Ortoceras is a genus of ammonite that lived during the Ordovician period, about 485–443 million years ago. This genus of ammonite is known for its elongated and thin shell, which was often used in the stratigraphic and chronological interpretation of rocks from that period.

Ammonites, including Ortoceras, were marine mollusks with spiral and segmented outer shells, which were often found as fossils in rock layers. These fossils are important for understanding the evolution of marine life and the geological history of the Earth.

Ortoceras shells are often long and cylindrical, with septa delineating the inner chambers of the shell. These sea creatures were carnivores, hunting other small organisms for food.

Orthoceras fossils are sought after by collectors and paleontology enthusiasts for their aesthetic beauty and for the valuable information they provide about life and the environment in which they lived in Earth's distant past.