Treptoceras fossil nautiloid (4.8 cm across) in calcisiltite from the Ordovician of Kentucky, USA.

This is a partial internal mold of a Treptoceras nautiloid cephalopod. The living animal was a squid-like creature that constructed a long, slightly tapering, aragonitic, hollow shell with regularly-spaced internal walls. The original aragonite shell has dissolved away, leaving an internal mold (impression of the shell's interior). Nautiloids were relatively common components of Paleozoic oceans. Most species had straight, slightly tapering shells, but some had loosely cosusiled or tightly coiled shells The entire group is represented in today's oceans by three living species of chambered nautilus - Nautilus pompilius, Nautilus macromphalus, and Allonautilus scrobiculatus.

Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea, Orthocerida

Stratigraphy: Kope Formation, Edenian Stage, lower Cincinnatian Series, Upper Ordovician