Birds and reptiles have a lot in common.  Similarities in their bones, digital claws, red blood cells, kidneys, penis, together with the reptilian appearance of bird embryos all speak to a common ancestor.[1]  A high calcium diet even causes birds to develop reptilian ankle bones.[2]

     Birds have even more in common with a certain type of reptile – the theropod dinosaurs.  They share the following characters:  fused clavicles, feet with three claws pointing forward and one backward, partially fused metatarsals, and a second set of ribs covering the front of the torso.[3]  The earliest bird, Archaeopteryx, even had three fingers with claws coming out the top of its wing, and it had teeth in its mouth instead of a toothless beak.  Theropod dinosaurs also had three fingers, all with claws, and sharp teeth in their mouths.

     One theropod in particular, Compsognathus, is often placed in museums next to the earliest bird, Archaeopteryx, to show their similarity.  At first glance, they look similar because they are both about the size of a chicken.  However, there are a number of substantial differences between Archaeopteryx and Compsognathus.  Archaeopteryx had a longer femur, thinner leg bones, and its tail vertebrae are of a different type than those of Compsognathus, having exchanged dinosaur features for bird-like features.[4]  Other characters of Archaeopteryx are distinctly more bird-like than dinosaur-like, including a larger brain size, the closeness of its teeth, the lack of dental serration, the nature of its shoulder girdle, its caudal maxillary sinus, fewer bones in the tail, its reduced prezygapophyses, its elongated prenarial, the break up of its postorbital bar, and the relationship between its caudal and columellar parts.[5]

     Archaeopteryx had full wings and tail feathers capable of flight.  In contrast, Compsognathus had no wings at all, and does not appear to have had any feathers – not even a few small ones on the skin for warmth.  In another gross morphological difference, Archaeopteryx possessed an ornithischian hip, wherein the pubis and the ischium both are positioned toward the posterior, unlike the saurischian hip of Compsognathus, with its forward placement of the pubis, as in all theropods.  On paper, this sounds obscure, but it is significant because if it weren't for other features, an ornithischian hip would normally make the bird more related to completely different looking dinosaurs such as the horned Triceratops and the plated Stegosaurus.

     So even though Compsognathus looks superficially similar to Archaeopteryx, it is actually a rather poor candidate for a "missing link."  We should look to other candidates.

     The coelurosaurian raptor dinosaurs make a better intermediary.  Unlike Compsognathus, many of the raptors had feathers, they had a stiffened tail, and their pubis was midway between the saurischian position and the ornithischian position.  Other characters shared with birds include a wishbone, birdlike feet, a carpus bone in the wrist, the social behavior of traveling in packs/flocks, and the presence of longer arms than those of other dinosaurs. 

     A wealth of such raptors has been uncovered from the Yixian Formation in Asia.  However, the age of the Yixian Formation is about 125 to 120 million years ago,[6] which presents a problem, because the first bird Archaeopteryx is known from a half-dozen specimens that are 25 to 30 million years older – found in the beginning of the Kimmeridgian of Bavaria, dating to 155 million years ago in the late Jurassic.  Thus, the feathered raptors of the Yixian Formation are too young to be the missing link between birds and dinosaurs.  Even though the feathered dinosaurs from the Yixian Formation might be related to the birds, they cannot be direct ancestors.  As Martin states,

 

The small coelurosaurian dinosaurs related to Archaeopteryx all occur in the fossil record after Archaeopteryx and so cannot be directly ancestral.[7]

 

     The dilemma concerning the origin of birds is similar to that of the snakes.  Raptors are to the birds what reduced-limbed lizards are to the snakes – an intermediary form that would be a good example of a missing link, except that it arrives too late in the fossil record to be a missing link.  Because they are intermediaries, they give evidence for the theory of descent in a general sense, yet because of their timing in the fossil record, they fail to provide true evidence for a gradual transition between forms over long time frames.  We are left with the reality of rapid evolution – the sudden evolution of new forms.  Raptors were evidently the descendents of missing links that evolved so quickly that they left little trace in the fossil record.      Luckily, there is at least some hope for finding a missing link.  A few raptor-like dinosaurs can be dated to around 160 to 165 million years ago, which is about 5 to 10 million years prior to Archaeopteryx.  Hence, it is still possible that the perfect intermediary might be discovered. 

     After Archaeopteryx, early birds retained certain reptilian characters for a long time, such as toothed mouths instead of the typical beaks of modern birds.  Their transition into truly modern forms was a long time in coming, and did not reach fruition until the Eocene and Oligocene – long after the dinosaurs became extinct.  Thus, the amount of time it took natural selection to perfect the new form was much longer than the time it took rapid evolution to build the basic structure of the new form.  As with the snakes, it appears that there are two mechanisms for evolution in play – rapid evolution of new forms by means of an unknown force, followed by gradual evolution of existing forms by means of natural selection. 

     The question is, what causes the rapid evolution?