TUBES, SCAFFOLDS AND CRUSTS: EDIACARAN METAZOAN BIOMINERALIZERS

Biomineralization in the terminal Ediacaran to early Cambrian metazoan fossil record coincides with a rapid increase in taxonomic disparity and diversity, suggesting that this innovation was a major factor in the rise of metazoans to ecological dominance. Ediacaran skeletal metazoans were diverse, and include poriferans (sponges), probable stem-group cnidarians, and possibly lophotrochozoans. Most taxa can be inferred to have been clonal.

Ediacaran biomineralization mechanisms were simple, and mainly represent biologically-induced styles. All these taxa can be inferred to have possessed pre-existing organic templates or scaffolds upon which calcification occurred, which likely represents an ancient mode of biomineralization.

Ediacaran skeletal metazoans were exclusively sessile, and restricted to carbonate habitats. But they could attach to both living microbial, as well as hard substrates, and also build reefs, often in association with microbialites. The ability to gain a secure and stable encrusting attachment to a hard substrate was a key innovation in the evolution of benthic metazoans, and created a competitive advantage in such settings. Ediacaran skeletal communities included both those dominated by organisms with short lifespans, high fecundity and rapid growth, as well as those with heavy calcification, high longevity, and an ability to recover from partial overgrowth. In particular, the modular and encrusting poriferan Namapoikia possessed indeterminate growth enabled via propagation of an organic scaffold from a basal pinacoderm prior to calcification. This imparted notable morphological flexibility. This morphological and ecological motif that appeared in the terminal Ediacaran was to dominate metazoan reef communities for most of the Paleozoic.