The first fossil of a sleeping non-avian dinosaur has been described by a pair of American Museum of Natural History paleontologists. The small bird-like dinosaur is preserved in a remarkable life-like pose, with its head tucked between its forearm and trunk with its tail encircling its body. The pose matches the typical sleeping or resting posture found in living birds and thereby supports the already established evolutionary connection between extinct dinosaurs and modern birds (which are living dinosaurs) and the occurrence of bird-like features in early dinosaurian evolution. It also supports the hypothesis that non-avian dinosaurs, like the modern birds that evolved after them, were warm-blooded.
Mark Norell, Chairman of the Division of Paleontology at the American Museum of Natural History, and Xu Xing, a post-doctoral fellow at the Museum who also is a curator at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences in Beijing, have named the new specimen Mei long, referring to the Chinese words for "soundly sleeping dragon." The new dinosaur fossil is described in the journal Nature by Drs. Norell and Xu.
"We're excited to have discovered this rare evidence of behavior, in this case sleeping, in an early fossilized dinosaur with bird-like features," Dr. Xu said.
Fossil evidence of dinosaur behavior such as sleeping or nesting is quite rare. In Mei long, the scientists have found a small bird-like theropod, called a troodontid, measuring less than two-feet long from about 130 million years ago. Characterized by large brains, stereoscopic vision, and distinctive teeth serrated like a steak knife, theropods, including Tyrannosaurus rex, are two-legged predators with bones strikingly similar to modern birds. Troodontids are one of the most avian-like dinosaur groups.
The fossil dinosaur, a juvenile that is nearly an adult, sits on long, folded hindlimbs. Its forelimbs are folded bird-like next to its body and its neck curves to the left so that its relatively small head lies between the left elbow and body, a posture that is identical to the "tuck-in" posture of many living birds. Living four-legged creatures rest and sleep in various postures, but only birds and a subset of mammals rest on folded limbs. And only birds, with their long, flexible necks, tuck their heads behind a forelimb or wing to rest. By doing so, birds ball up and conserve heat. "This specific heat-conserving pose that Mei long was found in provides support for the hypothesis that at least some non-avian dinosaurs, including this animal and its troodontid relatives, were warm-blooded as are today's birds," Dr. Norell said.
The fossil was discovered in layers of volcanic and former riverbed sediments in northeastern China's Liaoning Province, where numerous well-preserved non-avian dinosaur and bird fossils have been found in recent years. Sediment covered the animal while it was sleeping or resting on the ground, burying the animal alive. This is unlike most fossil animals which die and decompose or are scavenged before burial.
Mei long has large nostrils, a relatively small skull, long hindlimbs, numerous closely packed teeth in the middle of its jawbone, and a large U-shaped wishbone, features that distinguish it from most previously discovered troodontids. Incompletely fused bones indicate that it is nearly an adult specimen. Mei long's skeleton shares many features with dromaeosaurs (small carnivorous dinosaurs with large heads, sharp teeth, and clawed hands), avialans (the group that includes living birds), and Archaeopteryx (the "first bird"), such as a short snout, a long forehead, a large eye socket, a jawbone that becomes massive as it extends up toward the temple, the lack of a crest on the top of its head, unserrated teeth, a long and thin forearm (radius), an L-shaped bone at the shoulder joint, and a shoulder blade close to its spine. These shared features indicate that Mei longis a very primitive troodontid. In fact, analysis of the evolutionary relationships among these bird-like dinosaurs supports previous findings that troodontids, including Mei long, share a common ancestor with Avialae, the group that includes all modern birds. Thus, the tucking-in behavior for sleeping or resting in modern birds originated in small dinosaur precursors to modern birds. Drs. Xu and Norell state that Mei long's shared features with birds and its small size also support the theory that miniaturization was crucial to the development of flight.
The Evolutionary Link between Non-Avian Dinosaurs and 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.
The Liaoning Fossil Beds 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 Mei long, were formed by streams or rivers. Soft tissues like feathers are preserved, and the specimens are found in three dimensions.
The work on Mei long was funded by the National Natural Science Foundation of China, Special Funds for Major State Basic Research Projects of China, National Geographic Society, American Museum of Natural History, Chinese Academy of Sciences, and National Science Foundation of USA.
Image credits: Mick Ellison, AMNH
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. The exhibition also will feature a full-scale model of Mei long.
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.