Just over 500 million years ago, a diverse community of early animals lived on an algal reef high atop an undersea cliff off the west coast of North America. Periodically, the outer edges of the reefs would collapse and huge masses of mud would careen down to the base of the cliff entombing any animals caught in the flow. These mud flows would become what is one of the most important fossil localities in the world, the Burgess Shale.
Since its discovery more than 100 years ago, the rocks deposited during the Middle Cambrian Period and exposed in the Rocky Mountains of British Columbia have been famous for yielding hundreds of specimens of early animals. The Burgess Shale preserves soft bodied animals in extraordinary detail, making it a type of fossil deposit known as a laggerstätte. Lagerstätten are important to paleontologists because they preserve soft tissues, and sometimes whole organisms, that are normally destroyed in the fossilization process.
The Burgess Shale and other Cambrian sites from around the world show the rapid appearance of complex large-sized animal species in the fossil record in a short span of time, an event known as the Cambrian Explosion. Prior to the Cambrian, the fossil record of animals shows that they were rare, and oftentimes microscopic. The rate of evolution seen during the Cambrian Explosion was higher than what's seen in most other times throughout Earth history, but was not as rapid as some previous estimates. In fact, a great deal of the sudden diversity of animal life can be attributed to biomineralization, as well as the evolution of large body sizes.