The reasons of the evolution of plesiosaurs revealed

Illustration of Georgiasaurus, an extcint genus of plesiosaur from the Late Creataceous. ©Andrey Atuchin

An international team of researchers, led by Valentin Fischer (GEOLOGY Research Unit - Faculty of Sciences), recently studied the case of polycotylids, plesiosaurs who lived during the Cretaceous period, in order to understand the reasons for their rapid evolution, which does not seem, as the scientific community thought, to be linked to the extinction of their rivals.

P

lesiosaurs are large aquatic vertebrates that belong to the reptile class. Recognizable by their often elongated necks - which can sometimes reach several metres - these large vertebrates were the most diverse marine reptiles of all time and disappeared at the end of Cretaceous, along with most dinosaurs. Polycotylids are a family of plesiosaurs that entered the fossil record during the Early Cretaceous and quickly became diverse and abundant, without anyone clearly understanding why.

An international team, led by researchers from the EDDy Lab at the University of Liège and the Earth Science Department of the University of Oxford, looked at the evolution of polycotylids and in particular Thililua longicollis - a species discovered years ago in Morocco - in order to understand the rapid development and evolution of this group. The results of this research are published in the Royal Society's journal Open Science (1). « For this study, we chose Thililua longicollis, a very peculiar species of this family of plesiosaurs which holds the keys to understand their diversification, explains Valentin Fischer, researcher at the GEOLOGY Research Unit of the Faculty of Sciences at ULiège. We have studied the anatomical specificities of this group and quantified their evolution over time to understand why they may have evolved so rapidly, while many other marine reptiles disappeared around this time period ». Until then, scientists believed that the rise of polycotylids was due to the extinction of ichthyosaurs and pliosaurs, which were seen as direct competitors of polycotylids. But after numerous analyses, the team concluded that the polycotylids had diversified before their rivals disappeared, thus refuting the ‘competition’ hypothesis. The rapid evolution of polycotylids actually coincides with profound changes in marine ecosystems and climates." This clearly indicates that this group of marine animals reacted very differently to these events than their counterparts," explains the researcher.

Details of the analysis of Thililua longicollis skull @ULiège

These results indicate that sudden climate and environmental changes in the past can have a complex effect on the biodiversity of large predators: for some groups, these changes are beneficial while for others, they can result in total extinction. Valentin Fischer's team is currently working on other groups to determine the evolution of marine ecosystems through the major crises of the past.

Scientific reference

(1) Fischer V, Benson RBJ, Druckenmiller PS, Ketchum HF & Bardet N. 2018. The evolutionary history of polycotylid plesiosaurians. Royal Society Open Science 5 :172177.

Contact

GEOLOGY Research Unit I Evolution & Diversity Dynamics Lab

Valentin FISCHER - v.fischer@uliege.be  - +32 4366 5279

Valentin Fischer is a lecturer at the Faculty of Sciences and is, since 2015, the director of the EDDy Lab (Evolution & Diversity Dynamics Lab)