They resembled dolphons.
The Dugong is a great example of a creature that has most likely gone from the water and then back to it - by looking at the bones in it flipper like legs.
I need to re read your post and come back - but Ijust wanted to throw those names out there.
The 'big bang' theory I think more refers to the things that happened AFTER the expansion of the universe... it doesn't declare the before hand (someone correct me if that doesn't sound right)
Single celled organisms developed in the ocean because the environmet was suitable..all the chemicals, the temperature etc
Those single celled organisms began to mutate...in many different ways.
Like when you have moisture on your bathroom wall, heat and certain temperatures- mould appears.
I know of a quick little vid where David Attenborough talks about it with visuals if you would be interested at looking at it?
Cool if not.