They are the only members of the order Craniida, the monotypic suborder Craniidina, and the superfamily Cranioidea; consequently, the latter two taxa are at present redundant and rarely used.

[1] As adults, craniids either live freely on the ocean floor or, more commonly, cement themselves onto a hard object with all or part of the ventral valve.

In the Lower Ordovician, they were mostly restricted to peri-Gondwanan terranes (modern central Europe) in the South Polar region.

[4] Craniid diversity and abundance was respectable but still fairly low during the Late Ordovician, and even lower through the rest of the Paleozoic.

Genetic evidence has clarified this misconception, revealing that "Craniscus japonica" actually represents a misattributed species of Neoancistrocrania.