Dolphin evolution
95 million years ago, the forefather of the dolphins, the Mesonix, appear on the earth. It was a terrestrial animal that get into the water to feed, and he had limbs with bones to support his weight.
The following 30 million years, the Mesonix started the evolution process of the dolphin, changing to adapt to a marine life.
It is likely that conditions on earth were no longer fit enough, causing the change.