Shadows in the stone
The Burgess Shale is a deposited of shale that was formed from large underwater avalanches that berried organisms around a half-billion years ago, preserving the soft parts of the bodies. However, the fossils have been squeezed down to a thin layer of carbon, leaving shadows of what the creatures once were.
Here are a couple of links to more information about the Burgess Shale.
Here’s a link to an episode about the Cambrian explosion.