A cylinder of rock narrower than a grain of rice went into an X-ray beam outside Melbourne and came back holding an ocean.

Sitting inside it were more than 20 microscopic fossils, spread across eight species, one of them never described before. They had been in the dark for roughly 445 million years, locked in a chip of black shale drilled from the Sichuan Basin in southern China, a stretch of ancient seabed better known these days for natural gas.

Jiani Sheng and Jonathan Aitchison, an emeritus professor at the University of Queensland, published the find with their co-authors in Royal Society Open Science. The fossils are radiolarians, single-celled plankton that build ornate mineral skeletons and drift through the water inside them. The newcomer has been named Haplotaeniatum wufengensis, after the rock formation that hid it.

Why the acid never found them

Radiolarian hunting usually means dissolving the surrounding rock in hydrofluoric acid, then sieving out whatever glassy silica skeletons survive the bath.

That approach works beautifully right up until the skeletons are gone. Across hundreds of millions of years of burial and heating, the silica in these particular specimens dissolved away, leaving hollow moulds that then filled with bitumen and dolomite. Tar and mineral, in the exact shape of the organism. Drop one of these into acid and get nothing back. There is nothing solid left to dissolve free. Radiolarians, as the paper puts it, “can be abundant within organic-rich black shales yet remain invisible to conventional micropaleontology.”

That turns a preservation quirk into a counting problem. Acid digestion has been the workhorse of the field for decades. Any fossil it cannot recover has effectively been absent from the record, and the authors argue that Late Ordovician radiolarian diversity has probably been undercounted as a result. Nor is black shale a rare rock. It forms wherever organic-rich mud settles into oxygen-poor water, which happened repeatedly and on a large scale, and it turns up on every continent.

How the fossils were imaged without breaking the rock

In coverage by Phys.org, the find got compared to Pompeii, and the comparison holds. Bodies at Pompeii rotted away inside hardened ash and left voids behind, and archaeologists poured plaster into the voids to get the shapes back.

Here the pouring happened by itself, and the plaster was tar.

Reading a mould like that needs an instrument that can distinguish bitumen, dolomite and shale by density alone, in three dimensions, at a scale of micrometres. Sheng’s team used synchrotron micro-computed tomography at ANSTO’s Australian Synchrotron at Clayton, on a beamline delivered under the Project BRIGHT expansion, resolving detail finer than 1.5 micrometres. Nothing gets destroyed. That chip of shale goes back in its drawer afterwards, still full of fossils.

What plankton can say about a mass extinction

Timing is what makes this more than a taxonomic footnote.

Mud in the sample settled shortly before the peak of the Late Ordovician mass extinction, oldest of the big five, an event that eliminated roughly 85 per cent of marine species. Ancient oceans were already cooling and shedding oxygen while these particular organisms were alive. Alexandre Pohl and a large international team have spent years working out how that cooling reshaped ocean oxygen levels in the lead-up to the extinction, research summarised by the University of Ottawa.

Plankton matter to that picture because they respond fast. They sit near the base of the food web, they have no habitat to retreat into, and they register warming and deoxygenation almost immediately. Their skeletons are also durable enough to leave a long, continuous record, which is why micropalaeontologists lean on them to date rock and reconstruct old sea conditions at all. So if the standard laboratory method has been quietly blind to radiolarians in black shale everywhere on Earth, then existing counts of their Late Ordovician diversity run too low, and the baseline that recovery gets measured against needs adjusting.

One core, eight species

Restraint is in order here.

This is one paper, built on a single 2.5 millimetre core taken from a sample collected in late 2018. Eight species is a startling haul from that volume of rock, though one lucky pebble demonstrates a technique rather than rewriting a chapter of ocean history. Sheng and colleagues say as much, and want to scale the approach up to longer cores and far more of them.

There is an unexpected wrinkle in where the sample came from. Black shale gets drilled constantly, because black shale is where the gas is, and the Wufeng Formation sample exists in the first place thanks to exactly that commercial interest. Warehouses on several continents hold kilometres of core already pulled out of the ground, logged for hydrocarbon content and stacked on shelves in numbered boxes.

Nobody has to go back into the field. That archive is already sitting in storage, catalogued and indexed, waiting for someone to aim an X-ray at it.