Cleidoic egg
The cleidoic egg (from Greek kleistos = closed) is the type of egg which reptiles and birds lay. Mammalian reproduction has evolved from laying cleidoic eggs to live birth. The cleidoic egg is sometimes called the amniotic egg, because it is characteristic of amniotes.
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[change] Discussion
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These eggs have a protective shell, and are laid out of water. The shell is porous to air, and may be flexible or calcareous (hard). The eggs contain all the food the embryo needs to develop into a hatchling. As a result, the amphibian's larval stage, the tadpole, is not needed by reptiles or birds. The special membranes that support these eggs are typical traits of all amniotes, including mammals.[1]p105
The significance of the cleidoic egg is that it enables reproduction out of, and often away from, water. The cleidoic egg has been called "a private little pond", from the way it gives water and nutrition to the embryo. Cleidoic eggs must have evolved in the Lower Carboniferous, when early Amniotes evolved. Amniotes include mammals, and monotreme mammals do lay eggs. Marsupial and eutherian mammals have the same kind of eggs, but without the shell. They give birth to live young because the development takes place internally. We know that some fossil groups of reptiles, such as Ichthyosaurs, also gave birth to live young.
The basic reproduction strategy of amphibia (frogs, for example), is to lay a huge number of eggs, which are left to their own fate. The strategy of amniotes is to invest more in the production of eggs, lay fewer eggs and take some care of them. Because the eggs are laid on land, internal fertilisation is needed. Thus, from the initial step of a cleidoic egg comes a number of changes in behaviour. With the changes in behaviour comes better and more flexible brains. Some reptiles, for example, Crocodiles and birds,[2] look after and protect their young. Mammals are an extreme case: they carry the protection and nursing of the young much further. All mammals provide milk for their young, and many smaller mammals raise their young in burrows, providing a meaasure of security. Humans go even further still, with the long period of learning before their young are independent. Along with this, in human evolution, goes a reduction in inherited behaviours, and a great increase in learned behaviours and social life. These later adaptations were made possible by the earlier evolution of the cleidoic egg.
[change] Consequences
The cleidoic egg is a good example of an evolutionary innovation which permitted the invasion of a vast range of new habitats, and led to one of the greatest adaptive radiations in the history of the vertebrates. In comparison, the amphibia, whose reproduction is tied to water, are represented today by just a few highly specialised groups.
