Here is a series of 7 videos put on by the Discovery Channel that covers the five major extinctions that have occurred in the last 450 million years. Each video is about 45 minutes long so it take a bit of time to watch them all, but it's well worth the time.

http://www.youtube.com/watch?v=bs5S1...ure=plpp_video

http://www.youtube.com/watch?v=GipQe...feature=relmfu

http://www.youtube.com/watch?v=dASWv...feature=relmfu

http://www.youtube.com/watch?v=vv-X7...feature=relmfu

http://www.youtube.com/watch?v=IV5UJ...eature=related

http://www.youtube.com/watch?v=FAnug...feature=relmfu

http://www.youtube.com/watch?v=U5s6u...feature=relmfu