Just found this paper on Cephbase Lethaia Vol. 30, 1997 (http://www.cephbase.utmb.edu/refdb/pdf/7785.pdf) (large PDF) describing the beaks and radula of the Mississippian goniatite Girtyoceras, quite interesting.

The last paragraph
The radula remains stable during the ammonoid evolution and demonstrates a more or less distinct similarity with that in recent octopods.

8-)

Hello Kevin,

Quite interesting, but that quote sets a new standard for mixed signals:

...a more or less
"Iffy."

...distinct
"Definitely."

...similarity
"No, still iffy."

Cheers,
Clem

Thanks Kevin. I've added the paper to the 'Reference Papers' thread.