Cladogram of Dinosauria showing the four-phase model for evolution of avian air sacs and lung ventilation within Theropoda.

Phase I (Theropoda), variable posterior extension of paraxial cervical air sacs. Phase II (Tetanurae), elaboration of cranial (clavicular) and caudal (abdominal) intrathoracic air sac divisions and subcutaneous diverticulae. Phase III (Maniraptoriformes), primitive costosternal ventilatory pump. Phase IV (Maniraptora), advanced costosternal ventilatory pump. Abbreviations: aas, abdominal air sac; cas, cervical air sac; caas, caudal air sacs; clas, clavicular air sac; cor, coracoid; cras, cranial air sacs; lu, lung; sr, sternal rib; st, sternum; up, uncinate process; vr, vertebral rib. Bold arrow on lung indicates flow-through lung ventilation; question mark indicates uncertainty in the direction of air flow (uni- or bidirectional).