This thesis is centred around an anatomical study of the cranial osteology of the Mesozoic sphenodontidans, Asiacephalosaurus wangi, Rarojugalosaurus mcgilli, Dianosaurus petilus and Homoeosaurus maximiliani. The first two are newly discovered and have not been studied previously. The third, previously described only in a preliminary fashion, was misinterpreted as a protorosaur. Although many specimens of H. maximiliani have been described (Cocude-Michel, 1963; Faber, 1981) since the species was first named by Meyer (1845), the newly exposed dorsal part of the skull of a new specimen has provided many new cranial features. Consequently, it was necessary to examine, draw and describe all of these sphenodontidans. The descriptions are all original and present a virtually complete picture of the cranial osteology of these genera for the first time. In addition, the different patterns of the middle ear apparatus of the Sphenodontida and the functional problems of the temporal region in the Lepidosauria have been considered. Although use is made of the literature to obtain background data, the conclusions drawn are original unless explicitly stated otherwise. Finally, the detailed descriptions of the four genera have permitted a reassessment of the phylogeny of the Sphenodontida. After reviewing the literature, original conclusions are given.

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