English: Geodized fossil nautiloid in limestone from the Ordovician of Ohio, USA. (field of view ~15.3 centimeters across)

Nautiloids were relatively common components of Paleozoic oceans. They were basically a squid-in-a-shell. Most species had straight, slightly tapering shells, but some had loosely coiled or tightly coiled shells. The shells were mostly hollow, aragonitic, and had regularly-spaced internal walls. 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.

Shown here is partially geodized nautiloid. The crystal-lined cavity at right represents a void in the original shell after burial. Fluids filled the void and precipitated calcite crystals (CaCO3 - calcium carbonate). The medium gray-colored material immediately adjacent to the crystal-lined cavity is micrite (= fine-grained calcite sediment; calcitic mud) that accumulated in the open chambers of the nautiloid shell.

The presence of a geodized void above and micrite-filled chambers below makes this a geopetal structure - a geologic feature that indicates the original orientation of gravity-defined “up” and “down” at the time of rock formation. Which way was up? At the time of final deposition, the shell was oriented with the micrite at the bottom.

Classification: Animalia, Mollusca, Cephalopoda, Nautiloidea

Stratigraphy: float from the upper Waynesville Formation or the Liberty Formation or the lower Whitewater Formation, Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician

Locality: loose piece from Caesar Creek Lake's emergency spillway, northeastern Warren County, southwestern Ohio, USA