 Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors.The Phacopida and other "new" clades almost certainly had Cambrian forebearers, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly.Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician, allowing many families to continue into the Silurian with little disturbance.