ammonite

This term refers specifically to the extinct cephalopods of the subclass Ammonoidea. In a scientific or paleontological context, it is used to describe the biological organism, but in common usage, it almost always refers to the fossilized remains of the spiral shell.

Because these fossils are frequently used as index fossils for dating geological strata, the word often appears in discussions regarding the Mesozoic era. It is distinct from the modern nautilus, despite the similar shell shape, due to the internal structure of the septa and sutures.