(>2,000 Ma), the level was in the order of 10
À14 PAL; during the short overlapping
phase of Stage II (2,000–1,700 Ma), the oxygen level rose close to 5 Â 10
À2 PAL,
followed by a sharp increase at 1,500 Ma to 1ÂPAL (Stage III). Parallel with the
turn from the anoxic to the oxic phase, the ocean became depleted of iron, a process
which started at an atmospheric oxygen level of 2 Â 10
À3 PAL (Kasting 1984). In
the Meso-Proterozoic (until 1,000 Ma) and especially in the Neo-Proterozoic
Oceans (1,000–543 Ma) secular deflections of d
13 C carb could be measured that
are indicative of changes in the ratio of organic to inorganic C removed from the
oceans by (organic) burial in sediments (Anbar and Knoll 2002). Interestingly,
during that period, termed Cryogenian that lasted from 850 to 630 Ma, very cold
global climate episodes occurred with more than two major worldwide glaciations.
The Sturtian glaciation persisted from 750 to 700 Ma, and the Marinoan/Varanger
glaciation terminated at circa 635 Ma. Since the characteristic glacial deposits are
also found in places at low latitudes, the hypothesis of deeply frozen planetary
oceans and the expression “Snowball Earth” (Hoffman and Schrag 2002; reviewed
in M€ uller et al. 2007b) were coined. Between these two major ice ages, the
multicellular animals (Knoll and Carroll 1999) evolved among which only the
sponges (Porifera) developed, a genetic repertoire that allowed the survival through
those adverse events until today (Xiao et al. 2000); hence, these animals were
termed “living fossils” (M€ uller 1998; see Pilcher 2005).
All multicellular animals have developed both an organic and an inorganic solid
skeleton. The organic skeleton is based on collagen and the inorganic solid skeleton
comprises silicon or calcium as an inorganic element. Collagen formation requires
atmospheric oxygen atoms that are incorporated into Pro-Pro-Gly and Ile-Lys-Gly
tripeptides, catalyzed enzymatically by the by prolyl hydroxylase (Towe 1970;
Kikuchi et al. 1983). In sponges, collagen molecules exist already in several classes,
from the primitive fibrillar collagen to the vertebrate-type basement collagens (see
Garrone 1998). Comparative analyses supported the assumption that during the
emergence of the sponges in the Meso-Proterozoic, the required atmospheric
oxygen pressure had been reached. From collagen formation studies with presentday vertebrate cells, it is known that collagen formation requires a partial oxygen
(pO 2 ) pressure of 16.0 kPa (approx. 120 mmHg) in order to allow a rate of 60% for
the synthesis of collagenous proteins; in the human capillaries, a lower pressure of
5 kPa is reached. For the mesopelagic deep-sea fangtooth fish Anoplogaster
cornuta, the average critical oxygen tension (Pc) for the entire group has been
determined to be 35 mmHg (Gordon et al. 1976), a tension which had been also
measured in the deep sea ocean (Teal and Carey 1967). Hence, this oxygen pressure
corresponds to values estimated for those in the air during the Meso-Proterozoic
period (Kasting et al. 1992).
Collagen has an important role in the stabilization of the extracellular matrix in
the sponge body, the mesohyl (also termed collagenous tissue) (see Garrone 1978;
Simpson 1984; Francesco et al. 2001). A few sponge species comprise a cortex
consisting mainly of a fibrous internal stroma of collagen bundles like in
Chondrosia reniformis (Francesco et al. 2001), while most of them have only a
poorly developed or completely absent cortex (Garrone 1978). Driven and
256
W.E.G. M€ uller et al.
À14 PAL; during the short overlapping
phase of Stage II (2,000–1,700 Ma), the oxygen level rose close to 5 Â 10
À2 PAL,
followed by a sharp increase at 1,500 Ma to 1ÂPAL (Stage III). Parallel with the
turn from the anoxic to the oxic phase, the ocean became depleted of iron, a process
which started at an atmospheric oxygen level of 2 Â 10
À3 PAL (Kasting 1984). In
the Meso-Proterozoic (until 1,000 Ma) and especially in the Neo-Proterozoic
Oceans (1,000–543 Ma) secular deflections of d
13 C carb could be measured that
are indicative of changes in the ratio of organic to inorganic C removed from the
oceans by (organic) burial in sediments (Anbar and Knoll 2002). Interestingly,
during that period, termed Cryogenian that lasted from 850 to 630 Ma, very cold
global climate episodes occurred with more than two major worldwide glaciations.
The Sturtian glaciation persisted from 750 to 700 Ma, and the Marinoan/Varanger
glaciation terminated at circa 635 Ma. Since the characteristic glacial deposits are
also found in places at low latitudes, the hypothesis of deeply frozen planetary
oceans and the expression “Snowball Earth” (Hoffman and Schrag 2002; reviewed
in M€ uller et al. 2007b) were coined. Between these two major ice ages, the
multicellular animals (Knoll and Carroll 1999) evolved among which only the
sponges (Porifera) developed, a genetic repertoire that allowed the survival through
those adverse events until today (Xiao et al. 2000); hence, these animals were
termed “living fossils” (M€ uller 1998; see Pilcher 2005).
All multicellular animals have developed both an organic and an inorganic solid
skeleton. The organic skeleton is based on collagen and the inorganic solid skeleton
comprises silicon or calcium as an inorganic element. Collagen formation requires
atmospheric oxygen atoms that are incorporated into Pro-Pro-Gly and Ile-Lys-Gly
tripeptides, catalyzed enzymatically by the by prolyl hydroxylase (Towe 1970;
Kikuchi et al. 1983). In sponges, collagen molecules exist already in several classes,
from the primitive fibrillar collagen to the vertebrate-type basement collagens (see
Garrone 1998). Comparative analyses supported the assumption that during the
emergence of the sponges in the Meso-Proterozoic, the required atmospheric
oxygen pressure had been reached. From collagen formation studies with presentday vertebrate cells, it is known that collagen formation requires a partial oxygen
(pO 2 ) pressure of 16.0 kPa (approx. 120 mmHg) in order to allow a rate of 60% for
the synthesis of collagenous proteins; in the human capillaries, a lower pressure of
5 kPa is reached. For the mesopelagic deep-sea fangtooth fish Anoplogaster
cornuta, the average critical oxygen tension (Pc) for the entire group has been
determined to be 35 mmHg (Gordon et al. 1976), a tension which had been also
measured in the deep sea ocean (Teal and Carey 1967). Hence, this oxygen pressure
corresponds to values estimated for those in the air during the Meso-Proterozoic
period (Kasting et al. 1992).
Collagen has an important role in the stabilization of the extracellular matrix in
the sponge body, the mesohyl (also termed collagenous tissue) (see Garrone 1978;
Simpson 1984; Francesco et al. 2001). A few sponge species comprise a cortex
consisting mainly of a fibrous internal stroma of collagen bundles like in
Chondrosia reniformis (Francesco et al. 2001), while most of them have only a
poorly developed or completely absent cortex (Garrone 1978). Driven and
256
W.E.G. M€ uller et al.