English: Amphicyrtoceras pettiti (Billings, 1866) - fossil nautiloid in dolostone from the Silurian of Ohio, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

Nautiloids are squid-like creatures that construct long and slightly tapering or coiled, aragonitic, hollow shells with regularly-spaced internal walls. In the fossil record, the original aragonite shell has usually dissolved away, leaving an internal mold (an 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 coiled or tightly coiled shells. The entire group is represented in today's oceans by five living species of chambered nautilus, all of which have a coiled shell: Nautilus pompilius, Nautilus macromphalus, Nautilus stenomphalus, Nautilus belauensis, and Allonautilus scrobiculatus.

In the above photo, the lines at the top of the fossil are sutures, marking the intersection of internal walls (septa) and the external shell. The lower portion of the fossil lacks sutures - that’s the living chamber where the squid animal was located.

Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea, Oncocerida (= Oncoceratida), Acleistoceratidae

Stratigraphy: Lockport Dolomite, Niagaran Series, Middle Silurian

Locality: Rockford Stone Company Quarry (now abandoned & flooded), 1.5 miles northwest of the town of Rockford, northern Mercer County, northwestern Ohio, USA. (40° 42' 29.10" North latitude, 84° 40' 01.28" West longitude)