A Late Cretaceous mosasaur fossil from central Jordan reveals a tail with flippers that suggests these lizards swam like sharks. The work is reported this week in Nature Communications.

Mosasaurs were the dominant marine reptiles from around 98-66 million years ago. Conflicting theories regarding the ancestry of mosasaurs, combined with a lack of soft tissue evidence in the fossil record and incorrect descriptions and reconstructions of the mososaur body plan, had led to the pervasive hypothesis that these reptiles were slow-swimmers. Lindgren and colleagues analysed a fossil specimen with exceptionally well-preserved soft tissue and identified a crescent-shaped tail fin, similar to whales and ichthyosaurs (marine reptiles that resembled dolphins).

Comparison of the morphology of the fossil with modern sharks led to conclusion that mosasaurs were efficient swimmers. The research also suggests that this characteristic tail have evolved independently in several groups that occupied similar environments.