The skeletal anatomy of Paraplacodus broilii Peyer from the Grenzbitumen-ho
rizon (Anisian-Ladinian boundary) of Monte San Giorgio (Switzerland) is des
cribed and compared with that of other placodonts. Paraplacodus is found to
share a number of potential synapomorphies with Placodus which could poten
tially corroborate the monophyly of the Placodontoidea, but Placodus also s
hares an number of potential synapomorphies with the armored placodonts (Cy
amodontoidea) which are absent in Paraplacodus. Parsimony analysis rejects
the monophyly of the Placodontoidea, and places Paraplacodus at the root of
the placodont tree, as the sister-taxon of all the other representatives o
f the clade. This correlates with a configuration of the temporal region of
the skull that suggest the loss of the lower temporal arch in a diapsid sk
ull. The loss of the lower temporal arch is therefore recognized as a sauro
pterygian synapomorphy, and might even be a lepidosauromorph synapomorphy.
(C) 2000 The Linnean Society of London.