Scientists identify mass extinction events in the fossil record primarily by detecting a sudden and significant decrease in the diversity and abundance of fossilized life forms between adjacent geological rock layers. In strata corresponding to these events, older rock layers show a rich diversity of fossils, while the younger layers immediately above reveal a sharp decline in species variety and numbers

. Key indicators include:

• Dramatic changes in fossil diversity: A rapid drop in the number and variety of species preserved in successive rock layers signals a mass extinction

• Distinct changes in rock composition: The rock layers before and after the extinction event often differ in composition, reflecting environmental disturbances that caused or resulted from the extinction

• Geologically rapid transitions: Although exact timing can be difficult to pinpoint, mass extinctions occur over relatively short geological periods (sometimes less than 200,000 years), which is rapid compared to Earth's 3.5 billion-year history

• Radiometric dating: Scientists use radioisotopic dating techniques to determine the age of the rock layers containing fossils, helping to constrain when the extinction occurred

• Patterns of extinction selectivity: Analysis of which species went extinct versus which survived can reveal non-random patterns linked to ecological, physiological, or environmental factors, providing clues about the causes and nature of the extinction event

Together, these methods allow scientists to recognize mass extinction events as sudden, global-scale losses of biodiversity recorded in the fossil record, often associated with environmental catastrophes such as asteroid impacts or intense volcanic activity