These spiral shells are fossils of an extinct species of marine animals called ammonites. Ammonite fossils are fairly common and serve as excellent index fossils. Index fossils are fossils used to reliably identify geologic time periods. The presence of ammonite fossils usually indicates a Jurassic (200 million to 145 million years ago) or Triassic (250 million to 200 million years ago) time period.
Photograph by Brian Piltz, MyShot