Cretaceous-Paleogene extinction event

The Cretaceous-Paleogene extinction event was an extinction event that occured approximately 66 million years ago, ending the Mesozoic era. The most well-known victims of the event are the dinosaurs, along with pterosaurs and marine reptiles.

Theory
The most widely accepted theory is that a massive meteor hit earth. This meteor was 10km wide, and weighed about 20,000 tonnes. When it smashed into the earth, it made a crater 181km wide, and 30km deep. It hit on the shores of Mexico, and therefore started massive tsunamis, up to three miles high. A pyroclastic wall of doom travelled across land at astonishing speed, burning everything in its path. Some other side effects of the meteor strike include scale 13 earthquakes, felt around the world, worldwide forest fires, and raining debris from the original strike. This debris would be flaming rocks. Finally, ash blotted out the sun for thousands of years, causing the death of plants, followed by plant-eaters, followed by meat-eaters. That was the end of the dinosaurs.

Other Theories
There are several other theories, some serious and plausible while others deliberately unlikely and bizzare. Some of the most widely known include increased volcanic eruptions, rising sea levels, global warming or cooling, and new disease. Some other well-known theories are much less plausible: some of the many ideas include slipped vertebral discs, poisonous plants and fungi, radiation from a supernova, egg-eating mammals, loss of interest in sexual reproduction, abduction by aliens, lack of room on Noah's Ark, and racial senility (a hypothesis that percieves the bony protrusions on Late Cretaceous dinosaurs to be a result of excessive hormones).