Researchers, led by Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences in Beijing, have described the specimens as Similicaudipteryx dinosaurs from the Lower Cretaceous. The fossils were found in western Liaoning Province in China. The two small oviraptors were preserved at different developmental stages and showed distinct feather types and patterning—much more so than is seen in the various stages of modern birds' maturation.
"This find suggests that early feathers were developmentally more diverse than modern ones and that some developmental features, and the resultant morphotypes, have been lost in feather evolution," the researchers noted in their study, published online April 28 in Nature (Scientific American is part of Nature Publishing Group).
The smaller of the two specimens (known as STM4-1) was less than a third of the size of the elder Similicaudipteryx (referred to as STM22-6). It had short ribbon-like flight feathers on the front forearms and much longer ones on the tail, in addition to downy "plumulaceous" feathers covering the rest of its body. The bigger of the two, STM22-6, had much longer, more developed pennaceous flight feathers on its forearms and tail, "possibly reflecting an increase in the functional role of the [wing feathers] as the individual approached adulthood," the authors suggested. It also had quite long (five centimeter-long) plumulaceous feathers around its head and pelvis.
This complex pattern of feather development is "not known in any modern bird," concluded Xu and his colleagues, despite the fact that theropod dinosaurs are the ancestors of modern birds. The team speculated that unlike the early expression of the genes that initiate growth of mature pennaceous feathers in today's young birds, activation of similar genes in the younger Similicaudipteryx "was probably delayed and incomplete," producing the curious and diverse state of feather growth seen in the two specimens.
Image of artists' rendering of the two different Similicaudipteryx specimens courtesy of Xing Lida and Song Qijin