The braincase of Euparkeria, and the evolutionary relationships of birds and crocodilians

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.