Ammonites are an extinct type of cephalopod that lived in Earth’s oceans for over 300 million years.

While they were alive, the ammonoid body dangled out of the large opening of the shell. Ammonites were free swimmers and floaters. They used cavities in the shell for buoyancy. When they died, their shells sank to the sea floor where they preserved easily in the sediment.

Their shells typically form into planispirals, which led to their nickname “snakestones” since they were once thought to be the remains of coiled snakes. This specific specimen is from the genus perisphinctes and lived about 200 million years ago.

Ammonites became extinct in the K-Pg mass extinction event 66 million years ago that ended most life on Earth.