China's Feathered Dinosaurs

The Chinese Fossils

 

In one of the most electrifying paleontological discoveries of the century, fossils recently unearthed in northeastern China have revolutionized our understanding of the origin of feathers and flight in dinosaurs.

Exquisitely preserved skeletons of four new theropod dinosaurs — Sinosauropteryx, Protarchaeopteryx, Caudipteryx and Confuciusornis — bear remains of feathers, although only one of them — Confuciusornis— could actually fly.

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.

Sinosauropteryx (adult 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

 

This much larger Sinosauropteryx specimen of what appears to be a more mature or adult animal was discovered in 1997. The fossil contains the remains of the creature’s last meal, the jawbones of an early mammal.

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.

Skull with sharp-toothed jaws of Sinosauropteryx

 

But the teeth andjaws of bird-line archosaurs were well suited to seizing, subduing and ingesting large prey. As in theancestral archosaur, for example, the lower jaw was set lower on the skull, thus increasing the gape
of the bite. Its heavy jaw muscles were modified to magnify the speed and power of that bite.

Theropod dinosaurs added further refinements. One, a joint between the muscle- and tooth-bearing parts of the lower jaw, increased the width of the gape, facilitating the ingestion of even larger food items. Note that in Sinosauropteryx the lower jaws are not fused together at the chin as they are in modern birds; instead elastic ligaments enabled the jaws to spread apart at their tips, also contributing to a wider gape.

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.

Detail showing downy plumage along backbone of Sinosauropteryx


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.

Sinosauropteryx retains several primitive features—such as very short arms—indicating that it is the least bird-like of the theropod dinosaurs preserved in this ancient Chinese lake bed. Nevertheless, its body is already covered with downy plumes that provide a glimpse of the earliest known stage in feather evolution. Sinosauropteryx feathers are composed of fine filaments branching from hollow quills, rather like down feathers in birds today.

Although individual feathers in Sinosauropteryx are hard to pick out, they all appear about the same size and shape. This contrasts with feathers on the hand (remiges), tail (retrices), and body (contour) feathers that evolved later in dinosaurs. These feathers can vary considerably in size and shape on different parts of the body. And unlike Sinosauropteryx feathers, the filaments in “aerodynamic” feathers of living birds are tightly bound
together by tiny hooks, thus forming clean, sharp-edged outlines and broad, fixed, aerodynamic surfaces.

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.

Three-fingered left hand of Protarchaeopteryx


The thumb is on the bottom, the index fingeris the longest, and the middle finger lies behind the others.

 

Protarchaeopteryx has the raptorial forelimbs typical of maniraptor theropods, which originally used their arms to capture prey but later used them for flight. All maniraptors, flighted or not, possess a unique wrist bone, the semilunate carpal, that moves with the hand in a broad, flat, 190 degree arc. Powered by heavy chest muscles, the bones of the arm link together with the wrist so as to force the grasping hands to spread out toward the prey during the forestroke/downstroke and fold in on the prey during the backstroke/upstroke. As shown in the video in the adjoining room, these motions play crucial roles in the highly complex flight stroke of modern-day birds.

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


(Caw-DIP-ter-iks = “tail feather”)
120 to 150 million years before present
Yixian Formation
Sihetun, Liaoning Province, China
National Geological Museum of China

 

Caudipteryx has enlarged, more bird-like feathers on its hands and tail tip than Sinosauropteryx.
The feathers may have played a role in courtship, but they could also have enhanced mobility and
stability in a two-legged (bipedal) runner.

Although the tail and hand feathers of Caudipteryx could not support flight, their distinctive shape
and position on the body is no less striking. Even small increases in lifting surfaces, so long as they
are sufficiently far from—and well-placed relative to—the center of gravity, can dramatically improve
stability and maneuverability in a bipedal runner.

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.

This Caudipteryx fossil shows the arched head and neck of the avian death pose.

 

Caudipteryx is preserved in a typical avian death pose: the head and neck are arched over the back and the legs lie close together on the same side of the body.

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.

Side view of Caudipteryx skull showing its beak

 

Caudipteryx possesses several new features that demonstrate that it has already diverged from the main line of avian evolution. It has, for example, a relatively short tail and unusually short arms for a maniraptor. It also has only a few small teeth at the tip of its upper jaws, and may have had a partial beak. This combination—tooth reduction combined with beak formation—has arisen several times in dinosaur evolution, including at least three times within flying dinosaurs. Such changes in jaw structure may reflect a shift in dietary preferences away from large prey items.

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.

Confuciusornis


(Con-few-shus-OR-nis = “Confucius bird”)
120 to 150 million years before present
Yixian Formation
Sihetun, Liaoning Province, China
National Geological Museum of China

 

Confuciusornis is one of the most abundant animals preserved in the Liaoning deposits, suggesting that it might have lived in large colonies on the shores of this ancient Chinese lake. This flying dinosaur reveals several anatomical peculiarities, including flight feathers that are longer than its body, indicating that it had already diverged from the direct line leading to birds.

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.

The foot of Confuciusornis


Because of its abundance and fine preservation, we expect that Confuciusornis will eventually teach us a great deal about this stage in the evolutionary development of flight and many other aspects of dinosaur biology. For example, visible anatomical differences that reveal sex, also known as sexual dimorphism, are common among fossil and living dinosaurs. Confuciusornis is no exception as the males, who are larger than females, sport a pair of exceptionally long, narrow tail feathers.

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.

The Collector: Mabel Loomis Todd (1856–1932)

 

A wonderful picture of Mabel Loomis Todd dressed in a kimono and holding a parasol, summer 1896.

Yale University Archives

 

In 1964 Yale University received a vast quantity of manuscripts, photographs and three-dimensional objects from Millicent Todd Bingham, the only child of David Peck Todd and Mabel Loomis Todd. The manuscript collection has been the source of many books and papers, since the Todd family’s interaction with the Dickinsons of Amherst had been famous, and infamous in terms of Puritan ethics.