not dive as deeply, but might also be explained by a greater predation pressure during the later epochs, more often necessitating a fast flight to the surface. However, this last possibility is contradicted by the fact that, with modern animals, damage is not caused by a limited number of rapid ascension incidents, but by a gradual accumulation of non-invalidating degeneration during normal diving behaviour. Additional evidence is provided by the eyes of ichthyosaurs that among vertebrates are both relatively and absolutely the largest known. Modern leopard seals can dive to up to hunting on sight. Motani suggested that ichthyosaurs, with their relatively much larger eye sockets, should have been able to reach even greater depths. Temnodontosaurus, with eyes that had a diameter of twenty-five centimetres, could probably still see at a depth of 1,600 metres. At these depths, such eyes would have been especially useful to see large objects. Later species, such as Ophthalmosaurus, had relatively larger eyes, again an indication that diving capacity was better in late Jurassic and Cretaceous forms. Metabolism Similar to modern cetaceans, such as whales and dolphins, ichthyosaurs were air-breathing. Whales and dolphins are mammals and warm-blooded. Of ichthyosaurs it was traditionally assumed that they were cold-blooded, being reptiles. However, since the 1970s many dominant reptile groups of the Mesozoic, such as theropod dinosaurs, pterosaurs and plesiosaurs, have been considered warm-blooded, as this offers an elegant explanation of their dominance. Some direct evidence is available that ichthyosaurs too might have been endothermic. In 1990, Vivian de Buffrénil published a histological study, indicating that ichthyosaurs possessed a fibrolamellar bone structure, as with warm-blooded animals in general, typified by fast growth and a strong vascularisation. Early Triassic species already show these traits. In 2012, it was reported that even the very basal form Utatsusaurus had this bone type, indicating that the ancestors of ichthyosaurs were already warm-blooded. Additional direct proof for a high metabolism is the isotopes of oxygen ratio in the teeth, which indicates a body temperature of between 35 and 39 °C, about 20° higher than the surrounding seawater. Blubber is consistent with warm-bloodedness as the insulating qualities require the animal to generate its own heat. Indirect evidence for endothermy is provided by the body shape of derived ichthyosaurs, which with its short tail and vertical tail fin seems optimised for a high cruising speed that can only be sustained by a high metabolism: all