Paleontology and geology
A few rocks of Cretaceous age are exposed in the far southeastern part of the state, in an area known as the Mississippi Embayment. These rocks began as muddy and sandy sediments deposited along the shoreline and on the floor of the newly opened Gulf of Mexico. Fossils of various molluscs, including ammonites and nautiloids, occur in these rocks, along with the bones of mosasaurs (large marine reptiles). The only dinosaur fossils from Missouri were recovered from a Cretaceous clay: a portion of a hadrosaur skeleton and the tooth of a theropod. Fossil leaves from some of the first flowering plants have also been found in the state's Cretaceous rocks.