Iron Preserves Ancient Dinosaur Soft Tissue in Fossils

A 2005 claim that scientists had found a 68-million-year-old soft-tissue sample preserved in a Tyrannosaurus rex fossil proved controversial when the research was published because exactly how the tissue remained preserved for eons was not understood. But new follow-up research attempts to put some of that controversy to bed by explaining that iron may play a role in preserving ancient tissues within dinosaur fossils before it can decay.

Mary Schweitzer, a North Carolina State University molecular paleontologist, was behind the 2005 research and its subsequent follow-ups, including the latest research published in the Proceedings of the Royal Society B, which reports that the presence of hemoglobin -- the iron-containing molecule that transports oxygen in red blood cells -- may play a duel role in both preserving and concealing ancient proteins within fossils.

"Iron is necessary for survival, but it's also highly reactive and destructive in living tissues, which is why our bodies have proteins that transport iron molecules to where they are needed but protect us from unwanted reactions at the same time," Schweitzer said in a statement. "When we die, that protective mechanism breaks down and the iron is turned loose on our tissues -- and that destructive process can act in much the same way formaldehyde does to preserve the tissues and proteins."

Because both birds and crocodiles -- the closest living relatives of the dinosaurs -- both have large, nucleated red blood cells with more hemoglobin than common mammals, Schweitzer hypothesizes that dinosaurs' red blood cells have a similarly large proportion hemoglobin, which could amplify its preserving effect on tissue.

The soft, transparent T. rex tissue sample, Schweitzer and her colleagues concluded after several studies, was a collagen. The team tested other fossils dating back to the Jurassic Period 145.5 million to 199.6 million years ago, finding the presence of similar soft tissues in about half of the specimens examined.

"The problem is, for 300 years, we thought, 'Well, the organics are all gone, so why should we look for something that's not going to be there?' and nobody looks," Schweitzer told LiveScience.

But even if anyone had thought to look for soft tissue remaining in fossils nearly 200 million years old, they may not have found it, as the iron from the hemoglobin does just more than just preserve the tissue, it seems to mask it as well.

By removing the iron from the soft tissue samples from the T. rex fossil and one of a Brachyolophosaurus, Schweitzer and her team observed that the iron-less tissue reacted much more strongly to antibodies that detect the presence of protein, which suggests that the iron may be masking the protein's presence in the tissues.

The researchers tested their theory by taking blood vessels and cells from a modern ostrich bone and placing some in a solution of hemoglobin taken from red blood cells and others in water. The sample in water degraded in less than a week, while the sample placed in hemoglobin remained intact for at least two years.

"We know that iron is always present in large quantities when we find well-preserved fossils, and we have found original vascular tissues within the bones of these animals, which would be a very hemoglobin-rich environment after they died," Schweitzer said. "We also know that iron hinders just about every technique we have to detect proteins. So iron looks like it may be both the mechanism for preservation and the reason why we've had problems finding and analyzing proteins that are preserved."