What is the realtion between a dolphin and a shark?

In actuality, dolphins and sharks aren't related at all. A shark is a fish, while a dolphin is a mammal. They look similar because of a phenomenon called convergence. Convergence is when two unrelated organisms evolve similar mechanisms to cope with the same environmental structure. This means that sharks and dolphins look the same because that form is the best form for an aquatic predator. For another example, look at the extinct ichthyosaurs. They may look like dolphins and fish, but they are actually repitles that evolved from a land-based ancestor.

their fin bones