The Triassic reptile Euparkeria has been frequently given a pivotal positio
n in interpretations of the evolution of archosaurs. Most recently, Welman
(1995) has argued from braincase data that Euparkeria is more closely relat
ed to birds than are either theropod dinosaurs or crocodilians - a conclusi
on clearly at odds with the current orthodoxy. The braincase of a single sp
ecimen of Euparkeria is described in detail and compared with previous desc
riptions and with the braincases of other diapsids. Variations among the kn
own specimens are documented. The homology of various braincase structures
are reassessed in light of the study by Welman (1995). We argue that the br
aincase of Euparkeria has an undivided metotic fissure, an incompletely oss
ified medial wall of the otic capsule, a well-defined 'semilunar depression
', and posteroventrally positioned foramina in the parabasisphenoid for the
entrance of the cerebral branches of the internal carotid arteries. It lac
ks enclosure of the Eustachian system in bone, well-developed tympanic sinu
ses, or a well-defined recess for the lagena. A review of braincase morphol
ogy in extinct and extant diapsids suggests that braincase features of Eupa
rkeria are largely plesiomorphic for Archosauria. The evolutionary relation
ships between Euparkeria and extant archosaurs (birds and crocodilians) are
considered by reviewing braincase morphology in extant and extinct diapsid
s. No shared derived characters could be found that support the resolutions
(crocodilians (Euparkeria + birds)) or (birds (Euparkeria + crocodilians))
. Three derived characters shared by extant archosaurs support the resoluti
on (Euparkeria (crocodilians f birds)), but only the presence of laterally
positioned foramina in the parabasisphenoid for the entrance of the cerebra
l branches of the internal carotid arteries appears to represent strong evi
dence. The other two features are a degree of ossification (of the medial w
all of the otic capsule) that exhibits some homoplasy among archosaurs, and
an absence (of the 'semilunar depression'), and therefore do not represent
particularly robust hypotheses of homology. Our interpretation of the brai
ncase of Euparkeria is fully congruent with the consensus among recent expl
icit phylogenetic analyses that this taxon is close to, but not a member of
, the archosaur crown group. Birds and crocodilians share a number of other
derived similarities (subdivided metotic fissure, elongated and tubular co
chlear recess, enclosed Eustachian system, extensive tympanic sinuses, quad
rate-prootic articulation) that are probably not homologous because of thei
r absence in a number of non-avian dinosaurs and crocodilianline crown-grou
p archosaurs.