What environmental conditions and structural features of vertebrates were the prerequisites for their emergence on land?

The main conditions that “pushed” the vertebrates to leave the aquatic environment were: the accumulation of atmospheric oxygen in the ground-air environment, the deterioration of living conditions in water bodies (constant changes in the volume of water, a small amount of oxygen, great competition between populations). The most important aromorphoses that allowed these animals to live on land are the appearance of jaws in armored fish, a change in the breathing method for the lung, and a change in the structure of fins in cross-finned fish. Later, the appearance of internal fertilization and various membranes that protect the embryo from drying out, as well as the complication of the structure of many organs (for example, the heart) and skin, are noted.