An article published in the journal “Nature Ecology & Evolution” describes a study on marine reptiles that lived in a sea area that crossed today’s northern Europe called Jurassic Sub-Boreal Seaway during the Jurassic period. A team of paleontologists coordinated by the University of Edinburgh analyzed the shape and size of teeth that belonged to reptile that lived over a period of 18 million years, discovering that the marine predators living in deep waters prospered when sea levels rose while species they lived in shallow waters declined or even became extinct.
In a time that extended far beyond the Jurassic embracing a large part of the Mesozoic, many species of marine reptiles lived in the seas of that ancient world and in their ecosystems occupied roles that today are of crocodiles, large fish and cetaceans. At the time, they spread at such levels that there was a coexistence of many reptile groups over more than 50 million years, through major environmental changes.
So far one of the major problems in reconstructing the history of marine reptiles in the Mesozoic was the lack of knowledge of the ecological changes that took place in their ecosystems. A team led by Davide Foffa of the University of Edinburgh’s School of GeoSciences tried to reconstruct at least some of those changes by analyzing the fossils available focusing on a sea region that at that time was tropical that extended from today’s northern France to today’s north England.
Dr. Steve Brusatte, also of the University of Edinburgh and among the authors of the research, explained that teeth are humble fossils but reveal a great story of how marine reptiles evolved over millions of years while their environment changed. Together with his colleagues, he reconstructed those changes in Jurassic reptiles, finding parallels with those that are taking place today for example in dolphins while sea levels are rising.
In the Jurassic, when sea level rose, the reptile species that lived in shallow waters, and captured the fish with their thin knife-like teeth, declined drastically. At the same time, more massive species that lived in deeper waters off that sea, and captured their prey with their broad teeth that tore their prey to pieces, prospered.
According to the researchers, the changes in temperatures and chemical composition of the seas that occurred when their level rose gave an advantage to the species that lived in deep waters. That’s because those changes could have increased nutrients with an advantage for the whole ecosystem and therefore also for their prey.
This research was useful to reconstruct at least a part of the food chain that existed in that Jurassic ecosystem. The results showed similarities with that existing in the modern oceans, with many species cohabiting because they don’t compete for the same resources. They also showed what kind of decline can occur following changes in some ways similar to those we’re seeing today.