Wyoming, US

Paleontology and geology

The Precambrian: Precambrian metamorphic and igneous rocks are exposed along the length of the Rocky Mountains in Wyoming. Stromatolites and some possible trace fossils have been found in metamorphosed Precambrian sediments in the southeastern part of the state. These rocks were originally formed from limy sediments deposited on a shallow sea floor. The Paleozoic: The Paleozoic is characterized by fluctuating sea levels. A shallow sea covered Wyoming during the Early Paleozoic, and fossils of trilobites, brachiopods and other marine organisms are found in the sandstones, shales, and limestones laid down at this time. The sea retreated in the Silurian, and the state underwent an interval of uplift and erosion before marine conditions returned in the Devonian. A shallow sea persisted through much of the rest of the Paleozoic, and a diverse assemblage of invertebrate animals inhabited the marine environments. The sea began to retreat once again during the Permian. The Mesozoic: The sea continued to retreat during the Triassic. Sediments accumulated in nearshore marine environments and then on vast river floodplains that developed as the sea regressed. Eventually, wind-blown sand dunes formed. Jurassic sediments were deposited during several transgressions and regressions of shallow marine water over Wyoming. Marine rocks are rich in fossils of oysters, belemnite cephalopods, and other marine invertebrates, while bones and trackways of dinosaurs are common in the deposits of the vast river floodplains at the edge of the sea. The Western Interior Seaway spread through much of the state during the Cretaceous, and a series of mountain-building episodes (the Laramide Orogeny) began. Cretaceous fossils include fish, turtles, crocodiles, pterosaurs, mammals, birds, and, of course, dinosaurs. The Cenozoic: Early Cenozoic (Tertiary) climates favored the growth of luxurious forests, and fossil plants and coal deposits are common in many Tertiary formations. Lakes formed in many of the intermountain basins, and the resulting sediments often contain abundant, beautifully preserved fish fossils, including the Wyoming State Fossil, Knightia eocaena. The Rockies continued to rise and large volcanic eruptions periodically deposited thick layers of ash across the state. Late Cenozoic (Quaternary) rocks in Wyoming are predominantly the result of volcanic activity, but glaciation also produced sediments, predominantly in the western part of the state. Microfossils have been recovered from glacial lake sediment and fossils of numerous vertebrates have been found in northern Wyoming.