Paleontology and geology
The Precambrian: The oldest rocks in Missouri are igneous and metamorphic rocks approximately 1.8 billion years old. No fossils are known from these rocks.
The Paleozoic: Warm, shallow seas covered Missouri through much of the Paleozoic, up until the Late Carboniferous. Fossils of trilobites, brachiopods, molluscs, echinoderms, corals, and bryozoans are common in many of the state’s Paleozoic rocks. Sharks and various fishes have also left their remains in some of these rocks. Late in the Paleozoic, erosion of great mountains along the eastern part of the United States, formed during the Alleghenian Orogeny, dumped vast quantities of sediments westward into this shallow sea, creating huge deltas with swampy lowlands. By the end of the Paleozoic, most of the state was above sea level and erosion outpaced deposition.
The Mesozoic: Missouri was primarily above sea level throughout most of the Mesozoic, and erosion outpaced deposition. In Late Mesozoic (Cretaceous) rocks, fossils of molluscs and marine reptiles indicate that the sea flooded the far southeastern part of the state, in an area known as the Mississippi Embayment. The only known dinosaur fossils from Missouri come from a Cretaceous clay in this area. Fossil leaves from some of the first flowering plants have also been found in the state’s Cretaceous rocks.
The Cenozoic: Early Cenozoic deposits in Missouri consist of stream-deposited clays, sands, gravels, and a few poorly consolidated sandstones that formed along the shores of the Mississippi Embayment, where the early Gulf of Mexico flooded the region. Plant fossils indicate that this was a time with a relatively mild climate. During the Late Cenozoic (Pleistocene), glaciers covered parts of the state north of the Missouri River, leaving behind deposits of till, clay, gravel, and loess (wind-blown silt). Fossils of Missouri’s ice age mammals, particularly mastodons, are famous.