In this era, life in the sea flourished with the first molluscs, crustaceans and fish. These were the first animals with a backbone. Water splashed on to the rocks, and mosses grew on the seashore. Soon, the first plants and ferns of giant proportions appeared on land. The seas became so crowded that some fish had to adapt to life on land. They developed lungs for breathing, and short legs. They were the first amphibians. Many different insects were on Earth at that time. It was humid, warm and green.