It's pretty much an undisputed fact that nautilus are awesome, and the same goes for their siblings, the extinct ammonites. Both are cephalopods with external, chambered shells, but they fall into different subclasses: Nautiloidea and Ammonoidea. How can you tell the difference? As this awesome, easy-to-read page shows, nautilus hit the scene first in the Late Cambrian, and in the Devonian, ammonites evolved as an offshoot of that branch. (The first nautilus had straight shells!)
