How did scientists find soft tissue in dinosaur fossils?

These pieces of petrified wood, found in Petrified Forest National Park, used to be trees. Now they're rocks -- good for paperweights but bad for paper. See more dinosaur pictures.
iStockphoto/Tom Marvin

­­­Fossilization is a real-world Medusa -- it turns living things to stone. Take petrified wood as an example. If a tree dies and is buried in sediment, the sediment protects the wood while mineral-laden groundwater seeps through it. The minerals gradually r­eplace the wood, leaving a rocky facsimile behind.

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The same thing happens when more complex organisms, like animals, become fossils. Anything that's stretchy, squishy, watery or rubbery decays while minerals reinforce the bones, turning them to stone. Even in the case of "mummified" fossil finds, the preserved skin and other organs aren't soft anymore. In fossils like these, the body was protected from decay long enough for minerals to replace the soft tissues faster than they could decay. The soft tissues themselves are completely gone, and only stone remains.

­At least, that's the conventional wisdom. But in 2005, a paper appeared in the journal Science that challenged the basic principles of fossilization from­ its very first sentence: "Soft tissues are preserved within hindlimb elements of Tyrannosaurus rex" [source: Schweitzer, 3/25/2005]. The paper goes on to describe blood vessels, bone matrix and elastic tissues, all found somewhere they shouldn't be.

According to the long-held view of fossilization, the presence of those types of tissue in a fossil is impossible. The paper's primary author, Mary Higby Schweitzer, had come to this unconventional conclusion by approaching her research in an unconventional way. In addition to sparking controversy in the scientific community, her find had implications for some religious communities.

So how did she find stretchy tissue in a rocky bone, and what do ostriches have to do with it? What kind of analysis supports her assertion that she's found 65-million-year-old soft tissue? Next, we'll take a look at how Schweitzer unveiled her samples and what happenstance had to do with this groundbreaking find.


The Soft Tissue in Dinosaur Bone

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

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.


Young-Earth Creationism and Dinosaur Tissues

Millions of years is plenty of time for fossil specimens to be contaminated by everything from animals to fungi.
Millions of years is plenty of time for fossil specimens to be contaminated by everything from animals to fungi.
Philip and Karen Smith/Stone/Getty Images

Scientists agree on one aspect of Mary Schweitzer's research. The tissues she found shouldn't have been there, at least according to basic concepts of fossilization. Because of this, critics assert that what Schweitzer really found was a contaminated sample, not a breakthrough. Over 65 million years, there's plenty of time for other life forms to contaminate the bones of a dinosaur. Fossils also come into contact with human and other tissues during excavation. This presents a challenge for researchers trying to prove that a cell, tissue sample or DNA strand came from a specific extinct animal.

After Schweitzer's first paper appeared in Science, some critics suggested that she published it before conducting enough analysis. Schweitzer agreed with this claim at least in part. She explained that the team published its findings as step to securing funding for later work [source: Yeoman].

A response to Schweitzer's 2007 paper -- the one reporting the presence of protein -- points out several questions about the findings, including the likelihood of contamination. The comment, written by Mike Buckley and an array of co-authors, notes:

  • The likelihood of collagen breakdown
  • Tests that should have been performed but were not
  • The inability to perform standard analyses on fragmented peptide sequences [source: Buckley et al.].

In their response to the comment, John M. Asara and Schweitzer assert that Buckley and his co-authors misinterpreted the data [source: Asara].

Schweitzer's 2008 paper describing protein sequences adds some weight to the idea that the tissue belonged to the T. rex and not an unrelated contaminant. But some critics remain unconvinced. For example, researcher Christina Nielsen-Marsh was quoted in by National Geographic as saying that the sequences described "make no sense at all" [source: Norris]. In the minds of many, the presence of peptides in a specimen as old as a T. rex is impossible. This means the only option is that the protein came from another source.

