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Taken from a 70-million-year-old thighbone, the structures look like the blood vessels, cells, and proteins involved in bone formation.
Most fossils preserve an organism's hard tissues, such as shell or bone. Finding preserved soft tissue is unheard of in a dinosaur-age specimen.
"To my knowledge, preservation to this extent-where you still have original flexibility and transparency-has not been noted in dinosaurs before, so we're pretty excited by the find," said Mary H. Schweitzer, a paleontologist at North Carolina State University in Raleigh.
The findings may provide new insights into dinosaur evolution, physiology, and biochemistry. They could also increase our understanding of extinct life and change how scientists think about the fossilization process.
"Finding these tissues in dinosaurs changes the way we think about fossilization, because our theories of how fossils are preserved don't allow for this [soft-tissue preservation]," Schweitzer said.
Uncovering T. Rex
For three years scientists from the Museum of the Rockies in Bozeman, Montana, excavated the T. rex from sandstone at the base of the nearby Hell Creek formation. The dinosaur was relatively small and around 18 years old when it died.
"The dinosaur was under an incredible amount of rock," said Jack Horner, a curator of paleontology at the museum. "When it was collected, the specimen was very far away from a road, and everything had to be done by helicopter.
"The team made a plaster jacket to get part of the fossil out, and it was too big for the helicopter to lift. And so we had to take the fossil apart.
"In so doing, we had to break a thighbone in two pieces. When we did that, it allowed [Schweitzer] to get samples out of the middle of the specimen. You don't see that in most excavations, because every effort is made to keep the fossil intact," said Horner, a co-author of the study.
A certain amount of serendipity lead to the discovery.
Because the leg bone was deliberately broken in the field, no preservatives were added. As a result, the soft tissues were not contaminated.
The museum, which is a part of Montana State University, has a laboratory that specializes in cellular and molecular paleontology (the study of prehistoric life through fossil remains).
The study authors also looked at several other dinosaur fossils to see whether there was something unique about this particular T. rex fossil.
"There's nothing unique about the specimen other than the fact that it's the first that's been examined really well," Horner concluded. Other dinosaurs, in other words, are probably similarly preserved.
Soft Tissues
Schweitzer's background is in biology, and she performed a number of tests on the fossils that are common medical practices today.
The paleontologist and her colleagues removed mineral fragments from the interior of the femur by soaking it in a weak acid. The fossil dissolved, exposing a flexible, stretchy material and transparent vessels.
The vessels resemble blood vessels, cells, and the protein matrix that bodies generate when bones are being formed.
"Bone is living tissue, is very active tissue, and has its own metabolism and has to have a very good blood supply," Schweitzer said.
"So bone is infiltrated with lots and lots of blood vessels in its basic structure. When bone is formed, it's formed by cells that are specific for bone, that secrete proteins like collagen and form a matrix."
Further chemical analysis might enable the scientists to answer long-standing questions about the physiology of dinosaurs. For instance, were they warm-blooded, cold-blooded, or somewhere in between?
If protein sequences can be identified, they can be compared to those of living animals. This might allow a better understanding of how different groups of animals are related.
The find may potentially change field practices, perhaps by encouraging more scientists to reserve parts of fossils for cellular and molecular testing.