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

Abstract

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 (Seebohm, 1889; Shufeldt, 1901; Delacour, 1954; Johnsgard, 1965; Cracraft, 1981a, 1986; Schulin, 1987). Dzerzhinsky (1982, 1995) contradicted the proposal by Olson and Feduccia (1980a), interpreting the two processes as completely fused in Anseriformes, forming a unique 'sphenotemporal process,' which he averred to have been derived evolutionarily from the condition found in the Galliformes. In the present study, we examined skulls and jaw muscles of juvenile and adult specimens of selected taxa from both orders to test these opposing hypotheses, and found that: (a) the processus zygomaticus is small or lacking in adult Galliformes, and absent in all Anseriformes; (b) the processus zygomaticus is connected to the tip of the processus postorbitalis by an ossified aponeurosis of m. adductor mandibulae externus (aponeurosis zygomatica) in adults of most galliforms, whereas the aponeurosis zygomatica of anseriforms has a linear origin along the os squamosum as far as the processus postorbitalis; the aponeurosis zygomatica is ossified in Anhimidae and unossified in Anatidae; (c) a laterally exposed fossa of the temporal region (fossa musculorum temporalium) is reduced in Galliformes and absent in Anseriformes; (d) pars superficialis and pars zygomatica of m. adductor mandibulae externus are shifted rostrad in Galliformes and Anseriformes, and (e) pars articularis of m. adductor mandibulae externus is much enlarged in both orders. Based on these observations, we conclude that the parts of musculus adductor mandibulae externus of Anseriformes have been misinterpreted in a number of previous studies, perhaps reflecting confusion about associated processes and fossae. These findings are interpreted with respect to the homology of the osteological features and their associated muscles. The distribution of the included states supports the growing consensus for a sister relationship between the Galliformes and Anseriformes.