Ammonite

A ammonite in a strict sense is a invertebrate Mollusque Céphalopode Fossile characteristic of the Mesozoic . However, the subclass of the Ammonoïdes , which includes, inter alia, the Goniatite S, the Clyménie S and the cératite S, appeared with the Dévonien (Paléozoïque). The ammonites disappeared at the end from the Mesozoic , there are million years, it is thus for that which one finds them only under the state of fossils.

One knows ammonites only their shell, generally rolled up in plane spiral with jointed turns. Rolling up is known as évolute when the covering from one turn to another is weak. It is known as involute when covering is important, sometimes going until completion to mask the internal turns. There exist also ammonites whose turns are not jointed. They can even present in end rolling up a right part, the pole, often followed return towards the whorl, the stick. Others are not rolled up in plane, but helical spiral. Others only right or are slightly arched. All these ammonites are known as heteromorphous.

The shell of the ammonite is partitioned: it consists of a succession of cabins separated by partitions (called rooms). The animal lived in the last and more big room. It used the others like ballasts to control its buoyancy.

The nautile has a common ancestor with the ammonites, but is not downward. There exists still nowadays. In spite of their resemblance to Nautile, the ammonites are now regarded as closer to Coléoïdes (octopuses, cuttlefish) and, with Mesozoic, the Belemnite S. the first stages of the development of Ammonoïdes and Coléoïdes are very similar. It is what brings to think that they had perhaps comparable lifestyles (type of reproduction, food, stroke, behavior).

See too

Related articles

  • the ammonites were formerly called " horns of Ammon in reference to the god Ammon " ( Encyclopedia of Diderot )

  • Ammolite: formed gem of fossils of ammonites
  • Aptychus: bivalvular device with the opening of the ammonites
  • Flagstone with ammonites

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