What happened to the dinosaurs??

        The book T.Rex and the Crater of Doom is an account of the events which led to the theory that the dinosaurs were wiped out by the impact of an extra-terrestrial object (comet or asteroid) some 65 million years ago, written by one of the scientists who was involved. It begins with the investigation of a layer of clay that marks the boundary between lower layers containing dinosaur fossils and layers above which contain none, and ends up with the discovery of a huge buried crater** of the right age, size and location to have caused the clay layer. The composition of this clay layer alone makes it virtually certain that the dinos were hit by a big space rock:

Bonk!
     Cartoon copyright Tom Weller, from his book "Science Made Stupid"
        While the evidence for this impact and it's effect on the dinosaurs is quite compelling, the book also mentions a more far-fetched theory that the passage of our star system thru the galactic plane in it's orbit about the center of the galaxy triggers periodic showers of comets into the inner solar system resulting in periodic mass extinctions on Earth. This theory is supported by little evidence, as far as I know, beyond the fact that there is very rough agreement between the period of Earth's passage thru the galactic plane and a 'periodicity' of extinctions. However, deciding whether mass extinctions even have a periodicity is a matter of how one interprets the limited data...

**The crater is at the tip of the Yucatan Peninsula, Mexico.