The Cambrian period (542 – 488 mya) is characterized by an evolutionarily rapid diversification of metazoan life forms. It is during this Cambrian explosion that most of the modern phyla, including arthropoda, first make a definitive appearance in the fossil record. The biota of this period is spectacularly preserved in the renowned Burgess Shale of British Columbia as well as other formations in China, Greenland, and Sweden.
Following the Cambrian, and near the end of the Ordovician period (488 – 443 mya), there was another relative explosion in diversity of biological forms, termed the Great Ordovician Biodiversification Event. Whereas the Cambrian explosion saw the appearance of the major modern phyla, the Great Ordovician Biodiversification Event is characterized by a diversification of life forms within those phyla, and the abrupt absence of the prototypical Cambrian forms.
Previously, the early Ordovician period was conspicuously absence from the fossil record, obscuring the fate of the Cambrian evolutionary forms. Now this gap in the fossil record has been filled with the discovery of rich marine fossil beds in southeastern Morocco. These formations show that a wondrous diversity of Cambrian forms persisted into the Ordovician and formed a connective link up to the Great Ordovician Biodiversification Event.
Below are some examples of beautiful arthropod fossils from the Moroccan Ordovician formation.