What distinguishes amphibians from other tetrapods?

Amphibians are uniquely characterized by their dependence on water for reproduction, which significantly distinguishes them from other tetrapods such as reptiles, birds, and mammals. Unlike these other groups that may lay eggs in various environments or give birth to live young, amphibians typically require a moist environment or direct water source to lay their eggs. This is because their eggs are usually non-amniotic and lack the protective shell that allows for development in dry conditions.

The necessity for aquatic environments during their reproductive cycle is also tied to their underlying physiological requirements, as amphibian larvae (like tadpoles) typically undergo a metamorphosis that begins in water. Over the course of their lifecycle, they transition from an aquatic larval stage to a terrestrial adult form, which highlights their unique evolutionary adaptations. This reliance on water for breeding ties into the broader ecological strategies and life histories of amphibians, underlining their distinctive role among tetrapods.