A feathered dinosaur not much bigger than a chicken, recently discovered in northeast China, is expected to give science a clearer picture of the ancestors of today’s birds, according to Field Museum paleontologist Peter Mackovicky.
He and three co-authors describe the pint-size, two-legged meat-eater they named Sinovenator changii in Thursday’s issue of the British scientific journal Nature.
Evolving between 130 million and 144 million years ago, Sinovenator, which means “China hunter,” is one of the earliest, most primitive members of a group of dinosaurs known as troodontids, Mackovicky said.
Troodontids and birds branched off from the same common ancestor at roughly the same time. Various species of troodontids lived on until the extinction of dinosaurs 65 million years ago. Birds, of course, survived and live on today.
“Sinovenator comes along close to the point where troodontids split off from that ancestor and birds split off to evolve in a different direction,” Mackovicky said. “Sinovenator shares some very close resemblances to the earliest bird we know, Archaeopteryx.”
Crow-size Archaeopteryx fossils, complete with feather impressions, have been found in limestone formations in Germany dating back 144 million years.
Another group of dinosaurs, dromaeosaurs, also branched off from the same ancestor. One of the later, better-known dromaeosaurs was velociraptor.
The common ancestor, for which no fossils have been found, would have been “small-bodied creatures the size of crows or chickens,” said Mackovicky, assistant curator of dinosaurs at the Field Museum. “They would have looked much like Sinovenator.”
With a mouth full of razor-sharp, serrated teeth, Sinovenator walked and ran on two legs. It had a prominent wishbone and a pelvic bone shaped much like a bird’s, and the formation of the bones in its shoulder was much like those in birds. Although it was feathered, it could not fly.
Paleontologists from China’s Institute of Vertebrate Paleontology and Paleoanthropology found Sinovenator two years ago in an area 200 miles northeast of Beijing that in recent years has produced many dinosaur discoveries, including other feathered species.
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Xing Xu, lead author of the Nature article, and two of his Chinese colleagues invited Mackovicky and Mark Norell, a paleontologist at New York’s American Museum, to collaborate in studying two Sinovenator fossils.
Mackovicky, who is planning eventually to look for fossils in the same area, is particularly interested in the earliest, most primitive forms of dinosaur families.
Because most evolving life forms start out small and grow larger, he often is looking at smaller dinosaurs.
“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,” Mackovicky said.
The fossil field in China where Sinovenator was found is of great interest because it yields fossils living in the transition period between the Jurassic and Cretaceous eras, he said.
At that time, roughly 135 million years ago, huge dinosaur species such as Brachiosaurus and Allosaurus that dominated the Jurassic period began to disappear.
“A lot of smaller things that began to appear at this time in Asia then spread out through the rest of the world,” Mackovicky said. “We’re sampling the beginnings of animals [in China] that come to dominate later.”
Troodontids, however, never evolved to anything very big–no more than 6 to 8 feet long.
The team studying Sinovenator is also looking at other as yet undescribed animals found at the site.
“As we’re getting to the fauna of this time period, we’re realizing there were a lot of smaller species present, including mammals, other dinosaurs and probably lizards,” Mackovicky said. “That is probably what Sinovenator was eating. Some of the dinosaurs were probably too much for him to take on, but he could have been preying on their babies.”