Well known to collectors, ammonites are very common fossils in France. If most are only a few centimeters, the largest specimen found is almost 2 meters in diameter!
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Emblematic cephalopods of the seas of Jurassic and Cretaceous, ammonites are fossils that are found quite frequently in limestone series in France. These marine animals are among those that disappeared at the same time as the dinosaurs, when of the Cretaceous-Tertiary biological crisis, 65 million years ago. Their great morphological diversity as well as their very wide geographical distribution make them chronological markers par excellence. The characteristics of their shells (ornamentation, size, winding, etc.) make it possible to distinguish a very large number ofspeciesof which the duration of existence may have been relatively short. Thus, ammonites often make it possible to finely date sedimentary levels.
Very variable sizes: from tiny to monster
Ammonites are characterized by a univalve shell, coiled in different ways depending on the species. Their size is also very variable, usually ranging from a few centimeters to several decimeters in diameter. A specimen, however, acts as a giant. The largest ammonite in the world measures approximately 1.8 meters in diameter. The fossil is however not entirely complete and scientists assume that this specimen would have originally reached 2.5 to 3.5 meters in diameter! If the shell alone is estimated at 705 kgthe weight of the living animal, thus including soft parts not preserved by the process of fossilization, would have reached nearly 1.5 tons. This monster is exhibited at the museum of paleontology from Muenster, Germany. It is dated to the Upper Cretaceous.
While this is the largest ammonite fossil found intact, some fragments suggest that even larger specimens roamed the seas of the past.
A heavy but resistant shell against predators
It is hard to imagine how such watches could move without sinking. With arms, like cuttlefish and octopuses which are their closest cousins, the ammonites had no fins and certainly moved by propulsion, by expelling water through a kind of funnel called “hyponome”. Models taking into account the hydrodynamics of the shell suggest that the ammonites must not have been swimming very fast. This slowness preventing them from escaping faster predators was certainly compensated by the resistance of their shell, especially in very large specimens. Their movement was however controlled by the different lodges of their shell, which these animals used as ballast to adapt their depth.
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