The origin or evolutionary `success' of taxa is often attributed to key innovations-aspects of organismal phenotype that promote diversification. Different ways of delimiting taxa and measuring `success' (i.e. number or longevity of species, morphological variety or differential control of energy) give rise to different ideas of how key innovations might operate. Key innovations may enhance competitive ability, relax adaptive trade-offs or permit exploitation of a new productive resource base. Recent key innovation studies comparing species richness in extant sister clades may miss important observations possible only with consideration of the fossil record, traditional higher taxa and phenotypic diversity.