When did they really die out, the ichthyosaurs, these marine reptiles which populated the seas several million years ago? Up until very recently the specialists situated their decline at the end of the Jurassic. Valentine Fischer, a researcher at the University of Liège, has just demonstrated that their decline only took place at the beginning of the Upper Cretaceous, in other words around 50 million years later. But the causes of this extinction still remain very mysterious...
With its long snout (called a ‘rostrum’), its impressive size (up to twenty metres in length), its smooth skin, its dorsal, lateral, and caudal fins, the ichthyosaur resembles a dolphin like two peas in a pod. But don’t trust appearances: this reptile has no family ties with the mammal and is not its ancestor. Not even a distant cousin.
If the two species have so many points in common, it is because they shared the same marine environment. Living in water obviously requires the development of similar characteristics. But in reality several tens of millions of years separate the dolphin and the ichthyosaurs.
The latter made its first appearance on earth one day during the Mesozoic era, in other words around 250 million years before our times. They preceded the dinosaurs by a ‘little’ (that is, by...20 million years). No one really knows why these reptiles ended up leaving terra firma to live in the marine environment. They had a presence in the four corners of the planet, from Australia to the Middle East, as well as Europe and America, and adapted to every kind of climatic zone, even the most glacial. ‘Their size could range from less than 1m in length to giants measuring over 20m. They all gave birth to their offspring directly in the sea, and certain of them were fast swimmers which could dive to great depths, and had enormous eyeballs and a so-called ‘warm blooded’ physiology,’ points out Valentin Fischer, a FNRS Research Fellow at the ULg’s Geology Department and a palaeontology researcher at the Belgian Royal Institute of Natural Sciences.
They lived for a long time, for over one hundred and fifty million years. Nevertheless, researchers generally agreed in thinking that these strange beasts had, for the great majority, died out at the end of the Jurassic, up until the boundary with the Cretaceous, 145 million years ago. For what reasons? The most widely accepted hypotheses offer two elements by way of a response: on the one hand global climate change, responsible for disrupting the ecosystems, and on the other hand competition with the so-called ‘modern’ fishes, which appeared during the Middle Jurassic. The latter, being capable of reproducing very rapidly, ended up supplanting their rivals.
A version of events that has now been called into question by Valentin Fischer’s recent discoveries. The doctoral student has just published an article in the journal PLoS ONE which demonstrates that these marine reptiles remained diversified and profuse for a good deal longer than had been predicted.