describe the theories that scientists have developed to explain the events of the jurassic period.
You may be referring to the "anoxic event" which caused the disappearance of oxygen from oceans, or the "triassic-jurassic extinction" event, when most land life disappeared. You can read about them at
http://en.wikipedia.org/wiki/Anoxic_event
and
http://en.wikipedia.org/wiki/Triassic-Jurassic_extinction_event