The same features have emerged, and they are virtually indistinguishable from tissue samples from modern species.
A 300,000-year-old wooly mammoth fossil, for example, yielded flexible vessels containing what seem to be red blood cells that lack nuclei, like those of modern mammals.
The dinosaur remains include blood cell-like structures containing nuclei, like those of birds today.
To demonstrate, Schweitzer showed two microscope-generated photographs side by side.
"One of these cells is 65 million years old, and one is about 9 months old. Can anyone tell me which is which?"
The mystery of these structures won't be solved completely until scientists understand how the tissues were preserved.
Schweitzer said a central focus of her research is to explain this phenomenon, which was once thought to be impossible.
New findingsnot yet publishedhave led her to suggest one possible explanation. The key, she believes, may be the iron content of the blood and muscle proteins hemoglobin and myoglobin.
After an organism dies, iron released from these proteins as they degrade may trigger the formation of highly reactive forms of oxygen known as free radicals. Other heavy metals in the environment may produce the same effect.
Schweitzer thinks these metal-generated free radicals may trigger the formation of longer molecular chains, known as polymers, which essentially bind and lock remaining cellular structures in place.
"Eventually, the polymerized remains become inert, free from attack from the outside and further chemical change," Schweitzer said.
The researchers are now trying to obtain a pure sample of the blood cell-like structures. If successful, Schweitzer hopes to apply a technique known as Raman spectroscopy to search for the presence of hemoglobin.
In addition to testing her preservation theory, this analysis will help determine if identifiable protein fragments from the ancient animal are still present in the tissues. It's possibe, Schweitzer says, that some unknown form of geochemical replacement preserved the tissue structure but changed its molecular composition.
Like Schweitzer, Michigan State University zoologist Peggy Ostrom, who also spoke at the St. Louis meeting, is trying to tease out the molecular identity of ancient remains.
Jurassic Park notwithstanding, Ostrom believes that prehistoric proteinsnot DNAoffer the greatest potential for recapturing pieces of the biological past.
Protein chains are shorter and far more stable than DNA, and their study is less fraught with risk of bad data due to contamination.
In a sense, the ancient protein molecules Ostrom works with are very much like the hard fossils traditionally studied by paleontologists.
"They have form and they have function," she said. "And they persist over time."
Ostrom reported the recovery of a complete protein sequence from a 42,000-year-old fossil horse discovered in a cave in Wyoming.
She recovered not only the beadlike string of amino acids but part of its functionally important, three-dimensional structure.
"We're working our way back in time," said Ostrom, referring both to her own work and the broader, emerging field of paleoproteomics, the study of ancient proteins.
So far, Ostrom's molecular time travel has taken her back 500,000 yearsthe age of musk ox bones from which she was able to excavate fragments of a protein known as osteocalcin.
For the first time, Ostrom said, "we can actually look at the real molecules that existed half a million years ago."
Such a view can open new understandings in a number of areas, from evolutionary histories to ancient food-web relationships.
Ultimately, scientists hope that more refined versions of Ostrom's biochemical techniques can help unlock the secrets of dinosaur soft tissues.
While a vast gulf of time still separates Ostrom's oldest protein sequences from Schweitzer's T. rex remains, the two researchers appear to be on convergent paths.
"Peggy's techniques are definitely applicable to our stuff," Schweitzer said.
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