Ammonite Fossils

Ammonites are extinct marine animals. They first evolved in the Paleozoic Era during the late Silurian period or perhaps early Devonian period, about 400 million years ago. Over the succeeding ages, they continued to evolve and diversify, surviving the entire Mesozoic Era (the so called "Age of Reptiles"). The last Ammonites died out at the end of the Mesozoic Era, during the Cretaceous-Tertiary mass extinction, which occurred about 65 million years ago.

Ammonites were molluscs, and although they seem to have outwardly resembled the modern Nautilus, they are actually believed to be more closely related to cuttlefish, octopi, and squid. The fossils of Ammonites that are found, are very often, but not always, just their shells, since the soft body parts rarely fossilized. The best known Ammonite fossils are ridged tubes in a flat spiral, although there were also some types of Ammonites with other spiral shapes, with only partial coiling, or even completely uncoiled shells. The typical spiral fossil is about 9 inches (23 centimeters) or less in diameter, but there some Ammonite fossils have been found which are several feet in diameter, such as Parapuzosia seppenradensis which is found in Cretaceous rocks from Germany.

Since Ammonite fossils are found in many different rocks around the world, and since different species of Ammonites evolved and then went extinct in rapid succession, scientists often use them as "index fossils". In other words, by finding an Ammonite fossil and identifying its species, it is often possible to date the rock in which it is found.

By: Sunil Tanna

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