Hidden fossils reveal secrets of oceans before major mass extinction

A tiny pellet of ancient rock, a mere half the size of a grain of rice, has yielded 20 microscopic fossils representing eight different species, including one that is entirely new to science. The discovery will enhance our understanding of the second-largest known mass extinction. It also shows how new analytical techniques are unlocking parts of the fossil record that have previously gone overlooked.

Jonathan Aitchison at the University of Queensland, Australia, and his colleagues extracted the pellet from a rock that was collected in late 2018 from the Sichuan basin in China, about 300 kilometres south of Xian. The rock is445 million years old, which means it formed just before the Late Ordovician mass extinction – the second most severe to have occurred over the past 500 million years.

Inside the pellet, they found eight different species of radiolarians, which are single-celled plankton that make their shells from silica. Radiolarians are still found throughout the oceans today.

The fossils found in the grain-sized sample represent five genera, four families and three orders, including a new species that the researchers have named Haplotaeniatum wufengensis.

The specimens were so well preserved because both their exteriors and internal structures were completely surrounded by and filled in with bitumen, leaving perfect impressions.

Patrick Smith at the Geological Survey of New South Wales in Sydney, Australia, who was not part of the research, says the fossils come from a period before the extinction event was fully under way.

“The high number and diversity of fossils show that marine ecosystems, particularly microscopic plankton communities, were rich and active shortly before the extinction,” says Smith. “The Ordovician oceans were far richer biologically than previously recognised, especially at the microscopic level. These fossils reveal thriving communities of plankton at a time when Earth’s oceans were on the precipice of major environmental change.”

Traditionally, such tiny fossils are studied by dissolving the surrounding rocks with acid – an incredibly destructive method, says Aitchison.

Instead, the researchers used a powerful X-ray machine – the Australian Nuclear Science and Technology Organisation’s Synchrotron, located in Melbourne – to scan the rock pellet and, within seconds, generate detailed 3D scans of the fossils it contained.

“I grew up looking at Mad comics, and there were always advertisements in the back for X-ray glasses where you could see through things,” says Aitchison. “Well, we could see right through this sample. We didn’t even have to get them out of the rock. We could look right through the rock and see these radiolarian plankton.”

“This is the biggest technological advance I’ve ever encountered during my whole career,” he says.

Aitchison adds that the richness of life found in such a small sample suggests that the diversity of marine life in other rocks from the Late Ordovician might have been “grossly underestimated”.

Smith says one of the key messages from the work is that there is still a great deal of Earth’s fossils to explore – not because they are missing “but because our traditional methods haven’t been able to detect or recover them”.