The oldest-known animal eggs and embryos, whose first pictures made the cover of Nature in 1998, were so small they looked like bugs – which, it now appears, they may have been.From the press release:
This week, a study in the same prestigious journal presents evidence for reinterpreting the 600 million-year-old fossils from the Precambrian era as giant bacteria.
The discovery "complicates our understanding of microfossils thought to be the oldest animals," said lead author Jake Bailey.
Bailey made his discovery by combining two separate findings about Thiomargarita, the world's largest known living bacterium. In 2005, Thiomargarita discoverer Heide Schulz showed that the bacterium promotes deposition of phosphorite. As it turs out, the fossils identified as eggs and embryos in 1998 came from southern China's Doushantuo Formation, which is rich in phosphorite.
Also in 2005, University of Georgia marine biologists found that Thiomargarita can multiply by reductive cell division, a process rare among bacteria but typical of animal embryos. The fossils had been identified as embryos in part because they showed evidence of reductive cell division.
"When I put those two pieces together, I said … perhaps they're not animal embryos at all."