Stromatolite (40-120mm) is considered one of the oldest fossils in the world.

Stromatolites are a form of sedimentary rock that is often highly layered and contains a lot of lime. It is caused by cyanobacteria capturing and retaining the sediment. These then form layer upon layer in a manner similar to the way coral reefs are formed. Stromatolites are among the oldest fossils, found from approximately 3.5 billion years ago. They were common in the Precambrian, but today they are only found formed in a few special environments, especially in places where the water is very salty or alkaline. The reason for this is that in normal environments a stromatolite would be 'grazed' by animals much faster than it can be built up. A well-known location of stromatolites is Shark Bay in Western Australia.