida or by a change to deeper, less agitated water (Hexactinellida) that commenced even in Cambrian time (E).