Surviving groups of bristle worms flourish in the underground watery labyrinths beneath the Central Desert, 200 Million AD.
Several species live below the Central Desert and they are all descended from a single ancestor, a marine trichobranchid bristle worm that thrived in this region when it was covered by shallow seas. When sea levels dropped, the worms were trapped, but they survived and adapted to their new environment, slowly evolving to fill the different niches the cave system offered. This process is called adaptive radiation.
Most other bristle worms in the rest of the world were almost completely annihilated by the mass extinction that ended 100 million AD. But this trichobranchid indeed survived (being one of the surviving bristle worms of the world), living in the cave system. Trapped in its own dark world, that one kind evolved into many different forms.
Most of them live all their lives in the perpetual underground gloom.