The strata contain fossil corals, crinoids, sea urchins and ammonites. Further taxa to be starkly affected include the brachiopods, trilobites, ammonites, conodonts, and acritarchs. Gastropod fossils may sometimes be confused with ammonites or other shelled cephalopods. In the oceans, plesiosaurs, ichthyosaurs and ammonites were abundant. In the oceans, plesiosaurs, ichthyosaurs and ammonites dominated the seas. During the Cretaceous invertebrates such as ammonites and pearl oysters lived in California. The first ammonites, species of molluscs, appeared. During this time, marine life (including ammonites and bivalves) flourished. The chalk contains many fossils such as ammonites and nautilus. It is rich in ammonites and bivalves. They show primeval creatures like dinosaurs and ammonites. The eggcase curiously resembles the shells of extinct ammonites. Calcified aptychi only occur in ammonites from the Mesozoic era. Below the surface, the sea was populated primarily by invertebrates such as ammonites and squid. Many Pierre Shale ammonites, and indeed many ammonites throughout earth history, are found inside concretions.