Limestone forms when calcite from the water crystallizes or when fragments from coral and shells cement together. Limestone is a type of sedimentary rock that contains fossils in the form of sea creatures. Entire reef formations and communities of organisms are found preserved in limestone. The types of fossils found in limestone include coral, algae (see images), clams, brachiopods, bryozoa and crinoids. Most limestone forms in shallow tropical or subtropical seas. In some cases, fossils make up the entire structure of limestone.