Une nouvelle bête géante – l’un des plus gros animaux de tous les temps – a été découverte dans les Alpes

Les prédécesseurs de la Méditerranée, vieux de 200 millions d’années, ont survécu dans les Alpes suisses. Les ichtyosaures de la taille d’une baleine n’étaient qu’occasionnellement descendus de la haute mer. Crédit : © Jeannette Rüegg / Heinz Furrer, Université de Zurich

La plus grande dent d’ichtyosaure de tous les temps fait partie des découvertes de fossiles, y compris les restes d’une nouvelle bête plus grande qu’un bowling.

Les paléontologues ont découvert des ensembles de fossiles représentant trois nouveaux ichtyosaures qui pourraient avoir été l’un des plus grands animaux à avoir jamais vécu, selon un nouvel article de recherche publié dans une revue à comité de lecture le 27 avril 2022. Journal de paléontologie des vertébrés.

La découverte, fouillée dans les Alpes suisses entre 1976 et 1990, couvre la plus grande dent d’ichtyosaure jamais trouvée. La largeur de la racine de la dent est le double de celle de tout reptile aquatique connu, le plus grand précédent appartenant à un ichtyosaure de 15 mètres (50 pieds) de long.

D’autres restes squelettiques incomplets incluent le plus grand tronc d’Europe, qui est le deuxième ichthyosurus en concurrence avec le fossile du plus grand reptile connu aujourd’hui, 21 mètres (69 pieds) de long. Shastasaurus sikkaniensis Du Canada à la Colombie-Britannique.

Racine de dent d'Ihthyosaurus

Le diamètre de la racine de la dent trouvée est de 60 millimètres (~ 2,4 pouces). Cela en fait la dent d’ichtyosaure la plus épaisse trouvée à ce jour. Crédit : © Rosi Roth / Université de Zurich

Le Dr Heinz Furrer, co-auteur de cette étude, faisait partie des équipes qui ont découvert les fossiles lors de la cartographie géologique de la Formation de Kössen dans les Alpes. Il y a plus de 200 millions d’années, les couches rocheuses recouvraient encore les fonds marins. En repliant les Alpes, cependant, ils avaient atteint une altitude de 2 800 mètres (9 200 pieds) !

Aujourd’hui conservateur à la retraite de l’Institut de paléontologie et musée de l’Université de Zurich, le Dr Furrer s’est dit ravi d’avoir découvert “l’ichtyosaure le plus long du monde ; la dent la plus épaisse jamais trouvée et le plus grand tronc d’Europe !”

Heinz Furrer avec de grandes vertèbres d'ichtyosaurus

Heinz Furrer avec les plus grandes vertèbres vertébrales. Crédit : © Rosi Roth / Université de Zurich

L’auteur principal P. Martin Sandler de l’Université de Bonn espère que “peut-être plus de restes d’animaux marins géants sont cachés sous les glaciers”.

“Plus c’est gros, mieux c’est”, dit-il. “Les grandes tailles corporelles ont des avantages sélectifs évidents. La vie va là où elle peut. Il n’y avait que trois groupes d’animaux pesant plus de 10 à 20 tonnes : les dinosaures à long cou (sauropodes), les baleines et les ichtyosaures géants.[{” attribute=””>Triassic.”

These monstrous, 80-ton reptiles patrolled Panthalassa, the World’s ocean surrounding the supercontinent Pangea during the Late Triassic, about 205 million years ago. They also made forays into the shallow seas of the Tethys on the eastern side of Pangea, as shown by the new finds.

Ichthyosaurs first emerged in the wake of the Permian extinction some 250 million years ago, when some 95 percent of marine species died out. The group reached its greatest diversity in the Middle Triassic and a few species persisted into the Cretaceous. Most were much smaller than S. sikanniensis and the similarly-sized species described in the paper.

Roughly the shape of contemporary whales, ichthyosaurs had elongated bodies and erect tail fins. Fossils are concentrated in North America and Europe, but ichthyosaurs have also been found in South America, Asia, and Australia. Giant species have mostly been unearthed in North America, with scanty finds from the Himalaya and New Caledonia, so the discovery of further behemoths in Switzerland represents an expansion of their known range.

