Limestone fossil, close-up. This piece of calcite limestone (calcium carbonate) contains the fossil of a shell of a mollusc that lived in shallow marine conditions in the Mesozoic Era (150-65 million years ago). The rock formed around the shell, preserving its shape as the shell was incorporated into the rock. The fossil provides evidence for the environmental conditions that existed during the formation of the limestone. This sample is from Gurnigel, Switzerland.