Evolution of the Cleidoic Egg Among Reptilian Antecedents of Birds

Evolution of the avian egg from the naked amniotic egg of ancestral reptiles probably was the outcome of intense predation by soil invertebrates and microbes on a highly integrated and coadapted complex of characters. The calcareous shell which from its inception afforded a measure of protection to eggs against attacks by soil organisms became progressively thicker and more complex in the face of continuing selection for antipredator devices. However, increases in thickness and complexity of eggshells led to simultaneous reductions in the amount of liquid water that could be absorbed by incubating eggs from the substrate. Because embryos initially were dependent upon uptake of substantial quantities of water from the environment to satisfy their needs for this solvent, adaptive increases in thickness of the eggshell required coupled increases in the amount of water contained by eggs at oviposition, thereby reducing the degree of dependence of embryos upon external sources of water for successful completion of development. The rigid-shelled eggs resulting from this evolutionary sequence absorbed little (if any) liquid water during incubation, and the eggs contained sufficient water at oviposition to sustain embryos to hatching. Such eggs were functionally cleidoic and had attained an avian level of organization.