Dinosaurs are believed to have become extinct about 65 million years ago, at the end of the Cretaceous period. We know of their existence today because of fossilized remains -- mostly bones, but in a few cases impressions of skin, feathers, and even internal organs. It is impossible to know for sure what caused this sudden mass extinction, but the prevailing theory is that a massive meteor struck the earth about that time causing drastic climate changes and thus the extinction. Other theories of dinosaur extinction involve decreasing oxygen levels in the atmosphere, natural climate changes caused by increased volcanic activity, or perhaps a massive "belching" of methane gas from below the surface of the ocean.