Fossil sea urchin spine. Coloured scanning electron micrograph (SEM) of the fossilised spine of a sea urchin. Sea urchins are small, spiny, globular animals that constitute the class Echinoidea of the echinoderm phylum.The earliest echinoid fossils date to the upper part of the Ordovician period, and the taxon has survived to the present as a successful and diverse group of organisms. Isolated spines are common as fossils. Most of the fossil echinoids from the Paleozoic era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly in Devonian and Carboniferous rocks. Magnification:x 10 when printed at 10cm high.