Fossil Beetle (Buprestidae fam.) in the Green River Formation. This beetle, now extinct, is similar to modern jewel beetles. This specimen lived during Eocene time, about 50 million years ago. These fossils are unusually well preserved in laminated limestone precipitated from calcium-rich waters. The limestone is interbedded with many thin layers of volcanic ash and mudstone. Fossil Butte National Monument. Near Kemmerer, Lincoln Co., Wyo.