I found another type of ammonite, in the north Texas creek bed I was exploring, called Eopachydiscus sp. I found a large number of them, in fact, but as many were sizable and heavy I only packed out one. I saw some that were well over 18" in diameter, approaching 24"! The specimen I chose to keep is about 8-9" in diameter, 3" thick and had some of it's sutures showing as well as oyster spats glommed onto it. What I don't know is if the oysters colonized the actual shell surface or if the original shell had already fossilized, dissolved away and left a steinkern as a base for them.
This specimen comes from the Duck Creek formation which is Cretaceous (Albian stage) in age.