Researchers from the University of Bristol have used state-of-the-art statistical methods to study the locomotion of Mesozoic marine reptiles. In their new paper published in Palaeontology, the team collected measurements from 125 fossilized skeletons to explore changes in swimming styles within lineages and through time. They discovered a gradual diversification of locomotory modes, which peaked in the Cretaceous period. This contradicts previous theories that explosive radiation existed at the beginning of the Mesozoic.

The team found a connection between body size and the diversification of locomotory modes. They also explored the evolution of size, a feature related to locomotion, animal physiology, and ocean productivity. The researchers found that body size followed a similar trend to the diversification of locomotory modes, and the widest spread of body size also occurred in the Cretaceous.

The Mesozoic era saw many groups of reptiles adapt to life in the sea, with ichthyosaurs, plesiosaurs, crocodiles, and mosasaurs among them. The researchers found that ichthyosaurs were highly specialized for aquatic locomotion from early evolution. The team also discovered an overlap between mosasaurs and ichthyosaurs, indicating that mosasaurs evolved a swimming mode by oscillating flukes, different from the eel-like body undulation suggested in the past.

Changes in anatomy in land-to-sea transitions are intimately linked to the evolution of swimming. For example, sea lions’ flippers have relatively short forearms and large hands, very different from the walking legs of their ancestors. The rich fossil record of Mesozoic marine reptiles provided an excellent opportunity for researchers to study these transitions at a large scale. The study also included measurements from living aquatic animals, such as otters, seals, and turtles, of which researchers know their swimming behavior, to provide a functional reference for the ancient species with unknown swimming modes.

References

• Gutarra, S., Stubbs, T. L., Moon, B. C., Heighton, B. H., & Benton, M. J. (2023). The locomotor ecomorphology of Mesozoic marine reptiles. Palaeontology, 66(2), e12645. https://doi.org/10.1111/pala.12645
• University of Bristol. (2023, April 18). Swimming secrets of prehistoric reptiles unlocked by new study. University of Bristol; University of Bristol. https://www.bristol.ac.uk/news/2023/april/swimming-dinosaurs-study.html