The false extinction of ichthyosaurs

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.

A furnished genealogy

During work on his undergraduate degree final year dissertation he discovered a complete rostrum in the south of France dating from the Valanginian stage (Lower Cretaceous), in other words a few million years after the supposed extinction during the Upper Jurassic. In sifting through the existing literature, he discovered the trace of several species that had also survived across this famous boundary. He thus decided to examine the specimens of the Cretaceous era from the whole of Western Europe in order to determine how many species were still present during this period.

‘Up until now it had been thought that the ichthyosaurs of the Cretaceous constituted a restricted group. Less specialised forms which struggled along up until their definitive disappearance, during the Cenomanian (Upper Cretaceous),’ recounts Valentin Fischer. His work would reveal very different results.

With the help of English, Scottish, Russian and German researchers, he exhumed certain fossils buried in museum collections for a good number of years. New species of ichthyosaurs were discovered during the research. Notably one, found in Germany and England and baptised Acamptonectes densus, which means ‘rigid and compact swimmer.’ A name that indicates that the skull and vertebral column of this ichthyosaurs were firmly locked together, which allowed it to swim almost solely thanks to its caudal fin and its tail.

On the basis of these (re)discoveries he got down to the task of carrying out phylogenetic analyses, kinds of genealogical tree retracing the family relationships between the different species. ‘Phylogenetics allows the diversity of a group to be estimated and the relationships between its species to be decoded, even when not many fossils exist,’ explains the Liège researcher. ‘By using the principle of ‘parsimony,’ which involves looking for the tree with the least possible changes, we obtain an evolution hypothesis for the group being considered. This hypothesis is the statistically most probable evolution, even if we do not know exactly if that is what really happened.’ Once the different ‘families’, genera, and species had been established, he finally settled to the task of determining the survival rates (those which survived beyond the boundary) and extinction rates.

Conclusions: the survival rates turn out to be far greater than anticipated. Many lineages originating in the Jurassic subsequently continued into the Cretaceous. Thus, the ‘tree’ reveals that, contrary to what had been imagined, the ‘survivors’ were very numerous. ‘We weren’t expecting that at all. All of a sudden we moved from a small limited group to a multitude of very distant cousins. During the Cretaceous the diversity was even greater than during the Jurassic!’

The mysterious causes of a disappearance

These are results that really change how the situation is looked at. The major extinction of ichthyosaurs did not take place during the Jurassic. On the contrary! During this period, even if other marine animals did indeed disappear, our reptiles were in great shape.

Five to six different genera in reality continued to cohabit, before definitively disappearing during the end of the Cretaceous, around 94 million years ago. A pretty brutal end to existence. As a result, this ‘second extinction’ only becomes all the more severe...and still as mysterious. The appearance of ‘modern’ fishes, even if it certainly had an effect, cannot in itself be sufficient to explain their decline. No more than the decline of the belemnites (types of small cuttlefish on which the ichthyosaurs fed).

The explanation may actually be a broader one. Valentine Fischer has put his finger on an important event, of which the extinction of ichthyosaurs was in the end just one aspect. ‘During the same era all the marine groups seem to have been affected by a remarkable event. Even the plants underwent some changes.’ An event that is difficult to determine for the moment. Maybe the advent of humid tropical forests? Or one of the numerous climatic and oceanographic changes which took place during this period, which was without a doubt the warmest throughout the whole of geological history? Or perhaps the mechanisms involved in collisions between continents? No-one knows yet. ‘There are so many hypotheses...It is impossible to give the exact cause.’

The next step: identifying this mysterious event...