Geoscientist Online

New fossils from China have thrown new light on the developmental differences between ancient and modern birds, reports Harriet Jarlett*.

Geoscientist Online 19 May 2010

Two astonishingly well-preserved Similicaudipteryx fossils have been discovered in a Lower Cretaceous formation in China. As part of the Family Oviraptorosauria, they represent some of the earliest feathered dinosaurs. Xu Xing and colleagues have discovered that both juvenile specimens show several stages of feather development, suggesting that at some point during the evolution of feathers a profound genetic shift occurred1.

The two specimens, both juveniles, appear to provide new insight into early feather development. The smaller specimen is about the size of a magpie suggesting it is barely more than a chick. The other, more the size of a turkey, also appears not to have quite reached adulthood. The smaller specimen shows two types of feather: large “pennaceous” ones (resembling quill pens with a central shaft that runs through its entire length) along the vertebral column, and more downy, “plumulaceous” ones over the rest of the body. The wing and tail feathers of the more mature specimen are entirely pennaceous, whereas the feathers of the younger dinosaur display both a flat, ribbon-like stem with pennaceous morphology confined to the tip.

This development of a fully adult feather through adolescence is a phenomenon not seen in any modern bird, whose chicks show fully developed feathers, even while still within the egg. "This baby dinosaur has bizarre flight feathers, which are strikingly different from those of adults” says Prof. Xu Xing from the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing.

Ribbon-like pennaceous feathers have been described before in Confuciusornithids and other basal birds. This has prompted Xu Xing and colleagues to suggest that semi-developed feathers represent an early stage of feather evolution, not seen in birds today. This suggests that the maturation of the feathers of proto-birds was far more diverse, and spanned the growth of the organism. Modern birds have been lost these stages through evolution.

It is likely, the researchers suggest, that these partly developed feathers formed as a “low dose” of certain bird genes cause their feather’s keratin sheets not to divide into barbs. Such genes are activated very early in the growth of modern birds, but could have been suppressed until later during the growth of Similicaudipteryx, resulting in the partially pennaceous morphology of younger feathers.

This new evidence of feather development may force palaeontologists to look for new evidence for early feather evolution and morphological types to see whether or not these new ideas about the genetics of early birds are well grounded, or just another flight of fancy.

1. Xu, X., Zheng, X. & You, H. (2010) Exceptional dinosaur fossils show ontogenetic development of early feathers; Nature 464, 1338-1341