recognized 3 so-called evolutionary faunas, named the Cambrian fauna, the Palaeozoic fauna
and the Modern fauna. His initial work has subsequently been improved by other scientists
but his basic conclusions are still accepted by most workers. His diagrams show the diversity
of marine taxa at family level. It should be noted that a family may consist of only one
species or many species or genera and this is not shown. His diagram also shows the major mass
extinctions that occurred several times in the history of life.
Especially five such events were dramatic, namely at the end Ordovician,
in the late Devonian, end Permian, end Triassic and end Cretaceous. These are called the big five.
Overall the sepkoski diagram shows that the marine animals have become more and more diverse
through time. It is, however, discussed among biologists and palaeontologists how evolution works.
Darwin predicted a slow, gradual evolution, with slow accumulation of beneficial
mutations, others have pointed out that this fits poorly with the fossil record, mostly
showing rapid changes and long periods where no or only few changes take place. Fossil examples
of gradual changes are actually rare. This school of palaeontologists has suggested that evolution
takes place in steps, so-called punctualistic evolution. They further suggest that evolution
mostly takes place in small populations where mutations quickly can spread because of the
limited number of individuals. It is, in any case, obvious that mass extinctions recurrently
wiped out a significant proportion of the fauna, allowing for development of news forms
afterwards. A well-known example is the radiation of mammals after extinction of the dinosaurs at
the end of the Cretaceous some 66 mill yrs ago.