The textbook view of early vertebrate evolution posits that although vertebrates were present in the Ordovician and perhaps the Cambrian, they were rare and remained at low diversity until they underwent an explosive radiation during the Mid to Late Silurian. This canonical view has been overturned in recent years with the discovery of an entirely unanticipated diversity of vertebrates in the Cambrian and Ordovician, the reinterpretation of the affinity (phylogenetic relationship) of taxa that were long known but poorly understood, and the incorporation of all these data into new phylogenies.