Adaptive radiation
The evolution of a new feature may let a group diversify because it makes possible new ways of living. A most striking example is the cleidoic egg, which developed in early amniotes, and permitted vertebrates to invade the land. The cleidoic egg must have been developed in the latest Devonian or early Carboniferous. Amphibians, who branched off before this event, still lay their eggs in water, and so are limited in the extent to which they can exploit land environments.