Bird Theory in Flight

__"The Origin of Birds and Their Flight" by Kevin Padian and Luis M. Chiappe, in Scientific American (Feb. 1998), 415 Madison Ave., New York, N.Y. 10017–1111; "The Big Flap" by Larry D. Martin, in The Sciences (Mar.–Apr. 1998), New York Academy of Sciences, 2 E. 63rd St., New York, N.Y. 10021.__

Is that feathered creature outside your doubt," assert Padian, a professor of intewindow a dinosaur, or at least a descendant grative biology and curator in the Museum of one? Yes, beyond any "reasonable of Paleontology at the University of California, Berkeley, and Chiappe, a Fellow at the American Museum of Natural History in New York.

The long-running scientific debate over the origin of birds is now over, they claim: paleontologists have determined "that birds descend from ground-dwelling, meat-eating dinosaurs of the group known as theropods." However, Martin, a paleoornithologist and curator for vertebrate paleontology at the University of Kansas Natural History Museum, maintains not only that the debate is not over but that the bird-dinosaur link has become increasingly dubious.

The controversy began in 1870, when Thomas Henry Huxley, "Darwin’s bulldog," first suggested that theropods and birds were closely related. A century later, Yale University paleontologist John H. Ostrom revived Huxley’s idea. After studying the bones of the 150-million-year-old Archaeopteryx lithographica (unearthed in Germany in 1861 and considered the oldest known bird specimen), Ostrom explicitly proposed that birds were direct descendants of theropods.

His conclusion has been "strongly validated," Padian and Chiappe say, by cladistics, a new method of analyzing the nature of relationships among organisms. Unlike traditional techniques, which might exclude a species from a group solely because it had a trait not shared by others in the group, cladistics arranges organisms on the basis of whether they have a set of newly emerged heritable traits in common. Cladistic analysis, write Padian and Chiappe, "shows that birds are not only descended from dinosaurs, they are dinosaurs (and reptiles)—just as humans are mammals, even though people are as different from other mammals as birds are from other reptiles."

The evidence is not confined to shared skeletal features, Padian and Chiappe argue. Recent discoveries of nesting sites in Mongolia and Montana suggest some similar reproductive behaviors. Skeletons of the Cretaceous theropod Oviraptor

("egg stealer") recently found atop nests of eggs, for example, indicate that instead of living up to their name, the dinosaurs were protecting the eggs in very birdlike fashion.

But Martin and other investigators are skeptical. "In spite of recent fossil finds that might support a dinosaurian origin for birds," he says, "other new evidence contradicting that view is just as strong, if not stronger." Two studies published in Science last fall, he notes, one focusing on lungs and the other on limbs, both argued that dinosaurs are clearly distinct from birds.

Martin himself grew disenchanted with the dinosaurs-to-birds theory after comparing some 85 anatomical features the two vertebrates were said to share. "To my shock, virtually none of the comparisons held up," he writes. The confusion over anatomy is partly due, he believes, to gaps in the ornithological

literature about many aspects of the avian skeleton. Dinosaur specialists generally leave avian anatomy to the ornithologists, who usually prefer to study birds’ songs, plumage, and behavior rather than their bones and muscles. Existing anatomical knowledge of both dinosaurs and Archaeopteryx, meanwhile, is "just blurry enough" to justify bird-dinosaur comparisons of anatomical features that do not precisely match. "When the burden of ad hoc repairs became too heavy for me, I had to abandon the theory altogether," Martin writes. "It was a disappointment. How wonderful it would have been if dinosaurs had escaped extinction!"