New evidence raises questions about current theory
Biologists examining evidence for the claim that birds evolved from dinosaurs have reached some surprising new conclusions. However, they caution that "the problem of avian origins is far from being resolved." Their analysis is published online October 10, 2005 in the Journal of Morphology, published by John Wiley & Sons, Inc. and available via Wiley InterScience (www.interscience.wiley.com/journal/morphology).
Dinosaurs have long captured the imagination while their relationships have eluded full explanation. Innovative research and a comprehensive consideration of the old can also inspire new interpretations, as researchers recently found when examining the evidence supporting the current theory about feather origins and the relationships of birds and dinosaurs.
All experts agree that birds are related to theropod dinosaurs; however, debate has raged on over whether today's winged creatures are derived directly from advanced theropods, or from an earlier shared ancestor. The current theory supports direct derivation, but recent fossil discoveries in China have led to new questions about the claim. The Chinese discoverers reported finding all stages of feather evolution and ancestral birds, even though the deposits are at least 25 million years younger than those containing the earliest known bird Archaeopteryx.
Researchers, led by Alan Feduccia of the University of North Carolina at Chapel Hill, considered the new findings in the context of the existing literature and furthered the knowledge base with additional experiments. Theagarten Lingham-Soliar of the University of KwaZulu-Natal in South Africa examined the skin of modern reptiles, the effects of decomposition on skin, and the fossil evidence relating to alleged feather progenitors (protofeathers). Richard Hinchliffe of the University College of Wales also examined evidence relating to the tridactyl hand, which is composed of digits 1-2-3 in dinosaurs, the most critical characteristic linking birds to derived theropods.
Feduccia, Lingham-Soliar and Hinchliffe found no evidence for the existence of protofeathers on dinosaurs and no evidence in support of the morphogenesis of the feather from putative filamentous protofeathers. They suggest that 'protofeathers' described on fossil findings "are probably the remains of collagenous fiber 'meshworks' that reinforced the dinosaur integument." Based on their examination of fossilized remains of dinosaurs with no relationship to birds, they suggest that decomposition of skin can lead to patterns resembling feathers.
Birds have been thought to be related to theropod dinosaurs because both groups have a hand reduced to three digits. Theropods are known from fossil evidence to exhibit a hand with digits 1-2-3, the thumb and next two digits. However, the researchers found that the vast majority of evidence supports a 2-3-4 digit identity for bird wings. The bird hand "appears different from that in theropod dinosaurs," they say, and casts doubt upon the theropod derivation hypothesis. Finally, they discuss the significance of the Chinese discoveries with respect to bird origin and flight.
The authors emphasize that the totality of evidence from various branches of science must be included if we are ever to solve the mystery of bird origins and the origin of avian flight. From their careful examination of the available evidence they offer an interim attempt to define morphologically the most salient features of Aves: "Birds are mesotarsal bipedal archosaurs with pennaceous feathers, and a tridactyl avian hand composed of digits 2-3-4."
It is too early to abandon debate on the origin of birds, the authors say. However, mounting evidence suggests that "a possible solution to the disparate data is that Aves plus birdlike maniraptoran theropods (e.g., microraptors and others) may be a separate clade, distinctive from the main lineage of Theropoda, a remnant of early avian radiation, exhibiting all stages of flight and flightlessness."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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