Fossil Ammonites (Hoploscaphites & Sphenodiscus) - South Dakota

This is a 4.2" wide rock nodule that contains a JHoploscaphites (Jeletzkytes) spedeni ammonite and a small Sphenodiscus lenticularis ammonite, collected from the Fox Hills Formation of South Dakota. The largest ammonite is Jeletzkytes spedeni and it measures 3" wide. A wide variety of complete and fragmented shells can also be found throughout the rock.

There is a repaired crack through the largest ammonite and through what remains of the surrounding rock.

These 70 million year old ammonites lived when South Dakota was a shallow inland sea. They were found preserved in concretions when split open. They then had to be hand-prepared to remove the hard rock surrounding them from their shells, a very time consuming task.

About Ammonites

Ammonites were ancient marine cephalopods, similar to today's squids and octopuses, but with a defining feature: their distinctive, tightly coiled spiral shells. These shells, resembling those of modern nautiluses, served as both a protective home and a buoyancy aid, allowing ammonites to navigate the prehistoric seas with ease. First emerging around 240 million years ago in the Triassic Period, ammonites thrived for over 175 million years, adapting through numerous forms and sizes. As predatory creatures, they likely fed on smaller marine organisms, using their tentacles to capture prey. However, their long reign came to an end 65 million years ago at the close of the Cretaceous, coinciding with the mass extinction event that also eliminated the dinosaurs.

What an ammonite would have looked like while alive.

More about ammonites