The work presented here was carried out by M.E.H.J. during a BBSRC funded Ph.D. at University College London under the supervision of S.E.E. and by M.E.H.J. and N.C. during research funded by BBSRC grants (BB/E007465/1, BB/E009204/1 and BB/E007813/1) awarded to S.E.E. (UCL), M.J.F. (University of Hull) and P.O'H. (Hull York Medical School and University of Hull), respectively. For contributions toward research trips, M.E.H.J. thanks the Bogue Fellowship (UCL) and the Palaeontological Association (Sylvester-Bradley Award). For hospitality, cooperation and access to specimens we are grateful to C. McCarthy and A.C. Milner (The Natural History Museum, UK); H. Chatterjee, J. Ashby, M. Carnall, J. Hatton, S. Levey and C. Washbrook (Grant Museum of Zoology, UCL, UK); R.J. Symonds (Cambridge University Museum of Zoology, UK); R. Coory and A.J.D. Tennyson (both Museum of New Zealand Te Papa Tongarewa, NZ); B.J. Gill and R. Prasad (both Auckland Museum); N. Hudson (University of Auckland, NZ); P. Scofield (Canterbury Museum, NZ); J. Adams (Booth Museum of Natural History, Brighton and Hove, UK); A.R. Milner (Birkbeck College, University of London, UK); M. Nowak-Kemp and L. Conyers (Oxford University Museum of Natural History, UK); T. Koppe and A. Deutsch (Ernst Moritz Arndt University Greifswald); G. Sales (King's College London, UK); A. Rester, K. Kelly and A. Wolf (The Field Museum, Chicago, USA); J. Müller (Museum für Naturkunde Berlin, Germany); H. McGhie and R. Smith (The Manchester Museum, University of Manchester, UK); M. Reilly (Hunterian Museum, University of Glasgow, UK); J.M. Hay (Massey University, NZ); and the Digimorph website (Austin, Texas, USA). In particular, we thank L.K. Murray and C.J. Bell for access to CT data for Sphenodon specimen YPM 9194 and other specimens on loan to them from Yale. We also thank S. Taft for CT scanning specimen LDUCZ x036. For access to literature we thank P.M. Barrett (Natural History Museum), C.J. Bell, J. Clack, W. Kathe (previously University of Zürich, Switzerland), J. Klembara (Comenius University in Bratislava) and C.A. Hipsley (Museum für Naturkunde Berlin, Germany). For help checking abbreviations we thank K. W?odarek (UCL). A preliminary version of this work was presented at ICVM 7 in Paris (Jones 2007). For suggestions and helpful discussion M.E.H.J. thanks W. Kathe (previously University of Zürich, Switzerland), K. Bird (UCLan. UK), P. Johnston (University of Auckland, NZ), C. Holliday (Marshall University, USA), I. Werneberg (Paläontologisches Institut und Museum der Universität Zürich, Switzerland), one anonymous reviewer and Ph.D. examiners E.J. Rayfield (University of Bristol, UK) and J.C. Buckland-Wright (Guy's, King's College London, UK).