65 million years ago the Cretaceous period ended. Many believe it ended abruptly. Sources vary widely regarding the changes that occurred, but on average they state that between 50 and 75% of all plant and animal life perished at this time. Of the animals, only amphibians and mammals survived. Fossilized dinosaur remains were first discovered in the mid-1800s. Since then, the reason for their demise has intrigued both scientists and lay-people. The most reliable testable hypothesis concerning their extinction was proposed by Alvarez and Alvarez in 1980.

Walter Alvarez, a geologist, was collecting sedimentary samples near Gubbio, Italy, working as part of a team measuring iridium. Not commonly found in Earth's crust and mantle, iridium has been regularly detected in scientific samples from other parts of the solar system; in other words, the team was searching for cosmic dust. Alvarez discovered - again, sources vary - a layer between 1 and 2 centimeters thick at the point believed to mark the boundary between the Cretaceous and Tertiary periods. This layer had an iridium content 30 times higher than its surrounding layers.

Q1. Fossilized dinosaur remains were first discovered in