Nevada's sea level continued to drop during the Triassic period. However, the western part of the state was still relatively deep. It was home to a rich ammonite and ichthyosaur fauna. During the Triassic, many invertebrates lived and died in the area now occupied by the Shoshone Mountains. Their fossils are scientifically significant as many are now used as index fossils or were completely unknown to science before their discovery here. By the Jurassic, the only deep marine habitats of Nevada were in the northwestern part of the state. Central Nevada was only under shallow water and the eastern and southern parts of the state were characterized by other types of environment. During the Cretaceous, a volcanic island chain formed in far western Nevada. Contemporary plant fossils from Nevada include twigs of petrified wood preserved in Eureka County. These may be the remnants of ancient Sequoia trees. Local dinosaurs included armored dinosaurs, possible duck-billed dinosaurs and relatives of the horned dinosaurs.