Exceptional Fossils Record Dinosaur Feather Changes

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Over the past decade and a half paleontologists have found the remains of numerous feathered dinosaurs, but, as announced in this week's edition of Nature, a new pair of specimens may show how the feathers of some of these dinosaurs changed as they grew up.

Among birds, feather growth is relatively straightforward. They are covered in a downy coating of fuzz as hatchlings but quickly grow their adult plumage, and they remain at this stage of feather development for the rest of their lives. Dinosaurs may have been different. In scrutinizing a young juvenile and an older juvenile of the recently-discovered oviraptorosaur Similicaudipteryx, paleontologists Xing Xu, Xiaoting Zheng and Hailu You noticed that there was a significant difference in the feather types. The long feathers on the arms (remiges) and those on the tail (retrices) of the younger individual were wide and ribbon-like where they attached to the body, whereas on the older individual these feathers were connected by central shafts and resembled the same feathers seen in living birds.

What does this disparity in feather construction mean? The authors of the new study suggest that, rather than transitioning from down to fully-developed feathers, there was a longer period of feather change in Similicaudipteryx in which a successive series of molts allowed the dinosaurs to grow slightly different feathers (the feathers themselves are not changing, in other words, but are being replaced by different feathers after being shed). If this hypothesis is correct, then it is the first known indication that dinosaur feathers went through a longer stage of transitions than that seen in modern birds.

But there may be other explanations for the differences between the two fossils. It may be that the younger individual was molting at the time it died, meaning that the more ribbon-like feathers were ones which were just emerging and do not actually represent a different feather stage. This hypothesis is not beyond criticism, either, and to resolve the question more fossils from juvenile Similicaudipteryx will be needed to better understand the growth of these dinosaurs.

Regardless of which hypothesis turns out to be correct, this new study raises some interesting questions about the origins and growth of feathers among dinosaurs closely related to the ancestors of birds. Studies of the genetics and development of living birds will be just as important to resolving these issues as more fossils, and through the combination of these different lines of evidence scientists will not only be better able to identify transitions such as these, but they will be developing new ways to investigate how they actually happened.

For more on this study, see Ed Yong's post at Not Exactly Rocket Science.

Xu, X., Zheng, X., & You, H. (2010). Exceptional dinosaur fossils show ontogenetic development of early feathers Nature, 464 (7293), 1338-1341 DOI: 10.1038/nature08965

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