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.