Coralline demosponges ; a geobiological portrait

Abstract

The polyphyletic coralline demosponges possess a calcareous basal skeleton of 4 major morphotypes. Each has its own phylogenetic history, with different mechanisms of formation. One extant taxon of each skeletal type has been investigated, and its biochemical (e.g., intracrystalline organic matrix proteins), geochemical (e.g., stable isotopes), and histological properties described in detail. The thalamid Vaceletia shows similarities in its skeletal features to extinct archaeocyathid sponges due to the presence of special Ca2+ waste deposit chambers in the lower part of the skeleton. In our opinion this type is phylogenetically the most important one because it represents one possible evolutionary way of Ca2+ detoxification and iIIustrates one function of basic biomineralization (Ca2+-detoxification). More sophisticated biomineralization processes are developed in the agelasid Ceratoporella, the "chaetetid" hadromerid sponge Spirastrella (Acanthochaetetes) weil si, and the "stromatoporoid" agelasid Astrosclera willeyana. Each of these taxa shows a distinct process of formation with a unique composition of its intracrystalline organic matrix and geochemical features, here characterized in detail. A model of phylogenetic relationships and grades of development is proposed. The first metazoans with CaC03 biomineralization were the worm- like Cloudinidae from the late Sinian, which form a tube with a foliated structure. However, the taphonomy- controlled mode of basal skeleton formation in Archaeocyatha and Vaceletidae is the most ancient type of biologically- controlled metazoan biomineralization. In general, basal skeletons of coralline sponges represent the simplest biologically controlled mineralization, intermediate between biologically induced type (e.g., organomineralization) and the fully enzymatically- controlled mineralization of higher Metazoa.