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Ammonitida

An ammonite is a chambered shell of an extinct squid –like animal. The order of Ammonitida belongs to the same class a common day Squids ( Cephalopoda ) and the phylum of Molluscs (Mollusca) . The name itself, is derived from the Egyptian god Ammon, who was often portrayed with ram horns. Most ammonites resemble these curly horns.

Due to the fact that ammonite shells, mainly consisted of aragonite. It is very rare to find complete specimens with intact shells, since aragonite is very difficult to fossilize. In most cases, only the core of the ammonite is fossilized. sometimes, these poorly fossilized outer shells are covered with a pearly coating, hence giving the fossil an amazing color palette. Smaller ammonites however, are more likely to be found with a pyrite coating .

Ammonites were swimming organisms who used the slow movements of their tentacles  to swim and move around. Although they also had the ability to move away very fast, by ejecting water under great pressure through their channels,  in a similar way octopuses do nowadays.

Ammonites, were also able to move themselves vertically. the vertical movements were made by pumping Nitrogen gas in the old shell rooms (septa) . These rooms were all  interconnected by a tube ( siphon ), that allowed the nitrogen gas to be transported from the  animals body fluids, to the different rooms. The siphon channel runs along the ventral side ( outside section of the internal shell) The latest, or most recent chamber (septa) , houses the body of the animal , and is referred to as the living area.

Example of an ammonite (Pleuroceras)

The size of the ammonites ranged from less than an inch, to more than 2.5 meters in diameter . There were many types and varieties of ammonites . Most species were spirally wound , but there were also unfolded forms (called 'Heteromorphes '). Ammonites with preservation of the soft tissue have not been found so far, making reconstructions speculative to some extent.

 


On this ammonite from Toarciaan of the Rhone -Alpes , France , the complex suture lines are visible

 

The curly and sometimes complex partitions of the rooms are very typical for most ammonites. In most fossils these lines occur as a kind of tree sheped lines, called " suture lines " , These lines make up a very important identification feature. The complexity of the suture lines is also a convenient way to distinguish some large groups of cephalopden from each other ( see figure ). Ammonites lived from the upper Silurian, and like the non- avian dinosaurs, became extinct at the end of the Cretaceous.

 


An Impression of a heteromorphic ammonite ( © Lerouge F )

Ammonites  have evolved rapidly in the course of history . Together with the fact that they fossilize well , it makes them a great guide fossil. The stratigraphy of the Mesozoic is largely based on the occurrence of certain types of ammonites . Especially during the Jurassic and Cretaceous, ammonites were very numerous.

 

Suture lines in nautiloids , goniatides , ceratides and ammonites

 

 

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