Homology and phylogenetic implications of some enigmatic cranial features in galliform and anseriform birds

Two landmarks of the temporal region of the skull in most birds are the zygomatic process (processus zygomaticus) and the postorbital process (processus postorbitalis). The morphology and homology of these processes in gallinaceous birds (Galliformes) and waterfowl (Anseriformes), however, are not clear. Anseriformes usually are said to lack a processus zygomaticus. By contrast, the processus zygomaticus of many Galliformes often is described as connected to the tip of the processus postorbitalis, forming a temporal arch. Olson and Feduccia (1980a) cited these cranial differences as evidence opposing a hypothesis of sister relationship between the two orders, an hypothesis having a substantial history of advocacy