ABSTRACT
Ninety-nine families currently are widely recognised within Class Echinoidea. However, these include paraphyletic, polyphyletic, and monotypic as well as monophyletic groups of genera; they vary in diversity with steeply decreasing frequency from a single ascribed genus (22 families), 2 to 10 ascribed genera (57 families), 11 to 23 genera (16 families), to some 40 genera (4 families); and they exlude many genera still of uncertain assignment. First and last occurrences of these families can generally be dated to stage level in the stratigraphical record, but stages adopted as a global standard are known to vary widely in duration (e.g. from 1.5 to 15 million years in the Mesozoic), and first preserved occurrence does not necessarily coincide with first evolutionary appearance: some taxa have predictably lower preservation potential than others. Apparent patterns in global diversity derived from such data should therefore be interpreted with caution. An overall increase in echinoid diversity since the Paleozoic is seemingly valid, but peaks of radiation and extinction are more likely to represent fossilization than evolutionary events.