In an article published in the journal PLoS One on July 20, 2008, researchers Thomas G. Kaye, Gary Gaugler and Zbigniew Sawlowicz argue just that. This team conducted more than 200 hours of scanning electron microscope analysis on a variety of dinosaur fossils. It came to the conclusion that Schweitzer's samples contained framboids, and the apparent soft tissue was essentially pond scum. Through carbon dating, the team also determined that the material was modern, not prehistoric [source: Kaye et al.]. In statements made to National Geographic, Schweitzer stood by her findings, noting, among other things, that Kaye's team did not address more recent protein studies of her T. rex samples [source: Roach].

But to another group, Schweitzer's findings make perfect sense. In the view of young-Earth creationists, soft tissue is proof that fossils aren't as old as scientists report. After all, according to scientific estimates, T. rex fossils are 65 million years old. Soft tissue and amino acids should last only a fraction of that time. Someone who believes the Earth is less than 10,000 years old may see Schweitzer's find as compelling evidence for a young Earth rather than a cause to re-examine the nature of fossilization. However, analysis using radiometric dating -- the method scientists use to determine the age of fossils -- conflicts with the idea of a 10,000-year-old Earth.

In interviews, Schweitzer has commented that her discoveries have enriched rather than conflicted with her Christian faith [source: Yeoman, Fields]. Schweitzer offers hypotheses for how the tissue could have survived so long. One is that the densely mineralized bone, combined with as-yet-undiscovered geological or environmental processes, protected the structures within [source: Schweitzer, 3/25/2005]. And regardless of whether the paleontological community eventually embraces or refutes the tissue in question, the find does seem to make ideas that used to be impossible merely improbable.

To find out more about fossils, paleontological disagreements, unexpected discoveries and related topics, dig your way through the links on the next page.

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  • Asara, John and Mary H. Schweitzer."Response to Comment on 'Protein Sequences from Mastadon and Tyrannosaurus rex Revealed by Mass Spectrometry'" Science. Vol. 319. 1/4/2008.
  • Boswell, Evelyn. "Eastern Montana dinosaur yields new protein that's 68 million years old." Montana State University. 4/12/2007 (8/24/2008)
  • Buckley, Mike et al."Comment on 'Protein Sequences from Mastadon and Tyrannosaurus Rex Revealed by Mass Spectrometry'" Science. Vol. 319. 1/4/2008.
  • Fields, Helen. "Dinosaur Shocker." Smithsonian. May 2006 (8/24/2008)
  • Johnson, Brian."Spectrosaurus Rex." Spectroscopy. Vol. 22, no. 12. December 2007.
  • Kaye, Thomas G. et al. “Dinosaurian Soft Tissues Interpreted as Bacterial Biofilms.” PLoS One. Vol. 3, no. 7. 7/30/2008 (8/4/2008)
  • Norris, Scott. "Dinosaur Soft Tissues Sequenced; Similar to Chicken Proteins." National Geographic News. 4/12/2007 (8/24/2008)
  • Organ, Chris al. "Molecular Phylogenetics of Mastadon and Tyrannosaurus Rex Revealed by Mass Spectrometry." Science. Vol. 320. 4/25/2008.
  • Peake, Tracey. "Protein Sequences from T. rex Collagen Show Evolutionary Relationships of Dinosaurs." NCSU. 4/24/2008 (8/24/2008)
  • Roach, John. “Dinosaur Slime Sparks Debate over Soft-tissue Finds.” National Geographic News. 7/30/3008 (8/4/2008)
  • Schweitzer,Mary and Tracy Staedter. "The Real Jurassic Park." Earth. Vol. 6, no. 3. 6/1997.
  • Schweitzer,Mary H. et al. "Gender-specific Reproductive Tissue in Ratites and Tyrannosaurus Rex." Science. Vol. 308. 6/3/2005.
  • Schweitzer,Mary H. et al. "Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex. Science. Vol. 307. 3/25/2005.
  • Yeoman, Barry. "Schweitzer's Dangerous Discovery." Discover. 4/27/2006 (8/24/2008)