Mass extinctions are intervals in Earth history when a large fraction of species disappeared in a relatively short geologic time. They matter because they reshaped ecosystems, changed the direction of evolution, and opened space for new groups of organisms to diversify. Dinosaurs are strongly linked to the end-Cretaceous extinction, but that event was only one of several major biological crises recorded in rocks and fossils.
Paleontologists study these events to understand how life responds to rapid environmental change.
Key Facts
- A mass extinction is usually defined as a global loss of many species over a short interval of geologic time.
- The five major mass extinctions are the Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic, and Cretaceous-Paleogene events.
- The Permian-Triassic extinction was the largest known, eliminating about 90 percent of marine species.
- The Cretaceous-Paleogene extinction occurred about 66 million years ago and is linked to an asteroid impact and major climate disruption.
- Extinction rate = number of species lost / time interval.
- Recovery after a mass extinction can take millions of years as ecosystems rebuild and surviving lineages diversify.
Vocabulary
- Mass extinction
- A mass extinction is a global event in which a large percentage of species die out over a relatively short geologic time.
- Fossil record
- The fossil record is the preserved evidence of past life found in rocks, including bones, shells, footprints, and microscopic remains.
- K-Pg boundary
- The K-Pg boundary is the rock layer marking the transition from the Cretaceous Period to the Paleogene Period about 66 million years ago.
- Iridium anomaly
- An iridium anomaly is an unusually high concentration of iridium in a rock layer, often used as evidence for an asteroid impact.
- Adaptive radiation
- Adaptive radiation is the rapid diversification of surviving organisms into new species after ecological opportunities become available.
Common Mistakes to Avoid
- Thinking all dinosaurs went extinct at the K-Pg boundary is wrong because birds are living dinosaurs that survived and diversified after the event.
- Assuming a mass extinction happens in a single day is wrong because even impact-related extinctions can involve years to thousands of years of climate and ecosystem disruption.
- Using one fossil site to describe the whole planet is wrong because mass extinctions are global patterns that require evidence from many locations and environments.
- Confusing extinction with evolution is wrong because extinction is the disappearance of a lineage, while evolution is change in populations over generations.
Practice Questions
- 1 A fossil group had 240 known species before a mass extinction and 72 species after it. What percentage of the species went extinct?
- 2 If an extinction interval lasted 20,000 years and 600 species disappeared during that time, what was the average extinction rate in species per year?
- 3 Explain why the disappearance of non-avian dinosaurs allowed mammals and birds to diversify in the Paleogene Period.