Why are aquatic mammal tails horizontal and fish vertical?

Mammals have become adapted to running on land. To run on land on four legs, the vertebral column has to flex up and down, as the legs move forward and backward. Fish and reptiles, such as ichthyosaurs, snakes and lizards, have vertebral columns that flex side to side. Fish tail fins need to be vertically flat so that the suface of the tail can push against the water as it swims by moving the tail side to side. Mammals swim the same way they run on land, by flexing their vertebral column up and down. If mammals swim with their tails (many don't), their tails also beat up and down. For a mammalian tail to move the water, it has to be horizontally flat.