London: Scientists from the Chinese Academy of Sciences, Virginia Tech in the US, and Northwest University in Xi`an, China, have discovered almost 600 million-year-old seaweed fossils in Lantian, a small village in Anhui Province of South China.
The fossils indicate that morphological diversification of macroscopic eukaryotes may have occurred only tens of millions of years after the snowball earth event that ended 635 million years ago, just before the Ediacaran Period.
They also show that despite the overall oxygen-free conditions, brief oxygenation of the oceans did occur.
"It is clearly different in terms of the number of species compared to biotas preserved in older rocks. There are more species here and they are more complex and larger than what evolved before," said Shuhai Xiao, professor of geobiology in the College of Science at Virginia Tech.
"These rocks were formed shortly after the largest ice age ever, when much of the global ocean was frozen. By 635 million years ago, the snowball earth event ended and oceans were clear of ice. Perhaps that prepared the ground for the evolution of complex eukaryotes."
The black shale rocks are known to be able to preserve fossils very well. In most cases, dead organisms were washed in and preserved in black shales. In this case, we discovered fossils that were preserved in pristine condition where they had lived - some seaweeds still rooted," Xiao said.
The team suggests that the Lantian basin was largely without oxygen but was punctuated by brief oxic episodes that were opportunistically populated by complex new life forms, which were subsequently killed and preserved when the oxygen disappeared.
"Such brief oxic intervals demand high-resolution sampling for geochemical analysis to capture the dynamic and complex nature of oxygen history in the Ediacaran Period," said lead author Xunlai Yuan, professor of palaeontology with the Chinese Academy of Sciences.
"We will need to sample each layer to see whether there is any difference in oxygen contents between layers with fossils and those without" said co-author Chuanming Zhou, professor of palaeontology with the Chinese Academy of Sciences.
The study appears in Nature.