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).

last ichthyosaurs

“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”.

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

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