Amazing Ammonites Ammonites, with their curled up shells that remind us of snails, are a fascinating extinct subclass of cephalopod invertebrates. A very successful group, ammonites patrolled the oceans for nearly 350 million years and are currently known from the Devonian (about 400 million years ago) until the devastating Cretaceous-Paleogene extinction (about 66 million years ago). Though the fleshy parts of the animals were not preserved, the shells of these cephalopods are both scientifically valuable and artistically beautiful.
For the scientific and geologic communities, ammonite fossils are numerous and widespread enough to be used as index fossils. Index or indicator fossils are globally prevalent fossils that are utilized in a form of relative dating to help define geologic time periods. Essentially, sedimentary rock deposited and formed in different places can be temporally linked if they contain fossils of the same species. These sedimentary rocks may presently look different, be at different altitudes, and have shifted to various angles, but their index fossils allow scientists to place their deposition and formation during the same span of time. This information is used to map how both landscapes and life forms evolved. For this purpose, the best organisms are ones that have easily visible, unique characteristics and evolved quickly so speciation can be used to discern the various geologic periods. The numbers of chambers inside the shells of various ammonite species is one distinguishing feature that helps make them exceptionally useful for this dating process.
These chambers, technically known as phragmocones, are also one of the reasons why ammonite fossils are so captivating. Much in the same way that a geode is formed from a volcanic gas bubble, the phragmocones of ammonites often filled in over time with beautiful minerals and crystals. When we cut these fossils lengthwise millions of years later, the visibly distinct chambers contain minerals such as quartz, agate, opal, jasper, and even pyrite. Also, when tilted under a light, the outer surface of the ammonite shells often express the characteristic multi-colored iridescence of ammolite, a rare biogenic gemstone. Biogenic gemstones are those like pearl and amber that come from an organic source.
Ammonite fossils are commonly found alongside another cephalopod called Orthoceras (scientific name: Orthoceras regulare). First appearing in the fossil record a full 88 million years earlier than ammonites, O. regulare shells are long and slender with a sharply pointed end. Similarly to our ammonite friends, the soft body parts of these animals did not preserve, but their fossilized shells are found worldwide with the largest concentrations in Scandinavia.
Pliny the Elder, famous Roman historian and naturalist, called ammonites ammonis cornua meaning “horns of Ammun” because their shape reminded him of the ram horns that the Egyptian god Ammun wore. It was this description that ultimately gave them their name today. In medieval Europe, ammonite fossils were believed to be coiled snakes, and they were called “snakestones”. There is mythology and lore related to ammonites from nearly every corner of the Earth.
Here at Nature’s Art Village you can find stunning ammonite fossils ranging in size from small ones in the jewelry in our Artistic Jewelry shop to large ones over 1.5 feet in width displayed in our A to Z Minerals shop. I personally love ammonites for their intrinsic value to science and geologic and Earth histories as well as for their stunning mineral variations. They are like the geodes of the fossil world! Come to Nature’s Art Village today to learn more about ammonites, fossils, minerals and precious stones.
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