Aquatic vertebrates, which include most tetrapods during the Carboniferous, were recovering from the Late Devonian extinction, a major extinction event that preceded Romer's gap, one on par with that which killed the dinosaurs. In this Hangenberg event, most marine and freshwater groups became extinct or were reduced to a few lineages, although the precise mechanism of the extinction is unclear. Before the event, oceans and lakes were dominated by lobe-finned fishes and armored fishes called placoderms. After the gap, modern ray finned fish, as well as sharks and their relatives were the dominant forms. The period also saw the demise of the Ichthyostegalia, the early fish-like amphibians with more than five digits.