Unter und mittelcambrische Porifera (Chancelloriidae und Hexactinellida)

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 be Walcott) 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 the Protospongia 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).