Conulariida

One little mutation, and your soft body starts producing a hard shell.  Now you move much more slowly, but you are much harder to eat.

You start living the other way up to the other jellyfish, with a single anchoring structure at one end of your four-sided cone, and tentacles at the other.  It is thought that you stay still for periods of time, then move, living slowly, growing quite big in the rich waters of the Cambrian era.

Unfortunately, as the temperature cools between the Cambrian and Triassic eras, heavy shells cost too much to create and transport, and you are out-competed in this niche by other jellyfish who didn't grow a shell.

You became extinct in the Lower Triassic.  Sorry.

That's as far as you can evolve here!  
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