The first known bones found in 1822 in England are of the dinosaur called Iguanodon. This fossil is now located at the British Museum of Natural History in London, England. Great extinctions have occured at 26 million year intervals. This theory was proposed in 1980 by Luis and Walter Alvarez. This theory is based on unusually high concentrations of the rare metal, Iridium, near rock levels close to the level of last dinosaur remains.
Iridium is common in meteorites.
The collision caused a hugh cloud of dust that circled the earth. The dust blocked the sunlight for as long as 5 years. This killed many plants and the Dinosaurs died too. Every 26 million years it comes very close and disturbs the gravitational balance of the solar system. This imbalance causes showers of comets to crash into Earth.