Orthoceras is a very abundant, worldwide orthocone, which is sometimes confused with Baculites. Many orthocone nautiloid fossils have been found in limestone from Sweden.
The tube running along the back of Orthoceras is called the siphuncle, and the round air chambers are called the septa. The septa helped Orthoceras ascend and descend, and the siphuncle was the tube that led to the siphon and pushed water out to move Orthoceras through the water.
Orthoceras probably ate trilobites like Walliserops and Calymene.
Orthoceras was about 6" long and probably had to avoid giant predators such as Eusthenodon, Eusthenopteron, and Dunkleosteus.
Orthoceras was Ordovician to Devonian, and resembles other orthocones like Cameroceras, Endoceras, and Dawsonoceras. But Orthoceras was much smaller than them, because Orthoceras was only 6" compared to huge nautiloids like Cameroceras, Endoceras, and Dawsonoceras.
When Orthoceras was first discovered, people believed that all orthocones belonged to the genus Orthocerus, but now the term Orthoceras is only used for the species Orthoceras regulare.
Orthoceras means "straight horn."