Peroniceras haasi? from the Mancos Shale in Utah. Late Cretaceous (Coniacian) Scaphites preventricosus Zone. The inner whorls are broken and shifted so I dont think any more prep would help. Just something to post until some of this snow melts and I can get out and look for more. :boohoo:

Nice find Kevin, as ever!

That ammonite has a strongly defined keel. I wonder if that feature helped maintain directional stability in the water and acted as streamlining to a limited extent?

Thanks Phil. This genus actually had 3 keels (tricarinate), the shell on this specimen is gone but the internal mold shows remnants. Although there was probably not spines on the nodes they still probably offset any advantage the keels would provide.