Paleontologists confuse ‘ghost fossils’ – new form of fossil formation reveals unexpected resilience of ancient calcareous algae

Surprising discovery: By chance, paleontologists discovered an entirely new form of fossil – tiny imprints on ancient pollen, spores, and organic agglomerates. These “ghost fossils” come from calcareous algae, whose shells were once pressed into these substrates and which have remained as amazingly detailed imprints for millions of years. These microfossils only prove that calcareous algae survived even in acidic oceans in the past interglacial periods.

Microfossils are important witnesses to the past. Small fossils can reveal, for example, when the first cells or eukaryotic algae appeared on our planet. Scientists also found fossils of the first terrestrial creatures in the form of fungi.

Traces of coccyx shells on an 183-million-year-old piece of organic matter. © Slater, Bowen et al. / Science

On the calcareous algae trail

Fossil traces of shell-forming plankton such as foraminifera or coccolithophores are of particular interest beyond paleontology. Because these marine organisms, often grouped together as calcareous algae, react sensitively to changes in the temperature and acidity of seawater. Therefore, their fossils allow to draw conclusions about the climate of the past and especially the consequences of the prehistoric warm phases.

“Palaeontologists usually look for fossils of these sparrows – and if they don’t find any, they usually assume that these ancient plankton communities have collapsed,” explains lead author Vivi Vajda of the Swedish Museum of Natural History in Stockholm. In fact, so far it looked as if this calcareous algae had vanished in past times of extreme global warming and ocean acidification.

Discovery under an electron microscope

But the chance of finding them refutes this — and reveals an entirely new form of microfossil. This was discovered by Vajda, first author Sam Slater and their colleagues when they wanted to examine 183-million-year-old rock samples from Germany, Great Britain, New Zealand and Japan for pollen and other organic matter. To do this, they first treated the samples with acid, which dissolves the lime minerals and thus makes the organic traces more visible.

When the researchers then examined their samples under an electron microscope, they noticed something surprising: traces of smaller structures could be seen on the surface of tiny pollen, spores and organic agglomerates – scales of calcareous algae. Although the fossils of these organisms were destroyed long ago, impressions of the crust have survived for millions of years. “The discovery of ‘ghost fossils’ was totally unexpected!” says Slater.

Even the details are perfectly preserved

The detailed preservation of these fossil fingerprints is almost astonishing: “The ghost fossils are very small – five micrometers in size, and they are 15 times thinner than a human hair,” says co-author Paul Bowen of University College London. But details
The original cover panels are still fully visible. “You can even determine the type of coccolithophores based on the ribs, arches, and seams.

Then, to find out if these ghost fossils were a phenomenon that only occurred at a short time in Earth’s history and in certain places, the scientists also examined rock samples from other geological eras, including the Early and Middle Cretaceous. They found what they were looking for there as well. “We found imprint fossils during both warm Cretaceous periods,” the team says. “This demonstrates that this nanofossil preservation is not unique to the early Jurassic-Taurastinian period.”

The Recipe: Clay, Squeeze, and Acidic Pore Water

But how did these unusual fossil fingerprints appear? When calcareous algae died millions of years ago and sank to the sea floor, it was gradually covered with sediment along with the remains of other living things. For example, where there are remnants of calcareous algae near pollen or clumps of droppings, they are compressed together by the increasing pressure in the ground. As a result, the hard calcareous shells of the calcareous grains pressed against the softer surfaces of the traces adjacent to them.

The team found that the ghost fossils gather in layers of rock that were once formed from fine, organic-rich clay deposits. “This indicated that the organic matter was an important prerequisite, because it provided the deformable substrate on which the fingerprints were imprinted,” explain Slater and colleagues. “At the same time, this also explains why lime from the peel is then dissolved: High levels of organic matter can make pore water acidic during formation.”

warm phase
Ghost fossils reveal that calcareous algae survived even primitive warm periods. © Slater, Bowen et al. / Science

Limescale algae survived even the warm phases

According to the researchers, microprints not only reveal a new form of fossil formation, but also provide entirely new insights into Earth’s history and climate. “It soon became apparent that calcareous moss footprints were also common during periods when natural coccolithophores were scarce or absent—which was a complete surprise!” “Ghost fossils show that the fossil record sometimes deceives us, and that there are other ways in which calcareous nanoplankton can be preserved.”

Contrary to popular belief, calcareous algae populations did not collapse during the primitive warming phases of the Jurassic and Cretaceous periods. Instead, the calcareous flagella apparently continued to thrive even though the oceans were warming and acidifying at the time. “The fingerprints we discovered show the resilience of these nanoplankton communities during several previous warm periods,” Slater and his team say.

This could mean that calcareous algae are more adaptable to changes in ocean conditions than previously thought – and this may also be true in current climate change. “These fossils are superseding our understanding of how calcareous nanoplankton respond to warming events,” says co-author Richard Twitchett of the Natural History Museum in London. (Science, 2022; doi: 10.1126/science.abm7330)

Source: University College London

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