The history and homology of the os paradoxum or dumb-bell-shaped bone of the platypus Ornithorhynchus anatinus (Mammalia, Monotremata)

The os paradoxum or dumb-bell-shaped bone is a paired bone occurring in the middle of the specialized bill of the platypusOrnithorhynchus anatinus. It has been variously considered as a neomorph of the platypus, as the homologue of the paired vomer of sauropsids, or as a part of the paired premaxillae. A review of the near 200-year history of this element strongly supports the os paradoxum as a remnant of the medial palatine processes of the premaxillae given its ontogenetic continuity with the premaxillae and association with the vomeronasal organ and cartilage, incisive foramen, and cartilaginous nasal septum. In conjunction with this hypothesis, homologies of the unpaired vomer of extant mammals and the paired vomer of extant sauropsids are also supported. These views are reinforced with observations from CT scans ofO. anatinus, the Miocene ornithorhynchidObdurodon dicksoni, and the extant didelphid marsupialDidelphis marsupialis. At the choanae,Obdurodonhas what appears to be a separate parasphenoid bone unknown in extant monotremes.