Final answer:
The mass extinction at the end of the Cretaceous period, ending the Mesozoic era, was caused by the impact of a large asteroid or meteorite. This event led to the extinction of the dinosaurs and many other species and allowed mammals and birds to become dominant. Evidence includes a significant iridium layer and an impact crater dating to this period.
Step-by-step explanation:
The mass extinction that occurred at the end of the Cretaceous period, which marked the end of the Mesozoic era, is attributed to a cataclysmic impact by a large meteorite or asteroid. This impact off the coast of the Yucatán Peninsula led to a sharp spike in levels of iridium - a substance rare on Earth's surface but common in space debris. The impact created a thick cloud of dust, blocking sunlight and halting photosynthesis, which triggered a chain reaction resulting in the extinction of dinosaurs and many other species.
The discovery of a significant iridium layer in the geological strata at the Cretaceous-Paleogene boundary, combined with the finding of a large impact crater dating to the same period, provided strong evidence supporting the impact hypothesis. The aftermath of this event saw the extinction of all land animals over 25 kg and left an ecological niche for mammals and birds, the latter being the direct descendants of a theropod clade that survived the mass extinction.
The recovery after this mass extinction event was relatively faster compared to the end-Permian extinction, taking approximately 10 million years for biodiversity to recover.