Morphology of the quadrate in the Eocene anseriform Presbyornis and extant galloanserine birds.

Despite the notoriety, phylogenetic significance, and large number of available specimens of Presbyornis, its cranial anatomy has never been studied in detail, and its quadrate has been partly misinterpreted. We studied five quadrates of Presbyornis that reveal features hitherto unknown in the anseriforms but otherwise present in galliforms. As a result, we analyzed the variable quadrate characters among all extant galloanserine families and identified synapomorphies and other morphological variation among the major galloanserine clades. In terms of quadrate morphology, Presbyornis is more plesiomorphic than any extant anseriform (including the Anhimidae) and shares ancestral galloanserine characters with the Megapodiidae, the earliest branch of extant galliforms. The quadrate's morphology is inconsistent with the currently accepted anseriform phylogeny that nests Presbyornis within the crown-group as a close relative of the Anatidae. The presbyornithid quadrates exhibit an unusual variation in the presence of a caudomedial pneumatic foramen, which we interpret as a result of a discontinuous change in the growth path of the pneumatic diverticulum. Another episode of morphogenetic imbalance in the growth path of the pneumatic diverticulum may have accompanied the disappearance of the basiorbital pneumatic foramen (along with the pneumatization of the pterygoid) at the origin of the crown-group anseriforms. This episode is marked by the striking individual variation in the presence and location of pneumatic foramina in the mandibular part of the quadrate in the Anhimidae.