Monophyly of the Galloanseres is supported by virtually all molecular analyses of different kinds of data, including DNA-DNA hybridization as well as mitochondrial and nuclear gene sequences, and also resulted from analyses of morphological characters (Sibley and Ahlquist 1990; Mayr and Clarke 2003; Cracraft et al. 2004; Fain and Houde 2004; Ericson et al. 2006; Livezey and Zusi 2007). Morphological apomorphies of this clade mainly concern skull features such as the presence of sessile, i.e., stalkless, basipterygoid processes, long, blade-like retroarticular processes on the mandible, and a derived morphology of the quadrate (Mickoleit 2004). The postcra-nial skeleton of extant Galliformes and Anseriformes is quite different, but the plesio-morphic morphology of some Paleogene stem group representatives of the Galliformes bridges the morphological gap between the extant taxa (see later).
Various Paleogene taxa have been assigned to the Galloanseres, which greatly differ in morphology and presumed ecological preferences. If assignment of all of them is confirmed by future studies, the morphological and ecological diversity within Paleogene Galloanseres was extraordinarily high, including giant flightless groundbirds with greatly reduced wings, long-legged filter-feeders, and pelagic taxa with a wingspan of 4-5 m. Figure 6.1 depicts a hypothesis on the interrelationships of these birds.
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