I received these pictures of an ammonite from South Dakota with an inquiry about the hole pattern near the opening. I figured I'd throw it up on here and see what came back.
I can't give any specific locality other than east of Black Hills, north of The Badlands. Upper Cretaceous (maybe Taylor group) was the best I could find on formation info.
The ammonite is about 9 inches across and was found many years ago on private land.
The files are big, so I'll have to post them one at a time. I had