146
The types of hard egg shells are partially based on geometrical crystal growth
considerations, which have been proposed for the deposition of the crocodilian,
testudinian and avian eggshells (Silyn-Roberts and Sharp 1986 ). As described by
these authors:
“In each shell column, crystal deposition is initiated at a single location, from
which growth fans out at all angles to the shell normal. In both calcitic and aragonitic
shells, growth is in the [001] direction, resulting in an increase in the degree of (001)
preferred orientation with distance from nucleation. Where there is unhindered
crystal growth, the shells show crystalline fracture morphology, and the degree of
texture that develops is a simple function of the column radius. This type of
growth makes up the whole of the testudinian shell, the inner 30–40 % of the
thick ratite shells, and the cone layer of the other avian shells,” (Silyn-Roberts and
Sharp 1986 ).
Most physiological, biochemical, nutritional, structural and morphological
studies have been carried out on avian eggs, and mostly on the egg of the domestic
chicken. This is of course because of its ready availability and commercial importance as a nutritious food for human consumption. In contrast, there is much less
known regarding eggs and eggshell of other birds or nonavian animals (snakes,
lizards, turtles, crocodiles and dinosaurs) (Hincke et al. 2012 ).
3.4.1 Eggshells of Marine Reptilia
The egg is the most vulnerable stage in a reptile’s life. Immobile and often lacking
parental care, it is susceptible to predation and exposed to the prevailing environmental
conditions. Suitable substrate temperature and moisture are vital for successful
development of the embryo and subsequent hatchling performance and behaviour
(Packard and Packard 1988 ). Flexible-shelled eggs of turtles exchange water with
the environment of the nest cavity. These water exchanges dramatically affect changes
in mass of the eggs over the course of incubation. Eggs incubated on relatively wet
substrates gain water and mass during incubation, while those incubated on relatively
dry substrates lose water and mass over the course of development (Miller et al.
1987 ). Therefore, the reptilian advancement of the hard eggshell was a clear
advantage. For the fi rst time, the reproductive process was not tied to sources of
water. The development of the amniote eggshell made the impermeability to fl uids
possible. At the same time, pores allowed the embryo to inhale oxygen and exhale
carbon dioxide. Dinosaurs used this basic reptilian design, while the modern birds
took this same format with added complexity (Pollinger 1997 ).
The structural peculiarities of the turtles egg shells are well studied, and for
detailed analysis I recommend the following papers published by Solomon and
Baird 1976 , 1977 ; Packard and Packard 1979 , 1980 ; Packard 1980 ; Solomon
and Watt 1985 ; Acuña–Mesén 1984 , 1989 ; Woodall 1984 ; Chan and Solomon
1989 ; Carthy 1992 ; Sahoo et al. 1996a , b ; Mahanty and Sahoo 1999 ; Kitimasak
et al. 2003 ; Al-Bahry et al. 2009 . The fi nding of the double layered (calcareous and
3 Biocomposites and Mineralized Tissues
The types of hard egg shells are partially based on geometrical crystal growth
considerations, which have been proposed for the deposition of the crocodilian,
testudinian and avian eggshells (Silyn-Roberts and Sharp 1986 ). As described by
these authors:
“In each shell column, crystal deposition is initiated at a single location, from
which growth fans out at all angles to the shell normal. In both calcitic and aragonitic
shells, growth is in the [001] direction, resulting in an increase in the degree of (001)
preferred orientation with distance from nucleation. Where there is unhindered
crystal growth, the shells show crystalline fracture morphology, and the degree of
texture that develops is a simple function of the column radius. This type of
growth makes up the whole of the testudinian shell, the inner 30–40 % of the
thick ratite shells, and the cone layer of the other avian shells,” (Silyn-Roberts and
Sharp 1986 ).
Most physiological, biochemical, nutritional, structural and morphological
studies have been carried out on avian eggs, and mostly on the egg of the domestic
chicken. This is of course because of its ready availability and commercial importance as a nutritious food for human consumption. In contrast, there is much less
known regarding eggs and eggshell of other birds or nonavian animals (snakes,
lizards, turtles, crocodiles and dinosaurs) (Hincke et al. 2012 ).
3.4.1 Eggshells of Marine Reptilia
The egg is the most vulnerable stage in a reptile’s life. Immobile and often lacking
parental care, it is susceptible to predation and exposed to the prevailing environmental
conditions. Suitable substrate temperature and moisture are vital for successful
development of the embryo and subsequent hatchling performance and behaviour
(Packard and Packard 1988 ). Flexible-shelled eggs of turtles exchange water with
the environment of the nest cavity. These water exchanges dramatically affect changes
in mass of the eggs over the course of incubation. Eggs incubated on relatively wet
substrates gain water and mass during incubation, while those incubated on relatively
dry substrates lose water and mass over the course of development (Miller et al.
1987 ). Therefore, the reptilian advancement of the hard eggshell was a clear
advantage. For the fi rst time, the reproductive process was not tied to sources of
water. The development of the amniote eggshell made the impermeability to fl uids
possible. At the same time, pores allowed the embryo to inhale oxygen and exhale
carbon dioxide. Dinosaurs used this basic reptilian design, while the modern birds
took this same format with added complexity (Pollinger 1997 ).
The structural peculiarities of the turtles egg shells are well studied, and for
detailed analysis I recommend the following papers published by Solomon and
Baird 1976 , 1977 ; Packard and Packard 1979 , 1980 ; Packard 1980 ; Solomon
and Watt 1985 ; Acuña–Mesén 1984 , 1989 ; Woodall 1984 ; Chan and Solomon
1989 ; Carthy 1992 ; Sahoo et al. 1996a , b ; Mahanty and Sahoo 1999 ; Kitimasak
et al. 2003 ; Al-Bahry et al. 2009 . The fi nding of the double layered (calcareous and
3 Biocomposites and Mineralized Tissues