Pro tip: Did you know that these guys drop chitin? If you play on mobile, tame an Ichthyosaurus, get it saddled, find a Dunk and Attack it. (Atleast on mobile) The Dunkleosteos will not attack your ichthyosaurus back(Same thing goes for other marine creatures such as Megalodon and Mosasaurus). However don't attack it using a weapon like a Crossbow. It'll agro on you instead.