Fig. 69. Skulls (in some cases restored) of selected ichthyosaurs. Left column: dorsal views; right column: left lateral views.
dental grooves, (2) bony fixation, and (3) sockets (Table 1).
The three may show different character states within a
single jaw, so it is necessary to consider variations within
an individual (Motani, 1997A). Names have been given to
various combinations of the character-states (e.g., Romer,
1956; Edmund, 1960, 1969; Motani, 1997A). For example,
the subthecodont implantation is characterized by the
presence of all three (Table 1), and this combination is
plesiomorphic for ichthyopterygians. The socket and bony
attachment went through reduction along the stem leading
to the Parvipelvia, which reached a condition called aula-
codonty (Mazin, 1983B), where the teeth are implanted in
dental grooves without proper sockets or bone of attachment
(strictly speaking, bony attachment near the bottom
of the teeth has not been confirmed as being absent, and its
presence remains a possibility).
Apart from these two dominant implantation types,
two autapomorphic conditions occurred in ichthyopterygians.
The first type, exemplified by Mixosaurus, lost dental
grooves posteriorly, leading to a condition called anky-
losed thecodont in that part of jaw. The absence of dental
grooves seems to be restricted to the upper jaw in M. kuhn-
schnyderi and M. nordenskioeldii, whereas the lower jaw also
lacks dental grooves, at least posteriorly, in M.fraasi.