Thursday, October 23, 2008

More feathers are flying... or, not.

While it was often hypothesized in the 20th century that dinosaurs were the evolutionary ancestors of birds, it wasn't until the late 1980s that we found the first firm evidence of a dinosaur with feathers - specifically, quill knobs (which are strongly correlated with large and well-developed secondary feathers), like those recently found by Norell et al (2007) on an ulna of Velociraptor mongoliensis. But the question still remained as to why feathers evolved in the first place. Theories range from insulation to flight, and the answer is still hotly debated by paleontologists. While flight is an obvious advantage, some say that dinosaurs couldn't just 'choose' to suddenly have feathers so they could fly, and thus there must be some other explanation for the initial development of feathers. Or, put bluntly by Yale Ornathologist Dr. Richard Prum (2002), “Concluding that feathers evolved for flight is like maintaining that digits evolved for playing the piano.” What else, then, could have spurred the evolution of feathers?

The answer may lie in new findings by Zhang and colleagues from the Daohugou Formation of Ningcheng County, north-eastern China reported this week in Nature. The fossil they describe is of a tiny dinosaur (about the size of a pigeon) that lived at least 2 million years before the first known birds. The new species, Epidexipteryx hui, is unique to begin with, but what has really gotten the scientific community excited are four, long objects that protrude from the rear of the animal. What are they? In all likelihood, they are homologous structures to feathers, dubbed "Elongate Ribbon-Like Tail Feathers" (ETFs) by the authors. These structures are too short and not the right design to be tail feathers for flight. No, these were likely used for display, suggesting that the initial evolution of feathers was as some form of signal.

Dr Fucheng Zhang, from the Chinese Academy of Sciences in Beijing, explains:
Elongate tail feathers (ETFs) are a normal component of the ornamental plumage in extant (living) birds. In contrast to other feather types, ornamental plumage is used to send visual signals that are essential to a wide range of avian (bird) behaviour patterns, particularly relating to courtship... It is highly probably that the ETFs of Epidexipteryx similarly had display as their primary function, rather than serving other purposes such as flight or insulation.

Epidexipteryx hui is also covered with 'non-ETF' structures that are equally interesting. The species is covered in complex, filamentous structures, which fit nicely into a theory of feather evolution proposed by Prum and others (2002, 2003). They proposed feathers began as hollow cylinders (stage 1), became unbranched barbs (stage 2), and then became more branched (stage 3), resulting in the complex, vaned feathers in today's birds. The structures covering Epidexipteryx huiare very similar to the expected stage 2, thus supporting the model for feather development. They even seem to attach at a membranous base, which may be homologous to the follicle collar where current birds' feathers begin.

However, not all believe that feathers could have evolved for any reason other than aerodynamics. Luis Chiappe, a paleontologist with the Natural History Museum of Los Angeles County, responded to the new study by saying that "feathers could have served an aerodynamic function of some sort whether you fly or not. You could flap feathered wings and run faster."

The remains date back to 152 million to 168 million years ago, making the newfound creature the oldest therapod — a group of bipedal dinosaurs that includes Tyrannosaurus rex — to possess any kind of 'feathers.' This means that the new find is the oldest bird relative found to date. If scientists are right about the use of its ribbon-like structures, it is possible that the dinosaur ancestors of birds evolved their plumage first for display (think a peacock's tail), and only later did they modify these adaptations to fly.