the first echinoid fossils are from the lower ordovician period. the earliest forms had flexible tests, with plates that could slide over each other. echinoids were a relatively unimportant part of the biota in the palaeozoic. only one group survived the p/tr extinction event to form the basis of all later echinoids. the group which survived into the triassic, the cidaroids, radiated into all other modern groups, which are known as the euechinoids.