Fossil spines of the sea urchin Cidaris faujasi. Sea urchins are globular, spiny organisms belonging to the phylum Echinodermata, that also include starfish, crinoids and other marine, bottom dwelling organisms. Echinoderms have lived in profusion throughout the geological record from the Ordovician to the present. Actual length of the spines may range from 30 to 70 mm.