The shark is a fish while the dolphin is a mammal. these are two completely different types of animals, but they have similar characteristics due to their similar dwellings. these animals may have evolved their similarities by

Sharks evolved several million years ago, before dolphins. Sharks adapted to the water with their hydrodynamic bodies. Dolphins evolved long after sharks from land mammals and adapted to water. Dolphins are warm-blooded mamals while Sharks are cold - bodied mammals.