‘Ghost fossils’ baffle paleontologists – New form of fossil formation reveals unexpected resilience of ancient calcareous algae

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

Microfossils are important witnesses of the past. The tiny fossils can reveal, for example, when the first cells or first eukaryotic algae on our planet. Also fossils of the first terrestrial creatures in the form of mushrooms, scientists have already found.

Imprints of coccolith calcareous shells on a 183-million-year-old piece of organic material. © Slater, Bown et al. / Science

On the trail of the calcareous algae

The fossil relics of shell-forming plankton such as foraminifera or coccolithophores are of particular importance beyond paleontology. Because these often as Kalkalgen summarized marine organisms react sensitively to changes in temperature and acidity of seawater. Their fossils therefore allow conclusions to be drawn about the climate of the past and in particular the consequences of prehistoric warm phases.

“Normally, paleontologists look for the fossils of these coccoliths – and if they don’t find any, they usually assume that these ancient plankton communities must have collapsed,” explains senior author Vivi Vajda from the Swedish Museum of Natural History in Stockholm. In fact, until now it looked as if these calcareous algae had disappeared in past times of strong global warming and ocean acidification.

Discovery under the electron microscope

But a chance find refutes this – and reveals a whole new form of microfossils. 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 calcareous minerals and thus makes the organic relics more visible.

When the researchers then examined their samples under the electron microscope, they noticed something surprising: imprints of even smaller structures could be seen on the surface of the tiny pollen, spores and organic clumps – the shells of calcareous algae. Although the fossils of these organisms had long been destroyed, the shell impressions had survived for millions of years. “The discovery of these ‘ghost fossils’ was totally unexpected!” says Slater.

Even the details are perfectly preserved

Almost as surprising is the detailed preservation of these fossil imprints: “The ghost fossils are extremely small – at five micrometers in size, they are 15 times thinner than a human hair,” says co-author Paul Bown from University College London. “But the details of the
original shell plates are still perfectly visible.” You can even determine the type of coccolithophores based on the ribs, arches and seams.

Next, to find out if these ghost fossils are a phenomenon that occurred only in a short period of time and in specific locations, the scientists also examined rock samples from other geological eras, including the Early and Middle Cretaceous. They found what they were looking for there too. “We found imprint fossils during both Cretaceous warm periods,” the team reports. “This demonstrates that this nannofossil conservation is not unique to the Early Jurassic Toarcian.”

Recipe: Mud, pressure and acidic pore water

But how did these unusual fossil imprints come about? When the calcareous algae died millions of years ago and sank to the sea floor, they were gradually covered by sediment along with other remains of organisms. Where, for example, remains of calcareous algae lay close to pollen or clumps of excrement, they were pressed together by the increasing pressure in the subsoil. As a result, the hard calcareous shells of the coccolithophores pressed into the softer surfaces of their neighboring relics.

The team found that the ghost fossils cluster in layers of rock that were once formed from fine, silty sediment rich in organic matter. “This indicates that the organic material was an important prerequisite, as it provided the deformable substrate on which the imprints were imprinted,” explain Slater and his colleagues. “At the same time, this also explains why the lime from the shells was then dissolved: high levels of organic material can make the pore water acidic during diagenesis.”

Warmphase
The ghost fossils reveal that calcareous algae survived even primeval warm periods. © Slater, Bown et al. / Science

Limescale algae even survived warm phases

According to the researchers, the microprints not only reveal a new form of fossil formation, they also provide completely new insights into the history of the earth and climate. “It quickly became clear that the calcareous algae footprints were also common during periods when the normal coccolithophore fossils were rare or absent — that was a total surprise!” says Slater. “The ghost fossils show that the fossil record sometimes tricks us, and that there are other ways by which the calcareous nannoplankton can be preserved.”

Contrary to popular belief, the calcareous algae populations did not collapse during the primeval warm phases in the Jurassic and Cretaceous periods. Instead, calcareous flagellates apparently continued to thrive despite warming and acidification of the oceans of the time. “The imprints we discovered demonstrate the resilience of these nannoplankton communities during several past warm periods,” state Slater and his team.

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

Those: University College London

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