Orthoceras fossil bookends loaded with squid like animals.
Orthoceras (“straight horn”) is a genus of extinct nautiloid cephalopod restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros.
Orthoceras was formerly thought to have had a worldwide distribution due to the genus’ use as a wastebasket taxon for numerous species of conical-shelled nautiloids throughout the Paleozoic and Triassic. Since this work was carried out and re cataloging of the genus, Orthoceras sensu stricto refers to O. regulare, of Ordovician-aged Baltic Sea limestones of Sweden and neighboring areas.
These are 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. Many other very similar species are included under the genus Michelinoceras.