Nautilus is a non-extinct mollusk based on the Chambered Nautilus that's found in Dinosaur Island encountered after the Acanthostega and before the Helicoprion when you get to deep water, along with the Trilobite. It is a replaced the Ammonite (an extinct relative of the Nautilus). It takes three slicing powers to kill and it drops 100 coins. Though it's listed as a fish, the Nautilus isn't a fish, since it's an invertebrate.