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The false extinction of ichthyosaurs
2/2/12

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.

Ichtyosaure rechercheOn 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!’

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