A major reshuffling of the cards
Ichthyosaurs therefore did not become extinct at the end of the Jurassic period, unlike many other marine reptiles and they maintained a diversity that was as wide as during their major evolutionary radiations. Current understanding of the group makes it possible to establish that in all likelihood they became extinct “suddenly” (over a relatively “short” geological period, in the order of a few million years) during the Cenomanian age (95 million years ago, in the middle of the Cretaceous period). What was the reason for this?
Certainly, life on our planet has had to endure many mass extinctions, some of which were violent. There was one at the latter end of the Jurassic, and another, more well-known extinction which occurred 65 million years ago at the end of the Cretaceous. After a meteorite crashed into the Earth, many species (non-avian dinosaurs, groups of tortoises and crocodiles, pterosaurs…) disappeared quite suddenly, paving the way for small mammals to propagate, evolve and thrive up to the present day. Ichthyosaurs, which were not affected by the Jurassic extinction, disappeared some 30 million years earlier, at a time when, on the face of it, very little was happening from a geological and paleontological point of view.
It was easier to find convincing hypotheses when it was believed that ichthyosaurs were in decline during the Cretaceous. A harmless factor for a thriving species can be fateful for a species that is already becoming extinct. “These theories only invoked biological factors. First, there was the idea of competition with other faster animals that reproduced more quickly, like fish, or a reduction in the diversity of cephalopods, which was the principal food of the ichthyosaurs. However, it seems that the species of ichthyosaurs that lived during the Cretaceous had varied diets and they remained healthy for millions of years after the appearance of fishes and other groups of sea reptiles. These theories were no longer satisfactory for a group that was revealed to be so diverse”.
Another explanation needed to be found, and this was the subject of the last chapter of Valentin Fischer’s thesis (3). With the help of researchers who are specialized in this period, he painstakingly went through the literature dealing with what happened 95 million years ago and this proved to be a revelation. “We noticed that most of the marine groups were affected by something at this precise time. There were extinctions and explosions of diversity across the food chain and in a relatively short space of time”.
There was a great reorganization of ecosystems, which correlated with global warming that affected the temperature, salinity and movement of the oceans. “Studies have made it possible to estimate that ocean levels rose to 200 meters above the current level, and the surface water was sometimes 15 to 25 degrees hotter. There was no ice anywhere at the poles. Clearly, this change favored or disadvantaged the groups that lived in the oceans at that time. It is in this context that the extinction of the ichthyosaurs must be placed”. Yet this is only the first part of the solution to the puzzle. At the current time, it is impossible to determine what finally brought about the extinction of the ichthyosaurs. Was it solely because of the temperature, the extinction of another species, or some other cause? “Such a shake-up involves so many variations of simultaneous factors that it is difficult to know precisely what subset of factors worked against the ichthyosaurs”.
The fact remains that the ichthyosaurs disappeared at this time, leaving no descendants, after one hundred and fifty million years of existence, closing a chapter in the history of the Earth. It was a chapter in the true sense of the word as even their appearance remains a mystery. The first fossils studied, dating from the beginning of the Triassic, show that they had already become aquatic animals, bearing no resemblance to their land ancestors. No one knows where to place the animal within the genealogy of reptiles.
Outside evolution and outside time… Why did Malawania evolve so little in relation to its contemporaries? What were its real faculties, its habits, and its environment? What happened during the 70 million years that separate it from its best-known cousin? More broadly, what caused the sudden disappearance of an animal that moved seamlessly through a time period of several tens of millions of years? Analysis of this specimen opens up a new branch in the study of ichthyosaurs. In particular, it raises new questions and provides specialists with the key to new avenues of research on the animal.
(3) Fischer V. (2013) Origin, biodiversity, and extinction of Cretaceous ichthyosaurs, Doctoral thesis, University of Liege.
