Discovery Strengthens Evolutionary Link Between Birds And Dinosaurs
A team of scientists announced last week (June 23) in Nature the discovery in northeastern China of two 120-million-year-old dinosaur species, both of which show unequivocal evidence of true feathers. Both remarkable new creatures provide further support for the theory that birds evolved from small, meat-eating, ground-dwelling dinosaurs and give new insights into the origin of birds.
The discovery was announced by an international team of scientists: Ji Qiang, director of the National Geological Museum of China; Phil Currie, curator, Royal Tyrrell Museum of Paleontology in Drumheller, Canada; Mark Norell, chairman and associate curator, Department of Vertebrate Paleontology, American Museum of Natural History, New York; and Ji Shu-An, of the National Geological Museum of China. The find is the cover story of the July issue of National Geographic magazine.
One of the new creatures was named Caudipteryx zoui, or "tail feather," for the fan of plumes that is visible at the end of the animal's tail. Down-like feathers and "semi-plumes" are visible on the fossil, suggesting that most of its body was feather-covered. Unlike Archaeopteryx - long considered the earliest known bird - and modern birds, whose wing-feathers are asymmetrical, however, Caudipteryx's wing-feathers are symmetrical. A symmetrical feather lacks the aerodynamic qualities thought to be necessary for flight, so it is unlikely that Caudipteryx could fly.
The other new species, Protarchaeopteryx robusta, is so-named because it resembles Archaeopteryx, but is more primitive. Protarchaeopteryx is about the size of a modern-day turkey, and was close to maturity when it died. Like Caudipteryx, most of its body was probably covered with feathers, although no evidence of wing-feathers was preserved. The relatively long legs of both animals show that they were swift, ground-dwelling runners.
In order to determine precisely where the two new species fall on the family tree of birds and dinosaurs, the research team examined ninety separate physical characteristics seen in the fossils. The analysis showed that neither Caudipteryx nor Protarchaeopteryx was a true bird, but that both were dinosaurs that were very closely related to birds - indeed, Caudipteryx was determined to be one of the dinosaurs most closely related to true birds. Protarchaeopteryx was found to be more primitive, and may be a close relative to the Velociraptorinae, a group of dinosaurs that takes its name from its most famous member, Velociraptor, although that relationship is not yet fully resolved. "At this point, the evidence indicating that birds are derived from dinosaurs is absolutely overwhelming," says Dr. Norell. "We have dozens of anatomical attributes that unambiguously connect the theropod dinosaur lineage to that of birds. The discovery of the feathered dinosaurs from China provides more dramatic and compelling evidence that the dinosaurs did not become extinct, and in fact are more diverse today than at any other time in the Earth's history." The presence of feathers in animals that are not birds raises fundamental questions about how "birds" should be defined. Traditionally feathers have been considered a critical feature in identifying an animal as a bird. The new discovery shatters this long-standing convention, and proves that feathers can no longer be considered the central defining characteristic of what is and what is not a bird.
"The new discoveries made in the western part of Liaoning Province, northeast China, provide conclusive evidence that birds developed from small theropod dinosaurs," says Dr. Ji Qiang. "But the same fossils that solidify the relationship between birds and dinosaurs are blurring the definition of what precisely constitutes a 'bird.' Our continued research in the fossil-bearing beds of Liaoning Province will almost certainly yield more discoveries that will illuminate this complex question."
If neither Caudipteryx nor Protarchaeopteryx were able to fly, what possible purpose did their feathers serve? The most likely explanation is that the feathers arose not to power flight, but as insulation. This has interesting implications for the long-standing debate concerning whether some dinosaurs were warm-blooded. The potential use of feathers to retain body heat would argue against a strictly cold-blooded metabolism, although it does not mean that dinosaurs or early birds had metabolisms comparable to those of birds today. Recent discoveries indicate that they had an intermediate type of metabolism that could be considered neither simply cold- nor warm-blooded.
The discovery of the feathered dinosaurs adds momentum to a shift away from the belief in the primacy of the famous fossil Archaeopteryx as the key to understanding the origin of birds. Numerous significant fossil finds in the past decade, including the discoveries announced this week, have caused scientists to dramatically revise their view of the stages in the evolution of birds and the origin of flight.
One of the many specimens that has contributed to this new understanding was found in 1996 at the same site as Caudipteryx and Protarchaeopteryx, and was described and named by Dr. Ji Qiang, who called the animal Sinosauropteryx prima, or "first Chinese dragon feather." Sinosauropteryx, a ground-dwelling runner, had tiny fibers believed to be "proto-feathers" bristling down its back. These proto-feathers, coupled with the primitive true feathers of the fossils described this week, give unprecedented insight into the evolutionary development of this feature so long associated with birds.
The discovery in 1997 in Patagonia of the new species Unenlagia provided evidence for another stage in the dinosaur/bird transition. This eight-foot-long flightless creature was capable of moving its arms up and down in a manner that was a precursor to the flapping movement a bird uses in flight that seems to represent a critical step towards powered flight.
Discoveries made in the 1990s by Dr. Norell and the American Museum of Natural History/Mongolian Academy of Sciences expedition team in the Gobi Desert have also included a number of fossils that reinforce the link between birds and dinosaurs. Among these discoveries was an Oviraptor found nesting on its eggs just as modern birds do, providing the first direct evidence of parental care among dinosaurs. The team also discovered the two new species Mononykus and Shuvuiaa, both long-legged, long-tailed animals with stubby arms, which, despite their very odd appearance, are more closely related to modern birds than is Archaeopteryx.
"For more than a century, scientific understanding of bird evolution relied almost exclusively on a handful of fossils, all of which were discovered in the late 1800s," says Luis Chiappe, research associate in the American Museum of Natural History's Department of Ornithology and past member of the Gobi expedition team. "The last decade has brought an avalanche of new discoveries that have made the evolutionary link between dinosaurs and birds indisputable, and have made it possible to begin to understand the evolution of the wing and feathers, as well as the origin of flight. The recent progress in this field is unprecedented, and with the discovery of astonishingly rich new fossil sites, it promises to continue."
Materials provided by American Museum Of Natural History. Note: Content may be edited for style and length.
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