Soft tissue in fossils still mysterious

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Three years ago, a team of scientists rocked the paleontology world by reporting that they’d recovered flexible tissue resembling blood vessels from a 68-million-year-old dinosaur fossil. Now, another group suggests that such pliable material could be something much more mundane: a modern-day film of bacterial slime.

The variety of preserved tissues described in the 2005 report is impressive: After dissolving parts of a fossilized leg bone from a Tyrannosaurus rex, scientists found blood-vessel-like tubes; dark red spheres approximately the size of red blood cells; and small, elongated structures similar to osteocytes, the most common type of bone cell. Using the same technique, the team recovered similar structures from other dinosaur fossils as well. Subsequent analyses by many of the same scientists — including Mary H. Schweitzer, a paleontologist at North CarolinaStateUniversity in Raleigh — indicated that the fossil contained small bits of collagen, a fiber-forming protein that’s the largest non-mineral component of bone.

Many of the biochemical and genetic techniques used to assess such pliable material in fossils “are very novel to most of us in paleo,” comments Matthew T. Carrano, a paleontologist at the Smithsonian Institution in Washington, D.C. “It will take a while for paleontologists to sort through the arguments,” he adds.

Now, another team’s analysis of other fossils suggests that soft tissue in fossil bones could just as likely be modern contamination or have formed by other natural processes. Rather than dissolving fossil fragments and analyzing the residue, Thomas G. Kaye, a paleontologist at the University of Washington’s Burke Museum of Natural History and Culture in Seattle, and his colleagues simply cracked open dozens of old bones to see whether soft tissue was visible.

Some of the specimens that Kaye’s team analyzed were 65 million years old and from the same rock formation as the T. rex Schweitzer and her colleagues studied. Other specimens were 30 million years old and a mere 10,000 years old. The team reports its findings July 30 in PLoS ONE.

Kaye and his colleagues suggest that the small, blood-cell-like spheres in the bones they studied are tiny enigmatic structures called framboids, named for the French word for raspberry. The team found these berry-shaped microstructures in many of their samples. Framboids are typically made of iron sulfides, but those riddling the fossils analyzed by Kaye’s team — as well as those found by Schweitzer’s team in the 68-million-year-old T. rex leg bone — were instead composed of iron oxide.

Framboids can, in some cases and over long periods, undergo a chemical transformation to iron oxide yet retain their characteristic raspberry-like structure, Kaye says.

A variety of evidence suggests that pliable material found in fossils may be biofilms of modern-day bacteria rather than ancient cells and blood vessels. Many of the fossils analyzed by Kaye and his colleagues, including specimens recently unearthed from rocks several meters deep in a quarry, contained such flexible material. Carbon-dating analyses of some samples indicate that the material is very recent, forming after 1950, Kaye says.

Recent studies, Kaye’s team reports, show that some bacteria sport a collagen-like surface protein, which might trick a biochemical test designed to detect collagen. Also, they note, the infrared absorption spectrum of a modern biofilm more closely matches that of a pliable coating found in a fossil turtle shell they analyzed than it does modern collagen.

Schweitzer and her colleagues, of course, take issue with the new findings. “There really isn’t a lot new here, although I really welcome that someone is attempting to look at and repeat the studies we conducted,” she notes.

For one thing, says Schweitzer, she and her team dismissed bacterial biofilms as a possible cause of the tissues she and her team observed. Such coatings probably would be thicker along the lower surfaces of the vascular spaces, but the flexible structures that her team recovered had walls with an even thickness. Also, she notes, there’s no reported evidence that biofilms can produce branching, hollow tubes like those noted in her study.

The material purported to be T. rex collagen in the Schweitzer study had the appropriate microscopic structure. Tests also revealed the material’s similarities, such as its ratio of glycine and alanine, to chicken collagen. These results bolster the notion that dinosaurs are related to modern birds, Schweitzer and her colleagues reported.

Furthermore, says John M. Asara, an analytical chemist at Harvard Medical School in Boston and a colleague of Schweitzer, the type of collagen found was bone-specific and isn’t a common protein contaminant.

Nevertheless, the interpretation that the soft tissue in fossils is actually modern-day biofilms “is a reasonable alternate hypothesis,” says Carrano.

The idea that modern biofilms could contaminate ancient fossils “is an interesting wake-up call,” says Larry D. Martin, a paleontologist at the University of Kansas in Lawrence. Maybe the purported soft tissue “is just a badly misinterpreted artifact,” he cautions. Solving the debate will likely require additional analyses of samples from the T. rex leg bone, he adds.