You push off, and up, and use your tail as a rudder, and catch some tasty food.

It’s not a bad strategy, and over many thousands of generations, you get better and better adapted to it. Your legs shrink, and your tail gets longer and flatter, to act as a rudder.

You hunt small fish, in seawater, and then later in brackish and fresh water, adapting to a different environment.

You are called a sea scorpion, although you’re not closely related to the land scorpions.

Then a major extinction event occurs, at the end of the Devonian period, and every species of sea scorpion in the sea dies due to the changed climate. A few of you hang on, in fresh water, living through the Carboniferous and Permian eras until the Permian Triassic extinction event, probably a mega-eruption of volcanoes changing the atmosphere still further, at which point every last one of you dies.