Secrets of ancient fossils revealed by Irish scientist
AN IRISH SCIENTIST has successfully recovered preserved muscle tissue from an 18 million-year-old fossilised salamander. The work raises the possibility of finding soft tissue on any fossilised bones, even dinosaurs, writes DICK AHLSTROM
The techniques could be applied to fossils of any age, says Dr Patrick Orr, of University College Dublin’s school of geological sciences. It was simply a matter of opening up the drawers and examining the fossils, but using very advanced equipment.
Orr recently published details of his work on the salamander fossils in the Proceedings of the Royal Society B, along with lead author and UCD geologist Dr Maria McNamara and collaborators at the University of Bristol and in Spain.
His approach depends on the use of scanning electron microscopes and transmission electron microscopes. These devices can produce images of almost incomprehensibly small objects, with a resolution down to millionths of a millimetre, he says.
“The project looked at the preservation of fossils in three fossilised lakes in Spain. Quiet water lake settings are one of the places where exceptionally preserved fossils occur. It increases the likelihood of either a fully articulated skeleton or soft tissue preservation,” he says.
Lake sediments are ideal for preserving dead animals. Over time the sediment becomes rock, retaining the preserved fossil animals.
But Orr was not on a fossil collecting expedition as much as a fossil shopping expedition. The extensive collection of fossils held in Spain provided all the material he needed to test his techniques.
“In the past 20 years or so we have developed a much better understanding of the processes involved in exceptional preservation,” he says. This has been matched by advances in the latest imaging technologies that allow very close scrutiny of minute biological structures that might otherwise be missed.
“It is not easy becoming a fossil unless you are bone, tooth or shell,” he says. “Your chances of getting into the fossil record are slim if you are not one of these.”
The long-held assumption was that only these fossilised. But in cases of exceptional preservation, such as when an animal is deposited in soft lake sediments, even soft tissues can become fossils.
He searched fossil collections from the former lakebeds of Ribesalbes. “Because [the animal] decays so quickly it either is not preserved at all or is preserved with very good biological detail,” he says.
Fossils first undergo meticulous examination using conventional microscopes in a minute search for possible soft-tissue preservation. If this is found they switch to scanning electron microscopes which can see objects down to a few thousandths of a millimetre.
In the past, possible soft tissue structures would be “picked off the bone” for closer scrutiny, but Orr next used another modern technique, transmission electron microscopy. This sends a beam of electrons through a thin slice of a sample to render images with a resolution down to millionths of a millimetre.
If something promising is detected with the scanning microscope a very thin cross-sectional slice, no thicker than one thousandth the width of a human hair, is cut using a diamond scalpel and the transmission electron microscope is used to produce images from this cross-section.
The method delivers fine three-dimensional images of the structures involved. In the case of the now extinct salamander, it was muscle tissues including the sheath covering these cells.
There was no doubt about what was being observed, given modern salamanders have the same types of tissues, Orr says. He has used this method to identify small blood vessels complete with what at one stage must have been blood. The blood cells themselves don’t survive but some of the vessels have a “residue” that may be what is left of them, he adds.
He has fossils of reptiles, birds, frogs and tadpoles from the lake, but there are countless fossils of creatures stretching back millions of years even to the dinosaurs. Each may deliver stories of ancient life.
“It gives you a perspective of the evolutionary history of life on earth,” Orr says. “It gives you a much more complete sample of the original biodiversity.”