Some scientists blame comets for the lack of dinosaurs on earth today.
They hypothesize that a large comet collided with our planet long ago. This resulted in great clouds of dust in the air, which shielded the sun and cooled the earth's climate for many years. Since dinosaurs could not adjust to the colder weather, they perished. This is just one of many theories about the demise of the dinosaurs. The list of suggested catastrophes, both on the earth and in the sky, is considerable.