This is a rare, Hoploscaphites nicolletii from the Fox Hills Formation of South Dakota.
Specimen is approx 2.5" wide and the hard concretion surrounding it has been meticulously prepared to leave it on a free standing base of rock.
The ammonite displays beautiful oak leaf sutures with distinguished bumpy nodes along the outer edges.
Upper Maastrichtian, Late Cretaceous
These 70 million year old ammonites lived when South Dakota was a shallow inland sea. It was found preserved in a concretion that was split open. It then underwent many tedious hours of fossil preparation to remove the hard matrix surrounding the fossil.