Substitution (Iron pyrites) fossil of ammonite

Compare these images. All are cephalopods, same family as squids, Ammonites are extinct, nautilus has existed for 500 million years and is till around. Both controlled their buoyancy by filling the shell chambers with water or gas. The chambered shell is known as a phragmacone, in some of the segments of the nautilus you can see siphuncle, which connected each chamber and regulated the buoyancy. The ammonite keel would have floated horizontally, the nautilus floats with a vertical keel.

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