CHICAGO – The discovery and analysis of an early carnivorous dinosaur, Sinovenator changii, are clarifying the evolutionary relationship between dinosaurs and birds, according to a paper to be published in Nature Feb. 14, 2002.
The small, relatively complete fossil was found in the rich Yixian Formation of western Liaoning in China, where scientists have recently discovered many groundbreaking fossils, including feathered dinosaurs.
“This new dinosaur, which was probably feathered, is closely related to and almost the same age as the oldest known bird, Archaeopteryx,” says Peter Makovicky, PhD, assistant curator of dinosaurs at The Field Museum and co-author of the paper. “It demonstrates that major structural modifications toward birds occurred much earlier in the evolutionary process than previously thought.
“Furthermore, these findings help counter, once and for all, the position of paleontologists who argue that birds did not evolve from dinosaurs,” he adds.
The fossil is more than 130 million years old and sheds light on dinosaurs during the transition from the Jurassic period to the Cretaceous period. Sinovenator changii (sigh-no-ven-ay-tor chang-eye) is a troodontid (tro-don-tid), a type of theropod (tare-a-pod). Although many theropods, such as Tyrannosaurus, are large animals, theropods close to the ancestry of birds show an evolutionary trend toward small body size.
Accordingly, an adult Sinovenator changii would have been less than a meter long. This particular specimen, almost fully grown, is slightly larger than a chicken.
“Although big dinosaurs may be more spectacular, we can actually learn more about evolution from the often overlooked smaller dinosaurs because they tend to be more primitive,” says Dr. Makovicky. “Sinovenator changii is more basal or primitive than any other known troodontid.”
Troodontids are a type of theropod distinguished by a puzzling combination of features such as having large air-filled spaces surrounding the braincase, small teeth with unusually large serrations, and a large sickle claw of the foot. These various can occur in different types of theropods, so troodontids have been hard to place on the evolutionary tree. In the past, they have been classified as close relatives of therizinosaurs, ornithomimods and dromaeosaurs.
Being a primitive troodontid, Sinovenator shows some features more similar to dromaeosaurs than to advanced troodontids. Dromaeosaurs, which include Velociraptor, and troodontids are related to birds.
The Sinovenator changii fossil is preserved three-dimensionally because it was found in rocks deposited by a river. While this provides more data on the animal’s three-dimensional structure, it explains why Sinovenator’s feathers were not preserved. The feathered dinosaurs recently discovered in the same part of China were found in rocks at the bottoms of lakes that were derived from lake sediment.
Sinovenator changii is named after Dr. Meeman Chang, a leading Chinese paleontologist who spent more than a year time studying fossil fishes at The Field Museum on different occasions, most recently in 1998.
“As head of the Beijing’s Institute of Vertebrate Paleontology and Paleoanthropology for many years, Dr. Chang has helped raise the standards for paleontological inquiry in China,” says Lance Grande, PhD, curator of fossil fishes at The Field Museum. “She is dedicated to improving the quality and productivity of Beijing’s natural science institutions through training students, publishing, and improving collection and research facilities in China. She has also played an important role in making it easier for foreign paleontologists to work in China, thereby broadening the scope and importance of paleontological sciences there.”
Dr. Makovicky plans to conduct collaborative fieldwork in China to look for more fossils. “This area is yielding extremely important information on the evolution of dinosaurs, mammals, insects and flowering plants,” he says. “I hope to find even more primitive specimens than Sinovenator changii.”
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