Amniotes were the first true land animals. Amniotes are animals that produce eggs with internal membranes. The membranes let gases but not water pass through. Therefore, in an amniotic egg, an embryo can breathe without drying out. Amniotic eggs were the first eggs that could be laid on land.
Amniotes also have larger kidneys and intestines, this helps increase water absorption. But the most important adaptation amniotes have is the amniotic egg. This egg means they do not have to return to water for reproduction. Not all amniotes lay eggs, some reptiles retain their eggs, and most mammals develop inside of the mother.