Martin Sander and Michael Hautmann Schesaplana

Martin Sander and Michael Hautmann look over the discovery layers on the southern slope of Schesaplana, on the Graubünden/Vorarlberg border. Credit: © Jelle Heijne/University of Bonn

However, so little is known about these giants that there are mere ghosts. Tantalizing evidence from the UK, consisting of an enormous toothless jaw bone, and from New Zealand suggest that some of them were the size of blue whales. An 1878 paper credibly describes an ichthyosaur vertebrae 45 cm (~18 inches) in diameter from there, but the fossil never made it to London and may have been lost at sea. Sander notes that “it amounts to a major embarrassment for paleontology that we know so little about these giant ichthyosaurs despite the extraordinary size of their fossils. We hope to rise to this challenge and find new and better fossils soon.”

These new specimens probably represent the last of the leviathans. “In Nevada, we see the beginnings of true giants, and in the Alps the end,” says Sander, who also co-authored a paper last year about an early giant ichthyosaur from Nevada’s Fossil Hill. “Only the medium–to–large-sized dolphin – and orca-like forms survived into the Jurassic.”

Large Ichthyosaur Rib

Martin Sander with a rib of the larger skeleton. The estimated length of the animal is 20 meters. Credit: © Laurent Garbay/University of Bonn

While the smaller ichthyosaurs typically had teeth, most of the known gigantic species appear to have been toothless. One hypothesis suggests that rather than grasping their prey, they fed by suction. “The bulk feeders among the giants must have fed on cephalopods. The ones with teeth likely feed on smaller ichthyosaurs and large fish,” Sander suggests.

The tooth described by the paper is only the second instance of a giant ichthyosaur with teeth—the other being the 15-meter-long Himalayasaurus. These species likely occupied similar ecological roles to modern sperm whales and killer whales. Indeed, the teeth are curved inwards like those of their mammalian successors, indicating a grasping mode of feeding conducive to capturing prey such as giant squid.

“It is hard to say if the tooth is from a large ichthyosaur with giant teeth or from a giant ichthyosaur with average-sized teeth,” Sander wryly acknowledges. Because the tooth described in the paper was broken off at the crown, the authors were not able to confidently assign it to a particular taxon. Still, a peculiarity of dental anatomy allowed the researchers to identify it as belonging to an ichthyosaur.

“Ichthyosaurs have a feature in their teeth that is nearly unique among reptiles: the infolding of the dentin in the roots of their teeth,” explains Sander. “The only other group to show this are monitor lizards.”

The two sets of skeletal remains, which consist of a vertebrae and ten rib fragments, and seven asssociated vertebrae, have been assigned to the family Shastasauridae, which contains the giants Shastasaurus, Shonisaurus, and Himalayasaurus. Comparison of the vertebrae from one set suggests that they may have been the same size or slightly smaller than those of S. sikkanniensis. These measurements are slightly skewed by the fact that the fossils have been tectonically deformed—that is, they have literally been squashed by the movements of the tectonic plates whose collision led to their movement from a former sea floor to the top of a mountain.

Known as the Kössen Formation, the rocks from which these fossils derive were once at the bottom of a shallow coastal area—a very wide lagoon or shallow basin.

This adds to the uncertainty surrounding the habits of these animals, whose size indicates their suitability to deeper reaches of the ocean. “We think that the big ichthyosaurs followed schools of fish into the lagoon. The fossils may also derive from strays that died there,” suggests Furrer.

“You have to be kind of a mountain goat to access the relevant beds,” Sander laughs. “They have the vexing property of not occurring below about 8,000 feet, way above the treeline.”

“At 95 million years ago, the northeastern part of Gondwana, the African plate (which the Kössen Formation was part of), started to push against the European plate, ending with the formation of the very complex piles of different rock units (called “nappes”) in the Alpine orogeny at about 30–40 million years ago,” relates Furrer. So it is that these intrepid researchers found themselves picking through the frozen rocks of the Alps and hauling pieces of ancient marine monsters nearly down to sea level once again for entry into the scientific record.

Reference: “Giant Late Triassic ichthyosaurs from the Kössen Formation of the Swiss Alps and their paleobiological implications” by P. Martin Sander, Pablo Romero Pérez de Villar, Heinz Furrer and Tanja Wintrich, 27 April 2022, Journal of Vertebrate Paleontology.
DOI: 10.1080/02724634.2021.2046017

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