The ammonites were a group of cephalopods that lived from the Devonian to the Cretaceous period, 65 million years ago. In the episode "SB-129," one can be seen during the incursion of Squidward to the past. They are now an extinct group of mollusk animals.
In the show, they appear to be pink, dark pink, and dark red. They have shells with a swirl design on them, and appear to have tentacles as well. They are seen floating instead of walking.
The name was influenced by the spiral shape of their shells.