sentences:  By the upper Triassic, their numbers increased again.Cidaroids have changed very little since the Late Triassic, and are the only Paleozoic echinoid group to have survived.The euechinoids diversified into new lineages in the Jurassic and Cretaceous periods, and from them emerged the first irregular echinoids (the Atelostomata) during the early Jurassic.