Peering through a powerful microscope, the researchers were amazed to see the footprints left by single-celled plankton, or fossilized nannoplankton, that lived millions of years ago, especially since they were analyzing something else.
“The discovery of the ghost fossils came as a complete surprise,” said study author Sam Slater, a researcher at the Swedish Museum of Natural History in Stockholm.
“We were actually studying fossil pollen from the same rocks. I had never seen this style of fossil preservation before, and the finding was doubly surprising because the footprints were found in abundance in rocks where normal nannofossils are rare or totally absent.”
As the researchers examined the pollen under a scanning electron microscope, they saw “tiny potholes” on the surface of the pollen, Slater said. When they zoomed in to see the potholes using magnifications of thousands of times, they observed complex structures.
These structures were the imprints left by the exoskeletons of nannoplanktons called coccolithophores.
This microscopic plankton still exists today, and it supports marine food webs, provides oxygen, and stores carbon in seafloor sediments. A coccolithophore surrounds its cell with a coccolith, or hard calcareous plate, which can fossilize in rocks.
Although tiny as individuals, coccolithophores can produce cloud-like flowers in the ocean that can be seen from space. And once they die, their exoskeletons drift to rest on the seabed. As they accumulate, the exoskeletons can turn into rocks like chalk.
Ghost fossils were created when sediments on the seabed turned into rock. Layers of mud accumulated on the seabed pressed the hard plates of coccoliths together with other organic matter, such as pollen and spores. Over time, the acidic water trapped in the rocky spaces dissolved the coccoliths. All that was left was the impression in stone they had once made.
“The preservation of these ghost nannofossils is truly remarkable,” study co-author Paul Bown, professor of micropaleontology at University College London, said in a statement.
“The ghost fossils are extremely small – their length is about five thousandths of a millimeter, 15 times narrower than the width of a human hair! – but the detail of the original plates is still perfectly visible, pressed into the surfaces of the ‘former organic matter, even though the plates themselves have dissolved,’ Bown said.
Fill a gap
Previous research noted a decline in these fossils during past global warming events that impacted the oceans, leading scientists to believe that plankton were negatively affected by ocean acidification and climate change in general.
Ghost fossils tell an entirely different story, providing a record that shows coccolithophores were abundant in the ocean during three ocean warming events 94 million, 120 million and 183 million years ago, across the Jurassic and Cretaceous periods.
“Normally, paleontologists only look for the fossil coccoliths themselves, and if they don’t find any, they often assume that these ancient plankton communities have collapsed,” study co-author Vivi said. Vajda, a professor at the Swedish Museum of Natural History, in a statement. .
“These ghost fossils show us that sometimes the fossil record plays tricks on us and that there are other ways to preserve this calcareous nannoplankton, which must be considered when trying to understand responses to past climate change.”
The researchers initially focused on the Toarcian oceanic anoxic event, when volcanoes released an increase in carbon dioxide in the southern hemisphere and caused rapid global warming 183 million years ago at the start of the period of the Jurassic.
Scientists have unearthed ghost fossils in the UK, Japan, Germany and New Zealand associated with this event, as well as specimens found in Sweden and Italy linked to ocean warming 120 million years ago and 94 million years, respectively.
Understanding these ghost fossils can help researchers search for them in other gaps in the fossil record and better understand warming periods in Earth’s history.
Plankton was not only resilient in the face of rising temperatures, it actually diversified and thrived, which may not have been a good thing for other species.
Large plankton blooms are not a sign that an ecosystem is in trouble, but when a bloom dies and sinks to the sea floor, its decomposition uses up oxygen and depletes it of water, which can create areas where most species cannot survive.
“Rather than falling victim to these past warming events, our records indicate that plankton blooms contributed to the expansion of marine dead zones – regions where seabed oxygen levels were too low for most species survive,” Slater said.
“These conditions, along with expanding dead zones and plankton blooms, could become more prevalent in our globally warming oceans,” he added.
Current global warming is happening faster than these historic events, and Slater thinks this study shows that scientists need a more nuanced approach to predicting how different species will react as the global climate changes, because not all will react. not in the same way.