Reflections on the origin of the amniote egg in the light of reproductive strategies and shell structure

Abstract

On the basis of evolutionary pattern of reproductive strategies (r‐ and K‐selected animals) and the ultrastructure of modern amniote eggshells, a new model of the origin of the amniote egg is presented. In contrast to the well‐known idea of Romer (1957) that the egg came first while adult reptiles in the Carboniferous remained in water, it is suggested that the early evolution of the amniote egg (including the gradual formation of membranes) happened within the aquatic realm. Increasing enlargement of eggs and yolk is interpreted as an adaptation of reproduction strategies to lakes with poor nutrient contents. The first accumulation of Ca‐ions in the outer membrane, paralleled by many modern noncalcified lepidosaurian eggs, was a process of detoxification, according to new ideas in biomineralization. The function of the shell, to protect the embryo against microbial activity and to prevent water loss, which is necessary for the preamniote egg to become a fully terrestrial egg, was the terminal step in this story. Yolk‐rich eggs enclosed by a more or less calcified shell can be interpreted as an essential preadaptation for tetrapods to have become fully terrestrial during the Late Paleozoic.

Key words:

• Amniote egg
• tetrapods
• evolution
• reproductive strategies
• eggshell structure