About 115 million years ago, when car-size pterosaurs flew overhead and long-necked sauropods tromped about on Earth, a tiny mushroom no taller than a chess piece fell into a river and later fossilized — a feat that makes it the oldest-known fossilized mushroom on record, a new study finds.
Researchers discovered the remains of the Cretaceous-age mushroom preserved in limestone from northeast Brazil's Crato Formation. But during its brief life, the mushroom lived on Gondwana, a supercontinent that once existed in the Southern Hemisphere.
"Most mushrooms grow and are gone within a few days," study lead researcher Sam Heads, a paleontologist at Illinois Natural History Survey (INHS), said in a statement. "The fact that this mushroom was preserved at all is just astonishing.
After the mushroom fell into the river, it floated into a salty lagoon and sank to the bottom, where fine sediments began to cover it. Over time, the mushroom mineralized, and its tissues were replaced with pyrite, a mineral also known as fool's gold. Later, the pyrite transformed into the mineral goethite, the researchers said.
"When you think about it, the chances of this thing being here — the hurdles it had to overcome to get from where it was growing into the lagoon, be mineralized and preserved for 115 million years — have to be minuscule," said Heads, who found the mushroom while digitizing a collection of fossils from the Crato Formation.
Researchers found the mushroom fossil in northeast Brazil's Araripe Basin, in a layer of limestone known as the Crato Formation.
An electron microscope image revealed the mushroom had gills under its cap, instead of pores or spines (also called teeth). These gills, which release spores, helped the researchers place the mushroom in a scientific order of gilled mushrooms called Agaricales, they said.
Until now, the oldest fungi on record were 99-million-year-old specimens (Palaeoagaricites antiquus) trapped in amber from Burma (also known as Myanmar), said study co-author Andrew Miller, an INHS mycologist (someone who studies fungi).
"They were enveloped by a sticky tree resin and preserved as the resin fossilized, forming amber," Heads said. "This is a much more likely scenario for the preservation of a mushroom, since resin falling from a tree directly onto the forest floor could readily preserve specimens. This certainly seems to have been the case, given the mushroom fossil record to date."
In fact, there are only 10 fossils resembling modern-day gilled mushrooms on record, and all of them are preserved in amber, the researchers said. These include four unnamed mushrooms from Burmese amber, a 94-million-year-old mushroom (Archaeomarasmius leggetti) from New Jersey, a 45-million-year-old mushroom (Gerontomyces lepidotus) from the Samland Peninsula of Russia, three mushrooms (Aureofungus yaniguaensis, Coprinites dominicana and Protomycena electra) from the Dominican Republic that date to between 16 million and 18 million years ago, and the Myanmar mushroom.
The mushroom sprouted during the early Cretaceous period, when the supercontinent Gondwana was breaking apart.
Credit: Danielle Ruffatto
"Fungi evolved before land plants and are responsible for the transition of plants from an aquatic to a terrestrial environment," Miller said. "Associations formed between the fungal hyphae [tubes] and plant roots. The fungi shuttled water and nutrients to the plants, which enabled land plants to adapt to a dry, nutrient-poor soil, and the plants fed sugars to the fungi through photosynthesis.
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