The discovery of fossils – or "ghosts" – of plankton that lived millions of years ago is shedding new light on how they reacted to global warming events in the past.
An international team of scientists have discovered a new type of fossilisation that has remained almost entirely overlooked.
The fossils – 15 times smaller than the width of a human hair – are microscopic imprints, or "ghosts", of single-celled plankton called coccolithophores.
They lived in the seas millions of years ago, and the discovery is changing the understanding of how plankton in the oceans are affected by climate change.
Today the creatures are important in the oceans, providing much of the oxygen we breathe, supporting marine food chains, and locking carbon away in seafloor sediments.
They are a type of nannoplankton that surround their cells with intricate plates made of calcium carbonate, called coccoliths, and these are what normally fossilise in rocks.
A decrease in the number of normal coccolithophore fossils has been documented during multiple past global warming events, suggesting these plankton were severely affected by climate change.
But the new study presents records of abundant "ghost" fossils from these warming intervals, suggesting the creatures were more resilient to past climate change than was previously thought.
The fossils were discovered in rocks from three past warming events that took place during the Jurassic and Cretaceous periods, through the use of powerful microscopes.
Dr Sam Slater, from the Swedish Museum of Natural History, said: "The discovery of these beautiful ‘ghost’ fossils was completely unexpected.
"We initially found them preserved on the surfaces of fossilised pollen, and it quickly became apparent that they were extremely abundant during intervals where normal coccolithophore fossils were rare or absent – this was a total surprise."
Despite their tiny size, when coccolithophores multiply rapidly, they can become so abundant that they form blooms in the ocean that can be seen from space.
After the plankton die, their exoskeletons sink to the seafloor, accumulating in vast numbers, forming rocks such as chalk.
Professor Paul Bown, of UCL, said: "The preservation of these ‘ghost’ nannofossils is truly remarkable.
"The ‘ghost’ fossils are extremely small ‒ their length is approximately five thousandths of a millimetre, 15 times smaller than the width of a human hair ‒ but the detail of the original plates is still perfectly visible, even though the plates themselves have dissolved away, so we can easily tell which species were present."
The fossils formed while the sediments at the seafloor were being buried and turned into rock.
As more mud was gradually deposited on top, the resulting pressure squashed the coccolith plates and other organic remains together.
Professor Vivi Vajda, of the Swedish Museum of Natural History, said: "Normally, palaeontologists only search for the fossil coccoliths themselves, and if they don’t find any then they often assume that these ancient plankton communities collapsed.
"These ‘ghost’ fossils show us that sometimes the fossil record plays tricks on us and there are other ways that these calcareous nannoplankton may be preserved, which need to be taken into account when trying to understand responses to past climate change."