Invertebrates are simply explained as a group of animals without a vertebral column. This large group of animals currently represents over 90% of all animal species. Soft bodied invertebrates require such specialized conditions for fossilization that they are rarely found. The fossil record, however, paints a much clearer picture of the hard bodied invertebrates. These animals have what is called an exoskeleton, an outer covering produced with compound of biological materials and minerals (calcite, aragonite & chitin), which supports and protects the animal throughout its lifetime. Animals with exoskeletons include mollusks (snails, clams, brachiopods, ammonites, nautilus), echinoderms (starfish, crinoids and echinoids) and arthropods (insects, spiders, crustaceans, trilobites). Animals with this ‘external skeleton’ first appeared in the fossil record about 550 MYA, the development of which is believed to have been a driving role in the ‘Cambrian Explosion’.