A new fossil from northeastern Chinas Liaoning Province
offers the best evidence yet of feathered dinosaurs.
Story by Mark Norell ~ Images by Mick Ellison
Over the past ten years, discoveries from Chinas Liaoning Province have been giving us rare glimpses of a fossil community near the boundary of the Jurassic and Cretaceous periods. These glimpses just keep getting better. Known as the Jehol biota, these ancient plants and animals are embedded in fine-grained sediments that preserve details: the veins of leaves and insect wings, the patterning of skin, and the filaments of feathers. Some of the fossils are proving pivotal in testing the hypothesis that birds are the living descendants of dinosaurs.
Liaonings farmers have been collecting fossil fish and insects in the area for decades. The Chinese government now regulates fossil collection and while paleontological excavation has taken place sporadically for years, most specimens are still unearthed by local people. The Jehol fossils are enclosed in gray volcanic ash that was deposited on the bottom of shallow lakes. They are the remains of a varied community of plants and animals that perished in or near the lakes and were quickly buried. The most abundant fossils are arthropods, but plants and fishes are also common. Rarer fossils include dinosaurs, turtles, pterosaurs, lizards, and early mammals. Some specimens reveal fossilized stomach contents and skin shaded in patterns. We cannot tell what colors the patterns represent, but we do know that some Jehol animals, including insects, fish, and small dinosaurs, were spotted or striped, like their living relatives.
Among the first of the remarkable fossils of land-dwelling vertebrates to emerge at Liaoning in the 1990s were creatures called protobirds. They are more closely related to modern birds than is Archaeopteryx from southern Bavaria, but more primitive than birds alive today. Protobirds such as Confuciusornis had the same kind of feathers as modern birds; some specimens even display long tail feathers reminiscent of tropic birds and birds of paradise.
In 1996 the fossil of a small theropoda bipedal, birdlike dinosaurcame to light and made news in the popular press as well as the scientific community. Named Sinosauropteryx, this creature was the first nonbird whose fossil included featherlike structures. The subsequent discovery of other small dinosaurs with feathery appendagesCaudipteryx, Protarchaeopteryx, Beipaosaurus, and Sinornithosauruswas seen by most paleontologists as evidence supporting several hypotheses: birds are the living descendants of theropod dinosaurs; birds are not the sole feather-bearing creatures; featherlike structures preceded flight and hence did not evolve in connection with it. Some scientists accept the presence of feathers on Confuciusornis but reject the idea that other Jehol theropods were feathered. They suggest that these creatures are actually primitive birds, or that the featherlike impressions are from a bird that became mixed in with the skeleton during burial, or that they are internal structures related to tail or body musculature. To clinch the argument, we needed a fossil that unambiguously showed a nonavian dinosaur with a feathery body covering.
Determining the age of the Jehol fossils is problematic. Groups of researchers using various techniques have come up with conflicting results. The kinds of mammals and pterosaurs found in the Jehol fossils appear to be Late Jurassic, roughly the age of Archaeopteryx. However, measurements of radioactive decay give dates of both 147 and 124 million years, a period that straddles the boundary between Jurassic and Cretaceous. This may not reflect error; the samples taken for analysis are from the same area but not the same quarry. The shallow lakes that once dotted northern China (and are now the fossil beds) were repeatedly and over long periods of time filled in with debris from erupting volcanos. Sets of plant and animal communities of several ages may be represented.
The new fossil tells us that a body covering similar to feathers was present in nonbirds. If not connected with the ability to fly, could feathers have evolved to keep animals warm? (See First Came Feathers, September 1998.) Modern birds are warm-blooded, and feathers play an integral role in maintaining body heat. A reasonable idea, although difficult to verify, is that theropods developed featherlike structures in tandem with warm-bloodedness. Only later were the structures co-opted for flight and display.
As more evidence of ancient life comes to light, we can refine our vision of dinosaurs and birds-as-dinosaurs. If we could see the juvenile dromaeosaur that hunted along the lakes of Liaoning, the best way we could describe it would be: Like a bird. Strange, but like a bird.
Curator and chairman of the American Museum of Natural Historys Division of Paleontology, Mark Norell has been at the forefront of documenting the evolutionary relationship of theropod dinosaurs and birds. In Mongolia, he discovered the first embryo of a carnivorous dinosaur, the theropod Shuvuuia, and the strange Mongolian bird Mononykus (see New Limb on the Avian Family Tree, September 1993). Norell has also analyzed the new fossil birds and dinosaurs emerging from northeastern China. Illustrator and photographer Mick Ellison, who has visited the Chinese site five times, is principal artist in the Museums Division of Paleontology. In addition to his work illustrating fossil creatures, he enjoys painting portraits of people.
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