Ammonites are an extinct order of carnivorous mollusks that were closely related to the squid, octopus, cuttlefish, and the chambered nautilus. Ammonites mysteriously became extinct at the same time as the dinosaurs some 65 million years ago. The term "ammonite" means "Ammon's Stone." The name was given to these coiled fossils by the ancient Egyptians who noted their resemblance to the horns of their ram headed god, Ammon. Ammonites flourished in oceans for approximately 330 million years.
Ammonites are easily recognized by their beautiful geometrically coiled shells. These shell were secreted by the animal and continued to grow and provide protection for the creature throughout its life. The animal itself probably very closely resembled a modern day squid living in the end of the shell. An ammonite could control its buoyancy much like a submarine, filling chambers in the shell with either gas or fluid. Ammonites became so prolific that they filled nearly every niche in the ocean.
Ammonites - South Dakota