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Animal EvolutionInterrelationships of the Living Phyla$
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Claus Nielsen

Print publication date: 2011

Print ISBN-13: 9780199606023

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199606023.001.0001

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Phylum Silicea

Phylum Silicea

(p.20) 5 Phylum Silicea
Animal Evolution

Claus Nielsen

Oxford University Press

The Silicea (Demospongiae + Hexactinellida) is a phylum comprised of about 4,800 living species of sessile, aquatic metazoans. Demosponges possess sterols that have been found in Late Precambrian (Cryogenian) deposits, even as fossil spicules from both groups were first identified in the Lower Cambrian. The ‘sponge grade’ of organisation observed in Silicea, Calcarea, and Homoscleromorpha has often been viewed as an apomorphy of a monophyletic Porifera. Recent molecular studies, combined with studies on ultrastructure and biochemistry, suggest that the three groups are separate clades. Evidence from the whole-genome study of Amphimedon refutes the concept of a ‘diploblast’ clade of sponges, Trichoplax, ctenophores, and cnidarians. Although Demospongiae and Hexactinellida differ in terms of structure, their sister-group relationship is strongly supported by molecular phylogeny. The ciliary apparatus of choanoflagellates and choanocytes exhibits many detailed similarities.

Keywords:   metazoans, Silicea, sponges, Trichoplax, ctenophores, cnidarians, Demospongiae, Hexactinellida, molecular phylogeny, choanoflagellates

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