The following taxa demonstrate the morphological divergence of bird evolution following Archaeopteryx. Future additions to reptileevolution.com will hopefully take on bird evolution in more depth.
Ichthyornis dispar (Marsh 1872) Late Cretaceous ~90 mya, 24 cm in length, wingspan 43 cm was a deep-breasted, tern-like flying bird derived from a sister to Archaeopteryx and closely related to all modern birds.
Distinct from Archaeopteryx, the skull of Ichthyornis had a longer rostrum, or beak, with teeth largely missing from the anterior half of the upper rostrum. The premaxilla was extended to form a sharper tip. The naris was larger than the antorbital fenestra. Several cranial bones fused to one another or disappeared.
The cervicals increased in number and decreased in size, creating a longer, more flexible neck. The dorsals were reduced in number. The sacrals were coosified into a sacrum. The caudal vertebrae were fused to form a pygostyle from which the tail feathers fanned out. The dorsal ribs were elongated and tied together with short uncinate processes. The sternal ribs extended to below the acetabulum.
The scapula and coracoid were elongated and gracile. The sternum was greatly enlarged and provided with a deep keel as an anchor for powerful flight muscles, as in all flying birds. The forelimb was elongated such that the elbows were extended posterior to the knees. The hands were able to fold parallel in tight Z-pattern with the rest of the forelimb. Fingers 2 and 3 were fused together. Finger 1 was a vestige.
The pubis was further rotated posterior such that it paralleled the posteriorly rotated ischium. The femur was shortened, no longer deeper than the torso. The foot was relatively reduced.
The above pattern is similar to most living birds, whether flying forms or not.
Mononykus olecranus (Perle et al., 1993) Late Cretaceous ~70 mya, 1 m in length was a flightless bird, nearly contemporary with Ichthyornis. Standing four times taller than Archaeopteryx, the skull of Mononykus was not quite twice as large.
As in Ichthyornis the cervicals were increased in number and the dorsals were reduced in number.
Only digit I remained on the hand, which was otherwise stunted, except for the proximal ulna (the elbow) which was enlarged.
As in Ichthyorinis, the pubis and ischium were oriented posteroventrallly. The ilium was elongated posteriorly, reduced anteriorly.
The hindlimbs were hyper-elongated and the foot was not quite twice as large as in Archaeopteryx.
This alvarezsaurid pattern is characterized by the strong but stunted forelimbs of enigmatic use.