The origin of birds is one of the most interesting questions in palaeontology and evolution. Birds evolved from theropod dinosaurs during the Mesozoic era.
Thomas Henry Huxley ("Darwin's bulldog"), who was a comparative anatomist, made a study of this nearly 150 years ago. He compared the first fossil bird, Archaeopteryx, with a small theropod dinosaur, Compsognathus. They are two fossils from the same place and time: the Solnhofen limestone in Bavaria, Germany. The strata come from the end of the Jurassic period, about 144 million years ago.
Huxley's papers on Archaeopteryx, and the origin of birds, were of great interest then, and still are. He showed that the two fossils were almost identical, except for the front limbs and feathers of Archaeopteryx.
He united reptiles and birds under the title of Sauropsida, and thought that birds had evolved from small carnivorous dinosaurs.
Modern ideas
All palaeontologists since Huxley agreed that Archaeopteryx evolved from reptiles, but many thought it was derived from an earlier type than dinosaurs. It took a long time before they agreed with Huxley. Almost all now think that early members of a group of small carnivorous dinosaur were ancestral to birds.
The group most favoured as ancestral is the Paraves or Eumaniraptora, which includes the dromaeosaurs and the troodonts. The dromaeosaurs include Velociraptor and Deinonychus but, of course, it would have been an earlier type of dromaeosaur, perhaps from the lower Jurassic. Dromaeosaurs have several traits we used to think were special to birds: hollow bones with holes for air-sacs, limbs with many bones either lost or fused, clavicles fused into wishbones, and feathers.
The discovery of feathered dinosaurs and birds in the Lower Cretaceous of China was what finally convinced palaeontologists. What seems to have happened is this. Theropod dinosaurs evolved feathers for heat regulation rather than flight. From then, a small group began to use them for primitive flight. This group is represented in the Jurassic by Archaeopteryx and Anchiornis.
Mosaic of old and new
Huxley had pointed out that Archaeopteryx was a mixture of reptile and bird features. Without the feathers and arms, its skeleton looked just like that of Compsognathus. We now know its bone growth physiology was much slower than modern birds, and more like that of its dinosaur ancestors. This means it would take longer after hatching before it could fly. A modern precocial bird takes for 3–6 weeks from hatching to flying. In Archaeopteryx this milestone might have taken about 18 weeks. It might have taken two to three years to reach its final adult size. The evolution of the physiology of modern forms occurred later in the group's history. They have had over 140 million years to evolve after Archaeopteryx. This is an example of mosaic evolution, where some characters in a transitional form are basal, while others are remarkably advanced.
Chinese bird fossils
Jurassic fossils
Anchiornis huxlei is so far the only dinobird fossil known which lived before Archaeopteryx. It was found in Liaoning, China. The strata dates to the mid-late Jurassic period, 160 to 155 million years ago. There are also feather-bearing non-flying dinosaurs from the Jurassic of China, such as the maniraptoran Epidexipteryx. Epidexipteryx is the earliest known example of ornamental feathers in the fossil record, about 152 to 168 million years ago.
Anchiornis had large wings, made up of flight feathers attached to the arm and hand (as in modern birds) as well as flight feathers on the hind legs, forming an arrangement of fore and hind wings. The forewing of Anchiornis was composed of 11 primary feathers and 10 secondary feathers. The primary feathers in Anchiornis indicated poorer aerodynamic ability compared to its later relatives. In Microraptor and Archaeopteryx, the longest forewing feathers were closest to the tip of the wing, making the wings appear long, narrow, and pointed. However, in Anchiornis, the longest wing feathers anchored near the wrist, making the wing broadest in the middle and tapering near the tip for a more rounded, less flight-adapted profile.
So the animal could glide, but probably not fly.