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Invertebrata |
Department of Geology and Geography, Queen’s College, Flushing 67, N.Y., U.S.A.
Department of Geology, University of Queensland, St Lucia, Brisbane, Australia
Phylum PORIFERA Grant 1836 Introduction. The first fossil remains definitely referable to sponges are hexactinellid stauractin spicules and heteractinid octactin spicules, both known from L. Camb strata. Spicules, or spiculelike objects, have been reported from Precambrian rocks (viz., Treatise E, 33 and Doubinger and Eller, 1963) but they cannot be referred with certainty to sponges, to the exclusion of some other spicule-producing organism. The Camb sponge fauna is not very diversified. Beginning in the middle Camb (Burgess shale) the record of the demosponges commences with sponges bearing simple monaxons. They are followed probably in the upper Camb and certainly in the Arenigian by the first lithistids, namely orchocladines, whose dendroclones are derivable from simple monaxons. The earliest rhizomorine lithistids, whose spicules are also monaxonic, may be present at this time. The orchocladine lithistids expand rather suddenly in Ord Llvirn to Ord Carad, totalling some 30 genera, and constituting the first major expansion of the sponges. Reefs in the Chazy beds of North America were built partly by sponges during this time, along with the bryozoans, corals and stromatoporoids which subsequently tended to exclude the sponges from middle Palaeozoic reefs. The lithistids undergo a contraction following Silurian times, whilst the hexactinellids (some 40 genera) tend to dominate in the late Dev and early Carb. During the Carb most of the lithistid demosponge orders that will dominate the Mesozoic and Cenozoic have appeared.
The Class Heteractinida attain a modest climax of some 10 genera in Carb time, concluding their career shortly thereafter
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