Ammonites and Goniatites are both Ammonoids and are similar see the link for mor information Ammonite Goniatids, informally goniatites, are ammonoid cephalopods from the Middle Devonian some 390 million years ago (around Eifelian stage). Goniatites (goniatitids) survived the Late Devonian extinction to flourish during the Carboniferous and Permian only to become extinct at the end of the Permian some 139 million years later.
One of the way to tell them apart are the suture lines which are visible on the edge of the fossil. The goniatite suture lines are a lot less complex “wavy and sort of sharp than the ammonite’s complex suture lines that often appear to look like plant-shaped patterns. They both have a ventral siphuncle, this is specific for all the ammonoids (this includes: Bactrites, Goniatites, Ceratites, Ammonites)
Goniatites are only found in the Palaeozoic and Ammonites only in the Mesozoic