Spindle-shaped inclusions in three-billion-year-old rocks are microfossils of plankton（浮游生物） that probably inhabited the oceans around the globe during that time, according to an international team of researchers. "It is surprising to have large, potentially complex fossils that far back," said Christopher H. House, professor of geosciences, Penn State, and lead author.

However, the researchers not only showed that these inclusions in the rocks were biological in origin, but also that they were likely planktonic autotrophs（自养生物） -- free-floating, tiny ocean organisms that produce energy from their environment.

The researchers looked at marine sediment rocks from the Farrel Quartzite in Western Australia. Isotopic analysis using secondary ion mass spectrometry was carried out at UCLA. "Ken (Kenichiro Sugitani, professor, Graduate School of Environmental Studies, Nagoya University, Japan, and a co-author) discovered these unusually shaped microfossils embedded in really old rock," said House.

To determine if these inclusions were actually biological in origin, the researchers looked at 15 different samples of Farrel Quartzite and determined their stable carbon isotope ratios. The percentage of carbon 13 in the microfossils was indicative of material produced by biological processes. They found that the carbon 13 percentage in the background organic matter in the surrounding rock was different from that of the microstructures.

"When considered along with published morphological and chemical studies, these results indicate that the Farrel Quartzite microstructures are bona fide microfossils, and support the interpretation that the spindles（主轴，纱锭） were planktonic," the researchers report in the current issue of Geology. The morphological and chemical studies were done by Sugitani and Dorothy Oehler, who is also a co-author and research scientist, Astromaterials Research and Exploration Science Directorate, NASA -- Johnson Space Center.