Jamieson, B.G.M. and Healy, J.M. 1992b. The phylogenetic position of the tuatara, Sphenodon (Sphenodontida, Amniota), as indicated by cladistic analysis of the ultrastructure of spermatozoa. Philosophical Transactions of the Royal Society London, B 335, 207-219.

Sphenodon has traditionally been regarded as a little changed survivor of
  the Permo-Triassic thecodont or eosuchian 'stem reptiles' but has
  alternatively been placed in the Lepidosauria as the plesiomorphic or even
  apomorphic sister-taxon of the squamates. A cladistic analysis of 16
  characters from spermatozoal ultrastructure of Sphenodon and other
  amniotes unequivocally confirms its exceedingly primitive status. The
  analysis suggests that monotremes are the sister-group of birds; squamates
  form the sister-group of a bird + monotreme clade while the three sister
  -groups successively below the bird + monotreme + squamate assemblage are
  the caiman, the tuatara and the outgroup (turtles). The monotreme + bird
  couplet, supports the concept of the Haemothermia, but can only be
  regarded heuristically. The usual concept of mammals as a synapsid-derived
  outgroup of all other extant amniotes is not substantiated
  spermatologically. All cladistic analyses made, and a separate
  consideration of apomorphies, indicate that Sphenodon is spermatologically
  the most primitive amniote, excepting the Chelonia. It is advanced
  (apomorphic) for the amniotes in only two of the 16 spermatozoal
  characters considered. A close, sister-group relationship of Sphenodon
  with squamates is not endorsed.