The identification of strata by the fossils they contained was pioneered by William Smith, Georges Cuvier and others in the early 19th century. Geologists could divide Earth history more precisely. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today. Cuvier thought many of the geological features of the Earth could be explained by catastrophic events which had caused the extinction of many species of animals. Over the course of his career, Cuvier came to believe there had not been a single catastrophe, but several, resulting in a succession of different faunas.