The first protobirds (Archaeornithes) evolved from small Coelurosaurian theropods during the late Jurassic (probably Kimmeridgian age). They were feathered gliders or jumpers and weak flyers that may have taken to the trees in to avoid ground-based predators. The mid-Tithonian extinction event eliminated a lot of the big dinosaurs and produced some empty ecological niches. Some Archaeornithines exploited these niches by becoming secondarily flightless ground-based predators and omnivores. Others however evolved into more efficient and advanced fliers. Thus for the first five or ten million years of the Cretaceous there would have been a tremendous evolutionary radiation. The result was about half a dozen or more different orders of birds, ranging from very dinosaur-like to very (modern-) bird-like, coexisting and co-evolving together throughout the seventy or eighty million years of the Cretaceous.
The terrible extinction event at the end of the Mesozoic (possibly a large asteroid impact, preceded and succeed by an extended period of vulcanism with consequent nuclear winter, acid rain etc) put an end to this primordial garden of Eden. Many protozoa and invertebrates as well as dinosaurs, pterosaurs, marine reptiles, and a number of bird and mammal lineages were taken out by this traumatic extinction. The only lineage of endothermic archosaurs to survive into the Cenozoic were the Neornithes or "modern birds". These likewise underwent a huge evolutionary radiation during the early Tertiary, including some amazing secondarily flightless forms. They were however never as anatomically or (probably) metabolically diverse as the Cretaceous protoaves and veraves. Today the Neornithes remain one of the largest and most successful groups of tetrapods.
Archaeornithes: Archaeopteryx. Definition: Archaeopteryx > Neornithes. Padian et al. (1999).
Range: Late Jurassic of Europe & probably Early Cretaceous of China. If Unenlagia, then Late Cretaceous of South America.
Phylogeny: Aves: Metornithes + *.
Characters: Pigeon-sized extremely primitive bird, probably capable of flight. Skull moderately long and pointed; maxilla, premaxilla and dentary toothed; teeth sparse, small, with constriction between crown & roots and not laterally flattened or serrated; interdental plates present; nasals are primary rostral bones; nares large; antorbital fossa with 2 fenestrae(?), very large; orbit very large; jugal reduced almost to bar connecting maxilla and quadrate; frontal expanded laterally; probable very large brain; parietal markedly narrower; quadrate may be moveable; large surangular forming upper surface of posterior portion of lower jaw; angular forms lower surface; most other dermal skull components present but strongly reduced; small, posteriorly-directed cervical ribs; notarium absent; 4 sacral vertebrae; tail longer than dorsal spine; sternum absent; coracoid short; scapula long and narrow; furcula large; tridactyl wings with short, sharp, curved claws; radius & ulna unfused; flexible carpus; pelvis long and fairly shallow; angle of pubes disputed, but certainly not propubic; knee hinge-like; femur < tibia; fibula reduced; distal tarsals fused to metatarsals, which are partially fused proximally (i.e. tarsometatarsus present); digits anisodactyl.
Note: Note how few of the Neornithine bird adaptations are found in Archaeopteryx. See also image at entry for Ornithurae, comparing skulls of Columba and Archaeopteryx.
Neornithes: modern birds.
Range: from the Late(?) Cretaceous
Phylogeny: Carinatae: Ichthyornithiformes + *: (Paleognathae + Dromornithiformes) + (Galloanserae + Gruimorpha).
Characters: The more important anatomical characteristics of this group are discussed in the Overview. They include: horn beak; teeth absent; fused limb bones. In addition Neornithes have a fully-separated four-chambered heart and typically exhibit complex social behaviors.
The First Birds