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)