Magnosia decorata, Agassiz in Desor, 1846, Fossil echinoid Sea urchins are a group of marine invertebrates that today can be found in almost every major marine habitat from the poles to the equator and from the intertidal zone to depths of more than 5,000 metres. 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.