regulated among others by the “famous” family of Homeobox genes. Homeoboxrelated genes that have been identified in sponges display, however, a more general
function as transcription factors active in all sponge cells (Seimiya et al. 1998).
The body plan of sponges is defined and its orientation is fixed by an inorganic
skeleton. In most sponges, this solid support, the spicules, is composed of hydrated,
amorphous, noncrystalline silica (SiO 2 /H 2 O) as in the classes of Demospongiae
and Hexactinellida, or of calcium carbonate (CaCO 3 ) as in the class of Calcarea.
The secretion of spicules occurs in specialized cells, the sclerocytes. While in
demosponges and hexactinellids silica is deposited around an organic filament, no
organic axial structure is found in the spicules from Calcarea. Exciting new data on
spicule formation and the enzymatic silicatein-mediated biosynthesis of silica have
been summarized recently (M€ uller et al. 2005 and 2006b). It had been mainly the
siliceous skeleton that put the sponges into the position to survive adverse climatic
periods and to enable them as the first metazoans to occupy new aquatic biotopes.
9.2 The Key Innovation During the Proterozoic: The Skeleton
of Metazoa
9.2.1 Proterozoic, Silica-Rich Ocean
The “Precambrian-Cambrian” boundary interval was the crucial time for the metazoan evolution; it dates back to the Vendian [610–545 million years ago (Ma)
(Gaucher et al. 2004)]/Ediacaran [542–580 Ma (Gradstein et al. 2005)] period,
preceded by the Marinoan (635–660 Ma) and Sturtian glaciations (700–750 Ma);
Fig. 9.1. During this time, the Earth changed drastically with respect to (1)
biological events, (2) geochemical processes, and (3) environmental conditions.
Prior to the two ice periods the metazoans evolved, and the Vendian mega-continent
was formed (the early Gondwana) through the amalgamation of the micro-continents
West Gondwana (North and South America, Siberia, North China, Baltica) and
East Gondwana (India, Australia, East Antarctica, South China, and East Africa);
Shanker et al. (2001). The ocean of the Precambrian-Cambrian period had a higher
concentration of silica than the present-day marine environment (Simonson 1985;
Siever 1992). Two components are pertinent for the understanding of the ancient
ocean; (1) silica weathering and (2) sulfate reduction. These two parameters
controlled alkalinity, total CO 2 (TCO 2 ), and in turn also the saturation index of
the mineralic components.
The major proportion of dissolved silica in the Pre-Cambrian ocean came from
silica weathering processes that also influenced decisively the ocean’s alkalinity.
Decomposition of the continental crust, primarily of the tectosilicate minerals from
the feldspar family (the plagioclase) NaAlSi 3 O 8 –CaAl 2 Si 2 O 8, occurred. During
weathering and dissolution of those minerals, two alkaline conditions are formed,
the Ca-bound alkalinity and the Na-bound alkalinity (Kazmierczak et al. 2004).
9 The Unique Invention of the Siliceous Sponges
253
function as transcription factors active in all sponge cells (Seimiya et al. 1998).
The body plan of sponges is defined and its orientation is fixed by an inorganic
skeleton. In most sponges, this solid support, the spicules, is composed of hydrated,
amorphous, noncrystalline silica (SiO 2 /H 2 O) as in the classes of Demospongiae
and Hexactinellida, or of calcium carbonate (CaCO 3 ) as in the class of Calcarea.
The secretion of spicules occurs in specialized cells, the sclerocytes. While in
demosponges and hexactinellids silica is deposited around an organic filament, no
organic axial structure is found in the spicules from Calcarea. Exciting new data on
spicule formation and the enzymatic silicatein-mediated biosynthesis of silica have
been summarized recently (M€ uller et al. 2005 and 2006b). It had been mainly the
siliceous skeleton that put the sponges into the position to survive adverse climatic
periods and to enable them as the first metazoans to occupy new aquatic biotopes.
9.2 The Key Innovation During the Proterozoic: The Skeleton
of Metazoa
9.2.1 Proterozoic, Silica-Rich Ocean
The “Precambrian-Cambrian” boundary interval was the crucial time for the metazoan evolution; it dates back to the Vendian [610–545 million years ago (Ma)
(Gaucher et al. 2004)]/Ediacaran [542–580 Ma (Gradstein et al. 2005)] period,
preceded by the Marinoan (635–660 Ma) and Sturtian glaciations (700–750 Ma);
Fig. 9.1. During this time, the Earth changed drastically with respect to (1)
biological events, (2) geochemical processes, and (3) environmental conditions.
Prior to the two ice periods the metazoans evolved, and the Vendian mega-continent
was formed (the early Gondwana) through the amalgamation of the micro-continents
West Gondwana (North and South America, Siberia, North China, Baltica) and
East Gondwana (India, Australia, East Antarctica, South China, and East Africa);
Shanker et al. (2001). The ocean of the Precambrian-Cambrian period had a higher
concentration of silica than the present-day marine environment (Simonson 1985;
Siever 1992). Two components are pertinent for the understanding of the ancient
ocean; (1) silica weathering and (2) sulfate reduction. These two parameters
controlled alkalinity, total CO 2 (TCO 2 ), and in turn also the saturation index of
the mineralic components.
The major proportion of dissolved silica in the Pre-Cambrian ocean came from
silica weathering processes that also influenced decisively the ocean’s alkalinity.
Decomposition of the continental crust, primarily of the tectosilicate minerals from
the feldspar family (the plagioclase) NaAlSi 3 O 8 –CaAl 2 Si 2 O 8, occurred. During
weathering and dissolution of those minerals, two alkaline conditions are formed,
the Ca-bound alkalinity and the Na-bound alkalinity (Kazmierczak et al. 2004).
9 The Unique Invention of the Siliceous Sponges
253