Orthocera

Orthoceras fossil specimen
 
Orthoceras are extinct nautiloid cephalopods restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden. This extinct mollusk was believed to have had worldwide distribution due to the misclassification of numerous conical-shelled nautiloids that lived throughout the Paleozoic and Triassic; however, it is now restricted to Orthoceras regularein Baltic Sea limestone. Orthoceras first emerged almost 500 million years ago and went extinct 200 million years ago.