143
of the fact that AFPs can be used as a model platform to understand biomineralization
processes, there is lack of knowledge about their possible role in calcifi cation
phenomena at temperatures near the freezing point. The possible mechanisms and
kinetics of this psychrophilic calcifi cation in particular attracts my attention.
3.4 Egg Shells of Marine Vertebrates
Abstract Of the many marine vertebrates, only reptilia and birds lay eggs. Egg
shell is an example of a unique calcium carbonate-based composite with diverse
thicknesses and physical properties. Correspondingly, there are different categories
of egg shells with respect to their rigidity (soft, fl exible, rigid egg shells) and several
basic types (testudoid, crocodiloid, dinodauroid, ornitoid, geckoid). The types of
hard egg shells are partially based on geometrical crystal growth considerations.
In contrast to well-studied mineral components of marine reptilian egg shells, the
organic matrix proteins of these taxa are poorly investigated; despite the fact that
proteins play a crucial role in the biomineralization of eggshells.
The egg of marine vertebrates, like reptiles, birds and formerly dinosaurs, is “fi rst
and foremost an embryonic chamber, through which gas exchange must occur and
from which certain ions such as calcium have to be available for embryo development , ” (Solomon and Gain 1996 ; see also Deeming and Thompson 1991 ). Additional
functions such as protection against bacterial invasion and mechanical stress are
determined by the multi-layered egg shell construct (Bain 1990 ). The only biomineralized layered structure of the egg is the egg shell, whose ultrastructure varies
considerably (see for review Erben 1970 ; Schleich and Kastle 1988 ; Mikhailov
1991 ; Winkler 2006 ). Thus, the calcareous shell unit is an example of a “complex
bioceramic” (Hincke et al. 2012 ), is made of calcite, aragonite (Roberts and Sharp
1985 ) or vaterite (Lakshminarayanan et al. 2005 ), and is the most basic structural
unit of an amniote egg. The physical properties like structure, breaking strength and
rigidity of both organic and inorganic components are genetically controlled features of a shell (see for review Young 1950 ; Bain 1990 ; Hincke et al. 2012 ). However,
early amniote eggs apparently lacked a calcifi ed eggshell. From a phylogenetic
analysis of the eggshells of extant amniotes, Stewart ( 1997 ) concludes that deposition
of calcium carbonate crystals over the fi brous eggshell membrane is a derived
character of sauropsids (extant “reptiles” and birds), and not a primitive amniote
feature. A brittle calcifi ed eggshell has evolved independently in archosaurs (dinosaurs,
crocodilians, birds), turtles (chelonians), and some gekkonid lizards, leading to a
variety of eggshell structures and differences in crystalline form (Oftedal 2002 ).
Of course, to fi nd fossilized eggshell with exceptionally well preserved organic
matrix is an event in contrast to the calcium carbonate structures which are
usually well preserved in fossilized material. The conclusion that early amniote
eggs were not rigidly calcifi ed is consistent with the absence of fossil eggs from the
Carboniferous, Permian, and early Triassic, a period of about 100 Ma (Oftedal 2002 ).
3.4 Egg Shells of Marine Vertebrates
of the fact that AFPs can be used as a model platform to understand biomineralization
processes, there is lack of knowledge about their possible role in calcifi cation
phenomena at temperatures near the freezing point. The possible mechanisms and
kinetics of this psychrophilic calcifi cation in particular attracts my attention.
3.4 Egg Shells of Marine Vertebrates
Abstract Of the many marine vertebrates, only reptilia and birds lay eggs. Egg
shell is an example of a unique calcium carbonate-based composite with diverse
thicknesses and physical properties. Correspondingly, there are different categories
of egg shells with respect to their rigidity (soft, fl exible, rigid egg shells) and several
basic types (testudoid, crocodiloid, dinodauroid, ornitoid, geckoid). The types of
hard egg shells are partially based on geometrical crystal growth considerations.
In contrast to well-studied mineral components of marine reptilian egg shells, the
organic matrix proteins of these taxa are poorly investigated; despite the fact that
proteins play a crucial role in the biomineralization of eggshells.
The egg of marine vertebrates, like reptiles, birds and formerly dinosaurs, is “fi rst
and foremost an embryonic chamber, through which gas exchange must occur and
from which certain ions such as calcium have to be available for embryo development , ” (Solomon and Gain 1996 ; see also Deeming and Thompson 1991 ). Additional
functions such as protection against bacterial invasion and mechanical stress are
determined by the multi-layered egg shell construct (Bain 1990 ). The only biomineralized layered structure of the egg is the egg shell, whose ultrastructure varies
considerably (see for review Erben 1970 ; Schleich and Kastle 1988 ; Mikhailov
1991 ; Winkler 2006 ). Thus, the calcareous shell unit is an example of a “complex
bioceramic” (Hincke et al. 2012 ), is made of calcite, aragonite (Roberts and Sharp
1985 ) or vaterite (Lakshminarayanan et al. 2005 ), and is the most basic structural
unit of an amniote egg. The physical properties like structure, breaking strength and
rigidity of both organic and inorganic components are genetically controlled features of a shell (see for review Young 1950 ; Bain 1990 ; Hincke et al. 2012 ). However,
early amniote eggs apparently lacked a calcifi ed eggshell. From a phylogenetic
analysis of the eggshells of extant amniotes, Stewart ( 1997 ) concludes that deposition
of calcium carbonate crystals over the fi brous eggshell membrane is a derived
character of sauropsids (extant “reptiles” and birds), and not a primitive amniote
feature. A brittle calcifi ed eggshell has evolved independently in archosaurs (dinosaurs,
crocodilians, birds), turtles (chelonians), and some gekkonid lizards, leading to a
variety of eggshell structures and differences in crystalline form (Oftedal 2002 ).
Of course, to fi nd fossilized eggshell with exceptionally well preserved organic
matrix is an event in contrast to the calcium carbonate structures which are
usually well preserved in fossilized material. The conclusion that early amniote
eggs were not rigidly calcifi ed is consistent with the absence of fossil eggs from the
Carboniferous, Permian, and early Triassic, a period of about 100 Ma (Oftedal 2002 ).
3.4 Egg Shells of Marine Vertebrates