送交者: JZ 于 2005-3-24, 16:26:40:
Newly Found Dinosaur Tissue Raises Hope of Extracting DNA
By JOHN NOBLE WILFORD
Published: March 24, 2005
Alive as dinosaurs may seem to children, knowledge of them as living creatures is limited almost entirely to what can be learned from bones that have long since turned to stony fossils. Their soft tissues, when rarely recovered, have lost their original revealing form.
A 70-million-year-old Tyrannosaurus rex recently discovered in Montana, scientists reported today, has apparently yielded the improbable: soft tissues, including blood vessels and possibly cells, that "retain some of their original flexibility, elasticity and resilience."
In a paper being published on Friday in the journal Science, the discovery team said that the remarkable preservation of the tissue might open up "avenues for studying dinosaur physiology and perhaps some aspects of their biochemistry."
"Tissue preservation to this extent has not been noted before in dinosaurs," the team leader, Dr. Mary H. Schweitzer of North Carolina State University, said in a teleconference on Tuesday
The scientists said that an examination with a scanning electron microscope showed the dinosaur blood vessels to be "virtually indistinguishable" from those recovered from ostrich bones. The ostrich is today's largest bird, and many paleontologists think birds are living descendants of some dinosaurs.
Dr. Schweitzer and other scientists not connected with the research cautioned that further analysis of the specimens was required before they could be sure the tissues had indeed survived unaltered. They said the extraction of DNA for studies of dinosaur genetics and cloning experiments was only a long shot.
But in a separate article in Science, Dr. Lawrence Witmer, a paleontologist at Ohio University who had no part in the research, said: "If we have tissues that are not fossilized, then we can potentially extract DNA. It's very exciting."
If the tissues are as well preserved as they seem, the scientists held out some hope of recovering intact proteins, which are less fragile and more abundant DNA. Proteins might provide clues to the evolutionary relationship of dinosaurs to other animals and possibly help solve the puzzle of dinosaur physiology: whether, as argued, dinosaurs were unlike other reptiles in being warm-blooded.
"If we can isolate certain proteins, we can address the issue of the physiology of dinosaurs," Dr. Schweitzer said.
Excavations of dinosaur remains sometimes turn up preserved tissues other than bone, such as feathers, embryonic fragments and internal organs. But as Dr. Schweitzer's group noted, in those cases their shapes may be replicated but their original composition is not preserved "as still soft, pliable tissues."
The scientists said it was usually difficult to determine what such modified tissues were like in life if the fossils are more than a few million years old. The last of the dinosaurs died out 65 million years ago.
The T. rex with the soft tissue was found in 2003 by a fossil-hunting team led by John R. Horner, a paleontologist with the Museum of the Rockies at Montana State University. Mr. Horner is a co-author of the journal report, along with Jennifer L. Wittmeyer of North Carolina State and Jan K. Toporski of the Carnegie Institution of Washington.
The trials of fieldwork led to the discovery of soft tissue inside a thigh bone.
Geologically, the T. rex skeleton was excavated from the Hell Creek Formation, in sandstone laid down about 70 million years ago. Geographically, this was deep in a remote corner of the Charles M. Russell Refuge, in Montana. The only way to get the heavy fossils out was by helicopter.
Tyrannosaurs were famously huge predators. This one, estimated to have been 18 years old at death, was not as large as most. Its femur, or thigh bone, was 3 ½ feet long; some T-rex femurs are at least a foot longer. But the creature was large enough so that some of the rock-encased long bones had to be broken in half to fit a helicopter rig - not a thing paleontologists like to do.
At the laboratory in Bozeman, scientists inspected the broken thigh bone before preparators would have applied preserving chemicals, which would have contaminated the specimen. Inside the dense bone, Dr. Schweitzer and colleagues noticed unusual tissue fragments lining the marrow cavity. Fossilization had not been complete.
When fossilizing mineral deposits in the tissues were dissolved by a weak acid, the scientists were left with stretchy material threaded with what looked like tiny blood vessels. Further examination revealed reddish brown dots that the scientists said looked like the nuclei of cells lining the blood vessels.
Dr. Schweitzer said it was too early to draw any definitive conclusions about the lives of dinosaurs based on the laboratory analysis so far. "We are still in the process of analyzing the microstructures of these tissues," she said.
Mr. Horner said the discovery was likely to alter both the field and laboratory work of dinosaur scientists. Each limb bone will be handled as a possible repository of tissues bearing on what it was like to be a living tyrannosaur, down to its tiniest blood vessels.