Body evolution for underwater trek

[curioucity]

Hello

Long question short, I'm curious on why different organisms choose different bodies to travel underwater....
Fish, Amphibians and Reptiles in general have vertical tails whcih they use to wiggle horizontally. Moreover, some fish have rather flat bodies as well, which I assume to is to suport the way they swim.
But then we have mammals (and some birds too), which swim in a perpendicular way compared to the fish-amph-reptiles, that is, they often have horizontal tails and wiggles vertically......

Anyone knows why they choose these?

 

[spidergoat]

It has to do with the fact that the spine of mammals has evolved to deal with walking, usually on all four limbs, so it moves up and down instead of side to side. Other fish evolved flatness perhaps to lay on the sea floor to avoid predators, and their swimming style changed to suit their shape.