AMPHIBIAN EGGS AND THE MONTANE ENVIRONMENT

Abstract

While the question of the origin of the amniote egg is not susceptible to direct proof, it is at least possible, by a consideration of various indirect lines of evidence, to arrive at a conclusion as to the most probable cause of this major evolutionary step. The traditional viewpoint has been that the animals that developed the amniote egg were already fully terrestrial and that the selective advantage of the cleidoic egg lay in the release of the adults from the necessity of returning periodically to water to breed. Romer (1957) points out that the earliest reptiles were still semiaquatic as were their amphibious ancestors. His argument that the terrestrial egg evolved before the terrestrial adult seems valid. Romer suggests that the land egg arose under conditions of periodic drought through the selective pressure resulting from desiccation of the eggs and larvae in ponds subject to drying. Tihen (1960) accepts Romer's basic thesis that the terrestrial egg preceded the terrestrial adult, but points out the inherent improbability that semiaquatic animals would have survived for long in regions where there was not a sufficiently dependable water supply to allow for aquatic development of eggs and larvae. Tihen further stresses that the modern amphibians that have terrestrial reproduction live in humid environments and that the basic amphibian adaptation to an intermittent water supply seems to be an acceleration of the developmental period. Tihen believes that reptiles evolved in humid, tropical or subtropical swamps, and that the selective force leading to the development of the terrestrial egg was heavy predation on the vulnerable eggs and early larvae. This suggestion has also been made by Lutz