Demineralized fragments of tissues lining the marrow cavity of Tyrannosaurus rex femur.

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The Soft Tissue in Dinosaur Bone

In 2000, paleontologist Bob Harmon found a Tyrannosaurus rex specimen in the Hell Creek Formation, an area of eastern Montana full of Cretaceous fossils [source: Boswell]. This T. rex wasn't very big, at least as far as Tyrannosaurus fossils go. But once it was excavated and wrapped in plaster for shipping, it was too heavy for the waiting helicopter to carry. The team split the fossil in two, breaking one of its femurs in the process. Fragments of the femur made their way to Dr. Mary Schweitzer at North Carolina State University.

Schweitzer did the opposite of what most paleontologists do with their specimens. Instead of preserving and protecting it, she destroyed it by soaking it in a weak acid. If the entire fossil had been made of rock, it would have dissolved completely. But in the terms used in Schweitzer's paper -- co-authored by Jennifer L. Whittmeyer, John R. Horner and Jan K. Toporski -- the acid demineralized the specimen. After seven days, the demineralization process revealed several unexpected tissues, including:

  • Blood vessels
  • Bone matrix
  • Small objects that appeared to be osteocytes, the cells that build bone

Just like the blood vessels in your body, the ones Schweitzer discovered in the fossil were hollow, flexible and branched. They were also transparent and full of "small round microstructures" [source: Schweitzer, 3/25/2005]. These microstructures visually resembled red blood cells, but their precise nature is still unclear. The tissue Schweitzer found was fibrous, stretchy and resilient --after being stretched, it returned to its normal shape.

Because the prevailing scientific theory links dinosaurs and birds from an evolutionary standpoint, Schweitzer and her team compared their samples to the bones of a dead ostrich. They found the samples to be similar. When viewed with a scanning electron microscope, the dinosaur's cortical bone -- the dense part of the bone -- was almost indistinguishable from the ostrich's.

These aren't the only discoveries to have come from these particular fragments of T. rex bone. In a later paper, Schweitzer and her co-authors announced that they had found medullary bone [source: Schweitzer, 6/3/2005]. Medullary bone is a type of bone female birds use to store calcium for making eggshells. Birds have this bone only when producing eggs -- so the T. rex was apparently female, pregnant and in some ways like a bird.

Samples of medullary bone: from left to right, T. rex, emu and ostrich

Courtesy NCSU

The medullary bone was visible to the naked eye, but a later discovery from the sample wasn't. In 2007, Schweitzer and six co-authors announced that analysis of the sample had revealed presence of collagen, a protein that's a major component of bones and soft tissue. The team used a mass spectrometer, a device that analyzes the mass of atoms and particles with magnetic fields, to confirm the protein's presence [source: Schweitzer, 1997].

By 2008, the team was isolating amino acid sequences from the sample and comparing them to living organisms. What they found in the T. rex bone was similar to today's chickens. The researchers, this time led by Chris L. Organ, used the same techniques on a mastodon fossil and found it similar to today's elephants [source: Organ].

After isolating some of the fragmented amino acid chains, the team compared them to the amino acids of living animals. They found three chains that were similar to those found in chickens and two that were comparable to newts and frogs [source: Johnson].

Schweitzer and her colleagues have also gone on to try the same techniques on other fossils, with similar results. But her work is still controversial. Read on to learn about critics' and supporters' reactions to her research.

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