Seeleyosaurus is an extinct genus of plesiosaur. It is known from a large almost complete skeleton from the Upper Lias (Toarcian) of Württemberg, Germany.

Artists impression of Seeleyosaurus

There seems to be the impression of a rhomboidal flap of skin in a vertical plane; if so, many plesiosaurs may have been equipped in this way.

The holotype was destroyed in 1945 (WWII) and only a cast exists today. A second specimen, preserved in 3D, has survived until today.

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