The family tree of fishes, showing the evolution of major groups through geological time. Numerous lineages of extinct fishes are not shown. Widened areas in the lines of descent indicate periods of adaptive radiation and the relative number of species in each group. The lobe-finned fishes (sarcopterygians), for example, flourished in the Devonian period, but declined and are today represented by only four surviving genera (lungfishes and coelacanths). Sharks and rays, which radiated during the Carboniferous period, came dangerously close to extinction during the Permian period, but staged a recovery in the Mesozoic era and are a secure group today.
Photo © Hickman et al. (2003)