Foot-long section of a large straight shelled nautiloid, both sides and end view showing the siphuncle. Based on the size of the section and the taper of the shell, this thing was probably 6 feet long or longer. I don’t know the species, but it’s from the Southern Lake Michigan/Illinois/Indiana region, likely Ordovician or Silurian in age. If you know what it is, I’d be happy to hear from you.
Like their modern relative the chambered nautilus, Nautiloids had a soft body with tentacles that emerged from a hollow shell filled with gas for buoyancy. Unlike the Nautilus, which is admired for the beauty of its spiral shell, the Nautiloid shell came in a variety of shapes. Orthoceras (which translates as “straight horn”) Nautiloids had long, cone shaped shells, and must have swum like torpedoes through ancient seas. Their fossilized shells are common and are used as decorative stone all over the world. These distinctive black and white specimens come from Morocco, and are often quarried in slabs teeming with fossil Orthoceras shells. The originally hollow chambers of the shell have filled in with white calcite crystals, and in some pieces the siphuncle, a tube that ran through all the chambers of the shell, is visible.