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Feathered dinosaurs are regarded by many paleontologists as the "missing link" between birds and dinosaurs. It was already well known that ancient birds such as Archaeopteryx had many saurian characteristics, such as teeth, and claws on their fingers, and for many years it had been theorised that birds evolved from theropod dinosaurs. In the late 1990s, discoveries of feathered dinosaurs provided conclusive evidence of the connection, though the genealogical details are still being worked out.
Early theories Shortly after the 1859 publication of Charles Darwin's The Origin of Species, British biologist and evolution-defender Thomas Henry Huxley proposed that birds were descendants of dinosaurs. He cited skeletal similarities, particularly between some saurischian dinosaurs, fossils of what was considered the "first bird," Archaeopteryx, and modern birds. In 1868 he published "On the Animals which are Most Nearly Intermediate between Birds and Reptiles," making the case; but the leading dinosaur expert of the time, Richard Owen, disagreed, claiming Archaeopteryx as the first bird, outside dinosaur lineage.
For the next century, claims that birds were dinosaur descendants faded, with more popular bird-ancestry hypotheses including "crocodylomorph" and "thecodont" ancestors, rather than dinosaurs.
Then, in 1964, John Ostrom discovered a fossilized dinosaur he called Deinonychus antirrhopus, a theropod whose skeletal resemblance to birds seemed unmistakable. Ostrom has since become a leading proponent of the theory that birds are direct descendants of dinosaurs. Further comparisons of bird and dinosaur skeletons, as well as cladistic analysis strengthened the case for the link, particlarly for a branch of theropods called maniraptors. Skeletal similarities include: the neck, pubis, wrists (semi-lunate carpal), arm and pectoral girdle, shoulder blade, clavicle and breast bone. In all, over a hundred distinct anatomical features are shared by birds and theropod dinosaurs.
By the 1990s, most paleontologists considered birds in fact to be surviving dinosaurs, and referred to "non-avian dinosaurs" (those that went extinct) to distinguish them from birds (aves, or avian dinosaurs).
Direct evidence to support the theory was missing, however. Some mainstream ornithologists including Smithsonian Institute curator Storrs L. Olson disputed the links, citing the lack of fossil evidence for feathered dinosaurs.
Adding to the controversy, in 1999 a supposed 'missing link' fossil of an apparently feathered dinosaur named Archaeoraptor liaoningensis, found in Liaoning Province, northeastern China, turned out to be a fake.
Fossil evidence After a century of hypotheses without hard evidence, beautifully preserved - and legitimate - fossils of feathered dinosaurs were discovered during the 1990s and 2000s. The fossils were preserved in a Lagerstätte — a sedimentary deposit exhibiting remarkable richness and completeness in its fossils — in Liaoning, China. The area had repeatedly been smothered in volcanic ash produced by eruptions in Inner Mongolia 124 million years ago, during the early Cretaceous. The fine-grained ash preserved the living organisms that it buried in extraordinary detail. The area was teaming with life, with millions of leaves, the oldest known angiosperms, insects, fish, frogs, salamanders, the oldest known mammals, turtles, lizards and crocodilians having been discovered so far.
The most important discoveries at Liaoning have been a host of spectacular feathered dinosaur fossils, with a steady stream of new finds filling in the picture of the dinosaur-bird connection, and adding more to theories of the evolutionary development of feathers and flight.
Current knowledge A number of dinosaurs are now known to have been feathered (see Category:Feathered dinosaurs for a more complete list).
At present, the earliest (known) feathered dinosaur is Sinosauropteryx (Jurassic/Cretaceous, 150-120 mya), whose body was covered with feather-like structures that look like hollow tubes, or hairs. They may or may not have had barbs like downy (plumulaceous) feathers. Another early fossil, Dilong paradoxus (early Cretaceous), an ancestor of Tyrannosaurus rex, also had similar feather structures. These early fossils suggest that feathers originally developed as insulators to maintain body temperatures (and thus also provide evidence for warm-blooded dinosaurs). Flight would have been a later evolutionary adaptation of feathers.
The first known dinosaur with true flight-structured feathers (pennaceous feathers) is Caudipteryx (135-121 mya), although evidence for these is restricted to its tail, so it is unlikely that these feathers were used for flight; they were more likely used for display.
Microraptor, on the other hand, was covered with flight-feathers, both on its arms and legs, making it a four-winged theropod. Analysis indicates that this animal was a glider and not yet a flier, which has led to the speculation that Microraptor was arboreal (tree-dwelling).
Feathered dinosaur fossil finds to date, together with cladistic analysis, provide convincing evidence that birds are in fact descendents of dinosaurs. They also suggest that many theropods may have had feathers, not just those that are especially similar to birds. In particular the smaller theropod species may all have had feathers, and possibly even the the larger theropods (for instance T. rex) may have had feathers in their early stages of development after hatching. Large adult theropods are unlikely to have had feathers, however, as the need for insulation would be less important, since inertial heat retention would likely be sufficient to manage heat.
See also
External links - DinoBuzz (http://www.ucmp.berkeley.edu/diapsids/avians.html), dinosaur-bird controversy explained, by UC Berkley.
- Journal of Dinosaur Paleontology (http://www.dinosauria.com/jdp/jdp.htm), with many articles on dinosaur-bird links.
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