Figure 4. Major stages in the evolution of modern avian skeletal design and function. Many skeletal innovations of critical functional importance for flight arose for other purposes among early theropods, including (1) the hollowing of all long bones of the skeleton (Theropoda) and removal of pedal digit I from its role in weight support; (2) evolution of a rotary wrist joint to efficiently deploy a large grasping manus; (3) expansion of the coracoid and sternum for increased pectoral musculature and plumulaceous feathers for insulation (89); (4) the presence of vaned feathers arranged as primaries, secondaries, and rectrices for display or brooding or both; (5) shortening of the trunk and increased stiffness of the distal tail for balance and maneuverability. Archaeopteryx remains a pivotal taxon, documenting (6) the acquisition of basic flight and perching function before the close of the Jurassic (laterally facing shoulder joint, split propulsion-lift wing with asymmetric feathers, and reversed hallux). Key refinements of powered flight and perching in later birds include (7) the deep thorax with strut-shaped coracoid and pygostyle; (8) the triosseal canal for the tendon of the principal wing rotator (the supracoracoideus muscle), alular feathers for control of airflow at slow speeds, rectriceal fan for maneuverability and braking during landing, and fully opposable hallux for advanced perching; and (9) the elastic furcula and deep sternal keel for massive aerobic pectoral musculature (90). Ornithothoracine birds diverged early as Enantiornithes ("opposite birds") (68, 77), which prevailed as the predominant avians during the Cretaceous, and Euornithes ("true birds"), which underwent an explosive radiation toward the close of the Cretaceous that gave rise to all living avians (Neornithes, or "new birds").