The Cambrian Period is marked by the extensive radiation of many extant animal phyla. Indeed, the rather dramatic increase in fossil abundance at this time suggested a veritable "Cambrian Explosion" to early paleontologists. There are representatives of several soft-bodied modern phyla, like the annelid Canadia and the onychophoran Aysheaia. Other fossils are more difficult to classify, like Hallucigenia (which may be an ancestral onychophoran), Wiwaxia (which may be an annelid or a mollusc), and Opabinia and Anomalocaris (which may belong to a stem taxon called "lobopods"). There are also several species classified as primitive chordates, including Pikaia, which resembles extant Lancelets, and Haikouichthys and Myllokunmingia from China (which both appear to have cartilaginous skulls). The increase in fossil abundance is no doubt due, however, to the evolutionary innovation of exoskeletons and shells in several animal phyla--notably the arthropod clade (represented by familiar taxa like trilobites and less familiar taxa like Marella and Sidneyia), reef-building sponges like Archeocyanthans, echinoderms like the eocrinoid Gogia, and the snail-like helcionellid molluscs. Some molluscs may have ventured forth on land, such as Protichnites that left tracks as trace fossils. This radiation correlates with the rise of predators and the selective advantage that protective coverings provided. Obviously, the evolution of hard parts would cause a significant increase in the frequency of successful fossilization, and would be recorded as a dramatic change in the abundance of fossils compared to pre-cambrian strata. One of the most impressive deposits of Cambrian fossils is the Burgess Shale Formation in western Canada. Many of the fossils found here are found nowhere else, and the extraordinary diversity of organisms suggests a complex community of predators, grazers, and burrowers.

Above: Cambrian scene; from: Univ. of Wisconsin Below: Anomalocaris appendages, from: The Smithsonian Institution