The shape of an animal is a reflection of the way it interacts with the physical world around it. By studying the mechanical laws which influence the evolution of modern animals, we can better understand the lives of their ancestors. Hydrodynamics examines the movement of water in contact with an organism, and can include everything from body shape to blood flow. In this episode we spoke to Dr Tom Fletcher, University of Leicester, about hydrodynamics in palaeontology, and his research looking at fossil fishes and modern sharks. Tom and others have published a paper on the hydrodynamics of fossil fishes, and he continues to work on the biomechanics of fossil animals.