Is Dinosaur ‘Soft Tissue’ Really Slime?

Science, August 1, 2008

In 2005, researchers made headlines when they reported that they had found intact blood vessels from a 68-million-year-old Tyrannosaurus rex. The discovery raised hopes that paleontologists could get their hands on the flesh and blood of vanished animals. This week, however, other scientists challenged the results, arguing that the dinosaur flesh was in fact just coatings of young bacteria. But the original researchers stand by their results, calling the new argument weak. “There really isn’t a lot new here,” says Mary Schweitzer of North Carolina State University in Raleigh.

In 2003, a crew led by Jack Horner of the Museum of the Rockies in Bozeman, Montana, dug up an exquisitely preserved T. rex fossil. Schweitzer dissolved a fragment in weak acid. With the bone gone, transparent vessels were left behind (Science, 25 March 2005, p. 1852). Other fossils yielded branched tubes, spheres that resembled blood cells, and what appeared to be bone-forming cells known as osteocytes. Later, Schweitzer and colleagues isolated what they identified as collagen proteins from the T. rex and from a mastodon fossil. The sequence of amino acids in the mastodon collagen was closest to that of elephants; the T. rex collagen was most similar to that of birds, its closest living relatives.

These results inspired Thomas Kaye, a research associate at the Burke Museum of Natural History and Culture in Seattle, Washington, to look for soft tissue. He set out to use scanning electron microscopes to find it without having to dissolve the fossils first. “I thought, ‘We’ll just crack the bones open and take a look,’” he says.

The researchers selected a well-preserved turtle fossil and quickly found cell-like balls that turned out to be located in tubes. “We actually did a happy dance,” he said.

But the happiness turned to suspicion when Kaye kept finding the spheres in other fossils, even in badly degraded ones. “It was clear they weren’t blood cells,” he says. He and his colleagues suspect that the balls are geological formations called framboids.

Schweitzer’s tubes and osteocytes, they argue, are not blood vessels or cells but biofilms formed by bacteria that invaded the fossils after death. In a paper published Monday in the journal PLoS ONE, Kaye and colleagues report that carbon dating of one sample shows that the tubes are at most a few decades old and that their infrared spectra give a closer match to bacterial biofilms than to collagen. Troughs in the walls of the tubes resemble the track a microbe would make crawling through a biofilm, they note. “We think that’s one of the smoking guns,” Kaye says.

“This piece of work demonstrates just how careful we have to be when attempting to analyze fossil bones for traces of original molecules or biomarker molecules,” says David Martill of the University of Portsmouth, U.K., a paleobiologist not involved in either study. Other researchers are less impressed. “There are a number of misinterpretations and lack of basic data in this paper,” says Frances Westall, the director of the Centre de Biophysique Moléculaire-CNRS in Orléans, France.

Schweitzer says she welcomes skepticism but that Kaye and his team “only address aspects of our study that fit conveniently with their preconceived ideas.” They did not explain how proteins from a bacterial biofilm could be similar to bird or elephant proteins, for example. “They pick and choose what to focus on,” Schweitzer says, arguing that a rebuttal of her work must account for all of her evidence. She also doubts that bacteria could have formed the tubes.

Martill, however, thinks that the tubes might well be biofilms. He says his own experience shows how aggressively bacteria and fungi can invade fossils. Still, he considers Schweitzer’s research “incredibly impressive” and believes that at least some of the dinosaur material is genuine collagen. “This is why we should not abandon hope,” he says.