SOME SILURIAN NAUTILOIDS OF OHIO
The only nautiloid cephalopod living today is Nautilus, which has a planispirally coiled shell. In the Paleozoic fossil record, nautiloids are moderately common. Paleozoic nautiloids have shells than range from straight to slightly curved to strongly curved to coiled.
Shown below are a couple of typical nautiloids from the Silurian dolostone succession of Ohio.
Harrisoceras rectum - internal mold of a straight-shelled nautiloid, preserved in dolostone. The thin whitish lines in the center and right portions of the fossil are sutures, marking the intersection of internal walls (septa) and the external shell. The left portion of the fossil lacks sutures - that’s the living chamber where the squid animal was located.
Stratigraphy: unrecorded unit in the Silurian
Locality: unspecified location in Wyandot County, northern Ohio, USA
Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea, Orthocerida, Geisonoceratidae
OSU public display (Orton Geology Museum, Ohio State University, Columbus, Ohio, USA)
Graftonoceras - limonite-stained external mold in dolostone. This coiled nautiloid still shows the pattern of its original shell ornament, which consisted of curvilinear waves.
Stratigraphy: Lockport Dolomite, Niagaran Series, Middle Silurian
Locality: unrecorded locality (probably a quarry) at or near Coldwater, southern Mercer County, western Ohio, USA
Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea, Tarphycerida, Trocholitidae
OSU public display (OSU 24835, Orton Geology Museum, Ohio State University, Columbus, Ohio, USA)