The story of the Burgess Shale is beautifully told in Stephen Jay Gould's book "Wonderful Life - The Burgess Shale and the Nature of History". For a book that contains some quite deep scientific concepts (quite deep if you're not a biologist anyway), it's very easy to read, and to me it's one of the most treasured possessions on my bookshelf.

To me the truly amazing thing about the fossilized creatures found in the Burgess Shale is their sheer diversity: as SJG says it's almost as if nature went on a crazy orgy of experimentation. Of all the things found there, very few of them fit into any modern classification categories -- most of them being not just members of previously undiscovered species or genus, but of whole new orders of animal. Some of the things that once crawled along our ocean floors would be laughed at in astonishment if a movie producer attempted to put them into a modern science fiction film: the best example is the truly freaky animal named Hallucigenia -- a thing so beyond our usual definitions of life that nobody knows which is its head and which its tail, or even which way up the thing would have stood!

Gould rounds off his tale by explaining that of all the weird and wonderful beings found in the Burgess Shale, it would have been almost impossible to predict which ones would survive the great extinction event that occurred at the end of the pre-Cambrian period. The animals that did survive ultimately evolved into all life as we know it, including homo sapiens. The final mindfuck comes from the thought that if some of the other creatures had survived, the world as we know it could be so different as to be beyond comprehension.