Prionocyclus hyatti (robust form) from the Mancos Shale of New Mexico

These large, bumpy ammonites are the robust form of Prionocyclus hyatti from the Mancos Shale (Turonian stage of the Cretaceous) of Sandoval County, New Mexico. None of the sutures are visible but the form of the shell is what helps to ID it. The robust form of the shell is wider than the normal form and has distinct, knobby ribs. There is also a noticeable raised keel. The cross section of the chambers is roughly square.

Specimen #1 still has part of the nodule, that once surrounded it, still attached. Ammonites in the Mancos shale are often found in nodules. Sometimes they are septarianized as well.

Right side

Left side

Keel

Closer view of the right side

Specimen #2 is larger than the previous specimen and does show some septarian features.
Right side

Left side

View of the cross section of the last chamber

Keel

Part of the fossil shows septarian features like the "seam" seen below.

The fossil had broken while in transit which allows a view of the calcite filling the interior.

Thanks to Mike (PFooley on the Fossil Forum) for this fossil that he offered up as part of an auction to help defray some of the operating costs of the Fossil Forum website.