The researchers also tried to quantify the speed at which the ichthyosaurs evolved. How quickly did new characteristics appear over the course of time expressed in millions of years? How many new species? “Evolution rates during the Cretaceous are all very low, explains Valentin Fischer. “The rate of morphological evolution had been very low for a long time and the rate at which new species appeared was low only from the end of the Jurassic. Never before had we observed this combination of both slower appearance of novel species and of novel characteristics. The ichthyosaurs seem to have experienced an intense period of evolution at the beginning of their existence in the Triassic and Jurassic, because there were no more rapid evolutions in the Cretaceous. The Cretaceous is mainly the result of what occurred earlier. This is summarised in the first part of the heading of our article: extinction was accompanied by a slower evolution”.

But why did they disappear?

In summary: at this stage of our research, we thus had evidence for a group of animals that were both disparate and diversified – therefore able to overcome minor changes in the environment – and whose lack of evolution seemed to show that they were well adapted to their respective ecological niches. How then, can their disappearance be explained?

“A final part of the research consisted of examining a series of signals such as sea level, global temperatures and variations in the carbon cycle and comparing these with extinction rates. But we did this in a novel way. Studies of this kind generally focus on averages. Therefore, we generally compare the diversity of species and sea levels over parcels of several millions of years. By proceeding in this way, we do not take account of possible minute fluctuations that are likely to occur during such long periods. You may have important variation in sea levels while the curve of average values remains very smooth”, explains Valentin Fischer.

And the analysis paid off for two of the three variables, the temperature and level of the seas. The researchers therefore showed that the level of extinction of the ichthyosaurs was higher during periods where rapid and large fluctuations were observed. Therefore it is not the value for sea level or temperature per se that matters, but the fluctuations in these parameters. The fluctuations actually obscure changes to the climate such as an increase in ice levels or the extent to which ice melts, a slowing of ocean circulation, etc. It is not the increase in sea level in itself that killed the ichthyosaurs, but rather, abrupt climate change (albeit over several thousands of years…) which impacted on their reproduction zone and their food etc.

Two phases of extinction

By fine-tuning their analyses, the researchers also observed that the extinction of the ichthyosaurs seemed to occur over the course of two separate phases.
During the first of these, ecological diversity disappeared. All the fossils dating from the Upper Cretaceous only belonged to one niche: the ichthyosaurs with large teeth, those at the top of the food chain. The two other niches (populations with large eyes and small teeth and the intermediaries) disappeared.

An obvious question to ask is whether this is not translated by a bias in the fossil record. In other words, palaeontologists have perhaps quite simply not found the “good” fossils. To get around this, the researchers looked at other fossil records, those of other marine reptiles like plesiosaurs, marine turtles etc. The result of their research showed that the number of geological formations in which fossils of this type were found increased during the Cenomanian (period of the Cretaceous during which the extinction of the ichthyosaurs occurred, that is to say, from 100.5 to 93.9 million years ago). Only the number of ichthyosaur fossils diminished. The quality of the fossil record is therefore good but it was the proportion of ichthyosaur fossils that dropped dramatically, two times. Therefore there were indeed two phases of extinction extended over a period of 5-6 million years. It is a short and rather abrupt but not catastrophic extinction.