The idea, proposed in 1992 by Gordon Baird, that certain groups of species remain unaltered for tens of millions of years, then experience an episode of rapid extinction and the formation of new species. This resembles punctuated equilibrium acting at the level of communities and may occur because the species interact so closely they cannot evolve, instead responding to environmental change by moving as a group to a more hospitable location. The fossil record of animals dwelling in ocean-bottom muds in the Silurian to middle Devonian appears to support the idea, but it is not accepted by all palaeontologists.