Although there's disagreement in scientific circles, the most favored theory at present is the evidence for a large asteroid striking the Earth during the Cretaceous (145 to 65 mya) in the vicinity ot the Yucatan penisula. The resulting energy release sent millons of tons of debris into the atmosphere as well as causing devastating firestorms that killed vast numbers of various species, either outright or within the next few years due to the lowering of temperatures into a "global winter" because of blocked sunlight...