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Telltale tracer. The Hoxd13 gene is active in the first digit in developing wings (dark stain, right), but Hoxd12 isn't (left).

Let's Shake Wings

Molecular studies have smoothed a wrinkle in the assumption that modern birds had dinosaur ancestors. After tracing the expression of two genes important in the development of digits in wings and other limbs, researchers have concluded that the three digits in bird wings correspond to the three digits in dinosaurs' forelimbs.

Over the past decade, new fossils and analyses of family trees have convinced most paleontologists that birds evolved from dinosaurs. A few researchers disagree, and one tenet of their argument has to do with how to count fingers. Terrestrial vertebrates typically have five fingers. In both dinosaur fossils and birds, just three digits are fully developed, a trait that at first glance supports a dinosaur-bird connection. But dinosaur forelimbs have the first three digits, with stubs for the last two. In contrast, going by some embryological evidence, birds appear to have retained the middle three fingers.

There's a way to identify the real first digit in embryos; it hinges on two genes crucial for digit development, Hoxd13 and Hoxd12. In mice, the development of the first digit relies on Hoxd13 but not Hoxd12, whereas the other digits need both. The first digit also forms differently. With this in mind, biologists Alexander Vargas of the University of Chile in Santiago, and John Fallon of the University of Wisconsin, Madison, examined the development of chick embryos.

They found that the wing's initial digit--until now considered to be digit 2, especially by opponents of the bird-dinosaur theory--used Hoxd13 but not Hoxd12, indicating that it really is the first digit, developmentally speaking. Birds therefore have the same digits as dinosaurs, Vargas and Fallon conclude in the January issue of Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. In birds, the first digit is simply masquerading as the second one.

"This may settle a long-standing controversy and will strengthen the theropod [dinosaur]–bird link," says Sankar Chatterjee, a paleontologist at the Museum of Texas Tech University in Lubbock. But Frietson Galis, a functional morphologist at Leiden University in the Netherlands, is not convinced that the Hoxd13 and Hoxd12 pattern is meaningful. Galis says he recently obtained evidence from birds with abnormal digit patterns that the digits in bird wings are equivalent to digits 2, 3, and 4 in other animals.

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