Newly Discovered Primitive Tyrannosaur Found To Be Feathered
by AMNH on
Finding by American Museum of Natural History Paleontologists and Others
Underscores Evolutionary Links among Living Birds and Non-Avian Dinosaurs
Two American Museum of Natural History scientists and their colleagues in China have found evidence that a newly identified 130-million-year-old primitive tyrannosaur was covered with branched protofeathers—precursors to the feathers found on living birds. The team has named the new small dinosaur Dilong paradoxus, with the generic name derived from the Mandarin words for emperor and dragon and the specific name referring to the two-yard-long carnivore's unusual features. The finding is based on the fossils of two specimens, including a fragmented one, with evidence of protofeathers, that went unidentified until a more complete fossil of the same creature was studied and found to match the morphology of the earlier fragments. Both the fossils for the research were discovered in fossil beds of northeastern China's Liaoning Province, where researchers have discovered that the soft tissue features, such as feathers and beaks, of dozens of recently unearthed specimens have been preserved along with their bones. Scientists only started to discover evidence of soft tissue features in dinosaur fossils eight years ago, and this new work is the first finding of feather-like structures of any kind on tyrannosaurs, predominantly large dinosaurs with short forelegs that walked on Earth between 130 and 65 million years ago. This new finding suggests that other tyrannosaurs, such as Albertosaurus sarcophagus, Daspletosaurus torosus, and even the most fierce Tyrannosaurus rex, were covered with fluffy protofeathers at some stage in their lives.
The new finding is described in the journal Nature by Mark A. Norell, Curator and Chairman of the Division of Paleontology at the American Museum of Natural History; Xu Xing, Wang Xiaolin, Jia Chankai, and Zhao Qi of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing; and Kuang Xuewen of the Tianjin Museum of Natural History. Dr. Xu is also a post-doctoral fellow at the American Museum of Natural History.
Dilong is not the first tyrannosaur found in the Liaoning fossil beds, and it is more primitive than all other tyrannosaurs found in the world thus far. Along with the protofeathers, the dinosaur's unusual features include a primitive skeleton typical of a juvenile tyrannosaur attached to a skull that resembles those of advanced tyrannosaurs, higher up the family tree. "The skull of this primitive tyrannosaur, Dilong, is much more advanced than its relatively conservative body skeleton," said Dr. Xu, the lead author on the paper.
Dilong's advanced tyrannosaur skull features include a single nasal bone (rather than a pair of them as found in more primitive reptiles), a jawbone that gently curves outward and becomes massive as it extends up toward the temple, a thick bone under and a thin bone over its eye socket, a flat area on the top of its head separated by a low crest, and small, closely packed front teeth that cluster to a center point. Compared to advanced tyrannosaurs, its skull is small, its temple is long, its skulltop crest is low and atypically Y-shaped, and its nostrils and braincase are large. Also, a bone that projects crosswise beneath its jaw distinguishes it from its advanced relatives.
Along with the unexpected skull features, Dilong has a longer neck and trunk than advanced tyrannosaurs, as well as lower bumps on the top of the vertebrae to support muscles and ligaments, a smaller pelvis, and shorter lower legs than in similar-sized tyrannosaurs. Using the newly identified fossil, the team was able to identify a previously found fossil fragment asDilong paradoxus. This specimen includes three-fingered hands, such as are typical in other advanced theropods.
"The discovery of protofeathers in such a primitive tyrannosaur is giving us a much clearer picture than we had even five years ago of how these animals looked and provides even more evidence of the shared evolutionary features between non-avian dinosaurs and living birds," Dr. Norell said. "Along with the miniaturization in Dilong, these new findings show that that protofeathers and the feathers we have found in more advanced theropods are not an adaptation for flight."
The protofeathers are branched and three-quarters of an inch long. They were found on the tail and left jawbone of the earlier specimen which the researchers now are able to identify as Dilong due to its morphological resemblance to the more complete and recently discovered fossil. The question of which features distinctly define birds has become more complex in the context of recent discoveries of feathered dinosaurs. In 2001, Dr. Norell and his colleagues announced the discovery of a remarkably preserved, 130-million-year-old fossil dinosaur from the same Chinese fossil beds where Dilongwas found. Covered from head to tail with downy fluff and primitive feathers, this is the first dinosaur found with its entire body covering intact, providing the best evidence yet that animals developed feathers for warmth before they could fly. Dr. Xu has described several other feathered dinosaurs from this same area.
