Scientists have always debated what event or events caused the demise of the Dinosaurs. At the beginning of the Mesozoic Era Dinosaurs started to appear and dominated all forms of life. Fossil records shown in the lower strata of rock contains the earliest and most primitive species of Dinosaur, and the upper strata contains the newer species. But then at the Cretaceous-Tertiary, the Dinosaurs are nowhere to be found. This where the Alvarez hypothesis comes into effect.
Iridium is rare on Earth, but abundant in meteorites.
The Alvarezs' suggested that a huge asteroid or comet, perhaps miles in diameter, hit the Earth at that time. The result of such an impact would be an enormous explosion that would throw dust clouds into the sky darkening the planet. Massive forest fires, triggered by the hit, would add smoke to the sky. This would cool the planet causing the climatic changes observed. A crater, now worn down and partly under the ocean, was found along the Mexican Yucatan Peninsula.