Within the GCR, many of the more commonly occurring invertebrate fossils are described in the context of their use in the correlation or rocks between places and the relative dating of geological events - and their contribution to understanding ancient environments of sediment deposition (as "environmental indicators"). However, this volume addresses the evolution and diversity of a significant animal group, given a special status in the GCR aside from the stratigraphy sections because of the relative rarity of the fossils. Arthropods - animals with jointed legs - have existed for over 500 million years. Although the trilobites are described in the relevant 'Stratigraphy' volumes, the rarer arthropods, and the sites from which the fossils were recovered, are described in detail. A description of the various groups and their evolution is provided.