 Despite their rich fossil record with thousands of described genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties.Except possibly for the members of the orders Phacopida and Lichida (which first appear during the early Ordovician), nine of the eleven trilobite orders appear prior to the end of the Cambrian.Most scientists believe that order Redlichiida, more specifically its suborder Redlichiina, contains a common ancestor of all other orders, with the possible exception of the Agnostina.