Britannica Sources used in this answer

Why were the earliest tetrapods still dependent on water?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

The earliest tetrapods were not fully terrestrial animals; rather, they were primarily aquatic creatures that lived in shallow water and dense vegetation. Their dependence on water was fundamentally tied to their reproductive biology.12

Unlike later amniotes, which evolved protective extra-embryonic membranes (the amnion, chorion, and allantois) that allowed for the deposition of eggs in terrestrial environments, the earliest tetrapods were anamniotes. Because they lacked these specialized membranes, their eggs and larvae required a moist, aquatic habitat to survive and develop. This requirement for moisture-associated reproduction persisted in the lineage that became modern amphibians, whereas the evolution of the amniotic egg was the critical adaptation that eventually permitted tetrapods to become fully terrestrial.32