Mass extinctions decimate the planet’s biodiversity, and in doing so, they can change the composition of the planet’s biota.The biota that goes into a mass extinction is not the same as the one that emerges. The actual extinctions are over very quickly – 20,000 years in the case of the end-Permian. But the recovery takes much longer. It takes time for new species to evolve and the biosphere to recover – and the Earth System will not operate properly until both processes are complete. Detailed analysis of the fossil content of sediments deposited following the end-Cretaceous extinction event reveal a long-term ecological recovery that parallels the short-term ecological succession that follows modern environmental disasters such as fires and floods. But the succession that follows a mass extinction occurs on a global scale and over a much longer time frame – often millions of years. This new post-mass-extinction recovery phase has been dubbed the Earth System succession.

Starting in 1860, there have been many attempts to document levels of ancient biodiversity using the fossil record. I review four attempts, starting with John Phillips in 1860, followed by Jack Sepkoski’s classic study from 1984. John Alroy’s 2008 work updates Sepkoski’s work but presents problems of its own. The final review is of a study by Fan et al. (2020) who used a supercomputer to produce the most detailed analysis of ancient biodiversity so far. The four studies taken together demonstrate how palaeontologists are using increasingly sophisticated statistical techniques and an improved geological time scale to address the limitations of the fossil record. But more than that, they reveal mass extinctions as powerful agents of biotic change. These events directly influence not only the level of the planet’s biodiversity, but also the composition of its biota.