The ichthyosaur of La Robine

A marine reptile in the time of the dinosaurs

A link in the history of the Alps….

At the beginning of Secondary school, in the Lower Jurassic, the phenomenon of extension of Pangea had already produced tears, letting the sea penetrate far into the land.

In the Toarcien (-183 to -174 million years ago), a large part of Western Europe was thus flooded. An arm of the sea, prefiguring the North Sea, connects the waters of the two great oceans, the Panthalassa and the Téthys. From now on, the emerged lands of Pangea are no longer all contiguous: the face of the old single continent has changed a lot ...

Despite all these upheavals, Western Europe still belongs to Pangea. Above its continental crust, the sedimentary and marine history of the future southern Alps continues. Shallow seas, called epicontinentals (so rare at the end of the primary era), are veritable reservoirs of biodiversity.

The secondary era is also “the era of reptiles” with the famous herbivorous or carnivorous dinosaurs which abound on the land. But from the beginning of the Triassic (-250 million years ago), certain reptiles were brought to explore the aquatic environment to seek food due to strong competition on dry land. This is the case with ichthyosaurs and plesiosaurs which adapted to shallow marine conditions and subsequently rapidly colonized the vast, deeper ocean spaces.

The remarkable ichthyosaur of La Robine lived, in the Lower Toarcian (around -183 million years ago), in the sea basin of the south-east of the future France. At the site, the seabed was not flat. Very chaotic, it presented escarpments of faults, high points and collapsed areas. Tectonics, which remained very active at this time, participated in the famous extension of Pangea. But the Alpine Ocean is not yet open!

… In connection with 22 other geosites

Vertebrate fossils are rare, and even more so, whole specimens. The ichthyosaur skeleton owes its preservation to the very particular marine environment of the time. If the upper section of water was very favorable to an abundant life, the bottom was unlivable because oxygen was lacking. Unlivable… except for a few so-called anaerobic bacteria whose action precisely allows fossilization.

In such a confined environment, these bacteria degrade organic matter and develop veils on corpses sunk on the bottom. Bacterial activity within the sediments leads to their early induration, much faster than during the usual process of transforming a loose sediment into compact rock (diagenesis). The same goes for organic remains transformed more quickly into fossils (fossilization) and better protected from dislocation and underwater erosion.

The origin of the exceptional conservation of the Sirenians of Castellane is quite different. If the absence of oxygen is a favorable factor common to both sites for the preservation of organic remains and fossilization, the skeletons and corpses of sirenians have undergone very rapid burial. Indeed, during strong storms, the swell agitated and reshaped the seabed then pushed back towards the coast large quantities of sand, limestone mud and organic remains including carcasses of sirenians. Thrown against the rocks and retained in sediment traps (hollows and other crevices), they remained trapped there. This very particular context has allowed the conservation and exceptional fossilization of many sirenian skeletons, most of the time dislocated.

What we can decipher from the current landscape

The fossilized skeleton of the ichthyosaur of La Robine rests at the foot of an intensely gullied marly unit of gray / black color. This type of landscape bears the local name of robines.

Their black color is due to the presence of very many microscopic grains of pyrite, iron sulfide produced by the activity of bacteria (sulfo-reducing), developed in a medium with little or no oxygen.

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A marl is a sedimentary rock composed of a mixture of limestone and clay (for a proportion of 35 to 65%).

How to find an ichthyosaur?

To do this, look a little higher up for a dry rock. Sit down and have a picnic!

This is what Professor Marcel Lemoine did in the early 1960s when his attention was attracted by a strange rock ...