Ordovician
The renowned Burgess Shale fauna disappears in the Middle Cambrian. It is now known that it did not go extinct, but survived, and thrived where the circumstances were right. A recently discovered lagerstätte (a deposit of exceptionally preserved fossils) has been found in the Fezouata Formation, Morocco. The site contains remarkable fossils of soft-bodied animals from a muddy ocean floor. The fauna also includes some hard-bodied animals such as horseshoe crabs. Probably low oxygen conditions kept predators and scavengers to a minimum.