CHAPEL HILL -- Careful study of bird, alligator and turtle embryos at early stages offer convincing evidence that the "fingers" of bird wings correspond to the index, middle and ring fingers of humans, while the little finger and "thumb" have been lost.
Such developmental evidence of digit identity conflicts with the theory that modern birds arose from dinosaurs as some paleontologists have claimed since the 1970s. Dinosaurs had "fingers" corresponding to the first, second and third fingers on human hands, and as a result, it is almost impossible to envision how a bird wing could have evolved from a dinosaur hand.
That's the view expressed in Friday's (Oct. 24) issue of the journal Science.
Dr. Ann C. Burke, a developmental biologist who is assistant professor at the University of North Carolina at Chapel Hill, conducted the new studies and wrote the Science article with Dr. Alan Feduccia, professor and chair of biology at UNC-CH. Feduccia has been a strong critic of the belief that dinosaurs gave rise to birds.
"The theory that birds descended from dinosaurs has become dogma in the past 20 years or so, and yet a large number of people do not accept it because there are insurmountable problems with that theory," Feduccia said. "First, there is the time problem in that superficially bird-like dinosaurs occurred some 30 million to 80 million years after the earliest known bird, which is 150 million years old."
Most of the bird-like dinosaurs were "looking at the meteor some 65 million years ago," he said, a reference to the giant meteor believed to have struck the Earth then and killed off all dinosaurs within a short time.
"Second, flesh-eating dinosaurs thought to have given rise to birds were large earth-bound creatures with heavy balancing tails and short forelimbs. This is absolutely the worse body plan for the evolution of bird flight."
Third, he said, if one views a chicken skeleton and a dinosaur skeleton through binoculars they appear similar, but close and detailed examination reveals many differences. Theropod dinosaurs, for example, had curved, serrated teeth, but the earliest birds had straight, unserrated peg-like teeth.
All dinosaurs had a major joint in the lower jaw that early birds did not. Birds have a reversed rear toe that opposes the front three toes and allows birds to perch. Dinosaurs had no reversed toe. Birds grow a girdle of bone in their chests quite different from dinosaur chests.
The new work involved microscopic examination of early limb development in ostriches, chickens, cormorants, alligators and turtles and comparison of chick fore- and hindlimbs.
"We know that dinosaurs developed "hands" with digits one, two and three -- which are the same as the thumb, index and middle fingers of humans -- because digits four and five remain as tiny bumps or vestiges on early dinosaur skeletons," Feduccia said. "Apparently dinosaurs developed a very specialized, almost unique "hand" for grasping and raking.
"Our studies of bird embryos, however, show that only digits two, three and four develop, and this creates a new problem," he said. "How do you derive a bird "hand," for example, with digits two, three and four from a dinosaur hand that has only digits one, two and three" The answer is that you can't."
Findings from the examination of alligator and turtle embryos were consistent with those of birds, the biologist added.
Far more likely is that birds and dinosaurs had a much older common ancestor, he said. Many superficial similarities between birds and dinosaurs arose because both groups developed body designs for walking upright on two hind legs and began to resemble each other over millions of years.
"The dinosaurian origin of birds is based on sloppy science," Feduccia said. "It is a fantasy by which one believes it's possible vicariously to study dinosaurs at the backyard bird feeder."
In an accompanying commentary in Science titled "The Forward March of the Bird-Dinosaur Halted?", Dr. Richard Hinchliffe of the University of Wales said the new report calls into question the dinosaurs-to-birds idea and is a forceful statement of the opposing theory.
"...The present paper gives the developmental evidence a sharp focus which makes it a timely contribution to current debate on bird origins," wrote Hinchliffe, the world authority on vertebrate limb development. "This convincing evidence of 2-3-4 wing digit identity will not be to the liking of ... supporters of a dinosaur origin of birds.
"For the time being, this important developmental evidence that birds have a 2-3-4 digital formula, unlike the dinosaur 1-2-3, is the most important barrier to belief in the dinosaur-origin orthodoxy."
Materials provided by University Of North Carolina At Chapel Hill. Note: Content may be edited for style and length.
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