Orthoceras sp.
Nautiloid
Silurian
Massie Shale
Napoleon, Indiana, USA

Big 210mm orthocerid nautiloid on 350mm matrix.

Cephalopod.

Orthoceras is a genus of nautiloid with a long chronologic range (Ordovician to Triassic) and wide paleogeographic range (North Anerica, Europe, China, north Africa, Madagascar, Indonesia).

Orthoceras is perhaps best-known from the huge number of polished specimens sold to tourists and casual collectors at mineral shows and flea markets.  It’s true they are actual nautiloid specimens but researchers have determined that at least most of those specimens actually belong to the genus, Geisonoceras.

Nautiloids appeared in the Late Cambrian and survived the mass extinction at the end of the period.  They diversified in the Ordovician, some becoming the apex predators of the seas.  However, they declined in diversity during the Devonian but continued to be significant elements of marine faunas over the rest of the Paleozoic Era and even survived the end-Permian mass extinction.

Today, only a few species survive in the Indo-Paciifc region and mostly in deeper water.