A new technique allowing virtual dissections of half-billion-year-old fossil embryos is producing the first three-dimensional images of the dawn of life.
It reveals a universe of detail impossible using previous methods, and researchers said it was pushing back the frontiers of science much as the scanning electron microscope did half a century ago.
"We are looking at the dawn of life," said lead researcher Phil Donoghue, a paleontologist at Bristol University in England. "Because of their tiny size and precarious preservation, embryos are the rarest of all fossils. But these fossils are the most precious of all because they contain information about the evolutionary changes that have occurred in embryos over the past 500 million years."
For the past decade or so, paleontologists have been using acid to dissolve the embryo fossils out of rocks, focusing on specimens from the age when multicellular animals began to proliferate. Tons of rock must be dissolved to retrieve a few hundred embryos, none of which are more than a fraction of a millimeter across.
In contrast to those methods, synchroton-radiation X-ray tomographic microscopy, or SRXTM, leaves the tiny fossils untouched but gives graphic details of their structure.
“It’s an interesting technique,” said Andrew Knoll, a professor of natural history at Harvard University who was not a part of the research team. “You get an improved picture of what animal life was like in its earliest days.”
Scanning with a particle accelerator
The Chinese, Swedish, Swiss and British researchers on Donoghue's team used a 1,640-foot-wide (500-meter-wide) particle accelerator in Switzerland to scan the minute fossils, and then fed the information into a computer that generated complete 3-D images of the internal structures in fine detail.
"The best analogy is with a medical CT scan ... but at two to three thousand times the resolution," Donoghue said. "We can see details less than 1,000th of a millimeter in dimension. We can look at any and every part of the fossil — inside and out — without harming it and then virtually dissect it however we like."
The method can even produce time-lapse sequences of the embryo's virtual development.
"The results are truly orgasmic," Donoghue said.
Rolling back evolutionary history
The team, who published their findings in the science journal Nature, said their discoveries could roll back the evolutionary history of arthropods like insects and spiders.
"SRXTM provides a non-invasive method of analysis of small and microscopic fossil materials ... unlocking the finest details of preserved anatomy from fossilised remains," they wrote. "The method has wide applicability in the study of microscopic structures ... and may thus bring about a revolution in paleontology on a par with that once brought about by the scanning electron microscope."
The images show that one fossil embryo known by the scientific name Markuelia must be most closely related to a modern group of marine invertebrates known as the penis worms, based on the number of teeth it has and the way they are arranged.
The creature sits very near a three-way split on the evolutionary tree that separated the unsegmented nematode worms and their segmented cousins from the gigantic arthropod phylum, which includes crustaceans, insects and spiders.
Another image shows that a segmented creature known as Pseudooides had a very unusual means of assembling itself. Modern segmented animals either develop all of their segments early and then simply get bigger, or they grow by adding segments to their hind ends.
But Pseudooides added its segments in the middle, "which is really totally bizarre," Donoghue said.
Interestingly, paleontologists have yet to find any definitive examples of larval stages among their microfossils. That suggests either that larvae are especially difficult to fossilize or that development through juvenile stages developed later in evolutionary history.
This report includes information from Reuters and The Associated Press.
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