What exactly is ammonite?
There is no strict definition of ammonites

Ammonites are not strictly defined. One can reffer to ammonites as to the whole group of ammonoids (Ammonoidea, ammonites sensu latto). This is common, but not supported by most researchers. Researchers use the term "ammonoid" for paleozoic ammonoidea and "ammonite" for Jurassic and Cretaceous Ammonoidea (ammonites sensu stricto). Ammonites sensu stricto reffer to ammonoids with complex (ammonitic) sutures, which are typical for Jurassic and Cretaceous ages. However, complex ammonitic sutures are not exclusive for Jurassic and Cretaceous ammonoids. Ceratitic sutures are not exclusive for Triassic either and occur in Jurassic ammonoids, too.


  So caution must be exercised when using suture lines as a means of classification. To obviate some of the problems posed by this
  simplistic classification, Wedekind, in 1918, introduced the terms
  palaeo-, meso- and neoammo-noids to classify ammonoids better.


(PDF) Goniatites. Available from: https://www.researchgate.net/publication/262048294_Goniatites [accessed May 10 2019].