The Excavation Site

The feathered dinosaur fossils that made international headlines in 1998 were preserved in rock layers deposited in a lake that existed 120 million years ago in Liaoning Province in northeastern China.

Professional paleontologists and local villagers have excavated fossils of millions of leaves, insects, fish, frogs, salamanders, mammals, turtles, lizards and crocodilians from these rocks, but the most extraordinary
finds to date have been the remains of feathered dinosaurs. Liaoning farmer Li Yin Fang is the discoverer of Sinosauropteryx prima (“first Chinese dragon feather”), found at Sihetun in 1996. Local farmers and scientists from China and around the world have been excavating the site since 1994.

The feathered dinosaur fossils were deposited in a lakebed in Liaoning Province in what is now northeastern China. Over 100 million years ago this area teemed with life; volcanic ashfall from eruptions in Inner Mongolia
to the west covered many of these living things, preserving them almost perfectly. Because these beds have been overturned, going deeper at the site means coming closer to the present in time.

Remains of a mammal—two jawbones—eaten by Sinosauropteryx

Most early theropod dinosaurs retained the meat-eating habits inherited from the distant ancestor they share in common with living crocodilians. Sinosauropteryx was no exception: the large, sharply pointed, prehensile teeth that lined its jaws had serrated edges well suited to rending prey too large to be swallowed whole. By that standard modern-day birds have unusual diets of insects, fruits and seeds. But that variety seems less  unusual when birds, some species of which weigh less than a penny, are considered as part of a huge array of dinosaurs that have adapted to fill many ecological niches.

Sinosauropteryx (juvenile specimen)

(Sine-oh-sore-OP-ter-iks = “Chinese dragon wing”)
120 to 150 million years before present
Yixian Formation
Sihetun, Liaoning Province, China
National Geological Museum of China

Sinosauropteryx is the first non-flying dinosaur to have been found so far that shows traces of downy plumage, indicating that feathers arose before flight. Discovered only three years ago in 1996, this small Sinosauropteryx was the first non-flying dinosaur fossil found to have feather impressions. We think it was a juvenile because a similar looking but much larger individual was found nearby the following year.

The joint coincidence of small body size and downy plumage at such an early stage in theropod evolution —well before the evolution of the modern avian flight apparatus—suggests that the role of feathers in retaining body heat preceded their function in flight.

Protarchaeopteryx

(Prote-ark-ee-OP-ter-iks = “first ancient wing”)
120 to 150 million years before present
Yixian Formation
Sihetun, Liaoning Province, China
National Geological Museum of China

Protarchaeopteryx used its long arms, huge hands and sharp claws to capture prey. The forelimbs of this feathered dinosaur show that important parts of the avian flight stroke did not evolve for flight, but for seizing prey.

Known only from a single, maddeningly incomplete specimen, Protarchaeopteryx is the most poorly known theropod at the Liaoning site. While it is not as modified as Caudipteryx, there are nevertheless some shared new features, including general skull form and short tail, suggesting that they could be closely related. Like the ancestral dinosaur, Sinosauropteryx, Caudipteryx and Protarchaeopteryx have powerfully developed hind limbs.

Detail of Protarchaeopteryx left hand, showing close-up of the thumb bone that in life supported a hooked claw

Another commonality between Caudipteryx and Protarchaeopteryx is that their feet show none of the special modifications for grasping one would expect to find in arboreal creatures. For example, the first toe does not oppose the others and the toe bones shorten toward the claw, rather than lengthen as they do in birds with grasping feet.

Of course, just because specialized structures often perform specialized functions does not mean those functions cannot also be performed with less specialized structures. Thus the absence of grasping characteristics in Caudipteryx and Protarchaeopteryx does not mean that these small theropods could not climb trees—or at least scamper up through low brush. But to be unable to exclude the possibility of an arboreal stage in the evolution of flight is not the same as having positive evidence for it. So far as is known, the anatomy of basal maniraptors is consistent with that of ground-dwelling runners, not tree-dwelling climbers.

Caudipteryx tail with impressions of fan of tail feathers. 

Feathers (remiges) attaching to the second finger of the hand of Caudipteryx are distinctly more bird-like than are the downy feathers of Sinosauropteryx. As in Caudipteryx, a fan of feathers (retrices) may—there is some uncertainty on this point—attach to the tail tip in the larger Sinosauropteryx. But the smaller specimen has no fan of enlarged tail feathers. If the Sinosauropteryx specimens are adult and juvenile of the same species, that could explain the difference, namely, that the larger specimen is sexually mature and the smaller is not. That in turn suggests that the use of the tail in courtship, so distinctive of birds today, arose long before birds did.

The form of the remiges (flight feathers) and retrices (tail feathers) in Caudipteryx suggests that they possessed the tiny hooks that bind feather filaments into the aerodynamic structures common to flying dinosaurs. It is nonetheless clear that Caudipteryx could not fly: its arms were far too short and its flight feathers far too small to support its body weight. Moreover, flight feathers in flying dinosaurs have curved shafts and asymmetrical vanes (the web or expanded flat part of the feather). But the “flight” feathers in Caudipteryx are like those of non-flying birds, with straight shafts and symmetrical vanes.

A turkey skeleton in a typical avian death pose

The soft tissues that help dinosaurs support their long necks shrink after death, thus bending the head and neck backward over the body. This death pose is common in modern birds, and is evident in several other extinct dinosaurs in this exhibit. The legs are often preserved on the same side of the body as well, for theropod dinosaurs are obligate bipeds; like humans, they rely on their hind limbs for support and locomotion on the ground.

Stomach stones from the crop of Caudipteryx

Note the pile of stomach stones (gastroliths) preserved in the belly of Caudipteryx near the pubic bones. Archosaurs—a fundamentally carnivorous lineage that includes both crocodilians and birds—generally do not chew their food. Instead, they swallow stones into a muscularized foregut, the gizzard, which can then “chew” the food for them. Large gizzards filled with lots of stomach stones correlate with ingestion of tough foods, particularly plant material, that require extra processing to aid digestion.

The grasping hand and flight apparatus of Confuciusornis

Confuciusornis is the oldest flying dinosaur with a nearly modern flight apparatus. It offers exciting new evidence for how a grasping hand evolved into a flying hand.

The ancestral theropod dinosaur had three functional fingers in the hand: the thumb, index and middle fingers. Birds retain these three fingers, although they support flight rather than grasping. We have long wondered how dinosaurs made the transition from a grasping to a flying hand, and Confuciusornis gives us new insight into that problem. Confuciusornis still has fully functional raptorial claws on its thumb and middle fingers, but its index finger—the finger that supports the flight feathers—is composed of broad, flat bones and a reduced claw. As with other basal maniraptors, the thumb and middle fingers converge on one another while grasping in
Confuciusornis, enabling its hand to support flight while still retaining some grasping ability.

Sinosauropteryx (juvenile specimen) detail

These early Cretaceous fossils reveal three different stages in the evolution of theshoulders and forelimbs of theropod dinosaurs and provide crucial evidence for how grasping armstransformed into flying wings.

Scientific study of these specimens was under the leadership of Dr. Ji Qiang of the National Geological Museum of China, Beijing, where the fossils will be maintained on their return to China. He has supervised much of the fossil recovery work.