Complex sentence: The sea was occupied by mosasaurs, ichthyosaur, and plesiosaurs.iod plants were embedded in mud that hardened into rock and became the limestone that appears today on the sides of ravines and along the streams of Nebraska.
Split into simpler sentences: The sea was occupied by mosasaurs, ichthyosaur, and plesiosaurs. Fossil skeletons of these animals and period plants were embedded in mud that hardened into rock and became the limestone that appears today on the sides of ravines and along the streams of Nebraska.