During the Precambrian, Wyoming was covered by a shallow sea. Stromatolites formed there. Some Precambrian stromatolites remain preserved in the Medicine Bow Mountains. Some local geologic structures may be a record of the activities of other inhabitants of this sea preserved as trace fossils. During the early part of the Paleozoic, Wyoming was still covered by a shallow sea. The state was home to brachiopods and trilobites at this time. By the Late Cambrian, Wyoming was home to calcareous algae. Great masses of this algae were preserved in the Gros Ventre Formation. The area of Wyoming now characterized by the Bighorn Mountains was a marine environment during the Ordovician. Ostracoderms swam in this sea. During the Silurian, the sea withdrew from Wyoming and local sediments were eroded away. During the Devonian, the sea returned to the state and remained until the Permian when it started to withdraw once more.