To demonstrate that non-avian(non-flying) dinosaurs and birds are related, scientists needed fossils of both advanced dinosaurs and primitive birds to compare characters (scientific term for trait). But at the time the early bird Archaeopteryx was discovered, no good specimens of small, advanced dinosaurs had been found. When they were discovered at the end of the 20th century, they showed that, except for the relative length of the forelimb, there is very little difference between primitive birds like Archaeopteryx and advanced theropods(a group of carnivorous dinosaurs) like the dromaeosaurVelociraptor.
wrists that contain a crescent-shaped bone
hands like most other advanced theropods, with three fingers in which the middle finger is the longest
Part of the reason that Archaeopteryx was so important is that for a long time it was the only well-preserved early bird fossil. All of that is now changed. Several newly found fossils document transitional body plans from non-avian dinosaur to modern bird.
From the perspective of a family tree
The best way to look at these is from the perspective of a family tree. Moving up the genealogic ladder from Archaeopteryx to modern birds, the next common bird is Confuciosornis. This animal was found about 20 years ago. Now there are nearly a thousand specimens. Unlike Archaeopteryx, it lacks a long tail and has no teeth. Nevertheless, it still has three separate bones in its hand. Some Confuciusornis specimens preserve long tail feathers, like those of a bird of paradise. These feathers may represent the breeding plumage of males.
After Confuciusornis there is a line of primitive birds that leads directly to modern birds. Included in this diversity from the Mesozoic era are birds like Hesperornis—a 4-foot-tall water bird that lived about 70 million years ago in the seaway that bisected North America. Hesperornis could not fly, and it had teeth. Another particularly interesting animal is Apsaravis, a small, robin-size flying bird from Mongolia. This little animal was an active flyer that undoubtedly flew above the heads and skittered around the feet of its non-avian dinosaur relatives.
But the dominant group of birds that lived during the Mesozoic was the enantiornithines, otherwise known as “opposite birds.” These animals had an extensive distribution and diversity during the age of dinosaurs. Some of them had teeth, some were probably good flyers, and others more closely resembled ground birds.
The diversification of the kinds of birds that we see today occurred rather late in bird history. The first representatives of birds that belong to the modern groups are loons, grebes, and similar waterfowl, whose fossils have been found at the end of the Cretaceous (144 to about 66 million years ago), just before the non-avian dinosaurs became extinct.
Archaeopteryx is the most primitive known bird and therefore most primitive known avian dinosaur.
The first specimen was discovered in the 1850s, just about the same time that Charles Darwin published On the Origin of Species. This book was one of the most influential ever published in that it laid out the basic principles of evolution by natural selection. Darwin’s book stated that evolution was the great sum of small changes; in this way it predicted that transitional forms would be found between modern species. Archaeopteryx, as was immediately recognized by Thomas Huxley, was just one of these physical links between non-avian dinosaurs and birds. It was a close relative of birds in that it had a wishbone and a reversed first toe on its foot. (This is the toe that allows some birds to perch.) It also had feathers. Some of the feathers were asymmetric, meaning that the front edge of the feather was shorter than the back edge. Such feathers indicate that Archaeopteryx had some capacity for flight.
Yet at the same time, it had many characteristics that are more primitive and more like non-avian theropods. For example, Archaeopteryx had a long bony tail similar to dromeosaurs, while in modern birds this tail is shortened and fused into the triangular piece called the pygostyle. Archaeopteryx also had teeth, three separate front fingers, and a breastbone that lacked a keel, like many non-avian theropods (except T. rex and oviraptorids). In modern birds, the teeth are lost, and the three fingers and the crescent-shaped bone in the wrist are all fused into a single bone in the hand.