Wednesday, February 1, 2006

Did feathered dinosaurs exist? Are birds the living descendants of dinosaurs? Paleontologists have been asking themselves these questions for decades, and the debate surrounding them continues around the world.

British paleontologist Thomas Huxley started the debate in 1868 when he suggested that birds descended from dinosaurs because of their similar body structures, which included their identical ankle joints and backward-pointing phalanx, or fourth toe.

Paleontologists are still split on the issue. Many, like Matt T. Carrano, curator of Dinosauria at the Smithsonian’s National Museum of Natural History, say birds have descended from a subgroup of maniraptoran theropod dinosaurs that ate meat, stood on their two back legs and were lizard-hipped.

These scientists believe birds evolved from a maniraptoran theropod because the two have more characteristics in common — more than 20 — than dinosaurs and other animals, such as reptiles.

“For me, it’s not a single piece of evidence,” Mr. Carrano says. “There’s such a long and detailed list of similarities. It includes their skeleton, their reproduction.”

Thomas R. Holtz Jr., the senior lecturer of vertebra paleontology at the University of Maryland, says these similarities include the fact that these species have similar skeletal structures; enlarged, advanced brain cases that give them better spatial control; cervical vertebrae that enable them to stretch their necks into an S shape; pneumatization, or hollow air sacs, in their lungs; and eggshell microstructures.

“Today, with the current evidence, we can reject the idea that birds came from another group,” Mr. Holtz says.

Others, including Larry D. Martin, senior curator of paleontology at the University of Kansas’ Natural History Museum, say birds are not descendants of dinosaurs but share a common, still unknown, ancestor.

Their main argument is that birds could not have evolved from dinosaurs because they have different structures in their toes. Mr. Martin says birds have the second, third and fourth digits that humans have, while dinosaurs have the thumb, second and third digits.

The 10th specimen of Archaeopteryx, the 150-million-year-old creature that is considered the oldest, most primitive bird to be studied closely, has further complicated the debate because of its combination of bird and maniraptoran theropod features.

Paleontologists such as Mr. Carrano and Scott Hartman, science director at the Wyoming Dinosaur Center, consider this fossil to be further evidence that a group of maniraptoran theropods developed into modern-day birds.

This latest Archaeopteryx is seen as the link between birds and groups of maniraptoran theropods because though it is classified as a bird, it contains many features characteristic of these dinosaurs.

“This is a perfect example where we have an intermediary between one group and another,” Mr. Carrano says of the Archaeopteryx. “It’s on its way to being a modern bird, but it’s not there.”

The scientists in this group say this Archaeopteryx fossil shows how dinosaurs evolved into birds because though it has bird structures like feathers and a wishbone, its foot is more similar to a maniraptoran theropod than a bird.

Mr. Martin disputes Mr. Hartman and other scientists’ claims about the lack of a reversed toe. He says that by looking at a photograph of the fossil, he was able to see that the hallux of the newest Archaeopteryx was indeed reversed just as birds’ feet are.

“The claw on the back toe points forward,” Mr. Martin says. “It is still a typical bird toe.”

Mr. Hartman does not agree with Mr. Martin’s logic because he says in order to have a reversed toe, the metatarsal, the bone on the hind foot between the ankle and toes, would have to be bent.

“It would have to be twisted, and it just isn’t,” Mr. Hartman says of the metatarsal of the newest Archaeopteryx.

There also has been a debate among these groups of paleontologists over the discoveries of feathered theropod dinosaurs like the Protarchaeopteryx robusta and the Caudipteryx zoui. These were found in 1997 and 1998 in Liaoning Province, China, in a formation that housed fossil remains from the Cretaceous period.

Mr. Holtz argues that downy coverings that have been found on these specimens are indeed feathers, which validates the idea that feathers could have evolved on non-flying dinosaurs for purposes other than flying, such as keeping warm.

“The big things that are coming from their arms and tails are feathers,” Mr. Holtz says of the oviraptosaur dinosaurs he observed, which are part of the same maniraptoran theropod group as the Caudipteryx zoui. “They are feathers by everyone’s definition.”

Alan Feduccia, an evolutionary biologist and former chairman of the Biology Department at the University of North Carolina, does not believe the structures found on the Chinese fossils are feathers. He contends in his paper “Do Feathered Dinosaurs Exist? Testing the Hypothesis on Neontological and Paleontological Evidence” that the structures found in these dinosaurs are collagen fibers, which are proteins that often form the connective tissues of animals.

“I would contend that there’s no such thing as feathered dinosaurs,” Mr. Feduccia says. “They have a feathery appearance, but have nothing to do with feathers.”

Even though these two groups of paleontologists do not agree on the evidence of feathered dinosaurs, they are connected because they are both searching for an ancestor to birds.

Mr. Martin says birds’ ancestors were most likely small animals from the Triassic period that lived in trees and glided.

For Mr. Hartman, Mr. Holtz and Mr. Carrano’s group, the key to their search is finding which group of maniraptoran theropods birds evolved from.

The major candidates so far are the sharp-clawed and agile dromaeosaurs and the small-in-size but large-brained troodontids, although many paleontologists are still open to the possibility that these two groups of dinosaurs and today’s birds all descended from a theropod from the late Jurassic period that has not yet been discovered.

“We are trying to narrow down what group of dinosaurs is closest to birds,” Mr. Hartman says. “They may be the direct descendents of all these groups, but we haven’t proven that. It may be that there is one that is the ancestor of all three.”

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