Orthoceras

The Orthoceras is an ancient mollusk that lived during the Ordovician period (370 to 488 million years ago). The name Orthoceras means ‘straight horn’. They ranged in size from 2.5 centimeters to 4 meters and were distinctive by their long, straight, conical shells and long tentacles. The tentacles were used to move the mollusk by jet propulsion. Having lived in the deep ocean ecosystems, they ate smaller Orthocerida species, invertebrates, and fish.

photo of an ammonite [photo credit: Natural History Museum]