The dichotomy between short-necked, large-headed “pliosaurs” and long-necked, small-headed “plesiosaurs” has formed the basis of plesiosaur taxonomy for over one hundred years. Recent work has cast doubt on the taxonomic validity of this dichotomy, suggesting that the pliosaur morphotype may have evolved independently in more than one clade. This paper quantifies the variation in body proportion in the clade Plesiosauria using principal component analysis and demonstrates that the traditional plesiosaur/pliosaur dichotomy is an oversimplified view of the range of morphologies present in the group. The topology of the clade is mapped into the morphospace, demonstrating that the pliosaur morphotype evolved three times from two different regions of morphospace. Both the range of body morphologies displayed by plesiosaurs and the evolutionary history of those morphologies, are more complex than previously supposed.

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