Holometabolan insects, including beetles (Coleoptera), represent about half of all living organisms. Their development cycle, allowing larvae and adults to exploit distinct ecological niches, is traditionally advocated as a critical adaptation resulting in this extraordinary diversity. Herein I report the occurrence of a beetle from the Pennsylvanian deposit of Mazon Creek (IL, USA). It predates the diversification of Coleoptera by at least 65 million years. This identification supports the view that, early in their history, hyper-diverse extant holometabolan lineages were dominated by Palaeozoic lineages, mostly extinct. End-Permian environmental perturbations might have played an important role in the Triassic radiation of holometabolans. However, it is hypothesized that ecological preferences of holometabolans might have resulted in an earlier diversification of moderate importance, correlating to that of seed plants, their putative host plants. The evolutionary success of ‘holometaboly’ might be the result of a sequence of favorable events.