The Proof Is in the Plumage

Natural History, July, 2001 by Mark Norell

A new fossil from northeastern China's Liaoning Province offers the best evidence yet of feathered dinosaurs.

Over the past ten years, discoveries from China's 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.

Liaoning's 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 theropod--a bipedal, birdlike dinosaur--came 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 feather-like structures. The subsequent discovery of other small dinosaurs with feathery appendages--Caudipteryx, Protarchaeopteryx, Beipaosaurus, and Sinornithosaurus--was 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.

A new specimen--for now known as NGMC 91--is that kind of fossil. I examined this specimen with colleagues from the Chinese Academy of Sciences. We know that it is a dromaeosaur, a theropod and relative of Velociraptor. We are confident about this assignment because NGMC 91 has several features present only in dromaeosaurs, notably the second toe modified into a sickle claw and the series of elongated connections between tail segments, which serve as stiffening rods. The head, tail, and much of the body are covered with single small fibers. Other parts of the body are covered with tufts or sprays of filaments. On the back of the "arms," branched structures lie parallel to one another, just like the barbs of a modern bird feather.

Discovered by a farmer in the winter of 2000, the fossil was acquired by the National Geological Museum of China shortly thereafter. It is now on loan to the American Museum of Natural History and will be on display until the end of August 2001. The skeleton, including the tail, is only about twenty-four inches long and is preserved on a slab and counterslab, two halves of one sheet of rock. The size of the head--large in relation to the rest of the body--indicates that this was a young animal. The serrated teeth and sharp claws show that it was a predator, and the long hind limbs suggest that it was a fast, nimble runner.

Despite the specimen's fine preservation, we are not sure to what species it belongs. A similar dromaeosaur found in the same general area and first described in 1999 was given the scientific name Sinornithosaurus. A few characteristics seem to indicate that NGMC 91 is a different species, yet until further research is done, we will wait to name it. (In the meantime, we call it Dave, a name from an old Cheech and Chong routine.) We know that if measurements of the bones of Sinornithosaurus and NGMC 91 are placed in a mathematical model that describes the growth pattern of Archaeopteryx (the most primitive of the protobirds), both animals deviate from the Archaeopteryx trajectory in the same way. So while neither grew like the protobird, they seem to have grown like each other.