The Chondrichthyes developed in the ocean and are cartilaginous fish with jaws. Sharks, skates, and rays are modern representatives of this class. The Osteichthyes apparently remained in the estuaries and are bony fish with jaws. Some bony fish evolved a form of lungs that permitted them to survive in water with low oxygen by breathing air. Getting to air, however, was not easy because the only appendages fish had were fins.
Fish fins have no joints.
Some fish developed lobe fins, which are a jointed fin. This would have permitted some movement on land, possibly from pond to pond. Reptilia solved the water problem with waterproof egg shells and waterproof skin, as well as improved kidneys. But a more efficient heart and lungs lead in turn to other efficiencies, such as improved locomotion, increased body size, higher body temperature etc. Mammals reflect many of the trends in vertebrate evolution, such as the trend to more and more parental care for their young.