There are around 800 extant species and the group has a long and detailed fossil record stretching back about 450 million years ago to the Late Ordovician Period. They belong to the Echinoidea, one of ...

This is a synapomorphy for the crown group of the Echinoidea, and is also true for all post-Palaeozoic echinoids, bar one, Tiarechinus. By contrast almost all Palaeozoic echinoids have a test which ...