The end of the Mesozoic: impact, extinction and survival

The Mesozoic ended at the Cretaceous–Paleogene boundary, about 66 million years ago. This boundary is recorded in rocks around the world and marks a mass extinction. Non-avian dinosaurs disappeared, along with ammonites, many marine reptiles and numerous plankton groups. Birds, mammals, crocodilians, turtles, flowering plants and many other lineages survived, though survival did not mean that ecosystems remained unchanged.

The impact evidence

A thin boundary layer at many sites contains an unusual enrichment of iridium, an element scarce in Earth's crust but more common in some meteorites. Other deposits include shocked minerals and glassy spherules formed from material thrown out by an impact. The roughly 180-kilometre Chicxulub crater beneath the Yucatán region has the right age and geological structure to connect the impact to the global boundary layer.

The impact would have produced immediate regional destruction, earthquakes and tsunamis. Fine dust and sulfur-rich aerosols could then have reduced sunlight and disrupted photosynthesis. Sediment and fossil records document severe disruption to food webs on land and in the oceans. The timing and duration of each environmental effect differed, and researchers continue to test the details with rocks and climate models.

Deccan volcanism and the longer background

Large eruptions in what is now India produced the Deccan Traps before and after the impact. They released carbon dioxide and sulfur gases and altered climate and ocean chemistry. High-precision dating shows that the eruptions overlapped the extinction interval, but the relative contribution of volcanism and impact remains a subject of research. The impact is strongly linked to the abrupt global crisis; the volcanic episode may have shaped environmental stress and recovery.

This distinction matters because several causes can operate on different timescales. Long-term climate trends and volcanic emissions can change ecosystems over thousands to millions of years. The impact added a sudden shock. Evidence from fossils and dated rocks is needed to separate those effects.

Extinction was selective

Groups with different diets, habitats and life cycles did not fare alike. Many large land animals vanished, while small mammals and birds persisted. Among dinosaurs, the surviving birds were one branch of theropods. No non-avian dinosaur lineage is known to have crossed the boundary. In the sea, many plankton organisms and large reptiles disappeared, while other groups recovered or diversified later.

Recovery took time. Fossil communities above the boundary differ from those below it, and the rebuilding of food webs varied by region. The event did not instantly create modern ecosystems; it changed the opportunities available to the lineages that survived.

For the main extinction question, see why dinosaurs went extinct. The Cretaceous Period page places the crisis at the end of a much longer and diverse interval.