Many have unknown affiliations with extant taxa. In the first 14 My of the Cambrian, not much more happened. The Cambrian Period ended with a mass extinction for unknown reasons. Large reductions in the diversity of trilobites echinoderms. Metazoan phylogeny Phylogeny of phyla is poorly understood.
Morphology is so different that homologous characters are difficult to discern.
May or may not be monophyletic. Diversification was at the class and order taxonomic level. Many formerly abundant taxa never fully recovered, so that the character of the fauna was different. New trilobite, brachiopod, bryozoan, and bivalve orders appeared. Fish-like vertebrates appeared toward the end of the Ordovician.