The adaptive significance of the structural changes that led to the differentiation, during the Triassic, of two important stocks of reptiles, the Lacertilia (lizards) and the sphenodontid Rhynchocephalia, is discussed. This provides a context for the consideration of the peculiar combination of characters found in a reptile, Glevosaurus hudsoni, from the Upper Trias of the Bristol Channel area, that has some structural affinities with each of these stocks.

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