Ammonites

Ammonoids are extinct, typically coiled-shelled cephalopods composing the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which compose the clade Coleoidea) than they are to nautiluses (family Nautilidae), which they resemble. The earliest ammonoids appeared during the Emsian stage of the Early Devonian, around 410-408 million years ago, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event approximately 66 million years ago. They are often called ammonites, which is most frequently used for members of the

Ammolite is an organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is commonly unearthed by natural erosion or through the process of various mining practices, within the perimeter of an ancient sea bed called the Western Interior Seaway. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones; others include amber and pearl.

thumb|upright 2.0|Some examples of aptychi (top right: Oppelia from Late Jurassic of Solnhofen, Germany; bottom left: aptychi (recto and versus) from Late Jurassic of Lombardy, Italy), and conceptual scheme of their function if indeed they were used to close the shell aperture, as opposed to being jaws. thumb|One of what would have been a pair of aptychi (at first given the name Trigonellites|Trigonellites latus and described as a bivalve) from the [[Kimmeridge Clay Formation in England]]