Modern vertebrate faunas are dominated by ray-finned fishes (Actinopterygii), tetrapods and sharks (Chondrichthyes). However, biotas at the origin of tetrapods, during the Devonian period (420-359 million years ago, My) were defined by now-extinct or depauperate groups such as lobe-finned fishes (lungfish and coelacanths). It was long thought that this turnover was caused by gradual forces and the greater fitness of living lineages. A comprehensive global database for early vertebrate diversity, analysed using methods from community ecology, has pinned this turnover to the supposedly minor end-Devonian Hangenberg mass extinction event (359 My). Global vertebrate faunas were destroyed and 44% of major clades lost, restructuring ecosystems and permitting the rise of the modern vertebrate biota from scarce survivors. However, recovery from the Hangenberg event was long and arduous, coincident with “Romer’s Gap,” a previously unexplained lull in tetrapod diversity. While some groups thrived and radiated (e.g., actinopterygians), members of once-diverse groups became “dead clades walking,” the extinct equivalent of “living fossils.” In addition, there was little replacement of lost lineages: survivors tended toward novel forms and niches. Thus, conditions during aftermath of the Hangenberg extinction had as much of an impact on subsequent biodiversity as the event itself. The new challenge is explain this record of differential success. The new opportunity is to re-examine the impact of other, perhaps neglected external events on vertebrate biota past and present.