Our fine-feathered friends ... dinosaurs?
NEW HAVEN, CONN.
Did baby Tyrannosaurus rex tumble out of its eggshell as a downy fuzzball? Three years ago, that notion would have drawn snickers from many paleontologists. After all, most people held that feathers originated with birds.
Last June, that dogma crumbled when Chinese, Canadian, and American researchers unveiled the first unambiguous fossils of flightless feather-bearing dinosaurs, pulled from deposits in China's Liaoning Province.
Now, new studies of these fossils suggest that a more Barney-like image of T. rex Jr. may not be far from the truth. Detailed analyses of the initial Chinese fossils and of fresh specimens of the same species are prompting several paleontologists to move these feathered dinosaurs earlier on the avian ancestral tree.
As the gap widens between these creatures and the first known prehistoric bird, archaeopteryx, these researchers not only are more convinced than ever that birds are direct descendants of two-legged, meat-eating dinosaurs. They posit that a wide range of these prehistoric theropods sported feathers - either as juveniles or throughout their lives. That, in turn, is sending scientists on a search for fresh explanations for the evolution of feathers and flight.
"These Chinese feathered dinosaurs are opening new realms of research," says Thomas Holtz, a vertebrate paleontologist at the University of Maryland at College Park.
The new results - and speculation on their implications - formed the nucleus of a symposium held earlier this month at Yale University in New Haven, Conn. The meeting was held to honor John Ostrom, who is widely credited for research that revived the idea that birds are linked to theropod dinosaurs - a theory that emerged in the 1860s and quickly became mired in controversy.
Coat of hollow filaments
The fossil celebrities include caudipteryx and protoarchaepteryx, dating back 120 million years. First reported in the journal Nature eight months ago, each shows clear evidence of feathers. Another species of similar age, sinosauropteryx, appears to be covered with hollow filaments that many have interpreted to be forerunners to feathers.
As more specimens of these species have come to light, "we've got better information on their anatomy. They exhibit many characters that you wouldn't find in a bird," says Philip Currie, curator of dinosaurs and birds at the Royal Tyrrell Museum of Paleontology in Alberta, Canada.
Thus, although these fossils are younger than the 140 million-year-old archaeopteryx, their skeletal features are sufficiently different to suggest they come from more primitive branches of a group of theropods known as coelurasaurs.
In terms of their evolutionary history, Dr. Currie says, sinosauropteryx is one of the most primitive coelurasaurs, with caudipteryx about half way up that branch and proto-
archaeopteryx higher still. Yet even then, protoarchaeopteryx falls several steps below archaeopteryx.
Making such connections is no mean feat. Researchers spend countless hours poring over fossils, noting the tiniest differences in bones and joints, assigning each of these "characters" a code, then building a database. Computers spend weeks sorting through the information before they produce a family tree that establishes possible linkages.
Three researchers - Dr. Holtz, Paul Sereno of Chicago's Field Museum of Natural History, and Mark Norell of New York's American Museum of Natural History - are trying to work the Chinese dinosaurs into their family trees.
And while they differ in some key details, the different versions are beginning to merge. "We're recognizing some fundamental relationships independently," Holtz notes. "That suggests these are well-supported relationships."
But in all of these new family trees, "T. rex is inside sino-sauropteryx," notes Jacques Gauthier, curator of vertebrate paleontology at the Peabody (Mass.) Museum. That means that at some point in T. rex's life, "it's got feathers."
Origin of feathers
Beyond the fossils' implication for dinosaurs' sartorial splendor lies the story they tell about feather evolution. "These three dinosaurs in particular have given us a great deal more insight into the origin of feathers," Dr. Gauthier says.
Faced with these fossils, as well as fossil feathers from the same period from elsewhere around the world, "the implication is that feathers evolved very rapidly," says Alan Brush, professor emeritus of biology at the University of Connecticut in Storrs.
"They are a simple innovation with a huge potential for evolution" because of their highly adaptable properties - from providing body contour and aerodynamic properties to warmth. Indeed, Gauthier adds, feathers probably functioned initially as insulators, holding in body heat. Later, they became crucial to surviving in a theropod-eat-theropod world.
He notes that in the mid-1980s, a physicist pointed out that gliding creatures increased their surface area from the body outward, while flying creatures increased their surface area away from their center of gravity. This gave them more lift.
"He showed that if you can gain enough lift at your outstretched hand to raise 1 percent of your body weight, you gain a lot in maneuverability," Gauthier says. In the run-or-be-eaten race, "everybody else can outrun you, so what's your advantage? Maneuverability."
Others add that wing motions evolved from the hunting techniques of coelurosaurs, whose shoulders and arms were structured in ways that allowed them to swing their arms down from above and behind their shoulders to grab and capture prey.
These theories strongly imply that flight originated from the ground up, rather than from the trees down. Some researchers have claimed that archaeopteryx, for example, lived in trees and flew by jumping to get enough acceleration to generate lift. Others hold that by running and flapping wings in a way that increases their angle of attack on the down stroke, the first flying theropods could generate enough lift to get them off the ground - particularly if they were running downhill.
"Every feature found in archaeopteryx to bolster it as a tree dweller also was present in tyrannosaurus rex," Gauthier says. "T. rex? A tree dweller? That's ridiculous."
Although a few researchers remain skeptical of the link between known theropods and birds, it remains "the best game in town in terms of the data that support it," Holtz says.
Indeed, the link is so tight, Gauthier adds, that it's time to acknowledge that "members of the Dinosaur Society and Audubon Society belong to the same club."