The mystery of the extinction of the ichthyosaurs

An extremely dense study (1) published in Nature Communications and of which Valentin Fischer, a lecturer at the University of Liege, is the first author, removes some of the mystery surrounding the extinction of the ichthyosaurs at the end of the Cretaceous. The interest of this study, however, lies in the methods used to reach its conclusions: rather than considering averages based on variable factors over long periods (several million years), the researchers were able to avail of access to data on this occasion (sea levels and global temperatures), which enabled them to make a more in-depth analysis by taking account of the dynamics of environmental phenomena during the Cretaceous. They were able to deduce that the ichthyosaurs disappeared quickly (though not suddenly), in two phases that coincided with overwhelming changes in the marine environment. These changes were due, in particular, to the rapid separation of the continents 95 million years ago. Sea levels were up to 150 metres higher than today and all the ice had disappeared from our planet!

Valentin Fischer, a lecturer in the geology department of the University of Liege knows a lot about ichthyosaurs. He devoted his doctoral thesis to them (see: the false extinction of the ichthyosaurs and the timeless swimmer), by showing that they had not, in fact, massively disappeared at the end of the Jurassic (145 million years ago), but much later, at the end of the Cretaceous (95 million years ago).

“We knew a lot about the ichthyosaurs at the beginning of their reign, particularly in terms of their diversity, (they appeared in the Triassic-Jurassic – from -250 to -150 million years)”, recalls Valentin Fischer, and not in the Cretaceous. Palaeontologists had formulated three theories about the disappearance of the ichthyosaurs. The first of these was that they had experienced a slow decline, for no particular reason. The second was based on competition between organisms. The arrival of new, fast-moving fishes towards the middle of the Cretaceous and new marine reptiles such as the mosasaur, led ichthyosaurs to their demise. The third theory postulated that their food disappeared. Experts agreed on the fact that the ichthyosaurs only ate one type of food, small belemnite cephalopods and when these massively disappeared, the ichthyosaurs followed suit. “These three theories actually have one point in common”, explains Valentin Fischer: “they were based on the supposition that the ichthyosaurs were not diversified in the Cretaceous, that there were few species, and as a result of this, they were unable to respond to minor changes in competition and food”. In his doctoral thesis, Valentin Fischer examined a quantity of fossils from the Cretaceous to establish the diversity of the species at that time and to compare this with the Jurassic. What were the results of his study?

In contrast with what had previously been thought, many different species in terms of their size, the shape of their teeth and their skulls co-habited at the time of the Cretaceous. In the light of this observation, it was natural to deduce that the extinction of the ichthyosaurs happened quite abruptly, because there were many different species emanating from an evolutionary radiation that happened deeper in time and which produced a variety of forms and occupied different ecological niches. “My thesis tended to dwell on this observation”, explains Valentin Fischer. “During my post-doctoral work, first at Liege and then at Oxford, I decided to concentrate on what is known as quantitative palaeontology, which involves quantifying the evolution of diversity and extinction. Did the extinction of the ichthyosaurs happen abruptly and did it also affect their ecological niches”?

Three niches

The results of this research published in Nature Communications corroborates and completes the thesis of the researcher from Liege. It appears that there was indeed both a lot of diversity (the number of different species) and a lot of disparity (the number of different morphologies) during the Cretaceous. “In addition to this fact”, explains Valentin Fischer, “they had never been so disparate since the end of the Triassic (210, 220 million years ago). Moreover, this disparity seems to have lasted up to the middle of the Early Cretaceous: we observed a peak in the level of disparity around 120 to 130 million years ago. This tied in with the fact that there also seemed to be a lot of different species”.

The researchers also analysed the ecological niches of the ichthyosaurs of the Cretaceous by measuring skulls and teeth, and, in certain cases, by studying their stomach contents.  Three very distinct groups emerged from this analysis. The first was defined by a set of species showing large worn teeth that were sometimes broken. And it was only in animals belonging to this group that the stomach contents contained the remains of tortoises, large fishes or birds. Therefore they seemed to be at the top of the food chain. A second group was characterised by a population of ichthyosaurs that had small, very thin teeth, with no bearing on the size of the animal. Their stomachs contained the remains of soft cephalopods, though they almost certainly ate small fish too. These also had the biggest eyes (undoubtedly, the biggest in the animal kingdom at the time, with pupils that were more than 10 cm in diameter!), meaning they were possibly deep sea divers. Finally, a third group showed precise characteristics borrowed of the two other groups. “We therefore found ourselves faced with different species of ichthyosaurs, which, by convergence, occupied three distinct ecological niches”, summarises Valentin Fischer. “These niches lasted almost up to the moment of their extinction. In other words, for most of their reign and up to a short time before their disappearance, they were extremely diversified. This diversification invalidated the three previous theories, which, as we recall, were based on lack of diversity. Taken separately, they did not explain the disappearance of the ichthyosaurs”.

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.

This raises a final question: what happened during this latter phase with the last existing niche, the one which includes the ichthyosaurs with large teeth? “Their final extinction is associated with slower evolution and stong environmental instability, but we do not know which factors were precisely at play ”, says Valentin Fischer. “We return to the food theory. At that time, the ichthyosaurs only occupied one niche. They were more vulnerable. And at the end of the Cenomanian, there were extinctions of cephalopods and that could have had an impact on the ichthyosaurs but it is not an optimal answer as these last ichthyosaurs occupied a niche with many possible sources of prey. More importantly, there were a lot of changes in the entire marine biosphere”.

The end of the Cenomanian saw a large peak in temperatures. There was no longer any ice on Earth; water no longer cooled and ocean circulation slowed down. Almost all the marine groups experienced great upheavals. It was not a massive extinction because some groups disappeared but others adapted to these new conditions and evolutionary radiations occurred. New lineages of bony fish and sharks appeared, especially those that currently populate the present-day oceans. What could be a possible cause of this shake-up? The separation of the continents which had never been happening so quickly. This separation altered ocean currents, global ocean circulation and sea levels. Volcanic eruptions were frequent, which released a series of greenhouse gases into the atmosphere causing ice to melt and increasing sea levels again. All these factors were interconnected. Sulphur compounds were dissolved in the water and directly or indirectly increased its acidity. The sea beds seemed to be deprived of oxygen. “We therefore do not know exactly what happened to the last ichthyosaurs. There is undoubtedly no single cause that alone brought about their extinction”, says Valentin Fischer. “We will probably never know the full truth”.

Unless… In continued collaboration with the Matt Friedman of the University of Oxford, Valentin Fischer is going to abandon the ichthyosaurs to extend his research to other sea groups in order to have a more precise idea as to what happened at other levels of the food chain. In this context, the laboratory in Liege, renamed EDDy lab (Evolution and Diversity Dynamics lab) in tribute to the work of Professor Edouard Poty, who was succeeded by Valentin Fischer, is going to focus on the evolution of fish and reefs made up of bivalves (rudists and hippuritoids), which became dominant during the Cretaceous replacing coral reefs before these recovered and the bivalves disappeared…at the same time as the dinosaurs! Yet another story to be told.

(1) Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility, Valentin Fischer et al. Nature Communications 2016.