Answer:
Mass extinctions were first identified by the obvious traces they left in the fossil record. In the strata corresponding to these time periods, the lower, older rock layer contains a great diversity of fossil life forms, while the younger layer immediately above is depauperate in comparison. Often, the rock layers bookending the mass extinction are noticeably different in their compositions. These changes in the rocks show the effects of environmental disturbation that triggered the mass extinction and sometimes hint at the catastrophic cause of the extinction.