Here's some more interesting tidbits of information gleaned from the Bill Bryson book I am reading. The term 'dinosaur' was first coined by Richard Owen in 1841 and means 'terrible-lizard'. Dinosaurs are believed to have gone extinct about 65 million years ago when a comet collided with the Earth in the Yucatán Peninsula of Mexico. The comet's impact zone is called Chicxulub Crater and is more than 180km in diameter.

The reason people think this astrological event is the cause of the dinosaur's extinction is because of a geological signature known as the KT Boundary. The KT Boundary is a thin iridium rich layer of clay that separates the much thicker geological layers from the Cretaceous and Tertiary periods. The lower layer from the Cretaceous period has lots of dinosaur fossils while the upper layer from the Tertiary period has few if any. That seems to indicate that the dinosaurs became extinct at about the same time the KT Boundary was created.

The reason why scientists believe that the KT extinction event is related to a comet hitting the earth is that iridium can only be found in low concentrations on the Earth's crust but is found in very high concentrations on asteroids. This hypothesis was originally conceived in 1980 and has become known as the Alvarez hypothesis after the father and son team who instigated the research. However this remains a hypothesis only and is still subject to much debate.