Some of the most extraordinary changes in body shapes, sizes, and means of locomotion during evolution have occurred in animals which have adapted to aquatic life, such as modern whales, seals, or turtles. During the Mesozoic period (252 – 66 million years ago), while a large number of dinosaurs still roamed the land, many groups of reptiles took to the seas, including the iconic ichthyosaurs, plesiosaurs, mosasaurs, and crocodiles.
Now, by using state-of-the-art statistical methods, a team of paleobiologists led by the University of Bristol has conducted a large-scale quantitative study on the locomotion of Mesozoic marine reptiles in order to map the variety of body transformations that have occurred during this critical period in animal evolution.
The experts collected measurements from 125 fossilized skeletons to examine changes in swimming styles within various lineages and through time. The analysis revealed that, instead of an explosive radiation at the beginning of the Mesozoic, there was a gradual diversification of locomotory modes, peaking in the Cretaceous period.
“Changes in anatomy in land-to-sea transitions are intimately linked to the evolution of swimming. For example, sea lions’ flippers have relatively short forearm and large hands, very different from the walking legs of their ancestors. The rich fossil record of Mesozoic marine reptiles provided great opportunity to study these transitions at a large scale,” said lead author Susana Guatarra, a postdoctoral researcher at the Natural History Museum in London who conducted this study during her doctoral degree at Bristol.
The examination of the fossils shed new light into the swimming of specific groups. For instance, while ichthyosaurs, hupehsuchians, and mosasaurs seemed to be highly specialized for aquatic locomotion from very early in their evolution, acquiring the capacity for swimming by fluke oscillation, other groups such as the metriorhynchids retained quite primitive-looking hindlimbs that were incompatible with such advanced types of aquatic locomotion.
The experts also examined the evolution of body size, a characteristic closely linked to locomotion, animal physiology, and ocean productivity. “We know that transition to life in water is usually accompanied by an increase in body mass, as seen in cetaceans, and one of our previous studies shows that large sizes benefit aquatic animals in reducing the mass-specific costs of drag. Thus, it was essential to explore this trait in the wider ensemble of Mesozoic marine reptiles,” explained senior author Mike Benton, a professor of Vertebrate Paleontology at Bristol.
According to Dr. Gutarra, body size followed a similar trend to the diversification of locomotory modes, with the widest spread of body size occurring during the Cretaceous too. “The rate of increase and the maximum limits to body size seems to vary a lot between groups. This is a fascinating observation. We need to explore further what factors influence and limit the increase in body mass in each group,” she concluded.
The study is published in the journal Paleontology.