These are probably the most well preserved ammonites I find around here, at least the ones that have the most original shell preserved. Scaphites and Baculites are probably the most common ammonites in Cretaceous rocks in the Western Interior of the US and are used to correlate the Upper Cretaceous rocks. Luckily other forms are also found that also occur in Europe etc. and help correlate these rocks globally.
There is a "lineage" of Pteroscaphites in Late Turonian thru Middle Santonian rocks around here. However, the different species of Pteroscaphites have more in common with the associated species of Scaphites than they do with each other so they are probably dwarfs. And it is not just the Scaphites, dwarfs of other genera are being found and described.
For more information and a better explanation see; Landman, N. H. 1989. Iterative progenesis in Upper Cretaceous ammonites. Paleobiology 15(1), pp. 95-117.