Increase in the diversity of distinct types in one or more taxonomic categories (i.e. species, genus, etc., see classification). Phanerozoic, marine invertebrates with well-developed hard parts provide an illustration: their diversity at phylum level remains much the same throughout, whereas at family level there is a peak at the mid-Palaeozoic, and a trough at the Permo–Triassic boundary; after this there is a steady increase to a second, higher peak in the Cenozoic.