JOHN SIBBICK / SCIENCE PHOTO LIBRARY JOHN SIBBICK / SCIENCE PHOTO LIBRARY
Prehistoric marine reptiles, illustration. At upper left is Ceresiosaurus (4 metres long). At right is Nothosaurus (3 metres long). At lower left is the heavily armoured Placodus. These are earlier, more primitive forms of marine reptiles, with legs and webbed feet or webbed flippers, compared to the fully developed flippers of the plesiosaurs and ichthyosaurs. Ceresiosaurus had hyperphalangy, extra bones that made the hands and feet more flipper-like. All three existed during during the Middle Triassic, around 240 million years ago.
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