A new genus and species of primitive echinozoan, Bromidechinus rimaporus, is described from the Upper Ordovician of Oklahoma, USA. This has a unique plate arrangement. There is a single perradial series of imperforate plates bounded on either side by a column of perforate ambulacral plates. A double column of interambulacral plates separates ambulacral zones. The sparse record of Ordovician echinozoans is reviewed and cladistic analysis suggests that Bromidechinus represents a lineage that diverged prior to the split between bothriocidarids and main-line echinoids. This leads to a revised interpretation of the earliest stages of morphological evolution of echinoids.