The trilobites of Order Agnostida (pronounced Ag-nós-t-eh-da)appeared in the Lower Cambrian where they were diverse and ubiquitous, and declined to become rare in the Ordovician prior to their complete extinction by the end of the Ordovician. There is debate about whether agnostids were benthic or pelagic, and the systematic position of Order
Agnostida remains unsettled, if not contentious. The agnostids were mostly blind and exhibit a strong symmetry of cephalon and pygidium size, making it hard to distinguish front from rear.