144
Some authors proposed some kind of classifi cation of eggs that is based on the
physical properties of the eggshell. Thus, there are three categories known like soft,
fl exible, or rigid (see for review Packard et al. 1981 ; Hirsch 1983 ; Bain 1990 ;
Wangkulangkul et al. 2000 ). The proportion of inorganic to organic matter determined the eggshell rigidity. Rigid eggshell has more dominated calcium carbonate
phase in comparison to organic matter incorporated into this structure. Conversely,
eggshell that contains more organic matter than hard crystalline material are related
to the soft and fl exible categories.
Soft eggshell : Poorly mineralized eggshells with well-developed organic matrix.
Representatives of snakes, lizards, and tuataras lay such kind of eggs. “These
eggs collapse and shrivel after the animal hatches, and are therefore unlikely to
be identifi ed or even preserved in the fossil record,” (Wilson et al. 2014 ).
Flexible eggshell : These shells are more calcifi ed in comparison to soft shells.
Reptiles, including some snakes, lizards, and turtles, lay eggs with fl exible shells.
Rigid eggshell : Well mineralized, robust eggshells have been reported for eggs of
dinosaurs, all crocodilians, some turtles and geckos, as well as birds (Jackson
et al. 2004). One of the characteristic example—thick, multiple-shelled turtle
eggs (Ewert et al. 1984).
The diversity between these egg shell categories is intriguing even within one
group of marine vertebrates. Packard et al. ( 1982 ) divided the eggshells of turtles on
the basis of structural features into three categories:
1. Flexible-shelled eggs laid by sea turtles of the families Cheloniidae and
Dermochelyidae ;
2. Flexible- shelled eggs laid by most emydids and chelydrids;
3. Rigid shelled eggs laid by testudinids, trionychids, and kinosternids (Packard
et al. 1984 ).
According to Hincke et al. ( 2012 ), most generally, eggshell types can be categorized as membrane-like (snakes and lizards), pliable (most turtles) and rigid
(some turtles, some gecko, all crocodiles, all birds and dinosaurs). There are also
such defi nitions as pliable-shelled (see for examples Deeming and Whitfi eld 2010 )
and parchment-shelled eggs like those of pterosaurus species (Unwin and Deeming
2008 ; Lü et al. 2011 ). Most lizards and snakes have parchment-shelled eggs, with
scattered and variable sites of calcium deposition (Oftedal 2002 ). These eggs
increase in mass, volume, and surface area because of their ability to absorb water
during incubation time. Fossil egg shells have been reported nearly worldwide,
especially from Upper Cretaceous and Tertiary deposits, and they have been
assigned, according to their microstructure and biomineralization, to turtles,
crocodiles, dinosaurs and birds (reviewed by Hirsch and Packards 1987 ). Five
basic types of hard eggshells of fossilized and living amniotic vertebrates may be
distinguished (Mikhailov 1991 , 1997a , b ) (Fig. 3.25 ):
1. Testudoid type (Chelonia). The shell unit consists of a single ultrastructural zone
with regular spherulithic growth of aragonite crystals (radial aragonite structure)
and organic core in the base.
3 Biocomposites and Mineralized Tissues
Some authors proposed some kind of classifi cation of eggs that is based on the
physical properties of the eggshell. Thus, there are three categories known like soft,
fl exible, or rigid (see for review Packard et al. 1981 ; Hirsch 1983 ; Bain 1990 ;
Wangkulangkul et al. 2000 ). The proportion of inorganic to organic matter determined the eggshell rigidity. Rigid eggshell has more dominated calcium carbonate
phase in comparison to organic matter incorporated into this structure. Conversely,
eggshell that contains more organic matter than hard crystalline material are related
to the soft and fl exible categories.
Soft eggshell : Poorly mineralized eggshells with well-developed organic matrix.
Representatives of snakes, lizards, and tuataras lay such kind of eggs. “These
eggs collapse and shrivel after the animal hatches, and are therefore unlikely to
be identifi ed or even preserved in the fossil record,” (Wilson et al. 2014 ).
Flexible eggshell : These shells are more calcifi ed in comparison to soft shells.
Reptiles, including some snakes, lizards, and turtles, lay eggs with fl exible shells.
Rigid eggshell : Well mineralized, robust eggshells have been reported for eggs of
dinosaurs, all crocodilians, some turtles and geckos, as well as birds (Jackson
et al. 2004). One of the characteristic example—thick, multiple-shelled turtle
eggs (Ewert et al. 1984).
The diversity between these egg shell categories is intriguing even within one
group of marine vertebrates. Packard et al. ( 1982 ) divided the eggshells of turtles on
the basis of structural features into three categories:
1. Flexible-shelled eggs laid by sea turtles of the families Cheloniidae and
Dermochelyidae ;
2. Flexible- shelled eggs laid by most emydids and chelydrids;
3. Rigid shelled eggs laid by testudinids, trionychids, and kinosternids (Packard
et al. 1984 ).
According to Hincke et al. ( 2012 ), most generally, eggshell types can be categorized as membrane-like (snakes and lizards), pliable (most turtles) and rigid
(some turtles, some gecko, all crocodiles, all birds and dinosaurs). There are also
such defi nitions as pliable-shelled (see for examples Deeming and Whitfi eld 2010 )
and parchment-shelled eggs like those of pterosaurus species (Unwin and Deeming
2008 ; Lü et al. 2011 ). Most lizards and snakes have parchment-shelled eggs, with
scattered and variable sites of calcium deposition (Oftedal 2002 ). These eggs
increase in mass, volume, and surface area because of their ability to absorb water
during incubation time. Fossil egg shells have been reported nearly worldwide,
especially from Upper Cretaceous and Tertiary deposits, and they have been
assigned, according to their microstructure and biomineralization, to turtles,
crocodiles, dinosaurs and birds (reviewed by Hirsch and Packards 1987 ). Five
basic types of hard eggshells of fossilized and living amniotic vertebrates may be
distinguished (Mikhailov 1991 , 1997a , b ) (Fig. 3.25 ):
1. Testudoid type (Chelonia). The shell unit consists of a single ultrastructural zone
with regular spherulithic growth of aragonite crystals (radial aragonite structure)
and organic core in the base.
3 Biocomposites and Mineralized Tissues