SYDNEY: A newly found series of deep-water fossils may push the first structural changes to complex organisms back by more than 20 million years, suggesting that seaweed-like organisms were among Earth’s first multi-cellular creatures, researchers report.
The specimens – some of which resemble modern algae and worm-like animals – were recovered from a quiet niche under the sea near what is now Lantian, a small village in Anhui Province of South China, and might provide a more complex understanding around the presence of oxygen in oceans during this period.
“The importance of the new discovery is that it represents one of the oldest assemblages of macroscopic, multicellular, and morphological complex eukaryotes,” said study co-author Shuhai Xiao from Virginia Polytechnic Institute and State University in the United
States. “There are more species here and they are more complex and larger than what evolved before.”
One of the recovered fossils from Lantian. The specimen is roughly 600 million years old and approximately 4 cm in height. It is seen to resemble modern ‘seaweed’ with a stalk and a crown of tentacle-like structures.
The diversification timeline
Prior to these findings, recently published in Nature, the oldest record of multicellular organisms was the Avalon assemblage discovered in 580 million-year-old rocks in Newfoundland, Canada.
Both sets of fossils are from the Ediacaran geological period dating from 635 to 542 million years ago, which immediately preceded the Cambrian explosion.
According to researchers, the new fossils show a comparable level of diversity and complexity falling into five morphological types that could represent as many as 15 different species.
Ice age aftermath brings diversification
The fossils were unearthed in rocks that were deposited after a major glaciation period referred to as the ‘snowball Earth event,’ which ended 635 million years ago.
This period of the Earth’s history is hypothesised to have lasted for more than 150 million years, and is characterised by repeated and extreme glaciations, in which the planet’s surface was, at various times, covered from pole to pole by a thick sheet of ice.
Xiao said the Lantian fossil discovery implies that it is plausible that the diversification of complex eukaryotes – multi-cellular organisms that are visible to the naked eye – may be related to this ice age and its aftermath, which likely included a large increase in the amount of available oxygen.
Ocean oxygenation and moving forward
A team of palaeontologists excavated the fossils, which were preserved in carbonaceous black shales, which are thin, darkly coloured sedimentary rocks rich in carbon, sulphide and organic material. Once recovered the fossils were imaged and analysed using light microscopy.
The exceptional preservation of the Lantian fossils led researchers to conclude they existed in a deep-water environment at the site, however geochemical analyses of the rocks suggest this was an environment void of oxygen.