I miss the definition of ammonite. In my opinion, many authors confuse the term "Ammonite" and "Ammonoid". Ammonoids (Ammonoidea) are large group of cephalopods including for example goniatites (Goniatitida), ceratites (Ceratitida) and ammonites (Ammonitida?).
EDIT: If you think that ammonites and ammonoids are the same, you are not right. Notice that researchers try to avoid using the term ammonite when talking about paleozoic ammonoids. Ammonites are usually refered to jurassic and cretaceous ammonoids. "While goniatites proliferated during the Carboniferous, ammonites thrived in Mesozoic times." https://www.researchgate.net/publication/262048294_Goniatites
But how can we strictly define "true ammonite"? Is it because of the ammonitic suture? This way, many triassic, jurassic, creataceous and possibly paleogenic (danian) ammonoids could be called true ammonites, because they have complex (ammonitic) sutures.
Can we call all jurassic and cretaceous ammonoids (phylloceratina, psiloceratina, lytoceratina, ammonitina, haploceratina, perisphinctina and ancyloceratina) ammonites? Can you show me where ammonites are on phylogenetic tree?