A terrestrial environment for breeding
When the first amphibians appeared, the terrestrial environment would have been a much safer site for the development of their offspring than an aquatic environment which has many predators (especially fish). As a consequence anurans evolved mechanisms to exploit the terrestrial environment for the breeding of its young. The midwife toadAlytes obstetricans (Family Discoglossidae) of Europe lives in holes close to water and mates on land. After fertilization the long strands of eggs are twisted around the hind leg of the male toad. The male carries them around until the tadpoles are ready to hatch and then takes them to water.
The South American Centrolene frogs defend calling sites which are leaves overhanging streams. Such sites are where the eggs are laid, and parent frogs attend the eggs until they hatch and the tadpoles fall into the water below. In Africa some species of frog (e.g. Chiromantis) breed on branches of trees above ponds. The female excretes a liquid which is beaten into a ball of froth by the male frog. The eggs are then laid and the outside surfaces of the froth harden into a crust which retains moisture. The female frogs may bring up additional moisture to the nest. The eggs hatch and tadpoles develop within the hardened froth. The tadpoles are released when the lower part of the froth ball liquifies and they fall into the water below. Frogs producing foam nests occurs in five anuran families (Rhacophoridae, Hyperoliidae, Myobatrachidae, Hylidae and Leptodactylidae). In some tropical American frog species (Eleutherodactylus) a considerable yolk is provided in each egg which makes it possible for all stages of larval development to take place within the eggs and fully developed froglets emerge directly from them, this is term direct development and occurs in nine families of frogs.