Ornithorhynchus anatinus, MVZ MAMM 32885, bone isosurfaces derived from CT scans. A, skull in dorsal view; B, rostrum in dorsal view; C, right rostrum in oblique medial view; D-G, os paradoxum in D, dorsal, E, ventral, F, anterior, and G, oblique anteromedial views. Scale for A = 10 mm; scale for B–C = 10 mm; scale for D-G = 5 mm. Septomaxilla and premaxilla are fused but colored separately based on their inferred relationships. Abbreviations: cns, surface for cartilaginous nasal septum; ii, incisive incisure; iof, infraorbital foramen; mx, maxilla; na, nasal; ons, ossified nasal septum; pmx, premaxilla; psc, surface for paraseptal cartilage; s, sulcus; smx, septomaxilla; spmx, septal process of maxilla; vo, vomer.