What happened to marine reptiles before the mass extinction at the end of the Cretaceous?

What happened to the mosasaurs, the dominant marine reptiles at the end of the dinosaur era, before their extinction 66 million years ago? A study conducted by Jamie MacLaren - palaeobiologist and anatomist - and his colleagues from the EddyLab at the University of Liège (Belgium), published in the journal Proceedings of the Royal Society B: Biological Sciences, provides new food for thought.

We have all heard about the extinction that wiped out most of the dinosaurs 66 million years ago. However, few of us know how the diversity of life evolved before that event. “During the last 20 million years of the Mesozoic - also known as the age of the dinosaurs - the seas were full of large predatory reptiles, explains Jamie MacLaren, a researcher at the EddyLab and the first author of the paper. One of these groups, the mosasaurs, became some of the largest marine predators ever known, reaching up to seventeen metres in length! These ancient cousins of snakes and lizards had elongated, sinuous bodies with four oar-like limbs and large heads with teeth adapted to various ecological roles. They dispersed throughout the world's oceans before their demise during the Cretaceous-Paleogene mass extinction.”

In a new study just published in the journal Proceedings of the Royal Society B: Biological Sciences, a European team of researchers from Belgium, France and the UK has unravelled the evolution of mosasaurs in the late Cretaceous. To do this, the researchers studied ecological characteristics across time and space via a series of high-resolution three-dimensional (3D) scans of mosasaur skulls. "The 3D data collection effort for marine reptiles in this study is unprecedented," explains Professor Valentin Fischer, professor of palaeontology and director of the Eddy Lab at ULiège. With these very detailed scans, we can draw very precise conclusions about the ability of these animals to hunt and move when they were alive."

The study's most important finding is that, during the last five million years of their reign in the oceans, mosasaurs appear to have reduced their morphological diversity. Groups that were only loosely related began to look more and more like each other and hunt in much the same way. However, the devil is in the detail, as Jamie MacLaren explains: "Having several very well sampled geographical areas, we used a method from population ecology for this study, which is also a novelty." The new analysis allowed the team to show that mosasaurs living in areas such as North America, Europe and Australasia (a region that includes Australia and New Zealand) were experiencing a decline in diversity, while the shallow seas covering what is now North Africa acted as a crucible for ecological innovations. "Our study illustrates how important it is in palaeontological studies to not only look at global data but also to analyse different regions that may tell a very different story independently," says MacLaren.

The new information from this study is just the tip of the iceberg for mosasaur research at the University of Liège. "We also have ongoing studies comparing the biomechanics and relationships of mosasaurs..." explains Rebecca Bennion, PhD student at the EddyLab. Moreover, our collaborations with other laboratories in Belgium and abroad give us an entirely new vision of these extinct animals.” Research that promises to bring us significant discoveries in the future.

Participation

• EDDyLab (Evolution & Diversity Dynamics Lab) - University of Liège
• Functional Morphology Lab - Universiteit Antwerpen
• Royal Belgian Institute of Natural Sciences
• CR2P – Centre de Recherche en Paléontologie de Paris (Paris Paleontology Research Centre -CR2P) -CNRS-MNHN-SU