On February 14, 2002, Nature, the leading international weekly journal of science, published an article about the discovery of a basal troodontid dinosaur in Northeast China's Liaoning Province.
Discovery of the meat-eating dinosaur, named Sinovenator changii, a species of theropod, is the latest of a series of sensational discoveries related to the origin of birds in western Liaoning Province, which started in the 1990s.
Over the past decades, excavations of rock formations in western Liaoning have harvested a large number of fossil discoveries. The findings have offered paleontologists throughout the world rare glimpses into the exuberant fauna and flora living between the Late Jurassic (about 147-135 million years ago) and Early Cretaceous (135-120 million years ago) periods.
The fossil community is known as Jehol Biota, as the discoveries were first unearthed in 1928 in an area then under the jurisdiction of Jehol Province that was later incorporated into today's Liaoning Province in Northeast China.
Enclosed in grey volcanic ash deposited on the bottom of shallow lakes and rivers, the Jehol fossils include insects, plants, fish, frogs, turtles, lizards and early mammals.
The study of the Biota is considered one of the world's frontiers of knowledge about nature because of the remarkable fossil findings of creatures called primitive birds and bird-like theropods in the 1990s. These primitive birds are more closely related to modern birds than Archaeopteryx, or the earliest known bird that lived 150 million years ago, which was unearthed in 1861 in southern Bavaria in Germany.
Among the discovered fossils are the primitive bird Confuciusornis, which had feathers similar to modern birds; and the bird-like dinosaur Sinosauropteryx, the first non-avian animal whose fossil included feather-like structures. The list also contains Caudipteryx, Protarchaeopteryx, Beipaosaurus, Sinornithosaurus and Microraptor, which are small theropods with feathery appendages.
These finds were seen by most paleontologists as evidence supporting the hypothesis that birds evolved from small theropods.
That was true until the discovery of Sinovenator.
During a recent interview in Beijing, Xu Xing, the leading author of the story in Nature, told China Daily that Sinovenator, living about 130 million years ago, is the most primitive troodontid, one of the most bird-like dinosaur groups ever discovered.
According to Xu, a researcher with the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the dinosaur is a small theropod with an estimated length of less than one meter.
Its short forelimbs can flap and its legs are slender, which means that it can run fast, and it has small, sharp teeth used to prey on insects and small mammals. Its brain is quite big compared with other parts of its body, indicating a relatively high level of intelligence. Its second toe was modified into a sickle claw.
The first specimen of Sinovenator was found in Early Cretaceous formations in western Liaoning Province in 2000 and collected by the institute. More specimens were unearthed last year in the same region.
Last spring, Xu and his Chinese colleagues began to study the specimens in cooperation with Dr Mark A. Norell from the American Museum of Natural History; Dr Peter J. Makovicky from the Field Museum in Chicago; and Dr Wu Xiaochun from the Canadian Museum of Nature.
They found that Sinovenator possesses such features as the backward directed pubis bone, which are not seen in other troodontids, but do occur in birds and dromaeosaurids, the group of theropod dinosaurs which are considered closest to birds.
This helps paleontologists to ascertain the troodontids' position on the family tree of theropods.
Xu explained that for years, paleontologists worldwide have unearthed quite a number of Sinovenator "cousins," which they call derived troodontids. However, these cousins, discovered prior to Sinovenator in many regions of the world, displayed an unusual combination of characters, which had confused scientists in their efforts to determine the position of troodondits on the evolution tree of theropods.
"As the most primitive troodondit, Sinovenator stands out as important evidence that troodontids are a group of bird-like dinosaurs close to dromaeosaurids such as Sinornithosaurus," Xu said in his office packed with books, maps and fossil specimens.
As the bird-like theropods usually have filaments (primitive plumage) or feathery covering, Sinovenator is thought to have primitive feathers or even vaned feathers.
More importantly, the paleontologist said, the discovery of Sinovenator suggests that principal structural modifications of birds were acquired in the early stages of the evolution of small theropods.
"The small size of Sinovenator and the recent discoveries of small-sized primitive dromaeosaurids means the trend toward miniaturization in early bird evolution is a continuation of the earlier trend in the evolution of birdlike dinosaurs," he said.
"It strengthens the hypothesis that birds are the living descendants of theropods."
Critics of the theropod theory of bird origins often point to the lack of bird-like dinosaurs found in sediments the same age or older than those that contained Archaeopteryx, said Xu.
They argue that if small ground-dwelling theropods are the closest relatives to birds, albeit more primitive, they should have lived at an earlier time.
"Compared with Sinosauropteryx, Caudipteryx zoui and Protarchaeopteryx (which are all about 125 million years old), Sinovenator lived closer to Archaeopteryx," Xu Xing said.
"The finds really give me confidence about finding bird-like dinosaurs living with Archaeopteryx in the same age or earlier."
(China Daily February 26, 2002)
