UK and Chinese researchers have linked a fossil embryo to a particular animal species for the very first time.
BBC News Online science staff
Palaeontologists sifted 6,000 kilograms (13,200 lbs) of rock excavated from southern China, yielding 100 fossil embryos dated to 500 million years ago.
The fossils belong to an extinct species of worm and scientists now know more about its development than that of its closest living relatives.
Details of the research are published in the scientific journal Nature.
Prior to this research, there were only about 15 true fossil embryos known to science.
"People have found dinosaur embryos before, but they're not real embryos, they've got bits of bone and shell.
"But these are real embryos composed entirely of layers of cells," said co-author Dr Philip Donoghue, a palaeontologist at Bristol University.
"Clearly [embryonic] mechanisms had evolved 500 million years ago."
Grain by grain
The embryos, which belong to the species Markuelia hunanensis, come from rock excavated from the Bitiao Formation in Wangcun, Hunan, southern China, and date to Middle and Lower Cambrian times.
These Cambrian worms would have lived in a deep offshore marine setting, rich in organic matter and poor in oxygen.
Markuelia is a scalidophoran, a group which still has living representatives today. These include the priapulids and the loriciferans.
To get at the fossils, the scientists dipped the rocks in acetic acid to dissolve the calcium carbonate surrounding them. The embryos themselves are composed of calcium phosphate, which is not dissolved by the acid.
The researchers then had to sift grain-by-grain through the residue left behind to identify the embryos, a process Dr Donoghue admits was laborious and time-consuming.
The fossils each measure less than half a millimetre in length.
Dr Donoghue said he now wanted to study the development of Markuelia's closest living relatives in order to determine the alterations in gene expression responsible for differences in appearance between the extinct and living worm species.
"We want to unravel their development more fully so we can determine the mechanisms behind the evolution of particular anatomical features," Dr Donoghue explained.
"Embryos from rocks of this age have been known for a few years now. But it's still very important work. It's good stuff," said Professor John Peel, director of the Museum of Evolution at the University of Uppsala, Sweden.
He added that this kind of research was key to understanding relationships between groups of organisms, because differences between phylogenetic groups are often strongly reflected in the way their embryos develop.
Dr Donoghue claims the fossils have settled a decades-old debate about the common ancestor of two animal groups: arthropods and nematodes.
Some researchers believe this ancestral creature had a smooth, or unsegmented, body.
But the embryos, which are close in time to the arthropod-nematode ancestor, are segmented.
Donoghue thinks this suggests that nematodes lost their ancestral segmentation and evolved a smooth body plan, not vice versa.