Ammonites are relatives of octopuses, squids, and nautiloids. The first appeared in the Devonian Period around 400 million years ago and didn’t go extinct until the end of the Cretaceous Period 65 million years ago. They lived in open water and could have uncoiled or coiled shells, some of which grew up to 6 feet in diameter.


  • Title: Ammonite
  • Location: Fox Hills Formation, Dewey Co., South Dakota
  • Physical Dimensions: L: 38.5 cm W: 43 cm D: 13.5 cm
  • Type: Fossil
  • Rights: This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws. http://www.si.edu/termsofuse
  • External Link: View this object record in the Smithsonian Institution Collections Search Center
  • Weight: 22.7 kg
  • USNM Catalog Number(s): PAL431079
  • Scientific Name: <i>Sphenodiscus lenticularis</i>
  • Photo Credit: John Steiner, Smithsonian Institution National Museum of Natural History
  • Historic Period: Lived 71–66 million years ago
  • Geologic Age: Mesozoic - Cretaceous - Upper/Late - Maastrichtian
  • Field: Paleobiology

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