Dolphin tails vs fish tails
Fish have vertical tail fins, while whales and dolphins have horizontal ones. Why is that? These traits are probably the result of the evolution of locomotion. Fish derive from wormy creatures that slithered over the sea floor. This made it crucial to have vertical tails that moved from side to side.
Marine mammals derive from land mammals that walked on four limbs. These animals move by flexing the spine up and down. The surrounding muscles devloped to aid this movement. Marine mammals move in the same way as these four-limbed land animals: dolphins and whales swim by moving their tail fin and rear body up and down, and the surrounding muscles support this movement.
by Judith Kok