A small, clustered tower of Orthoceras fossils in sea-bed matrix. 250 to 750 million years ago, the orthoceras mollusk lived in abundance during the Paleozoic Era. Orthoceras is a member of the class Cephalopoda and is closely related to the extinct giant Cephalopod and present-day chambered nautilus. Orthoceras ("straight horn") is a slender, elongate shells with the middle of the body chamber transversely constricted, and a subcentral orthochoanitic siphuncle. The surface is ornamented by a network of fine lirae.