It is unclear if all large, advanced tyrannosaurs bore scales, feathers, or both, but the discovery of protofeathers onDilong suggests a couple of alternatives. Dilong's protofeathers and the fact that living birds vary widely in their skin covering suggest that the appendages of large advanced tyrannosaurs might have been both scaly and feathered. On the other hand, it is possible that large tyrannosaurs lose their feathers as they mature, and no longer require them for insulation. In either case, Drs. Xu and Norell and their colleagues assert that feathers evolved originally to help animals regulate their body temperature.
How Are Dinosaurs Related to Birds?
In the last two decades, other bird-like dinosaurs and dinosaur-like birds have been unearthed at fossil sites around the world, including those in Madagascar, Mongolia, Patagonia, and Spain. Together with the Chinese fossils, they provide strong evidence that birds evolved from theropod dinosaurs.
The link between dinosaurs and birds was first noted in the mid-1800s by naturalist Thomas Henry Huxley, who observed that birds were built much like reptiles, but with a beak instead of teeth and with three reptilian fingers hidden inside their wings.
Today we know that theropod dinosaurs and birds share more than 100 anatomical features, including a wishbone, swiveling wrists, and three forward-pointing toes.
A Treasure Trove of Fossils in China
Consisting of layers of volcanic and sedimentary rock, the Yixian Formation in China's Liaoning Province has yielded an enormous variety of fossil fish, birds, insects, reptiles, shrimp, flowers, mammals, and dinosaurs dating back to the late Jurassic and early Cretaceous periods—more than 128 million years ago. At that time, the region was dotted with freshwater lakes, streams, rivers, and volcanoes. Volcanic explosions rained fine ash into the lakes, and animals that died or fell into the water were quickly buried in the fine-grained sediment at the bottom. Because they were buried so quickly, with so little oxygen available to promote decay, the fossil animals found in the Yixian Formation have delicate features almost impossibly preserved from feathers and fish scales to patterns on insect wings. Other deposits, like the one that produced Dilong, were formed by streams or rivers. Soft tissues like feathers are preserved, and the specimens are found in three dimensions.
"With the discovery of Dilong paradoxus and all of the other exciting fossils recently found in Northern China, Liaoning Province is now the most important place in the world for fossils that are relevant to the origin of birds," Dr. Xu said.
The work on Dilong paradoxus was funded by the Special Funds for Major State Basic Research Projects of China, the National Natural Science Foundation of China, the National Geographic Society, the Chinese Academy of Sciences, the National Science Foundation of the USA, and the American Museum of Natural History.
Upcoming Exhibition and New Dinosaur Discoveries
A new groundbreaking exhibition, Dinosaurs: Ancient Fossils, New Discoveries, will open at the Museum on May 14, 2005, curated by Dr. Norell. On view through January 8, 2006, the exhibition will reveal how current thinking about dinosaur biology has evolved and changed dramatically over the past two decades, and will highlight ongoing cutting-edge research by Museum scientists and other leading paleontologists around the world. Dinosaurs will present the most up-to-date look at how scientists are reinterpreting many of the most persistent and puzzling mysteries of the dinosaurs-how they looked, how they behaved, how they moved-including, ultimately, the complex and hotly debated theories of why they became extinct.
Among the highlights of Dinosaurs will be a number of recently uncovered fossils, including a remarkably preserved, 130-million-year-old psittacosaur, a 2.5-foot-long parrot-beaked dinosaur from China's Liaoning Province; a 3-foot-long tyrannosaur limb bone; a partial skeleton of a juvenile albertosaur; enormous apatosaur bones including vertebrae and tail portions; a Protoceratops skull; and a preserved cicada.
In the past decade, Dr. Norell has also been making annual visits to China to confer with paleontology colleagues at Beijing University; the Chinese Academy of Geological Sciences; and the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. During these visits, he studies the newest fossils collected from Liaoning Province and other recently discovered rich fossil beds in China. These visits also enhance a strong and highly productive informal exchange of scientists and research that has developed in recent decades between these Chinese institutions and the American Museum of Natural History.
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