Trilobites appeared some 600 million years ago during the Cambrian period. They belonged (probably) to the phylum Arthropoda. This phylum includes the most successful (78%) of all animal life forms, including crabs, centipedes, spiders, shrimps and insects. The trilobites, living in shallow seas, flourished as swimmers, crawlers and burrowers for some 350 million years. Over time, they evolved rapidly into a wide variety of strange and beautiful forms.