The most widely accepted theory is the impact event, which proposes an asteroid collided with Earth in the late Cretaceous Period, 65 million years ago, and triggered a chain of events that led to mass extinction. The asteroid would have been 5 to 15 kilometres wide, and it created a 180 km crater found in Mexico – the Chicxulub Crater. This theory is supported in part by a layer of increased iridium, measured in rock stratum in the same geological time period – iridium is a transition metal that is very rare on Earth, but found in meteorites.
The asteroid landed in the ocean and caused megatsunamis, and also released sulfiruc acid aerosols into the air, possibly causing acid rain. The debris created a massive dust cloud that blocked out the sun for up to a year, inhibiting photosynthesis and killing off sunlight-dependent organisms, which would have in turn affected the creatures reliant on them for food. The debris falling back down through the atmosphere may also have caused pulses of infrared radiation, killing exposed creatures, and an intense greenhouse gas effect increased the temperatures of the Earth’s atmosphere and possibly caused global firestorms - the oxygen-rich atmosphere was ideal for combustion…but, alas, not ideal for the survival of the dinosaurs.
Over half of the world’s species went extinct – plankton took a huge hit, the last of the non-avian dinosaurs disappeared, vegetation withered, and even sea sponges were affected. The speed of extinction would have been for the most part very rapid.
Scientists are yet unsure whether the Chicxulub Crater was the only cause of the extinction, or whether there were several impacts, or whether it was instead caused by massive volcanic activity - likely the Deccan Traps - which would have brought about much the same events. Some think that both contributed - but either way, without the Cretaceous-Paleogene extinction event, mammals would never have risen to take their place and flourish, and we wouldn’t be here today.