For many years a fossil discovered in a Bavarian limestone quarry was considered to be the first bird. Archeopteryx lithographica (ancient wing of limestone) lived about 150 million years ago and had many reptilian and avian characteristics; of the latter, feathers were the most notable. Its discovery was a very clear indication that birds evolved from reptiles and is considered to be a classic intermediate evolutionary form. Discovery of other fossils over the years and some very recent evidence indicates that Archeopteryx is probably not a good first bird, though. One clue: analysis of its bone growth indicates that it grew slowly, more like a reptile than a bird.
The most recent fossil evidence of avian origins was found in 2011 in western Liaoning, China and dates from about 161 to 145 million years ago. This fossil, Xiaotingia zhengi, is similar to Archeopteryx in that it had feathers, possessed claws on the ends of its forelimbs, sharp teeth, long finger bones and wishbone. But both are apparently more closely related to reptiles like Velociraptor and Microraptor than birds. Like many ideas in science, this interpretation is new and only tentative and surely new discoveries will shed yet more light on the subject.
What’s also interesting about Archeopteryx is the attention it has gotten from creationists over the years. For ages, creationists have argued against the existence of any organism that is truly intermediate between two major groups. I could go on for pages listing the characteristics of Archeopteryx that apply to reptiles only, birds only, or both. It truly is an intermediate form.
Out of curiosity I looked at some creationist websites. Some creation sites agree that Archeopteryx is an intermediate but say that doesn’t mean anything because the bible (in a very convoluted way) mentions (according to the writer’s interpretation) an intermediate between birds and reptiles. Other sites argue that Archeopteryx is just a bird because it had legs like a perching bird and could fly (both wrong.) And yet more creationist sites argue that the fossil was a reptile because earlier dinosaur-like fossils had feathers (which argues for their being intermediates as well.)
Feathers evolved about the time ancient reptiles were developing homeothermy (warm-bloodedness) and perhaps had some value as insulation. Scales grew longer and eventually served that purpose; later they became feathers with the purposes they have now: insulation, decoration, waterproofing, and flight. Recent evidence, however, suggest that these feathers evolved as communication – signals, as they were colored and showed patterns. We’ll probably find more as the fossil record is always being added onto; but fossils are hard to find, often in pieces, and typically only consist of hard parts – bones, teeth, scales – and rarely provide information about internal organs, color, metabolism, or behavior.
Science is a process, always open to additional evidence. Conclusions are usually temporary (which could be a long time.) But there’s enough evidence about most areas of scientific inquiry that changes are typically small tweaks; rarely do major changes come about. A century from now we’ll know a lot more about the evolution of birds but I bet we still won’t have all the answers.