Didymoceras cheyennense (Meek & Hayden, 1856) - fossil ammonite from the Cretaceous of South Dakota, USA.

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

Ammonites get their name from the coiled shell shape being reminiscent of a ram’s horn. The ancient Egyptian god Amun (“Ammon” in Greek) was often depicted with a ram’s head & horns. Pliny’s Natural History, book 37, written in the 70s A.D., refers to these fossils as “Hammonis cornu” (the horn of Ammon), and mentions that people living in northeastern Africa perceived them as sacred. Pliny also indicates that ammonites were often pyritized.

Ammonites typically have a planispirally coiled, calcareous shell. The fossil shown above has a bizarre and irregularly shaped shell. Such examples are called heteromorph ammonites.

Classification: Animalia, Mollusca, Cephalopoda, Ammonoidea, Ancyloceratina, Nostoceratidae

Stratigraphy: Pierre Shale, Upper Cretaceous

Locality: unrecorded/undisclosed site in Custer County, southwestern South Dakota, USA

See info. at: <a href="https://en.wikipedia.org/wiki/Didymoceras" rel="nofollow">en.wikipedia.org/wiki/Didymoceras</a> and